1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 #![allow(non_snake_case)]
13 register_long_diagnostics! {
16 A pattern used to match against an enum variant must provide a sub-pattern for
17 each field of the enum variant. This error indicates that a pattern attempted to
18 extract an incorrect number of fields from a variant.
22 Apple(String, String),
27 Here the `Apple` variant has two fields, and should be matched against like so:
31 Apple(String, String),
35 let x = Fruit::Apple(String::new(), String::new());
39 Fruit::Apple(a, b) => {},
44 Matching with the wrong number of fields has no sensible interpretation:
48 Apple(String, String),
52 let x = Fruit::Apple(String::new(), String::new());
56 Fruit::Apple(a) => {},
57 Fruit::Apple(a, b, c) => {},
61 Check how many fields the enum was declared with and ensure that your pattern
66 Each field of a struct can only be bound once in a pattern. Erroneous code
76 let x = Foo { a:1, b:2 };
78 let Foo { a: x, a: y } = x;
79 // error: field `a` bound multiple times in the pattern
83 Each occurrence of a field name binds the value of that field, so to fix this
84 error you will have to remove or alter the duplicate uses of the field name.
85 Perhaps you misspelled another field name? Example:
94 let x = Foo { a:1, b:2 };
96 let Foo { a: x, b: y } = x; // ok!
102 This error indicates that a struct pattern attempted to extract a non-existent
103 field from a struct. Struct fields are identified by the name used before the
104 colon `:` so struct patterns should resemble the declaration of the struct type
114 let thing = Thing { x: 1, y: 2 };
117 Thing { x: xfield, y: yfield } => {}
121 If you are using shorthand field patterns but want to refer to the struct field
122 by a different name, you should rename it explicitly.
126 ```compile_fail,E0026
132 let thing = Thing { x: 0, y: 0 };
147 let thing = Thing { x: 0, y: 0 };
150 Thing { x, y: z } => {}
156 This error indicates that a pattern for a struct fails to specify a sub-pattern
157 for every one of the struct's fields. Ensure that each field from the struct's
158 definition is mentioned in the pattern, or use `..` to ignore unwanted fields.
162 ```compile_fail,E0027
168 let d = Dog { name: "Rusty".to_string(), age: 8 };
170 // This is incorrect.
176 This is correct (explicit):
184 let d = Dog { name: "Rusty".to_string(), age: 8 };
187 Dog { name: ref n, age: x } => {}
190 // This is also correct (ignore unused fields).
192 Dog { age: x, .. } => {}
198 In a match expression, only numbers and characters can be matched against a
199 range. This is because the compiler checks that the range is non-empty at
200 compile-time, and is unable to evaluate arbitrary comparison functions. If you
201 want to capture values of an orderable type between two end-points, you can use
204 ```compile_fail,E0029
205 let string = "salutations !";
207 // The ordering relation for strings can't be evaluated at compile time,
208 // so this doesn't work:
210 "hello" ... "world" => {}
214 // This is a more general version, using a guard:
216 s if s >= "hello" && s <= "world" => {}
223 This error indicates that a pointer to a trait type cannot be implicitly
224 dereferenced by a pattern. Every trait defines a type, but because the
225 size of trait implementors isn't fixed, this type has no compile-time size.
226 Therefore, all accesses to trait types must be through pointers. If you
227 encounter this error you should try to avoid dereferencing the pointer.
229 ```compile_fail,E0033
230 # trait SomeTrait { fn method_one(&self){} fn method_two(&self){} }
231 # impl<T> SomeTrait for T {}
232 let trait_obj: &SomeTrait = &"some_value";
234 // This tries to implicitly dereference to create an unsized local variable.
235 let &invalid = trait_obj;
237 // You can call methods without binding to the value being pointed at.
238 trait_obj.method_one();
239 trait_obj.method_two();
242 You can read more about trait objects in the [Trait Objects] section of the
245 [Trait Objects]: https://doc.rust-lang.org/reference/types.html#trait-objects
249 The compiler doesn't know what method to call because more than one method
250 has the same prototype. Erroneous code example:
252 ```compile_fail,E0034
263 impl Trait1 for Test { fn foo() {} }
264 impl Trait2 for Test { fn foo() {} }
267 Test::foo() // error, which foo() to call?
271 To avoid this error, you have to keep only one of them and remove the others.
272 So let's take our example and fix it:
281 impl Trait1 for Test { fn foo() {} }
284 Test::foo() // and now that's good!
288 However, a better solution would be using fully explicit naming of type and
302 impl Trait1 for Test { fn foo() {} }
303 impl Trait2 for Test { fn foo() {} }
306 <Test as Trait1>::foo()
323 impl F for X { fn m(&self) { println!("I am F"); } }
324 impl G for X { fn m(&self) { println!("I am G"); } }
329 F::m(&f); // it displays "I am F"
330 G::m(&f); // it displays "I am G"
336 It is not allowed to manually call destructors in Rust. It is also not
337 necessary to do this since `drop` is called automatically whenever a value goes
340 Here's an example of this error:
342 ```compile_fail,E0040
354 let mut x = Foo { x: -7 };
355 x.drop(); // error: explicit use of destructor method
361 You can't use type parameters on foreign items. Example of erroneous code:
363 ```compile_fail,E0044
364 extern { fn some_func<T>(x: T); }
367 To fix this, replace the type parameter with the specializations that you
371 extern { fn some_func_i32(x: i32); }
372 extern { fn some_func_i64(x: i64); }
377 Rust only supports variadic parameters for interoperability with C code in its
378 FFI. As such, variadic parameters can only be used with functions which are
379 using the C ABI. Examples of erroneous code:
382 #![feature(unboxed_closures)]
384 extern "rust-call" { fn foo(x: u8, ...); }
388 fn foo(x: u8, ...) {}
391 To fix such code, put them in an extern "C" block:
401 Items are missing in a trait implementation. Erroneous code example:
403 ```compile_fail,E0046
411 // error: not all trait items implemented, missing: `foo`
414 When trying to make some type implement a trait `Foo`, you must, at minimum,
415 provide implementations for all of `Foo`'s required methods (meaning the
416 methods that do not have default implementations), as well as any required
417 trait items like associated types or constants. Example:
433 This error indicates that an attempted implementation of a trait method
434 has the wrong number of type parameters.
436 For example, the trait below has a method `foo` with a type parameter `T`,
437 but the implementation of `foo` for the type `Bar` is missing this parameter:
439 ```compile_fail,E0049
441 fn foo<T: Default>(x: T) -> Self;
446 // error: method `foo` has 0 type parameters but its trait declaration has 1
449 fn foo(x: bool) -> Self { Bar }
455 This error indicates that an attempted implementation of a trait method
456 has the wrong number of function parameters.
458 For example, the trait below has a method `foo` with two function parameters
459 (`&self` and `u8`), but the implementation of `foo` for the type `Bar` omits
462 ```compile_fail,E0050
464 fn foo(&self, x: u8) -> bool;
469 // error: method `foo` has 1 parameter but the declaration in trait `Foo::foo`
472 fn foo(&self) -> bool { true }
478 The parameters of any trait method must match between a trait implementation
479 and the trait definition.
481 Here are a couple examples of this error:
483 ```compile_fail,E0053
492 // error, expected u16, found i16
495 // error, types differ in mutability
496 fn bar(&mut self) { }
502 It is not allowed to cast to a bool. If you are trying to cast a numeric type
503 to a bool, you can compare it with zero instead:
505 ```compile_fail,E0054
508 // Not allowed, won't compile
509 let x_is_nonzero = x as bool;
516 let x_is_nonzero = x != 0;
521 During a method call, a value is automatically dereferenced as many times as
522 needed to make the value's type match the method's receiver. The catch is that
523 the compiler will only attempt to dereference a number of times up to the
524 recursion limit (which can be set via the `recursion_limit` attribute).
526 For a somewhat artificial example:
528 ```compile_fail,E0055
529 #![recursion_limit="2"]
541 // error, reached the recursion limit while auto-dereferencing &&Foo
546 One fix may be to increase the recursion limit. Note that it is possible to
547 create an infinite recursion of dereferencing, in which case the only fix is to
548 somehow break the recursion.
552 When invoking closures or other implementations of the function traits `Fn`,
553 `FnMut` or `FnOnce` using call notation, the number of parameters passed to the
554 function must match its definition.
556 An example using a closure:
558 ```compile_fail,E0057
560 let a = f(); // invalid, too few parameters
561 let b = f(4); // this works!
562 let c = f(2, 3); // invalid, too many parameters
565 A generic function must be treated similarly:
568 fn foo<F: Fn()>(f: F) {
569 f(); // this is valid, but f(3) would not work
575 The built-in function traits are generic over a tuple of the function arguments.
576 If one uses angle-bracket notation (`Fn<(T,), Output=U>`) instead of parentheses
577 (`Fn(T) -> U`) to denote the function trait, the type parameter should be a
578 tuple. Otherwise function call notation cannot be used and the trait will not be
579 implemented by closures.
581 The most likely source of this error is using angle-bracket notation without
582 wrapping the function argument type into a tuple, for example:
584 ```compile_fail,E0059
585 #![feature(unboxed_closures)]
587 fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) }
590 It can be fixed by adjusting the trait bound like this:
593 #![feature(unboxed_closures)]
595 fn foo<F: Fn<(i32,)>>(f: F) -> F::Output { f(3) }
598 Note that `(T,)` always denotes the type of a 1-tuple containing an element of
599 type `T`. The comma is necessary for syntactic disambiguation.
603 External C functions are allowed to be variadic. However, a variadic function
604 takes a minimum number of arguments. For example, consider C's variadic `printf`
608 use std::os::raw::{c_char, c_int};
611 fn printf(_: *const c_char, ...) -> c_int;
615 Using this declaration, it must be called with at least one argument, so
616 simply calling `printf()` is invalid. But the following uses are allowed:
619 # #![feature(static_nobundle)]
620 # use std::os::raw::{c_char, c_int};
621 # #[cfg_attr(all(windows, target_env = "msvc"),
622 # link(name = "legacy_stdio_definitions", kind = "static-nobundle"))]
623 # extern "C" { fn printf(_: *const c_char, ...) -> c_int; }
626 use std::ffi::CString;
628 let fmt = CString::new("test\n").unwrap();
629 printf(fmt.as_ptr());
631 let fmt = CString::new("number = %d\n").unwrap();
632 printf(fmt.as_ptr(), 3);
634 let fmt = CString::new("%d, %d\n").unwrap();
635 printf(fmt.as_ptr(), 10, 5);
640 // ^ Note: On MSVC 2015, the `printf` function is "inlined" in the C code, and
641 // the C runtime does not contain the `printf` definition. This leads to linker
642 // error from the doc test (issue #42830).
643 // This can be fixed by linking to the static library
644 // `legacy_stdio_definitions.lib` (see https://stackoverflow.com/a/36504365/).
645 // If this compatibility library is removed in the future, consider changing
646 // `printf` in this example to another well-known variadic function.
649 The number of arguments passed to a function must match the number of arguments
650 specified in the function signature.
652 For example, a function like:
655 fn f(a: u16, b: &str) {}
658 Must always be called with exactly two arguments, e.g. `f(2, "test")`.
660 Note that Rust does not have a notion of optional function arguments or
661 variadic functions (except for its C-FFI).
665 This error indicates that during an attempt to build a struct or struct-like
666 enum variant, one of the fields was specified more than once. Erroneous code
669 ```compile_fail,E0062
677 x: 0, // error: field `x` specified more than once
682 Each field should be specified exactly one time. Example:
690 let x = Foo { x: 0 }; // ok!
