1 // ignore-tidy-linelength
2 // ignore-tidy-filelength
4 #![allow(non_snake_case)]
6 register_long_diagnostics! {
9 A pattern used to match against an enum variant must provide a sub-pattern for
10 each field of the enum variant. This error indicates that a pattern attempted to
11 extract an incorrect number of fields from a variant.
15 Apple(String, String),
20 Here the `Apple` variant has two fields, and should be matched against like so:
24 Apple(String, String),
28 let x = Fruit::Apple(String::new(), String::new());
32 Fruit::Apple(a, b) => {},
37 Matching with the wrong number of fields has no sensible interpretation:
41 Apple(String, String),
45 let x = Fruit::Apple(String::new(), String::new());
49 Fruit::Apple(a) => {},
50 Fruit::Apple(a, b, c) => {},
54 Check how many fields the enum was declared with and ensure that your pattern
59 Each field of a struct can only be bound once in a pattern. Erroneous code
69 let x = Foo { a:1, b:2 };
71 let Foo { a: x, a: y } = x;
72 // error: field `a` bound multiple times in the pattern
76 Each occurrence of a field name binds the value of that field, so to fix this
77 error you will have to remove or alter the duplicate uses of the field name.
78 Perhaps you misspelled another field name? Example:
87 let x = Foo { a:1, b:2 };
89 let Foo { a: x, b: y } = x; // ok!
95 This error indicates that a struct pattern attempted to extract a non-existent
96 field from a struct. Struct fields are identified by the name used before the
97 colon `:` so struct patterns should resemble the declaration of the struct type
107 let thing = Thing { x: 1, y: 2 };
110 Thing { x: xfield, y: yfield } => {}
114 If you are using shorthand field patterns but want to refer to the struct field
115 by a different name, you should rename it explicitly.
119 ```compile_fail,E0026
125 let thing = Thing { x: 0, y: 0 };
140 let thing = Thing { x: 0, y: 0 };
143 Thing { x, y: z } => {}
149 This error indicates that a pattern for a struct fails to specify a sub-pattern
150 for every one of the struct's fields. Ensure that each field from the struct's
151 definition is mentioned in the pattern, or use `..` to ignore unwanted fields.
155 ```compile_fail,E0027
161 let d = Dog { name: "Rusty".to_string(), age: 8 };
163 // This is incorrect.
169 This is correct (explicit):
177 let d = Dog { name: "Rusty".to_string(), age: 8 };
180 Dog { name: ref n, age: x } => {}
183 // This is also correct (ignore unused fields).
185 Dog { age: x, .. } => {}
191 In a match expression, only numbers and characters can be matched against a
192 range. This is because the compiler checks that the range is non-empty at
193 compile-time, and is unable to evaluate arbitrary comparison functions. If you
194 want to capture values of an orderable type between two end-points, you can use
197 ```compile_fail,E0029
198 let string = "salutations !";
200 // The ordering relation for strings can't be evaluated at compile time,
201 // so this doesn't work:
203 "hello" ..= "world" => {}
207 // This is a more general version, using a guard:
209 s if s >= "hello" && s <= "world" => {}
216 This error indicates that a pointer to a trait type cannot be implicitly
217 dereferenced by a pattern. Every trait defines a type, but because the
218 size of trait implementors isn't fixed, this type has no compile-time size.
219 Therefore, all accesses to trait types must be through pointers. If you
220 encounter this error you should try to avoid dereferencing the pointer.
222 ```compile_fail,E0033
223 # trait SomeTrait { fn method_one(&self){} fn method_two(&self){} }
224 # impl<T> SomeTrait for T {}
225 let trait_obj: &SomeTrait = &"some_value";
227 // This tries to implicitly dereference to create an unsized local variable.
228 let &invalid = trait_obj;
230 // You can call methods without binding to the value being pointed at.
231 trait_obj.method_one();
232 trait_obj.method_two();
235 You can read more about trait objects in the [Trait Objects] section of the
238 [Trait Objects]: https://doc.rust-lang.org/reference/types.html#trait-objects
242 The compiler doesn't know what method to call because more than one method
243 has the same prototype. Erroneous code example:
245 ```compile_fail,E0034
256 impl Trait1 for Test { fn foo() {} }
257 impl Trait2 for Test { fn foo() {} }
260 Test::foo() // error, which foo() to call?
264 To avoid this error, you have to keep only one of them and remove the others.
265 So let's take our example and fix it:
274 impl Trait1 for Test { fn foo() {} }
277 Test::foo() // and now that's good!
281 However, a better solution would be using fully explicit naming of type and
295 impl Trait1 for Test { fn foo() {} }
296 impl Trait2 for Test { fn foo() {} }
299 <Test as Trait1>::foo()
316 impl F for X { fn m(&self) { println!("I am F"); } }
317 impl G for X { fn m(&self) { println!("I am G"); } }
322 F::m(&f); // it displays "I am F"
323 G::m(&f); // it displays "I am G"
329 It is not allowed to manually call destructors in Rust. It is also not
330 necessary to do this since `drop` is called automatically whenever a value goes
333 Here's an example of this error:
335 ```compile_fail,E0040
347 let mut x = Foo { x: -7 };
348 x.drop(); // error: explicit use of destructor method
354 You can't use type or const parameters on foreign items.
355 Example of erroneous code:
357 ```compile_fail,E0044
358 extern { fn some_func<T>(x: T); }
361 To fix this, replace the generic parameter with the specializations that you
365 extern { fn some_func_i32(x: i32); }
366 extern { fn some_func_i64(x: i64); }
371 Rust only supports variadic parameters for interoperability with C code in its
372 FFI. As such, variadic parameters can only be used with functions which are
373 using the C ABI. Examples of erroneous code:
376 #![feature(unboxed_closures)]
378 extern "rust-call" { fn foo(x: u8, ...); }
382 fn foo(x: u8, ...) {}
385 To fix such code, put them in an extern "C" block:
395 Items are missing in a trait implementation. Erroneous code example:
397 ```compile_fail,E0046
405 // error: not all trait items implemented, missing: `foo`
408 When trying to make some type implement a trait `Foo`, you must, at minimum,
409 provide implementations for all of `Foo`'s required methods (meaning the
410 methods that do not have default implementations), as well as any required
411 trait items like associated types or constants. Example:
427 This error indicates that an attempted implementation of a trait method
428 has the wrong number of type or const parameters.
430 For example, the trait below has a method `foo` with a type parameter `T`,
431 but the implementation of `foo` for the type `Bar` is missing this parameter:
433 ```compile_fail,E0049
435 fn foo<T: Default>(x: T) -> Self;
440 // error: method `foo` has 0 type parameters but its trait declaration has 1
443 fn foo(x: bool) -> Self { Bar }
449 This error indicates that an attempted implementation of a trait method
450 has the wrong number of function parameters.
452 For example, the trait below has a method `foo` with two function parameters
453 (`&self` and `u8`), but the implementation of `foo` for the type `Bar` omits
456 ```compile_fail,E0050
458 fn foo(&self, x: u8) -> bool;
463 // error: method `foo` has 1 parameter but the declaration in trait `Foo::foo`
466 fn foo(&self) -> bool { true }
472 The parameters of any trait method must match between a trait implementation
473 and the trait definition.
475 Here are a couple examples of this error:
477 ```compile_fail,E0053
486 // error, expected u16, found i16
489 // error, types differ in mutability
490 fn bar(&mut self) { }
496 It is not allowed to cast to a bool. If you are trying to cast a numeric type
497 to a bool, you can compare it with zero instead:
499 ```compile_fail,E0054
502 // Not allowed, won't compile
503 let x_is_nonzero = x as bool;
510 let x_is_nonzero = x != 0;
515 During a method call, a value is automatically dereferenced as many times as
516 needed to make the value's type match the method's receiver. The catch is that
517 the compiler will only attempt to dereference a number of times up to the
518 recursion limit (which can be set via the `recursion_limit` attribute).