696 This error indicates that during an attempt to build a struct or struct-like
697 enum variant, one of the fields was not provided. Erroneous code example:
699 ```compile_fail,E0063
706 let x = Foo { x: 0 }; // error: missing field: `y`
710 Each field should be specified exactly once. Example:
719 let x = Foo { x: 0, y: 0 }; // ok!
725 The left-hand side of a compound assignment expression must be a place
726 expression. A place expression represents a memory location and includes
727 item paths (ie, namespaced variables), dereferences, indexing expressions,
728 and field references.
730 Let's start with some erroneous code examples:
732 ```compile_fail,E0067
733 use std::collections::LinkedList;
735 // Bad: assignment to non-place expression
736 LinkedList::new() += 1;
740 fn some_func(i: &mut i32) {
741 i += 12; // Error : '+=' operation cannot be applied on a reference !
745 And now some working examples:
754 fn some_func(i: &mut i32) {
761 The compiler found a function whose body contains a `return;` statement but
762 whose return type is not `()`. An example of this is:
764 ```compile_fail,E0069
771 Since `return;` is just like `return ();`, there is a mismatch between the
772 function's return type and the value being returned.
776 The left-hand side of an assignment operator must be a place expression. An
777 place expression represents a memory location and can be a variable (with
778 optional namespacing), a dereference, an indexing expression or a field
781 More details can be found in the [Expressions] section of the Reference.
783 [Expressions]: https://doc.rust-lang.org/reference/expressions.html#places-rvalues-and-temporaries
785 Now, we can go further. Here are some erroneous code examples:
787 ```compile_fail,E0070
793 const SOME_CONST : i32 = 12;
795 fn some_other_func() {}
798 SOME_CONST = 14; // error : a constant value cannot be changed!
799 1 = 3; // error : 1 isn't a valid place!
800 some_other_func() = 4; // error : we can't assign value to a function!
801 SomeStruct.x = 12; // error : SomeStruct a structure name but it is used
806 And now let's give working examples:
813 let mut s = SomeStruct {x: 0, y: 0};
815 s.x = 3; // that's good !
819 fn some_func(x: &mut i32) {
820 *x = 12; // that's good !
826 You tried to use structure-literal syntax to create an item that is
827 not a structure or enum variant.
829 Example of erroneous code:
831 ```compile_fail,E0071
833 let t = U32 { value: 4 }; // error: expected struct, variant or union type,
834 // found builtin type `u32`
837 To fix this, ensure that the name was correctly spelled, and that
838 the correct form of initializer was used.
840 For example, the code above can be fixed to:
848 let u = Foo::FirstValue(0i32);
856 #### Note: this error code is no longer emitted by the compiler.
858 You cannot define a struct (or enum) `Foo` that requires an instance of `Foo`
859 in order to make a new `Foo` value. This is because there would be no way a
860 first instance of `Foo` could be made to initialize another instance!
862 Here's an example of a struct that has this problem:
865 struct Foo { x: Box<Foo> } // error
868 One fix is to use `Option`, like so:
871 struct Foo { x: Option<Box<Foo>> }
874 Now it's possible to create at least one instance of `Foo`: `Foo { x: None }`.
878 #### Note: this error code is no longer emitted by the compiler.
880 When using the `#[simd]` attribute on a tuple struct, the components of the
881 tuple struct must all be of a concrete, nongeneric type so the compiler can
882 reason about how to use SIMD with them. This error will occur if the types
885 This will cause an error:
888 #![feature(repr_simd)]
891 struct Bad<T>(T, T, T);
897 #![feature(repr_simd)]
900 struct Good(u32, u32, u32);
905 The `#[simd]` attribute can only be applied to non empty tuple structs, because
906 it doesn't make sense to try to use SIMD operations when there are no values to
909 This will cause an error:
911 ```compile_fail,E0075
912 #![feature(repr_simd)]
921 #![feature(repr_simd)]
929 When using the `#[simd]` attribute to automatically use SIMD operations in tuple
930 struct, the types in the struct must all be of the same type, or the compiler
931 will trigger this error.
933 This will cause an error:
935 ```compile_fail,E0076
936 #![feature(repr_simd)]
939 struct Bad(u16, u32, u32);
945 #![feature(repr_simd)]
948 struct Good(u32, u32, u32);
953 When using the `#[simd]` attribute on a tuple struct, the elements in the tuple
954 must be machine types so SIMD operations can be applied to them.
956 This will cause an error:
958 ```compile_fail,E0077
959 #![feature(repr_simd)]
968 #![feature(repr_simd)]
971 struct Good(u32, u32, u32);
976 Enum discriminants are used to differentiate enum variants stored in memory.
977 This error indicates that the same value was used for two or more variants,
978 making them impossible to tell apart.
980 ```compile_fail,E0081
998 Note that variants without a manually specified discriminant are numbered from
999 top to bottom starting from 0, so clashes can occur with seemingly unrelated
1002 ```compile_fail,E0081
1009 Here `X` will have already been specified the discriminant 0 by the time `Y` is
1010 encountered, so a conflict occurs.
1014 An unsupported representation was attempted on a zero-variant enum.
1016 Erroneous code example:
1018 ```compile_fail,E0084
1020 enum NightsWatch {} // error: unsupported representation for zero-variant enum
1023 It is impossible to define an integer type to be used to represent zero-variant
1024 enum values because there are no zero-variant enum values. There is no way to
1025 construct an instance of the following type using only safe code. So you have
1026 two solutions. Either you add variants in your enum:
1036 or you remove the integer represention of your enum:
1044 Too many type parameters were supplied for a function. For example:
1046 ```compile_fail,E0087
1050 foo::<f64, bool>(); // error, expected 1 parameter, found 2 parameters
1054 The number of supplied parameters must exactly match the number of defined type
1059 You gave too many lifetime parameters. Erroneous code example:
1061 ```compile_fail,E0088
1065 f::<'static>() // error: too many lifetime parameters provided
1069 Please check you give the right number of lifetime parameters. Example:
1079 It's also important to note that the Rust compiler can generally
1080 determine the lifetime by itself. Example:
1088 // it can be written like this
1089 fn get_value<'a>(&'a self) -> &'a str { &self.value }
1090 // but the compiler works fine with this too:
1091 fn without_lifetime(&self) -> &str { &self.value }
1095 let f = Foo { value: "hello".to_owned() };
1097 println!("{}", f.get_value());
1098 println!("{}", f.without_lifetime());
1104 Not enough type parameters were supplied for a function. For example:
1106 ```compile_fail,E0089
1110 foo::<f64>(); // error, expected 2 parameters, found 1 parameter
1114 Note that if a function takes multiple type parameters but you want the compiler
1115 to infer some of them, you can use type placeholders:
1117 ```compile_fail,E0089
1118 fn foo<T, U>(x: T) {}
1122 foo::<f64>(x); // error, expected 2 parameters, found 1 parameter
1123 foo::<_, f64>(x); // same as `foo::<bool, f64>(x)`
1129 You gave too few lifetime parameters. Example:
1131 ```compile_fail,E0090
1132 fn foo<'a: 'b, 'b: 'a>() {}
1135 foo::<'static>(); // error, expected 2 lifetime parameters
1139 Please check you give the right number of lifetime parameters. Example:
1142 fn foo<'a: 'b, 'b: 'a>() {}
1145 foo::<'static, 'static>();
1151 You gave an unnecessary type parameter in a type alias. Erroneous code
1154 ```compile_fail,E0091
1155 type Foo<T> = u32; // error: type parameter `T` is unused
1157 type Foo<A,B> = Box<A>; // error: type parameter `B` is unused
1160 Please check you didn't write too many type parameters. Example:
1163 type Foo = u32; // ok!
1164 type Foo2<A> = Box<A>; // ok!
1169 You tried to declare an undefined atomic operation function.
1170 Erroneous code example:
1172 ```compile_fail,E0092
1173 #![feature(intrinsics)]
1175 extern "rust-intrinsic" {
1176 fn atomic_foo(); // error: unrecognized atomic operation
1181 Please check you didn't make a mistake in the function's name. All intrinsic
1182 functions are defined in librustc_codegen_llvm/intrinsic.rs and in
1183 libcore/intrinsics.rs in the Rust source code. Example:
1186 #![feature(intrinsics)]
1188 extern "rust-intrinsic" {
1189 fn atomic_fence(); // ok!
1195 You declared an unknown intrinsic function. Erroneous code example:
1197 ```compile_fail,E0093
1198 #![feature(intrinsics)]
1200 extern "rust-intrinsic" {
1201 fn foo(); // error: unrecognized intrinsic function: `foo`
1211 Please check you didn't make a mistake in the function's name. All intrinsic
1212 functions are defined in librustc_codegen_llvm/intrinsic.rs and in
1213 libcore/intrinsics.rs in the Rust source code. Example:
1216 #![feature(intrinsics)]
1218 extern "rust-intrinsic" {
1219 fn atomic_fence(); // ok!
1231 You gave an invalid number of type parameters to an intrinsic function.
1232 Erroneous code example:
1234 ```compile_fail,E0094
1235 #![feature(intrinsics)]
1237 extern "rust-intrinsic" {
1238 fn size_of<T, U>() -> usize; // error: intrinsic has wrong number
1239 // of type parameters
1243 Please check that you provided the right number of type parameters
1244 and verify with the function declaration in the Rust source code.
1248 #![feature(intrinsics)]
1250 extern "rust-intrinsic" {
1251 fn size_of<T>() -> usize; // ok!
1257 This error means that an incorrect number of lifetime parameters were provided
1258 for a type (like a struct or enum) or trait:
1260 ```compile_fail,E0107
1261 struct Foo<'a, 'b>(&'a str, &'b str);
1262 enum Bar { A, B, C }
1265 foo: Foo<'a>, // error: expected 2, found 1
1266 bar: Bar<'a>, // error: expected 0, found 1
1272 You tried to give a type parameter to a type which doesn't need it. Erroneous
1275 ```compile_fail,E0109
1276 type X = u32<i32>; // error: type parameters are not allowed on this type
1279 Please check that you used the correct type and recheck its definition. Perhaps
1280 it doesn't need the type parameter.
1285 type X = u32; // this compiles
1288 Note that type parameters for enum-variant constructors go after the variant,
1289 not after the enum (`Option::None::<u32>`, not `Option::<u32>::None`).
1293 You tried to give a lifetime parameter to a type which doesn't need it.
1294 Erroneous code example:
1296 ```compile_fail,E0110
1297 type X = u32<'static>; // error: lifetime parameters are not allowed on
1301 Please check that the correct type was used and recheck its definition; perhaps
1302 it doesn't need the lifetime parameter. Example:
1305 type X = u32; // ok!
1310 You can only define an inherent implementation for a type in the same crate
1311 where the type was defined. For example, an `impl` block as below is not allowed
1312 since `Vec` is defined in the standard library:
1314 ```compile_fail,E0116
1315 impl Vec<u8> { } // error
1318 To fix this problem, you can do either of these things:
1320 - define a trait that has the desired associated functions/types/constants and
1321 implement the trait for the type in question
1322 - define a new type wrapping the type and define an implementation on the new
1325 Note that using the `type` keyword does not work here because `type` only
1326 introduces a type alias:
1328 ```compile_fail,E0116
1329 type Bytes = Vec<u8>;
1331 impl Bytes { } // error, same as above
1336 This error indicates a violation of one of Rust's orphan rules for trait
1337 implementations. The rule prohibits any implementation of a foreign trait (a
1338 trait defined in another crate) where
1340 - the type that is implementing the trait is foreign
1341 - all of the parameters being passed to the trait (if there are any) are also
1344 Here's one example of this error:
1346 ```compile_fail,E0117
1347 impl Drop for u32 {}
1350 To avoid this kind of error, ensure that at least one local type is referenced
1354 pub struct Foo; // you define your type in your crate
1356 impl Drop for Foo { // and you can implement the trait on it!
1357 // code of trait implementation here
1358 # fn drop(&mut self) { }
1361 impl From<Foo> for i32 { // or you use a type from your crate as
1363 fn from(i: Foo) -> i32 {
1369 Alternatively, define a trait locally and implement that instead:
1373 fn get(&self) -> usize;
1377 fn get(&self) -> usize { 0 }
1381 For information on the design of the orphan rules, see [RFC 1023].
1383 [RFC 1023]: https://github.com/rust-lang/rfcs/blob/master/text/1023-rebalancing-coherence.md
1387 You're trying to write an inherent implementation for something which isn't a
1388 struct nor an enum. Erroneous code example:
1390 ```compile_fail,E0118
1391 impl (u8, u8) { // error: no base type found for inherent implementation
1392 fn get_state(&self) -> String {
1398 To fix this error, please implement a trait on the type or wrap it in a struct.
1402 // we create a trait here
1403 trait LiveLongAndProsper {
1404 fn get_state(&self) -> String;
1407 // and now you can implement it on (u8, u8)
1408 impl LiveLongAndProsper for (u8, u8) {
1409 fn get_state(&self) -> String {
1410 "He's dead, Jim!".to_owned()
1415 Alternatively, you can create a newtype. A newtype is a wrapping tuple-struct.
1416 For example, `NewType` is a newtype over `Foo` in `struct NewType(Foo)`.
1420 struct TypeWrapper((u8, u8));
1423 fn get_state(&self) -> String {
1424 "Fascinating!".to_owned()
1431 An attempt was made to implement Drop on a trait, which is not allowed: only
1432 structs and enums can implement Drop. An example causing this error:
1434 ```compile_fail,E0120
1437 impl Drop for MyTrait {
1438 fn drop(&mut self) {}
1442 A workaround for this problem is to wrap the trait up in a struct, and implement
1443 Drop on that. An example is shown below:
1447 struct MyWrapper<T: MyTrait> { foo: T }
1449 impl <T: MyTrait> Drop for MyWrapper<T> {
1450 fn drop(&mut self) {}
1455 Alternatively, wrapping trait objects requires something like the following:
1460 //or Box<MyTrait>, if you wanted an owned trait object
1461 struct MyWrapper<'a> { foo: &'a MyTrait }
1463 impl <'a> Drop for MyWrapper<'a> {
1464 fn drop(&mut self) {}
1470 In order to be consistent with Rust's lack of global type inference, type
1471 placeholders are disallowed by design in item signatures.
1473 Examples of this error include:
1475 ```compile_fail,E0121
1476 fn foo() -> _ { 5 } // error, explicitly write out the return type instead
1478 static BAR: _ = "test"; // error, explicitly write out the type instead
1483 You declared two fields of a struct with the same name. Erroneous code
1486 ```compile_fail,E0124
1489 field1: i32, // error: field is already declared
1493 Please verify that the field names have been correctly spelled. Example:
1504 It is not possible to define `main` with generic parameters.
1505 When `main` is present, it must take no arguments and return `()`.
1506 Erroneous code example:
1508 ```compile_fail,E0131
1509 fn main<T>() { // error: main function is not allowed to have generic parameters
1515 A function with the `start` attribute was declared with type parameters.
1517 Erroneous code example:
1519 ```compile_fail,E0132
1526 It is not possible to declare type parameters on a function that has the `start`
1527 attribute. Such a function must have the following type signature (for more
1528 information: http://doc.rust-lang.org/stable/book/first-edition/no-stdlib.html):
1532 fn(isize, *const *const u8) -> isize;
1541 fn my_start(argc: isize, argv: *const *const u8) -> isize {
1548 This error means that an attempt was made to match a struct type enum
1549 variant as a non-struct type:
1551 ```compile_fail,E0164
1552 enum Foo { B { i: u32 } }
1554 fn bar(foo: Foo) -> u32 {
1556 Foo::B(i) => i, // error E0164
1561 Try using `{}` instead:
1564 enum Foo { B { i: u32 } }
1566 fn bar(foo: Foo) -> u32 {
1575 Explicitly implementing both Drop and Copy for a type is currently disallowed.
1576 This feature can make some sense in theory, but the current implementation is
1577 incorrect and can lead to memory unsafety (see [issue #20126][iss20126]), so
1578 it has been disabled for now.
1580 [iss20126]: https://github.com/rust-lang/rust/issues/20126
1584 An associated function for a trait was defined to be static, but an
1585 implementation of the trait declared the same function to be a method (i.e. to
1586 take a `self` parameter).
1588 Here's an example of this error:
1590 ```compile_fail,E0185
1598 // error, method `foo` has a `&self` declaration in the impl, but not in
1606 An associated function for a trait was defined to be a method (i.e. to take a
1607 `self` parameter), but an implementation of the trait declared the same function
1610 Here's an example of this error:
1612 ```compile_fail,E0186
1620 // error, method `foo` has a `&self` declaration in the trait, but not in
1628 Trait objects need to have all associated types specified. Erroneous code
1631 ```compile_fail,E0191
1636 type Foo = Trait; // error: the value of the associated type `Bar` (from
1637 // the trait `Trait`) must be specified
1640 Please verify you specified all associated types of the trait and that you
1641 used the right trait. Example:
1648 type Foo = Trait<Bar=i32>; // ok!
1653 Negative impls are only allowed for auto traits. For more
1654 information see the [opt-in builtin traits RFC][RFC 19].
1656 [RFC 19]: https://github.com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md
1660 #### Note: this error code is no longer emitted by the compiler.
1662 `where` clauses must use generic type parameters: it does not make sense to use
1663 them otherwise. An example causing this error:
1670 #[derive(Copy,Clone)]
1675 impl Foo for Wrapper<u32> where Wrapper<u32>: Clone {
1680 This use of a `where` clause is strange - a more common usage would look
1681 something like the following:
1688 #[derive(Copy,Clone)]
1692 impl <T> Foo for Wrapper<T> where Wrapper<T>: Clone {
1697 Here, we're saying that the implementation exists on Wrapper only when the
1698 wrapped type `T` implements `Clone`. The `where` clause is important because
1699 some types will not implement `Clone`, and thus will not get this method.
1701 In our erroneous example, however, we're referencing a single concrete type.
1702 Since we know for certain that `Wrapper<u32>` implements `Clone`, there's no
1703 reason to also specify it in a `where` clause.
1707 A type parameter was declared which shadows an existing one. An example of this
1710 ```compile_fail,E0194
1712 fn do_something(&self) -> T;
1713 fn do_something_else<T: Clone>(&self, bar: T);
1717 In this example, the trait `Foo` and the trait method `do_something_else` both
1718 define a type parameter `T`. This is not allowed: if the method wishes to
1719 define a type parameter, it must use a different name for it.
1723 Your method's lifetime parameters do not match the trait declaration.
1724 Erroneous code example:
1726 ```compile_fail,E0195
1728 fn bar<'a,'b:'a>(x: &'a str, y: &'b str);
1733 impl Trait for Foo {
1734 fn bar<'a,'b>(x: &'a str, y: &'b str) {
1735 // error: lifetime parameters or bounds on method `bar`
1736 // do not match the trait declaration
1741 The lifetime constraint `'b` for bar() implementation does not match the
1742 trait declaration. Ensure lifetime declarations match exactly in both trait
1743 declaration and implementation. Example:
1747 fn t<'a,'b:'a>(x: &'a str, y: &'b str);
1752 impl Trait for Foo {
1753 fn t<'a,'b:'a>(x: &'a str, y: &'b str) { // ok!
1760 Safe traits should not have unsafe implementations, therefore marking an
1761 implementation for a safe trait unsafe will cause a compiler error. Removing
1762 the unsafe marker on the trait noted in the error will resolve this problem.
1764 ```compile_fail,E0199
1769 // this won't compile because Bar is safe
1770 unsafe impl Bar for Foo { }
1771 // this will compile
1772 impl Bar for Foo { }
1777 Unsafe traits must have unsafe implementations. This error occurs when an
1778 implementation for an unsafe trait isn't marked as unsafe. This may be resolved
1779 by marking the unsafe implementation as unsafe.
1781 ```compile_fail,E0200
1784 unsafe trait Bar { }
1786 // this won't compile because Bar is unsafe and impl isn't unsafe
1787 impl Bar for Foo { }
1788 // this will compile
1789 unsafe impl Bar for Foo { }
1794 It is an error to define two associated items (like methods, associated types,
1795 associated functions, etc.) with the same identifier.
1799 ```compile_fail,E0201
1803 fn bar(&self) -> bool { self.0 > 5 }
1804 fn bar() {} // error: duplicate associated function
1809 fn baz(&self) -> bool;
1815 fn baz(&self) -> bool { true }
1817 // error: duplicate method
1818 fn baz(&self) -> bool { self.0 > 5 }
1820 // error: duplicate associated type
1825 Note, however, that items with the same name are allowed for inherent `impl`
1826 blocks that don't overlap:
1832 fn bar(&self) -> bool { self.0 > 5 }
1836 fn bar(&self) -> bool { self.0 }
1842 Inherent associated types were part of [RFC 195] but are not yet implemented.
1843 See [the tracking issue][iss8995] for the status of this implementation.
1845 [RFC 195]: https://github.com/rust-lang/rfcs/blob/master/text/0195-associated-items.md
1846 [iss8995]: https://github.com/rust-lang/rust/issues/8995
1850 An attempt to implement the `Copy` trait for a struct failed because one of the
1851 fields does not implement `Copy`. To fix this, you must implement `Copy` for the
1852 mentioned field. Note that this may not be possible, as in the example of
1854 ```compile_fail,E0204
1859 impl Copy for Foo { }
1862 This fails because `Vec<T>` does not implement `Copy` for any `T`.
1864 Here's another example that will fail:
1866 ```compile_fail,E0204
1873 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
1874 differs from the behavior for `&T`, which is always `Copy`).
1879 An attempt to implement the `Copy` trait for an enum failed because one of the
1880 variants does not implement `Copy`. To fix this, you must implement `Copy` for
1881 the mentioned variant. Note that this may not be possible, as in the example of
1883 ```compile_fail,E0205
1889 impl Copy for Foo { }
1892 This fails because `Vec<T>` does not implement `Copy` for any `T`.
1894 Here's another example that will fail:
1896 ```compile_fail,E0205
1904 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
1905 differs from the behavior for `&T`, which is always `Copy`).