520 For a somewhat artificial example:
522 ```compile_fail,E0055
523 #![recursion_limit="5"]
533 let ref_foo = &&&&&Foo;
535 // error, reached the recursion limit while auto-dereferencing `&&&&&Foo`
540 One fix may be to increase the recursion limit. Note that it is possible to
541 create an infinite recursion of dereferencing, in which case the only fix is to
542 somehow break the recursion.
546 When invoking closures or other implementations of the function traits `Fn`,
547 `FnMut` or `FnOnce` using call notation, the number of parameters passed to the
548 function must match its definition.
550 An example using a closure:
552 ```compile_fail,E0057
554 let a = f(); // invalid, too few parameters
555 let b = f(4); // this works!
556 let c = f(2, 3); // invalid, too many parameters
559 A generic function must be treated similarly:
562 fn foo<F: Fn()>(f: F) {
563 f(); // this is valid, but f(3) would not work
569 The built-in function traits are generic over a tuple of the function arguments.
570 If one uses angle-bracket notation (`Fn<(T,), Output=U>`) instead of parentheses
571 (`Fn(T) -> U`) to denote the function trait, the type parameter should be a
572 tuple. Otherwise function call notation cannot be used and the trait will not be
573 implemented by closures.
575 The most likely source of this error is using angle-bracket notation without
576 wrapping the function argument type into a tuple, for example:
578 ```compile_fail,E0059
579 #![feature(unboxed_closures)]
581 fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) }
584 It can be fixed by adjusting the trait bound like this:
587 #![feature(unboxed_closures)]
589 fn foo<F: Fn<(i32,)>>(f: F) -> F::Output { f(3) }
592 Note that `(T,)` always denotes the type of a 1-tuple containing an element of
593 type `T`. The comma is necessary for syntactic disambiguation.
597 External C functions are allowed to be variadic. However, a variadic function
598 takes a minimum number of arguments. For example, consider C's variadic `printf`
602 use std::os::raw::{c_char, c_int};
605 fn printf(_: *const c_char, ...) -> c_int;
609 Using this declaration, it must be called with at least one argument, so
610 simply calling `printf()` is invalid. But the following uses are allowed:
613 # #![feature(static_nobundle)]
614 # use std::os::raw::{c_char, c_int};
615 # #[cfg_attr(all(windows, target_env = "msvc"),
616 # link(name = "legacy_stdio_definitions", kind = "static-nobundle"))]
617 # extern "C" { fn printf(_: *const c_char, ...) -> c_int; }
620 use std::ffi::CString;
622 let fmt = CString::new("test\n").unwrap();
623 printf(fmt.as_ptr());
625 let fmt = CString::new("number = %d\n").unwrap();
626 printf(fmt.as_ptr(), 3);
628 let fmt = CString::new("%d, %d\n").unwrap();
629 printf(fmt.as_ptr(), 10, 5);
634 // ^ Note: On MSVC 2015, the `printf` function is "inlined" in the C code, and
635 // the C runtime does not contain the `printf` definition. This leads to linker
636 // error from the doc test (issue #42830).
637 // This can be fixed by linking to the static library
638 // `legacy_stdio_definitions.lib` (see https://stackoverflow.com/a/36504365/).
639 // If this compatibility library is removed in the future, consider changing
640 // `printf` in this example to another well-known variadic function.
643 The number of arguments passed to a function must match the number of arguments
644 specified in the function signature.
646 For example, a function like:
649 fn f(a: u16, b: &str) {}
652 Must always be called with exactly two arguments, e.g., `f(2, "test")`.
654 Note that Rust does not have a notion of optional function arguments or
655 variadic functions (except for its C-FFI).
659 This error indicates that during an attempt to build a struct or struct-like
660 enum variant, one of the fields was specified more than once. Erroneous code
663 ```compile_fail,E0062
671 x: 0, // error: field `x` specified more than once
676 Each field should be specified exactly one time. Example:
684 let x = Foo { x: 0 }; // ok!
690 This error indicates that during an attempt to build a struct or struct-like
691 enum variant, one of the fields was not provided. Erroneous code example:
693 ```compile_fail,E0063
700 let x = Foo { x: 0 }; // error: missing field: `y`
704 Each field should be specified exactly once. Example:
713 let x = Foo { x: 0, y: 0 }; // ok!
719 The left-hand side of a compound assignment expression must be a place
720 expression. A place expression represents a memory location and includes
721 item paths (ie, namespaced variables), dereferences, indexing expressions,
722 and field references.
724 Let's start with some erroneous code examples:
726 ```compile_fail,E0067
727 use std::collections::LinkedList;
729 // Bad: assignment to non-place expression
730 LinkedList::new() += 1;
734 fn some_func(i: &mut i32) {
735 i += 12; // Error : '+=' operation cannot be applied on a reference !
739 And now some working examples:
748 fn some_func(i: &mut i32) {
755 The compiler found a function whose body contains a `return;` statement but
756 whose return type is not `()`. An example of this is:
758 ```compile_fail,E0069
765 Since `return;` is just like `return ();`, there is a mismatch between the
766 function's return type and the value being returned.
770 The left-hand side of an assignment operator must be a place expression. A
771 place expression represents a memory location and can be a variable (with
772 optional namespacing), a dereference, an indexing expression or a field
775 More details can be found in the [Expressions] section of the Reference.
777 [Expressions]: https://doc.rust-lang.org/reference/expressions.html#places-rvalues-and-temporaries
779 Now, we can go further. Here are some erroneous code examples:
781 ```compile_fail,E0070
787 const SOME_CONST : i32 = 12;
789 fn some_other_func() {}
792 SOME_CONST = 14; // error : a constant value cannot be changed!
793 1 = 3; // error : 1 isn't a valid place!
794 some_other_func() = 4; // error : we can't assign value to a function!
795 SomeStruct.x = 12; // error : SomeStruct a structure name but it is used
800 And now let's give working examples:
807 let mut s = SomeStruct {x: 0, y: 0};
809 s.x = 3; // that's good !
813 fn some_func(x: &mut i32) {
814 *x = 12; // that's good !
820 You tried to use structure-literal syntax to create an item that is
821 not a structure or enum variant.
823 Example of erroneous code:
825 ```compile_fail,E0071
827 let t = U32 { value: 4 }; // error: expected struct, variant or union type,
828 // found builtin type `u32`
831 To fix this, ensure that the name was correctly spelled, and that
832 the correct form of initializer was used.
834 For example, the code above can be fixed to:
842 let u = Foo::FirstValue(0i32);
850 #### Note: this error code is no longer emitted by the compiler.
852 You cannot define a struct (or enum) `Foo` that requires an instance of `Foo`
853 in order to make a new `Foo` value. This is because there would be no way a
854 first instance of `Foo` could be made to initialize another instance!
856 Here's an example of a struct that has this problem:
859 struct Foo { x: Box<Foo> } // error
862 One fix is to use `Option`, like so:
865 struct Foo { x: Option<Box<Foo>> }
868 Now it's possible to create at least one instance of `Foo`: `Foo { x: None }`.
872 #### Note: this error code is no longer emitted by the compiler.