1910 You can only implement `Copy` for a struct or enum. Both of the following
1911 examples will fail, because neither `[u8; 256]` nor `&'static mut Bar`
1912 (mutable reference to `Bar`) is a struct or enum:
1914 ```compile_fail,E0206
1915 type Foo = [u8; 256];
1916 impl Copy for Foo { } // error
1918 #[derive(Copy, Clone)]
1920 impl Copy for &'static mut Bar { } // error
1925 Any type parameter or lifetime parameter of an `impl` must meet at least one of
1926 the following criteria:
1928 - it appears in the self type of the impl
1929 - for a trait impl, it appears in the trait reference
1930 - it is bound as an associated type
1934 Suppose we have a struct `Foo` and we would like to define some methods for it.
1935 The following definition leads to a compiler error:
1937 ```compile_fail,E0207
1940 impl<T: Default> Foo {
1941 // error: the type parameter `T` is not constrained by the impl trait, self
1942 // type, or predicates [E0207]
1943 fn get(&self) -> T {
1944 <T as Default>::default()
1949 The problem is that the parameter `T` does not appear in the self type (`Foo`)
1950 of the impl. In this case, we can fix the error by moving the type parameter
1951 from the `impl` to the method `get`:
1957 // Move the type parameter from the impl to the method
1959 fn get<T: Default>(&self) -> T {
1960 <T as Default>::default()
1967 As another example, suppose we have a `Maker` trait and want to establish a
1968 type `FooMaker` that makes `Foo`s:
1970 ```compile_fail,E0207
1973 fn make(&mut self) -> Self::Item;
1982 impl<T: Default> Maker for FooMaker {
1983 // error: the type parameter `T` is not constrained by the impl trait, self
1984 // type, or predicates [E0207]
1987 fn make(&mut self) -> Foo<T> {
1988 Foo { foo: <T as Default>::default() }
1993 This fails to compile because `T` does not appear in the trait or in the
1996 One way to work around this is to introduce a phantom type parameter into
1997 `FooMaker`, like so:
2000 use std::marker::PhantomData;
2004 fn make(&mut self) -> Self::Item;
2011 // Add a type parameter to `FooMaker`
2012 struct FooMaker<T> {
2013 phantom: PhantomData<T>,
2016 impl<T: Default> Maker for FooMaker<T> {
2019 fn make(&mut self) -> Foo<T> {
2021 foo: <T as Default>::default(),
2027 Another way is to do away with the associated type in `Maker` and use an input
2028 type parameter instead:
2031 // Use a type parameter instead of an associated type here
2033 fn make(&mut self) -> Item;
2042 impl<T: Default> Maker<Foo<T>> for FooMaker {
2043 fn make(&mut self) -> Foo<T> {
2044 Foo { foo: <T as Default>::default() }
2049 ### Additional information
2051 For more information, please see [RFC 447].
2053 [RFC 447]: https://github.com/rust-lang/rfcs/blob/master/text/0447-no-unused-impl-parameters.md
2057 This error indicates a violation of one of Rust's orphan rules for trait
2058 implementations. The rule concerns the use of type parameters in an
2059 implementation of a foreign trait (a trait defined in another crate), and
2060 states that type parameters must be "covered" by a local type. To understand
2061 what this means, it is perhaps easiest to consider a few examples.
2063 If `ForeignTrait` is a trait defined in some external crate `foo`, then the
2064 following trait `impl` is an error:
2066 ```compile_fail,E0210
2067 # #[cfg(for_demonstration_only)]
2069 # #[cfg(for_demonstration_only)]
2070 use foo::ForeignTrait;
2071 # use std::panic::UnwindSafe as ForeignTrait;
2073 impl<T> ForeignTrait for T { } // error
2077 To work around this, it can be covered with a local type, `MyType`:
2080 # use std::panic::UnwindSafe as ForeignTrait;
2081 struct MyType<T>(T);
2082 impl<T> ForeignTrait for MyType<T> { } // Ok
2085 Please note that a type alias is not sufficient.
2087 For another example of an error, suppose there's another trait defined in `foo`
2088 named `ForeignTrait2` that takes two type parameters. Then this `impl` results
2089 in the same rule violation:
2091 ```ignore (cannot-doctest-multicrate-project)
2093 impl<T> ForeignTrait2<T, MyType<T>> for MyType2 { } // error
2096 The reason for this is that there are two appearances of type parameter `T` in
2097 the `impl` header, both as parameters for `ForeignTrait2`. The first appearance
2098 is uncovered, and so runs afoul of the orphan rule.
2100 Consider one more example:
2102 ```ignore (cannot-doctest-multicrate-project)
2103 impl<T> ForeignTrait2<MyType<T>, T> for MyType2 { } // Ok
2106 This only differs from the previous `impl` in that the parameters `T` and
2107 `MyType<T>` for `ForeignTrait2` have been swapped. This example does *not*
2108 violate the orphan rule; it is permitted.
2110 To see why that last example was allowed, you need to understand the general
2111 rule. Unfortunately this rule is a bit tricky to state. Consider an `impl`:
2113 ```ignore (only-for-syntax-highlight)
2114 impl<P1, ..., Pm> ForeignTrait<T1, ..., Tn> for T0 { ... }
2117 where `P1, ..., Pm` are the type parameters of the `impl` and `T0, ..., Tn`
2118 are types. One of the types `T0, ..., Tn` must be a local type (this is another
2119 orphan rule, see the explanation for E0117). Let `i` be the smallest integer
2120 such that `Ti` is a local type. Then no type parameter can appear in any of the
2123 For information on the design of the orphan rules, see [RFC 1023].
2125 [RFC 1023]: https://github.com/rust-lang/rfcs/blob/master/text/1023-rebalancing-coherence.md
2130 You used a function or type which doesn't fit the requirements for where it was
2131 used. Erroneous code examples:
2134 #![feature(intrinsics)]
2136 extern "rust-intrinsic" {
2137 fn size_of<T>(); // error: intrinsic has wrong type
2142 fn main() -> i32 { 0 }
2143 // error: main function expects type: `fn() {main}`: expected (), found i32
2150 // error: mismatched types in range: expected u8, found i8
2160 fn x(self: Rc<Foo>) {}
2161 // error: mismatched self type: expected `Foo`: expected struct
2162 // `Foo`, found struct `alloc::rc::Rc`
2166 For the first code example, please check the function definition. Example:
2169 #![feature(intrinsics)]
2171 extern "rust-intrinsic" {
2172 fn size_of<T>() -> usize; // ok!
2176 The second case example is a bit particular : the main function must always
2177 have this definition:
2183 They never take parameters and never return types.
2185 For the third example, when you match, all patterns must have the same type
2186 as the type you're matching on. Example:
2192 0u8...3u8 => (), // ok!
2197 And finally, for the last example, only `Box<Self>`, `&Self`, `Self`,
2198 or `&mut Self` work as explicit self parameters. Example:
2204 fn x(self: Box<Foo>) {} // ok!
2211 You used an associated type which isn't defined in the trait.
2212 Erroneous code example:
2214 ```compile_fail,E0220
2219 type Foo = T1<F=i32>; // error: associated type `F` not found for `T1`
2226 // error: Baz is used but not declared
2227 fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
2231 Make sure that you have defined the associated type in the trait body.
2232 Also, verify that you used the right trait or you didn't misspell the
2233 associated type name. Example:
2240 type Foo = T1<Bar=i32>; // ok!
2246 type Baz; // we declare `Baz` in our trait.
2248 // and now we can use it here:
2249 fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
2255 An attempt was made to retrieve an associated type, but the type was ambiguous.
2258 ```compile_fail,E0221
2274 In this example, `Foo` defines an associated type `A`. `Bar` inherits that type
2275 from `Foo`, and defines another associated type of the same name. As a result,
2276 when we attempt to use `Self::A`, it's ambiguous whether we mean the `A` defined
2277 by `Foo` or the one defined by `Bar`.
2279 There are two options to work around this issue. The first is simply to rename
2280 one of the types. Alternatively, one can specify the intended type using the
2294 let _: <Self as Bar>::A;
2301 An attempt was made to retrieve an associated type, but the type was ambiguous.
2304 ```compile_fail,E0223
2305 trait MyTrait {type X; }
2308 let foo: MyTrait::X;
2312 The problem here is that we're attempting to take the type of X from MyTrait.
2313 Unfortunately, the type of X is not defined, because it's only made concrete in
2314 implementations of the trait. A working version of this code might look like:
2317 trait MyTrait {type X; }
2320 impl MyTrait for MyStruct {
2325 let foo: <MyStruct as MyTrait>::X;
2329 This syntax specifies that we want the X type from MyTrait, as made concrete in
2330 MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct
2331 might implement two different traits with identically-named associated types.
2332 This syntax allows disambiguation between the two.
2336 You attempted to use multiple types as bounds for a closure or trait object.
2337 Rust does not currently support this. A simple example that causes this error:
2339 ```compile_fail,E0225
2341 let _: Box<std::io::Read + std::io::Write>;
2345 Auto traits such as Send and Sync are an exception to this rule:
2346 It's possible to have bounds of one non-builtin trait, plus any number of
2347 auto traits. For example, the following compiles correctly:
2351 let _: Box<std::io::Read + Send + Sync>;
2357 An associated type binding was done outside of the type parameter declaration
2358 and `where` clause. Erroneous code example:
2360 ```compile_fail,E0229
2363 fn boo(&self) -> <Self as Foo>::A;
2368 impl Foo for isize {
2370 fn boo(&self) -> usize { 42 }
2373 fn baz<I>(x: &<I as Foo<A=Bar>>::A) {}
2374 // error: associated type bindings are not allowed here
2377 To solve this error, please move the type bindings in the type parameter
2382 # trait Foo { type A; }
2383 fn baz<I: Foo<A=Bar>>(x: &<I as Foo>::A) {} // ok!
2386 Or in the `where` clause:
2390 # trait Foo { type A; }
2391 fn baz<I>(x: &<I as Foo>::A) where I: Foo<A=Bar> {}
2396 This error indicates that not enough type parameters were found in a type or
2399 For example, the `Foo` struct below is defined to be generic in `T`, but the
2400 type parameter is missing in the definition of `Bar`:
2402 ```compile_fail,E0243
2403 struct Foo<T> { x: T }
2405 struct Bar { x: Foo }
2410 This error indicates that too many type parameters were found in a type or
2413 For example, the `Foo` struct below has no type parameters, but is supplied
2414 with two in the definition of `Bar`:
2416 ```compile_fail,E0244
2417 struct Foo { x: bool }
2419 struct Bar<S, T> { x: Foo<S, T> }
2424 A cross-crate opt-out trait was implemented on something which wasn't a struct
2425 or enum type. Erroneous code example:
2427 ```compile_fail,E0321
2428 #![feature(optin_builtin_traits)]
2432 impl !Sync for Foo {}
2434 unsafe impl Send for &'static Foo {}
2435 // error: cross-crate traits with a default impl, like `core::marker::Send`,
2436 // can only be implemented for a struct/enum type, not
2440 Only structs and enums are permitted to impl Send, Sync, and other opt-out
2441 trait, and the struct or enum must be local to the current crate. So, for
2442 example, `unsafe impl Send for Rc<Foo>` is not allowed.
2446 The `Sized` trait is a special trait built-in to the compiler for types with a
2447 constant size known at compile-time. This trait is automatically implemented
2448 for types as needed by the compiler, and it is currently disallowed to
2449 explicitly implement it for a type.
2453 An associated const was implemented when another trait item was expected.
2454 Erroneous code example:
2456 ```compile_fail,E0323
2465 // error: item `N` is an associated const, which doesn't match its
2466 // trait `<Bar as Foo>`
2470 Please verify that the associated const wasn't misspelled and the correct trait
2471 was implemented. Example:
2481 type N = u32; // ok!
2495 const N : u32 = 0; // ok!
2501 A method was implemented when another trait item was expected. Erroneous
2504 ```compile_fail,E0324
2515 // error: item `N` is an associated method, which doesn't match its
2516 // trait `<Bar as Foo>`
2520 To fix this error, please verify that the method name wasn't misspelled and
2521 verify that you are indeed implementing the correct trait items. Example:
2541 An associated type was implemented when another trait item was expected.
2542 Erroneous code example:
2544 ```compile_fail,E0325
2553 // error: item `N` is an associated type, which doesn't match its
2554 // trait `<Bar as Foo>`
2558 Please verify that the associated type name wasn't misspelled and your
2559 implementation corresponds to the trait definition. Example:
2569 type N = u32; // ok!
2583 const N : u32 = 0; // ok!
2589 The types of any associated constants in a trait implementation must match the
2590 types in the trait definition. This error indicates that there was a mismatch.
2592 Here's an example of this error:
2594 ```compile_fail,E0326
2602 const BAR: u32 = 5; // error, expected bool, found u32
2608 The Unsize trait should not be implemented directly. All implementations of
2609 Unsize are provided automatically by the compiler.
2611 Erroneous code example:
2613 ```compile_fail,E0328
2616 use std::marker::Unsize;
2620 impl<T> Unsize<T> for MyType {}
2623 If you are defining your own smart pointer type and would like to enable
2624 conversion from a sized to an unsized type with the
2625 [DST coercion system][RFC 982], use [`CoerceUnsized`] instead.
2628 #![feature(coerce_unsized)]
2630 use std::ops::CoerceUnsized;
2632 pub struct MyType<T: ?Sized> {
2633 field_with_unsized_type: T,
2636 impl<T, U> CoerceUnsized<MyType<U>> for MyType<T>
2637 where T: CoerceUnsized<U> {}
2640 [RFC 982]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md
2641 [`CoerceUnsized`]: https://doc.rust-lang.org/std/ops/trait.CoerceUnsized.html
2645 // Associated consts can now be accessed through generic type parameters, and
2646 // this error is no longer emitted.
2648 // FIXME: consider whether to leave it in the error index, or remove it entirely
2649 // as associated consts is not stabilized yet.
2652 An attempt was made to access an associated constant through either a generic
2653 type parameter or `Self`. This is not supported yet. An example causing this
2654 error is shown below:
2663 impl Foo for MyStruct {
2664 const BAR: f64 = 0f64;
2667 fn get_bar_bad<F: Foo>(t: F) -> f64 {
2672 Currently, the value of `BAR` for a particular type can only be accessed
2673 through a concrete type, as shown below:
2682 fn get_bar_good() -> f64 {
2683 <MyStruct as Foo>::BAR
2690 An attempt was made to implement `Drop` on a concrete specialization of a
2691 generic type. An example is shown below:
2693 ```compile_fail,E0366
2698 impl Drop for Foo<u32> {
2699 fn drop(&mut self) {}
2703 This code is not legal: it is not possible to specialize `Drop` to a subset of
2704 implementations of a generic type. One workaround for this is to wrap the
2705 generic type, as shown below:
2717 fn drop(&mut self) {}
2723 An attempt was made to implement `Drop` on a specialization of a generic type.
2724 An example is shown below:
2726 ```compile_fail,E0367
2729 struct MyStruct<T> {
2733 impl<T: Foo> Drop for MyStruct<T> {
2734 fn drop(&mut self) {}
2738 This code is not legal: it is not possible to specialize `Drop` to a subset of
2739 implementations of a generic type. In order for this code to work, `MyStruct`
2740 must also require that `T` implements `Foo`. Alternatively, another option is
2741 to wrap the generic type in another that specializes appropriately:
2746 struct MyStruct<T> {
2750 struct MyStructWrapper<T: Foo> {
2754 impl <T: Foo> Drop for MyStructWrapper<T> {
2755 fn drop(&mut self) {}
2761 This error indicates that a binary assignment operator like `+=` or `^=` was
2762 applied to a type that doesn't support it. For example:
2764 ```compile_fail,E0368
2765 let mut x = 12f32; // error: binary operation `<<` cannot be applied to
2771 To fix this error, please check that this type implements this binary
2775 let mut x = 12u32; // the `u32` type does implement the `ShlAssign` trait
2780 It is also possible to overload most operators for your own type by
2781 implementing the `[OP]Assign` traits from `std::ops`.
2783 Another problem you might be facing is this: suppose you've overloaded the `+`
2784 operator for some type `Foo` by implementing the `std::ops::Add` trait for
2785 `Foo`, but you find that using `+=` does not work, as in this example:
2787 ```compile_fail,E0368
2795 fn add(self, rhs: Foo) -> Foo {
2801 let mut x: Foo = Foo(5);
2802 x += Foo(7); // error, `+= cannot be applied to the type `Foo`
2806 This is because `AddAssign` is not automatically implemented, so you need to
2807 manually implement it for your type.
2811 A binary operation was attempted on a type which doesn't support it.
2812 Erroneous code example:
2814 ```compile_fail,E0369
2815 let x = 12f32; // error: binary operation `<<` cannot be applied to
2821 To fix this error, please check that this type implements this binary
2825 let x = 12u32; // the `u32` type does implement it:
2826 // https://doc.rust-lang.org/stable/std/ops/trait.Shl.html
2831 It is also possible to overload most operators for your own type by
2832 implementing traits from `std::ops`.
2834 String concatenation appends the string on the right to the string on the
2835 left and may require reallocation. This requires ownership of the string
2836 on the left. If something should be added to a string literal, move the
2837 literal to the heap by allocating it with `to_owned()` like in
2838 `"Your text".to_owned()`.
2843 The maximum value of an enum was reached, so it cannot be automatically
2844 set in the next enum value. Erroneous code example:
2847 #[deny(overflowing_literals)]
2849 X = 0x7fffffffffffffff,
2850 Y, // error: enum discriminant overflowed on value after
2851 // 9223372036854775807: i64; set explicitly via
2852 // Y = -9223372036854775808 if that is desired outcome
2856 To fix this, please set manually the next enum value or put the enum variant
2857 with the maximum value at the end of the enum. Examples:
2861 X = 0x7fffffffffffffff,
2871 X = 0x7fffffffffffffff,
2877 When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
2878 definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
2879 `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
2880 definition, so it is not useful to do this.
2884 ```compile_fail,E0371
2885 trait Foo { fn foo(&self) { } }
2889 impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
2890 impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
2891 impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
2892 impl Baz for Bar { } // Note: This is OK
2897 A struct without a field containing an unsized type cannot implement
2899 [unsized type](https://doc.rust-lang.org/book/first-edition/unsized-types.html)
2900 is any type that the compiler doesn't know the length or alignment of at
2901 compile time. Any struct containing an unsized type is also unsized.
2903 Example of erroneous code:
2905 ```compile_fail,E0374
2906 #![feature(coerce_unsized)]
2907 use std::ops::CoerceUnsized;
2909 struct Foo<T: ?Sized> {
2913 // error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
2914 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
2915 where T: CoerceUnsized<U> {}
2918 `CoerceUnsized` is used to coerce one struct containing an unsized type
2919 into another struct containing a different unsized type. If the struct
2920 doesn't have any fields of unsized types then you don't need explicit
2921 coercion to get the types you want. To fix this you can either
2922 not try to implement `CoerceUnsized` or you can add a field that is
2923 unsized to the struct.
2928 #![feature(coerce_unsized)]
2929 use std::ops::CoerceUnsized;
2931 // We don't need to impl `CoerceUnsized` here.
2936 // We add the unsized type field to the struct.
2937 struct Bar<T: ?Sized> {
2942 // The struct has an unsized field so we can implement
2943 // `CoerceUnsized` for it.
2944 impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
2945 where T: CoerceUnsized<U> {}
2948 Note that `CoerceUnsized` is mainly used by smart pointers like `Box`, `Rc`
2949 and `Arc` to be able to mark that they can coerce unsized types that they
2954 A struct with more than one field containing an unsized type cannot implement
2955 `CoerceUnsized`. This only occurs when you are trying to coerce one of the
2956 types in your struct to another type in the struct. In this case we try to
2957 impl `CoerceUnsized` from `T` to `U` which are both types that the struct
2958 takes. An [unsized type] is any type that the compiler doesn't know the length
2959 or alignment of at compile time. Any struct containing an unsized type is also
2962 Example of erroneous code:
2964 ```compile_fail,E0375
2965 #![feature(coerce_unsized)]
2966 use std::ops::CoerceUnsized;
2968 struct Foo<T: ?Sized, U: ?Sized> {
2974 // error: Struct `Foo` has more than one unsized field.
2975 impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
2978 `CoerceUnsized` only allows for coercion from a structure with a single
2979 unsized type field to another struct with a single unsized type field.
2980 In fact Rust only allows for a struct to have one unsized type in a struct
2981 and that unsized type must be the last field in the struct. So having two
2982 unsized types in a single struct is not allowed by the compiler. To fix this
2983 use only one field containing an unsized type in the struct and then use
2984 multiple structs to manage each unsized type field you need.
2989 #![feature(coerce_unsized)]
2990 use std::ops::CoerceUnsized;
2992 struct Foo<T: ?Sized> {
2997 impl <T, U> CoerceUnsized<Foo<U>> for Foo<T>
2998 where T: CoerceUnsized<U> {}
3000 fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
3001 Foo { a: 12i32, b: t } // we use coercion to get the `Foo<U>` type we need
3005 [unsized type]: https://doc.rust-lang.org/book/first-edition/unsized-types.html
3009 The type you are trying to impl `CoerceUnsized` for is not a struct.
3010 `CoerceUnsized` can only be implemented for a struct. Unsized types are
3011 already able to be coerced without an implementation of `CoerceUnsized`
3012 whereas a struct containing an unsized type needs to know the unsized type
3013 field it's containing is able to be coerced. An
3014 [unsized type](https://doc.rust-lang.org/book/first-edition/unsized-types.html)
3015 is any type that the compiler doesn't know the length or alignment of at
3016 compile time. Any struct containing an unsized type is also unsized.
3018 Example of erroneous code:
3020 ```compile_fail,E0376
3021 #![feature(coerce_unsized)]
3022 use std::ops::CoerceUnsized;
3024 struct Foo<T: ?Sized> {
3028 // error: The type `U` is not a struct
3029 impl<T, U> CoerceUnsized<U> for Foo<T> {}
3032 The `CoerceUnsized` trait takes a struct type. Make sure the type you are
3033 providing to `CoerceUnsized` is a struct with only the last field containing an
3039 #![feature(coerce_unsized)]
3040 use std::ops::CoerceUnsized;
3046 // The `Foo<U>` is a struct so `CoerceUnsized` can be implemented
3047 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}
3050 Note that in Rust, structs can only contain an unsized type if the field
3051 containing the unsized type is the last and only unsized type field in the
3056 You tried to implement methods for a primitive type. Erroneous code example:
3058 ```compile_fail,E0390
3064 // error: only a single inherent implementation marked with
3065 // `#[lang = "mut_ptr"]` is allowed for the `*mut T` primitive
3068 This isn't allowed, but using a trait to implement a method is a good solution.
3080 impl Bar for *mut Foo {
3087 This error indicates that a type or lifetime parameter has been declared
3088 but not actually used. Here is an example that demonstrates the error:
3090 ```compile_fail,E0392
3096 If the type parameter was included by mistake, this error can be fixed
3097 by simply removing the type parameter, as shown below:
3105 Alternatively, if the type parameter was intentionally inserted, it must be
3106 used. A simple fix is shown below:
3114 This error may also commonly be found when working with unsafe code. For
3115 example, when using raw pointers one may wish to specify the lifetime for
3116 which the pointed-at data is valid. An initial attempt (below) causes this
3119 ```compile_fail,E0392
3125 We want to express the constraint that Foo should not outlive `'a`, because
3126 the data pointed to by `T` is only valid for that lifetime. The problem is
3127 that there are no actual uses of `'a`. It's possible to work around this
3128 by adding a PhantomData type to the struct, using it to tell the compiler
3129 to act as if the struct contained a borrowed reference `&'a T`:
3132 use std::marker::PhantomData;
3134 struct Foo<'a, T: 'a> {
3136 phantom: PhantomData<&'a T>
3140 [PhantomData] can also be used to express information about unused type
3143 [PhantomData]: https://doc.rust-lang.org/std/marker/struct.PhantomData.html
3147 A type parameter which references `Self` in its default value was not specified.
3148 Example of erroneous code:
3150 ```compile_fail,E0393
3153 fn together_we_will_rule_the_galaxy(son: &A) {}
3154 // error: the type parameter `T` must be explicitly specified in an
3155 // object type because its default value `Self` references the
3159 A trait object is defined over a single, fully-defined trait. With a regular
3160 default parameter, this parameter can just be substituted in. However, if the
3161 default parameter is `Self`, the trait changes for each concrete type; i.e.