874 When using the `#[simd]` attribute on a tuple struct, the components of the
875 tuple struct must all be of a concrete, nongeneric type so the compiler can
876 reason about how to use SIMD with them. This error will occur if the types
879 This will cause an error:
882 #![feature(repr_simd)]
885 struct Bad<T>(T, T, T);
891 #![feature(repr_simd)]
894 struct Good(u32, u32, u32);
899 The `#[simd]` attribute can only be applied to non empty tuple structs, because
900 it doesn't make sense to try to use SIMD operations when there are no values to
903 This will cause an error:
905 ```compile_fail,E0075
906 #![feature(repr_simd)]
915 #![feature(repr_simd)]
923 When using the `#[simd]` attribute to automatically use SIMD operations in tuple
924 struct, the types in the struct must all be of the same type, or the compiler
925 will trigger this error.
927 This will cause an error:
929 ```compile_fail,E0076
930 #![feature(repr_simd)]
933 struct Bad(u16, u32, u32);
939 #![feature(repr_simd)]
942 struct Good(u32, u32, u32);
947 When using the `#[simd]` attribute on a tuple struct, the elements in the tuple
948 must be machine types so SIMD operations can be applied to them.
950 This will cause an error:
952 ```compile_fail,E0077
953 #![feature(repr_simd)]
962 #![feature(repr_simd)]
965 struct Good(u32, u32, u32);
970 Enum discriminants are used to differentiate enum variants stored in memory.
971 This error indicates that the same value was used for two or more variants,
972 making them impossible to tell apart.
974 ```compile_fail,E0081
992 Note that variants without a manually specified discriminant are numbered from
993 top to bottom starting from 0, so clashes can occur with seemingly unrelated
996 ```compile_fail,E0081
1003 Here `X` will have already been specified the discriminant 0 by the time `Y` is
1004 encountered, so a conflict occurs.
1008 An unsupported representation was attempted on a zero-variant enum.
1010 Erroneous code example:
1012 ```compile_fail,E0084
1014 enum NightsWatch {} // error: unsupported representation for zero-variant enum
1017 It is impossible to define an integer type to be used to represent zero-variant
1018 enum values because there are no zero-variant enum values. There is no way to
1019 construct an instance of the following type using only safe code. So you have
1020 two solutions. Either you add variants in your enum:
1030 or you remove the integer represention of your enum:
1037 // FIXME(const_generics:docs): example of inferring const parameter.
1039 #### Note: this error code is no longer emitted by the compiler.
1041 Too many type arguments were supplied for a function. For example:
1043 ```compile_fail,E0107
1047 foo::<f64, bool>(); // error: wrong number of type arguments:
1048 // expected 1, found 2
1052 The number of supplied arguments must exactly match the number of defined type
1057 #### Note: this error code is no longer emitted by the compiler.
1059 You gave too many lifetime arguments. Erroneous code example:
1061 ```compile_fail,E0107
1065 f::<'static>() // error: wrong number of lifetime arguments:
1066 // expected 0, found 1
1070 Please check you give the right number of lifetime arguments. Example:
1080 It's also important to note that the Rust compiler can generally
1081 determine the lifetime by itself. Example:
1089 // it can be written like this
1090 fn get_value<'a>(&'a self) -> &'a str { &self.value }
1091 // but the compiler works fine with this too:
1092 fn without_lifetime(&self) -> &str { &self.value }
1096 let f = Foo { value: "hello".to_owned() };
1098 println!("{}", f.get_value());
1099 println!("{}", f.without_lifetime());
1105 #### Note: this error code is no longer emitted by the compiler.
1107 Too few type arguments were supplied for a function. For example:
1109 ```compile_fail,E0107
1113 foo::<f64>(); // error: wrong number of type arguments: expected 2, found 1
1117 Note that if a function takes multiple type arguments but you want the compiler
1118 to infer some of them, you can use type placeholders:
1120 ```compile_fail,E0107
1121 fn foo<T, U>(x: T) {}
1125 foo::<f64>(x); // error: wrong number of type arguments:
1126 // expected 2, found 1
1127 foo::<_, f64>(x); // same as `foo::<bool, f64>(x)`
1133 #### Note: this error code is no longer emitted by the compiler.