3162 `i32` will be expected to implement `A<i32>`, `bool` will be expected to
3163 implement `A<bool>`, etc... These types will not share an implementation of a
3164 fully-defined trait; instead they share implementations of a trait with
3165 different parameters substituted in for each implementation. This is
3166 irreconcilable with what we need to make a trait object work, and is thus
3167 disallowed. Making the trait concrete by explicitly specifying the value of the
3168 defaulted parameter will fix this issue. Fixed example:
3173 fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!
3178 You implemented a trait, overriding one or more of its associated types but did
3179 not reimplement its default methods.
3181 Example of erroneous code:
3183 ```compile_fail,E0399
3184 #![feature(associated_type_defaults)]
3192 // error - the following trait items need to be reimplemented as
3193 // `Assoc` was overridden: `bar`
3198 To fix this, add an implementation for each default method from the trait:
3201 #![feature(associated_type_defaults)]
3210 fn bar(&self) {} // ok!
3216 The functional record update syntax is only allowed for structs. (Struct-like
3217 enum variants don't qualify, for example.)
3219 Erroneous code example:
3221 ```compile_fail,E0436
3222 enum PublicationFrequency {
3224 SemiMonthly { days: (u8, u8), annual_special: bool },
3227 fn one_up_competitor(competitor_frequency: PublicationFrequency)
3228 -> PublicationFrequency {
3229 match competitor_frequency {
3230 PublicationFrequency::Weekly => PublicationFrequency::SemiMonthly {
3231 days: (1, 15), annual_special: false
3233 c @ PublicationFrequency::SemiMonthly{ .. } =>
3234 PublicationFrequency::SemiMonthly {
3235 annual_special: true, ..c // error: functional record update
3236 // syntax requires a struct
3242 Rewrite the expression without functional record update syntax:
3245 enum PublicationFrequency {
3247 SemiMonthly { days: (u8, u8), annual_special: bool },
3250 fn one_up_competitor(competitor_frequency: PublicationFrequency)
3251 -> PublicationFrequency {
3252 match competitor_frequency {
3253 PublicationFrequency::Weekly => PublicationFrequency::SemiMonthly {
3254 days: (1, 15), annual_special: false
3256 PublicationFrequency::SemiMonthly{ days, .. } =>
3257 PublicationFrequency::SemiMonthly {
3258 days, annual_special: true // ok!
3266 The length of the platform-intrinsic function `simd_shuffle`
3267 wasn't specified. Erroneous code example:
3269 ```compile_fail,E0439
3270 #![feature(platform_intrinsics)]
3272 extern "platform-intrinsic" {
3273 fn simd_shuffle<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3274 // error: invalid `simd_shuffle`, needs length: `simd_shuffle`
3278 The `simd_shuffle` function needs the length of the array passed as
3279 last parameter in its name. Example:
3282 #![feature(platform_intrinsics)]
3284 extern "platform-intrinsic" {
3285 fn simd_shuffle8<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3291 A platform-specific intrinsic function has the wrong number of type
3292 parameters. Erroneous code example:
3294 ```compile_fail,E0440
3295 #![feature(repr_simd)]
3296 #![feature(platform_intrinsics)]
3299 struct f64x2(f64, f64);
3301 extern "platform-intrinsic" {
3302 fn x86_mm_movemask_pd<T>(x: f64x2) -> i32;
3303 // error: platform-specific intrinsic has wrong number of type
3308 Please refer to the function declaration to see if it corresponds
3309 with yours. Example:
3312 #![feature(repr_simd)]
3313 #![feature(platform_intrinsics)]
3316 struct f64x2(f64, f64);
3318 extern "platform-intrinsic" {
3319 fn x86_mm_movemask_pd(x: f64x2) -> i32;
3325 An unknown platform-specific intrinsic function was used. Erroneous
3328 ```compile_fail,E0441
3329 #![feature(repr_simd)]
3330 #![feature(platform_intrinsics)]
3333 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3335 extern "platform-intrinsic" {
3336 fn x86_mm_adds_ep16(x: i16x8, y: i16x8) -> i16x8;
3337 // error: unrecognized platform-specific intrinsic function
3341 Please verify that the function name wasn't misspelled, and ensure
3342 that it is declared in the rust source code (in the file
3343 src/librustc_platform_intrinsics/x86.rs). Example:
3346 #![feature(repr_simd)]
3347 #![feature(platform_intrinsics)]
3350 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3352 extern "platform-intrinsic" {
3353 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3359 Intrinsic argument(s) and/or return value have the wrong type.
3360 Erroneous code example:
3362 ```compile_fail,E0442
3363 #![feature(repr_simd)]
3364 #![feature(platform_intrinsics)]
3367 struct i8x16(i8, i8, i8, i8, i8, i8, i8, i8,
3368 i8, i8, i8, i8, i8, i8, i8, i8);
3370 struct i32x4(i32, i32, i32, i32);
3372 struct i64x2(i64, i64);
3374 extern "platform-intrinsic" {
3375 fn x86_mm_adds_epi16(x: i8x16, y: i32x4) -> i64x2;
3376 // error: intrinsic arguments/return value have wrong type
3380 To fix this error, please refer to the function declaration to give
3381 it the awaited types. Example:
3384 #![feature(repr_simd)]
3385 #![feature(platform_intrinsics)]
3388 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3390 extern "platform-intrinsic" {
3391 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3397 Intrinsic argument(s) and/or return value have the wrong type.
3398 Erroneous code example:
3400 ```compile_fail,E0443
3401 #![feature(repr_simd)]
3402 #![feature(platform_intrinsics)]
3405 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3407 struct i64x8(i64, i64, i64, i64, i64, i64, i64, i64);
3409 extern "platform-intrinsic" {
3410 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i64x8;
3411 // error: intrinsic argument/return value has wrong type
3415 To fix this error, please refer to the function declaration to give
3416 it the awaited types. Example:
3419 #![feature(repr_simd)]
3420 #![feature(platform_intrinsics)]
3423 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3425 extern "platform-intrinsic" {
3426 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3432 A platform-specific intrinsic function has wrong number of arguments.
3433 Erroneous code example:
3435 ```compile_fail,E0444
3436 #![feature(repr_simd)]
3437 #![feature(platform_intrinsics)]
3440 struct f64x2(f64, f64);
3442 extern "platform-intrinsic" {
3443 fn x86_mm_movemask_pd(x: f64x2, y: f64x2, z: f64x2) -> i32;
3444 // error: platform-specific intrinsic has invalid number of arguments
3448 Please refer to the function declaration to see if it corresponds
3449 with yours. Example:
3452 #![feature(repr_simd)]
3453 #![feature(platform_intrinsics)]
3456 struct f64x2(f64, f64);
3458 extern "platform-intrinsic" {
3459 fn x86_mm_movemask_pd(x: f64x2) -> i32; // ok!
3465 The `typeof` keyword is currently reserved but unimplemented.
3466 Erroneous code example:
3468 ```compile_fail,E0516
3470 let x: typeof(92) = 92;
3474 Try using type inference instead. Example:
3484 A non-default implementation was already made on this type so it cannot be
3485 specialized further. Erroneous code example:
3487 ```compile_fail,E0520
3488 #![feature(specialization)]
3495 impl<T> SpaceLlama for T {
3496 default fn fly(&self) {}
3500 // applies to all `Clone` T and overrides the previous impl
3501 impl<T: Clone> SpaceLlama for T {
3505 // since `i32` is clone, this conflicts with the previous implementation
3506 impl SpaceLlama for i32 {
3507 default fn fly(&self) {}
3508 // error: item `fly` is provided by an `impl` that specializes
3509 // another, but the item in the parent `impl` is not marked
3510 // `default` and so it cannot be specialized.
3514 Specialization only allows you to override `default` functions in
3517 To fix this error, you need to mark all the parent implementations as default.
3521 #![feature(specialization)]
3528 impl<T> SpaceLlama for T {
3529 default fn fly(&self) {} // This is a parent implementation.
3532 // applies to all `Clone` T; overrides the previous impl
3533 impl<T: Clone> SpaceLlama for T {
3534 default fn fly(&self) {} // This is a parent implementation but was
3535 // previously not a default one, causing the error
3538 // applies to i32, overrides the previous two impls
3539 impl SpaceLlama for i32 {
3540 fn fly(&self) {} // And now that's ok!
3546 The number of elements in an array or slice pattern differed from the number of
3547 elements in the array being matched.
3549 Example of erroneous code:
3551 ```compile_fail,E0527
3552 let r = &[1, 2, 3, 4];
3554 &[a, b] => { // error: pattern requires 2 elements but array
3556 println!("a={}, b={}", a, b);
3561 Ensure that the pattern is consistent with the size of the matched
3562 array. Additional elements can be matched with `..`:
3565 #![feature(slice_patterns)]
3567 let r = &[1, 2, 3, 4];
3569 &[a, b, ..] => { // ok!
3570 println!("a={}, b={}", a, b);
3577 An array or slice pattern required more elements than were present in the
3580 Example of erroneous code:
3582 ```compile_fail,E0528
3583 #![feature(slice_patterns)]
3587 &[a, b, c, rest..] => { // error: pattern requires at least 3
3588 // elements but array has 2
3589 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3594 Ensure that the matched array has at least as many elements as the pattern
3595 requires. You can match an arbitrary number of remaining elements with `..`:
3598 #![feature(slice_patterns)]
3600 let r = &[1, 2, 3, 4, 5];
3602 &[a, b, c, rest..] => { // ok!
3603 // prints `a=1, b=2, c=3 rest=[4, 5]`
3604 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3611 An array or slice pattern was matched against some other type.
3613 Example of erroneous code:
3615 ```compile_fail,E0529
3618 [a, b] => { // error: expected an array or slice, found `f32`
3619 println!("a={}, b={}", a, b);
3624 Ensure that the pattern and the expression being matched on are of consistent
3631 println!("a={}, b={}", a, b);
3638 The `inline` attribute was malformed.
3640 Erroneous code example:
3642 ```ignore (compile_fail not working here; see Issue #43707)
3643 #[inline()] // error: expected one argument
3644 pub fn something() {}
3649 The parenthesized `inline` attribute requires the parameter to be specified:
3663 Alternatively, a paren-less version of the attribute may be used to hint the
3664 compiler about inlining opportunity:
3671 For more information about the inline attribute, read:
3672 https://doc.rust-lang.org/reference.html#inline-attributes
3676 An unknown argument was given to the `inline` attribute.
3678 Erroneous code example:
3680 ```ignore (compile_fail not working here; see Issue #43707)
3681 #[inline(unknown)] // error: invalid argument
3682 pub fn something() {}
3687 The `inline` attribute only supports two arguments:
3692 All other arguments given to the `inline` attribute will return this error.
3696 #[inline(never)] // ok!
3697 pub fn something() {}
3702 For more information about the inline attribute, https:
3703 read://doc.rust-lang.org/reference.html#inline-attributes
3707 The `export_name` attribute was malformed.
3709 Erroneous code example:
3711 ```ignore (error-emitted-at-codegen-which-cannot-be-handled-by-compile_fail)
3712 #[export_name] // error: export_name attribute has invalid format
3713 pub fn something() {}
3718 The `export_name` attribute expects a string in order to determine the name of
3719 the exported symbol. Example:
3722 #[export_name = "some_function"] // ok!
3723 pub fn something() {}
3730 An unknown field was specified into an enum's structure variant.
3732 Erroneous code example:
3734 ```compile_fail,E0559
3739 let s = Field::Fool { joke: 0 };
3740 // error: struct variant `Field::Fool` has no field named `joke`
3743 Verify you didn't misspell the field's name or that the field exists. Example:
3750 let s = Field::Fool { joke: 0 }; // ok!
3755 An unknown field was specified into a structure.
3757 Erroneous code example:
3759 ```compile_fail,E0560
3764 let s = Simba { mother: 1, father: 0 };
3765 // error: structure `Simba` has no field named `father`
3768 Verify you didn't misspell the field's name or that the field exists. Example:
3776 let s = Simba { mother: 1, father: 0 }; // ok!
3781 If an impl has a generic parameter with the `#[may_dangle]` attribute, then
3782 that impl must be declared as an `unsafe impl.
3784 Erroneous code example:
3786 ```compile_fail,E0569
3787 #![feature(dropck_eyepatch)]
3790 impl<#[may_dangle] X> Drop for Foo<X> {
3791 fn drop(&mut self) { }
3795 In this example, we are asserting that the destructor for `Foo` will not
3796 access any data of type `X`, and require this assertion to be true for
3797 overall safety in our program. The compiler does not currently attempt to
3798 verify this assertion; therefore we must tag this `impl` as unsafe.
3802 The requested ABI is unsupported by the current target.
3804 The rust compiler maintains for each target a blacklist of ABIs unsupported on
3805 that target. If an ABI is present in such a list this usually means that the
3806 target / ABI combination is currently unsupported by llvm.
3808 If necessary, you can circumvent this check using custom target specifications.
3812 A return statement was found outside of a function body.
3814 Erroneous code example:
3816 ```compile_fail,E0572
3817 const FOO: u32 = return 0; // error: return statement outside of function body
3822 To fix this issue, just remove the return keyword or move the expression into a
3828 fn some_fn() -> u32 {
3839 In a `fn` type, a lifetime appears only in the return type,
3840 and not in the arguments types.
3842 Erroneous code example:
3844 ```compile_fail,E0581
3846 // Here, `'a` appears only in the return type:
3847 let x: for<'a> fn() -> &'a i32;
3851 To fix this issue, either use the lifetime in the arguments, or use
3856 // Here, `'a` appears only in the return type:
3857 let x: for<'a> fn(&'a i32) -> &'a i32;
3858 let y: fn() -> &'static i32;
3862 Note: The examples above used to be (erroneously) accepted by the
3863 compiler, but this was since corrected. See [issue #33685] for more
3866 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
3870 A lifetime appears only in an associated-type binding,
3871 and not in the input types to the trait.
3873 Erroneous code example:
3875 ```compile_fail,E0582
3877 // No type can satisfy this requirement, since `'a` does not
3878 // appear in any of the input types (here, `i32`):
3879 where F: for<'a> Fn(i32) -> Option<&'a i32>
3886 To fix this issue, either use the lifetime in the inputs, or use
3890 fn bar<F, G>(t: F, u: G)
3891 where F: for<'a> Fn(&'a i32) -> Option<&'a i32>,
3892 G: Fn(i32) -> Option<&'static i32>,
3899 Note: The examples above used to be (erroneously) accepted by the
3900 compiler, but this was since corrected. See [issue #33685] for more
3903 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
3907 This error occurs when a method is used on a type which doesn't implement it:
3909 Erroneous code example:
3911 ```compile_fail,E0599
3915 x.chocolate(); // error: no method named `chocolate` found for type `Mouth`
3916 // in the current scope
3921 An unary operator was used on a type which doesn't implement it.
3923 Example of erroneous code:
3925 ```compile_fail,E0600
3931 !Question::Yes; // error: cannot apply unary operator `!` to type `Question`
3934 In this case, `Question` would need to implement the `std::ops::Not` trait in
3935 order to be able to use `!` on it. Let's implement it:
3945 // We implement the `Not` trait on the enum.
3946 impl Not for Question {
3949 fn not(self) -> bool {
3951 Question::Yes => false, // If the `Answer` is `Yes`, then it
3953 Question::No => true, // And here we do the opposite.
3958 assert_eq!(!Question::Yes, false);
3959 assert_eq!(!Question::No, true);
3964 An attempt to index into a type which doesn't implement the `std::ops::Index`
3965 trait was performed.
3967 Erroneous code example:
3969 ```compile_fail,E0608
3970 0u8[2]; // error: cannot index into a value of type `u8`
3973 To be able to index into a type it needs to implement the `std::ops::Index`
3977 let v: Vec<u8> = vec![0, 1, 2, 3];
3979 // The `Vec` type implements the `Index` trait so you can do:
3980 println!("{}", v[2]);
3985 A cast to `char` was attempted on a type other than `u8`.
3987 Erroneous code example:
3989 ```compile_fail,E0604
3990 0u32 as char; // error: only `u8` can be cast as `char`, not `u32`
3993 As the error message indicates, only `u8` can be cast into `char`. Example:
3996 let c = 86u8 as char; // ok!
4000 For more information about casts, take a look at The Book:
4001 https://doc.rust-lang.org/book/first-edition/casting-between-types.html
4005 An invalid cast was attempted.
4007 Erroneous code examples:
4009 ```compile_fail,E0605
4011 x as Vec<u8>; // error: non-primitive cast: `u8` as `std::vec::Vec<u8>`
4015 let v = 0 as *const u8; // So here, `v` is a `*const u8`.
4016 v as &u8; // error: non-primitive cast: `*const u8` as `&u8`
4019 Only primitive types can be cast into each other. Examples:
4025 let v = 0 as *const u8;
4026 v as *const i8; // ok!
4029 For more information about casts, take a look at The Book:
4030 https://doc.rust-lang.org/book/first-edition/casting-between-types.html
4034 An incompatible cast was attempted.
4036 Erroneous code example:
4038 ```compile_fail,E0606
4039 let x = &0u8; // Here, `x` is a `&u8`.
4040 let y: u32 = x as u32; // error: casting `&u8` as `u32` is invalid
4043 When casting, keep in mind that only primitive types can be cast into each
4048 let y: u32 = *x as u32; // We dereference it first and then cast it.
4051 For more information about casts, take a look at The Book:
4052 https://doc.rust-lang.org/book/first-edition/casting-between-types.html
4056 A cast between a thin and a fat pointer was attempted.
4058 Erroneous code example:
4060 ```compile_fail,E0607
4061 let v = 0 as *const u8;
4065 First: what are thin and fat pointers?
4067 Thin pointers are "simple" pointers: they are purely a reference to a memory
4070 Fat pointers are pointers referencing Dynamically Sized Types (also called DST).
4071 DST don't have a statically known size, therefore they can only exist behind
4072 some kind of pointers that contain additional information. Slices and trait
4073 objects are DSTs. In the case of slices, the additional information the fat
4074 pointer holds is their size.
4076 To fix this error, don't try to cast directly between thin and fat pointers.
4078 For more information about casts, take a look at The Book:
4079 https://doc.rust-lang.org/book/first-edition/casting-between-types.html
4083 Attempted to access a non-existent field in a struct.
4085 Erroneous code example:
4087 ```compile_fail,E0609
4088 struct StructWithFields {
4092 let s = StructWithFields { x: 0 };
4093 println!("{}", s.foo); // error: no field `foo` on type `StructWithFields`
4096 To fix this error, check that you didn't misspell the field's name or that the
4097 field actually exists. Example:
4100 struct StructWithFields {
4104 let s = StructWithFields { x: 0 };
4105 println!("{}", s.x); // ok!
4110 Attempted to access a field on a primitive type.
4112 Erroneous code example:
4114 ```compile_fail,E0610
4116 println!("{}", x.foo); // error: `{integer}` is a primitive type, therefore
4117 // doesn't have fields
4120 Primitive types are the most basic types available in Rust and don't have
4121 fields. To access data via named fields, struct types are used. Example:
4124 // We declare struct called `Foo` containing two fields:
4130 // We create an instance of this struct:
4131 let variable = Foo { x: 0, y: -12 };
4132 // And we can now access its fields:
4133 println!("x: {}, y: {}", variable.x, variable.y);
4136 For more information about primitives and structs, take a look at The Book:
4137 https://doc.rust-lang.org/book/first-edition/primitive-types.html
4138 https://doc.rust-lang.org/book/first-edition/structs.html
4142 Attempted to dereference a variable which cannot be dereferenced.
4144 Erroneous code example:
4146 ```compile_fail,E0614
4148 *y; // error: type `u32` cannot be dereferenced
4151 Only types implementing `std::ops::Deref` can be dereferenced (such as `&T`).
4157 // So here, `x` is a `&u32`, so we can dereference it:
4163 Attempted to access a method like a field.
4165 Erroneous code example:
4167 ```compile_fail,E0615
4176 let f = Foo { x: 0 };
4177 f.method; // error: attempted to take value of method `method` on type `Foo`
4180 If you want to use a method, add `()` after it:
4183 # struct Foo { x: u32 }
4184 # impl Foo { fn method(&self) {} }
4185 # let f = Foo { x: 0 };
4189 However, if you wanted to access a field of a struct check that the field name
4190 is spelled correctly. Example:
4193 # struct Foo { x: u32 }
4194 # impl Foo { fn method(&self) {} }
4195 # let f = Foo { x: 0 };
4196 println!("{}", f.x);
4201 Attempted to access a private field on a struct.
4203 Erroneous code example:
4205 ```compile_fail,E0616
4208 x: u32, // So `x` is private in here.
4212 pub fn new() -> Foo { Foo { x: 0 } }
4216 let f = some_module::Foo::new();
4217 println!("{}", f.x); // error: field `x` of struct `some_module::Foo` is private
4220 If you want to access this field, you have two options:
4222 1) Set the field public:
4227 pub x: u32, // `x` is now public.
4231 pub fn new() -> Foo { Foo { x: 0 } }
4235 let f = some_module::Foo::new();
4236 println!("{}", f.x); // ok!
4239 2) Add a getter function:
4244 x: u32, // So `x` is still private in here.
4248 pub fn new() -> Foo { Foo { x: 0 } }
4250 // We create the getter function here:
4251 pub fn get_x(&self) -> &u32 { &self.x }
4255 let f = some_module::Foo::new();
4256 println!("{}", f.get_x()); // ok!
4261 Attempted to pass an invalid type of variable into a variadic function.
4263 Erroneous code example:
4265 ```compile_fail,E0617
4267 fn printf(c: *const i8, ...);
4271 printf(::std::ptr::null(), 0f32);
4272 // error: can't pass an `f32` to variadic function, cast to `c_double`
4276 Certain Rust types must be cast before passing them to a variadic function,
4277 because of arcane ABI rules dictated by the C standard. To fix the error,
4278 cast the value to the type specified by the error message (which you may need
4279 to import from `std::os::raw`).
4283 Attempted to call something which isn't a function nor a method.
4285 Erroneous code examples:
4287 ```compile_fail,E0618
4292 X::Entry(); // error: expected function, found `X::Entry`
4296 x(); // error: expected function, found `i32`
4299 Only functions and methods can be called using `()`. Example:
4302 // We declare a function:
4303 fn i_am_a_function() {}
4311 #### Note: this error code is no longer emitted by the compiler.
4312 The type-checker needed to know the type of an expression, but that type had not
4315 Erroneous code example:
4321 // Here, the type of `v` is not (yet) known, so we
4322 // cannot resolve this method call:
4323 v.to_uppercase(); // error: the type of this value must be known in
4330 Type inference typically proceeds from the top of the function to the bottom,
4331 figuring out types as it goes. In some cases -- notably method calls and
4332 overloadable operators like `*` -- the type checker may not have enough
4333 information *yet* to make progress. This can be true even if the rest of the
4334 function provides enough context (because the type-checker hasn't looked that
4335 far ahead yet). In this case, type annotations can be used to help it along.