1135 You gave too few lifetime arguments. Example:
1137 ```compile_fail,E0107
1138 fn foo<'a: 'b, 'b: 'a>() {}
1141 foo::<'static>(); // error: wrong number of lifetime arguments:
1142 // expected 2, found 1
1146 Please check you give the right number of lifetime arguments. Example:
1149 fn foo<'a: 'b, 'b: 'a>() {}
1152 foo::<'static, 'static>();
1158 You gave an unnecessary type or const parameter in a type alias. Erroneous
1161 ```compile_fail,E0091
1162 type Foo<T> = u32; // error: type parameter `T` is unused
1164 type Foo<A,B> = Box<A>; // error: type parameter `B` is unused
1167 Please check you didn't write too many parameters. Example:
1170 type Foo = u32; // ok!
1171 type Foo2<A> = Box<A>; // ok!
1176 You tried to declare an undefined atomic operation function.
1177 Erroneous code example:
1179 ```compile_fail,E0092
1180 #![feature(intrinsics)]
1182 extern "rust-intrinsic" {
1183 fn atomic_foo(); // error: unrecognized atomic operation
1188 Please check you didn't make a mistake in the function's name. All intrinsic
1189 functions are defined in librustc_codegen_llvm/intrinsic.rs and in
1190 libcore/intrinsics.rs in the Rust source code. Example:
1193 #![feature(intrinsics)]
1195 extern "rust-intrinsic" {
1196 fn atomic_fence(); // ok!
1202 You declared an unknown intrinsic function. Erroneous code example:
1204 ```compile_fail,E0093
1205 #![feature(intrinsics)]
1207 extern "rust-intrinsic" {
1208 fn foo(); // error: unrecognized intrinsic function: `foo`
1218 Please check you didn't make a mistake in the function's name. All intrinsic
1219 functions are defined in librustc_codegen_llvm/intrinsic.rs and in
1220 libcore/intrinsics.rs in the Rust source code. Example:
1223 #![feature(intrinsics)]
1225 extern "rust-intrinsic" {
1226 fn atomic_fence(); // ok!
1238 You gave an invalid number of type parameters to an intrinsic function.
1239 Erroneous code example:
1241 ```compile_fail,E0094
1242 #![feature(intrinsics)]
1244 extern "rust-intrinsic" {
1245 fn size_of<T, U>() -> usize; // error: intrinsic has wrong number
1246 // of type parameters
1250 Please check that you provided the right number of type parameters
1251 and verify with the function declaration in the Rust source code.
1255 #![feature(intrinsics)]
1257 extern "rust-intrinsic" {
1258 fn size_of<T>() -> usize; // ok!
1264 This error means that an incorrect number of generic arguments were provided:
1266 ```compile_fail,E0107
1267 struct Foo<T> { x: T }
1269 struct Bar { x: Foo } // error: wrong number of type arguments:
1270 // expected 1, found 0
1271 struct Baz<S, T> { x: Foo<S, T> } // error: wrong number of type arguments:
1272 // expected 1, found 2
1274 fn foo<T, U>(x: T, y: U) {}
1278 foo::<bool>(x); // error: wrong number of type arguments:
1279 // expected 2, found 1
1280 foo::<bool, i32, i32>(x, 2, 4); // error: wrong number of type arguments:
1281 // expected 2, found 3
1287 f::<'static>(); // error: wrong number of lifetime arguments:
1288 // expected 0, found 1
1295 You tried to provide a generic argument to a type which doesn't need it.
1296 Erroneous code example:
1298 ```compile_fail,E0109
1299 type X = u32<i32>; // error: type arguments are not allowed for this type
1300 type Y = bool<'static>; // error: lifetime parameters are not allowed on
1304 Check that you used the correct argument and that the definition is correct.
1309 type X = u32; // ok!
1310 type Y = bool; // ok!