4337 To fix this error, just specify the type of the variable. Example:
4340 let mut x: Vec<String> = vec![]; // We precise the type of the vec elements.
4343 v.to_uppercase(); // Since rustc now knows the type of the vec elements,
4344 // we can use `v`'s methods.
4352 A cast to an unsized type was attempted.
4354 Erroneous code example:
4356 ```compile_fail,E0620
4357 let x = &[1_usize, 2] as [usize]; // error: cast to unsized type: `&[usize; 2]`
4361 In Rust, some types don't have a known size at compile-time. For example, in a
4362 slice type like `[u32]`, the number of elements is not known at compile-time and
4363 hence the overall size cannot be computed. As a result, such types can only be
4364 manipulated through a reference (e.g., `&T` or `&mut T`) or other pointer-type
4365 (e.g., `Box` or `Rc`). Try casting to a reference instead:
4368 let x = &[1_usize, 2] as &[usize]; // ok!
4373 An intrinsic was declared without being a function.
4375 Erroneous code example:
4377 ```compile_fail,E0622
4378 #![feature(intrinsics)]
4379 extern "rust-intrinsic" {
4380 pub static breakpoint : unsafe extern "rust-intrinsic" fn();
4381 // error: intrinsic must be a function
4384 fn main() { unsafe { breakpoint(); } }
4387 An intrinsic is a function available for use in a given programming language
4388 whose implementation is handled specially by the compiler. In order to fix this
4389 error, just declare a function.
4393 A private item was used outside of its scope.
4395 Erroneous code example:
4397 ```compile_fail,E0624
4406 let foo = inner::Foo;
4407 foo.method(); // error: method `method` is private
4410 Two possibilities are available to solve this issue:
4412 1. Only use the item in the scope it has been defined:
4422 pub fn call_method(foo: &Foo) { // We create a public function.
4423 foo.method(); // Which calls the item.
4427 let foo = inner::Foo;
4428 inner::call_method(&foo); // And since the function is public, we can call the
4429 // method through it.
4432 2. Make the item public:
4439 pub fn method(&self) {} // It's now public.
4443 let foo = inner::Foo;
4444 foo.method(); // Ok!
4449 This error indicates that the struct or enum must be matched non-exhaustively
4450 as it has been marked as `non_exhaustive`.
4452 When applied within a crate, downstream users of the crate will need to use the
4453 `_` pattern when matching enums and use the `..` pattern when matching structs.
4455 For example, in the below example, since the enum is marked as
4456 `non_exhaustive`, it is required that downstream crates match non-exhaustively
4459 ```rust,ignore (pseudo-Rust)
4460 use std::error::Error as StdError;
4462 #[non_exhaustive] pub enum Error {
4467 impl StdError for Error {
4468 fn description(&self) -> &str {
4469 // This will not error, despite being marked as non_exhaustive, as this
4470 // enum is defined within the current crate, it can be matched
4473 Message(ref s) => s,
4474 Other => "other or unknown error",
4480 An example of matching non-exhaustively on the above enum is provided below:
4482 ```rust,ignore (pseudo-Rust)
4485 // This will not error as the non_exhaustive Error enum has been matched with a
4488 Message(ref s) => ...,
4494 Similarly, for structs, match with `..` to avoid this error.
4498 This error indicates that the struct or enum cannot be instantiated from
4499 outside of the defining crate as it has been marked as `non_exhaustive` and as
4500 such more fields/variants may be added in future that could cause adverse side
4501 effects for this code.
4503 It is recommended that you look for a `new` function or equivalent in the
4504 crate's documentation.
4508 This error indicates that there is a mismatch between generic parameters and
4509 impl Trait parameters in a trait declaration versus its impl.
4511 ```compile_fail,E0643
4513 fn foo(&self, _: &impl Iterator);
4516 fn foo<U: Iterator>(&self, _: &U) { } // error method `foo` has incompatible
4517 // signature for trait
4523 It is not possible to define `main` with a where clause.
4524 Erroneous code example:
4526 ```compile_fail,E0646
4527 fn main() where i32: Copy { // error: main function is not allowed to have
4534 It is not possible to define `start` with a where clause.
4535 Erroneous code example:
4537 ```compile_fail,E0647
4541 fn start(_: isize, _: *const *const u8) -> isize where (): Copy {
4542 //^ error: start function is not allowed to have a where clause
4549 `export_name` attributes may not contain null characters (`\0`).
4551 ```compile_fail,E0648
4552 #[export_name="\0foo"] // error: `export_name` may not contain null characters
4557 This error indicates that the numeric value for the method being passed exists
4558 but the type of the numeric value or binding could not be identified.
4560 The error happens on numeric literals:
4562 ```compile_fail,E0689
4566 and on numeric bindings without an identified concrete type:
4568 ```compile_fail,E0689
4570 x.neg(); // same error as above
4573 Because of this, you must give the numeric literal or binding a type:
4578 let _ = 2.0_f32.neg();
4581 let _ = (2.0 as f32).neg();
4586 A struct with the representation hint `repr(transparent)` had zero or more than
4587 on fields that were not guaranteed to be zero-sized.
4589 Erroneous code example:
4591 ```compile_fail,E0690
4592 #[repr(transparent)]
4593 struct LengthWithUnit<U> { // error: transparent struct needs exactly one
4594 value: f32, // non-zero-sized field, but has 2
4599 Because transparent structs are represented exactly like one of their fields at
4600 run time, said field must be uniquely determined. If there is no field, or if
4601 there are multiple fields, it is not clear how the struct should be represented.
4602 Note that fields of zero-typed types (e.g., `PhantomData`) can also exist
4603 alongside the field that contains the actual data, they do not count for this
4604 error. When generic types are involved (as in the above example), an error is
4605 reported because the type parameter could be non-zero-sized.
4607 To combine `repr(transparent)` with type parameters, `PhantomData` may be
4611 use std::marker::PhantomData;
4613 #[repr(transparent)]
4614 struct LengthWithUnit<U> {
4616 unit: PhantomData<U>,
4622 A struct with the `repr(transparent)` representation hint contains a zero-sized
4623 field that requires non-trivial alignment.
4625 Erroneous code example:
4627 ```compile_fail,E0691
4628 #![feature(repr_align, attr_literals)]
4631 struct ForceAlign32;
4633 #[repr(transparent)]
4634 struct Wrapper(f32, ForceAlign32); // error: zero-sized field in transparent
4635 // struct has alignment larger than 1
4638 A transparent struct is supposed to be represented exactly like the piece of
4639 data it contains. Zero-sized fields with different alignment requirements
4640 potentially conflict with this property. In the example above, `Wrapper` would
4641 have to be aligned to 32 bytes even though `f32` has a smaller alignment
4644 Consider removing the over-aligned zero-sized field:
4647 #[repr(transparent)]
4648 struct Wrapper(f32);
4651 Alternatively, `PhantomData<T>` has alignment 1 for all `T`, so you can use it
4652 if you need to keep the field for some reason:
4655 #![feature(repr_align, attr_literals)]
4657 use std::marker::PhantomData;
4660 struct ForceAlign32;
4662 #[repr(transparent)]
4663 struct Wrapper(f32, PhantomData<ForceAlign32>);
4666 Note that empty arrays `[T; 0]` have the same alignment requirement as the
4667 element type `T`. Also note that the error is conservatively reported even when
4668 the alignment of the zero-sized type is less than or equal to the data field's
4674 A method was called on a raw pointer whose inner type wasn't completely known.
4676 For example, you may have done something like:
4679 # #![deny(warnings)]
4681 let bar = foo as *const _;
4687 Here, the type of `bar` isn't known; it could be a pointer to anything. Instead,
4688 specify a type for the pointer (preferably something that makes sense for the
4689 thing you're pointing to):
4693 let bar = foo as *const i32;
4699 Even though `is_null()` exists as a method on any raw pointer, Rust shows this
4700 error because Rust allows for `self` to have arbitrary types (behind the
4701 arbitrary_self_types feature flag).
4703 This means that someone can specify such a function:
4705 ```ignore (cannot-doctest-feature-doesnt-exist-yet)
4707 fn is_null(self: *const Self) -> bool {
4708 // do something else
4713 and now when you call `.is_null()` on a raw pointer to `Foo`, there's ambiguity.
4715 Given that we don't know what type the pointer is, and there's potential
4716 ambiguity for some types, we disallow calling methods on raw pointers when
4717 the type is unknown.
4722 register_diagnostics! {
4723 // E0035, merged into E0087/E0089
4724 // E0036, merged into E0087/E0089
4730 // E0122, // bounds in type aliases are ignored, turned into proper lint
4735 // E0159, // use of trait `{}` as struct constructor
4736 // E0163, // merged into E0071
4739 // E0172, // non-trait found in a type sum, moved to resolve
4740 // E0173, // manual implementations of unboxed closure traits are experimental
4742 // E0182, // merged into E0229
4744 // E0187, // can't infer the kind of the closure
4745 // E0188, // can not cast an immutable reference to a mutable pointer
4746 // E0189, // deprecated: can only cast a boxed pointer to a boxed object
4747 // E0190, // deprecated: can only cast a &-pointer to an &-object
4748 // E0196, // cannot determine a type for this closure
4749 E0203, // type parameter has more than one relaxed default bound,
4750 // and only one is supported
4752 // E0209, // builtin traits can only be implemented on structs or enums
4753 E0212, // cannot extract an associated type from a higher-ranked trait bound
4754 // E0213, // associated types are not accepted in this context
4755 // E0215, // angle-bracket notation is not stable with `Fn`
4756 // E0216, // parenthetical notation is only stable with `Fn`
4757 // E0217, // ambiguous associated type, defined in multiple supertraits
4758 // E0218, // no associated type defined
4759 // E0219, // associated type defined in higher-ranked supertrait
4760 // E0222, // Error code E0045 (variadic function must have C or cdecl calling
4761 // convention) duplicate
4762 E0224, // at least one non-builtin train is required for an object type
4763 E0227, // ambiguous lifetime bound, explicit lifetime bound required
4764 E0228, // explicit lifetime bound required
4767 // E0235, // structure constructor specifies a structure of type but
4768 // E0236, // no lang item for range syntax
4769 // E0237, // no lang item for range syntax
4770 // E0238, // parenthesized parameters may only be used with a trait
4771 // E0239, // `next` method of `Iterator` trait has unexpected type
4775 // E0245, // not a trait
4776 // E0246, // invalid recursive type
4778 // E0248, // value used as a type, now reported earlier during resolution as E0412
4780 E0307, // invalid method `self` type
4781 // E0319, // trait impls for defaulted traits allowed just for structs/enums
4782 // E0372, // coherence not object safe
4783 E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
4784 // between structures with the same definition
4785 E0533, // `{}` does not name a unit variant, unit struct or a constant
4786 // E0563, // cannot determine a type for this `impl Trait`: {} // removed in 6383de15
4787 E0564, // only named lifetimes are allowed in `impl Trait`,
4788 // but `{}` was found in the type `{}`
4789 E0587, // type has conflicting packed and align representation hints
4790 E0588, // packed type cannot transitively contain a `[repr(align)]` type
4791 E0592, // duplicate definitions with name `{}`
4792 // E0611, // merged into E0616
4793 // E0612, // merged into E0609
4794 // E0613, // Removed (merged with E0609)
4795 E0627, // yield statement outside of generator literal
4796 E0632, // cannot provide explicit type parameters when `impl Trait` is used in
4797 // argument position.
4798 E0634, // type has conflicting packed representaton hints
4799 E0640, // infer outlives requirements
4800 E0641, // cannot cast to/from a pointer with an unknown kind
4801 E0645, // trait aliases not finished
4802 E0698, // type inside generator must be known in this context