1313 Note that generic arguments for enum variant constructors go after the variant,
1314 not after the enum. For example, you would write `Option::None::<u32>`,
1315 rather than `Option::<u32>::None`.
1319 #### Note: this error code is no longer emitted by the compiler.
1321 You tried to provide a lifetime to a type which doesn't need it.
1322 See `E0109` for more details.
1326 You can only define an inherent implementation for a type in the same crate
1327 where the type was defined. For example, an `impl` block as below is not allowed
1328 since `Vec` is defined in the standard library:
1330 ```compile_fail,E0116
1331 impl Vec<u8> { } // error
1334 To fix this problem, you can do either of these things:
1336 - define a trait that has the desired associated functions/types/constants and
1337 implement the trait for the type in question
1338 - define a new type wrapping the type and define an implementation on the new
1341 Note that using the `type` keyword does not work here because `type` only
1342 introduces a type alias:
1344 ```compile_fail,E0116
1345 type Bytes = Vec<u8>;
1347 impl Bytes { } // error, same as above
1352 This error indicates a violation of one of Rust's orphan rules for trait
1353 implementations. The rule prohibits any implementation of a foreign trait (a
1354 trait defined in another crate) where
1356 - the type that is implementing the trait is foreign
1357 - all of the parameters being passed to the trait (if there are any) are also
1360 Here's one example of this error:
1362 ```compile_fail,E0117
1363 impl Drop for u32 {}
1366 To avoid this kind of error, ensure that at least one local type is referenced
1370 pub struct Foo; // you define your type in your crate
1372 impl Drop for Foo { // and you can implement the trait on it!
1373 // code of trait implementation here
1374 # fn drop(&mut self) { }
1377 impl From<Foo> for i32 { // or you use a type from your crate as
1379 fn from(i: Foo) -> i32 {
1385 Alternatively, define a trait locally and implement that instead:
1389 fn get(&self) -> usize;
1393 fn get(&self) -> usize { 0 }
1397 For information on the design of the orphan rules, see [RFC 1023].
1399 [RFC 1023]: https://github.com/rust-lang/rfcs/blob/master/text/1023-rebalancing-coherence.md
1403 You're trying to write an inherent implementation for something which isn't a
1404 struct nor an enum. Erroneous code example:
1406 ```compile_fail,E0118
1407 impl (u8, u8) { // error: no base type found for inherent implementation
1408 fn get_state(&self) -> String {
1414 To fix this error, please implement a trait on the type or wrap it in a struct.
1418 // we create a trait here
1419 trait LiveLongAndProsper {
1420 fn get_state(&self) -> String;
1423 // and now you can implement it on (u8, u8)
1424 impl LiveLongAndProsper for (u8, u8) {
1425 fn get_state(&self) -> String {
1426 "He's dead, Jim!".to_owned()
1431 Alternatively, you can create a newtype. A newtype is a wrapping tuple-struct.
1432 For example, `NewType` is a newtype over `Foo` in `struct NewType(Foo)`.
1436 struct TypeWrapper((u8, u8));
1439 fn get_state(&self) -> String {
1440 "Fascinating!".to_owned()
1447 An attempt was made to implement Drop on a trait, which is not allowed: only
1448 structs and enums can implement Drop. An example causing this error:
1450 ```compile_fail,E0120
1453 impl Drop for MyTrait {
1454 fn drop(&mut self) {}
1458 A workaround for this problem is to wrap the trait up in a struct, and implement
1459 Drop on that. An example is shown below:
1463 struct MyWrapper<T: MyTrait> { foo: T }
1465 impl <T: MyTrait> Drop for MyWrapper<T> {
1466 fn drop(&mut self) {}
1471 Alternatively, wrapping trait objects requires something like the following:
1476 //or Box<MyTrait>, if you wanted an owned trait object
1477 struct MyWrapper<'a> { foo: &'a MyTrait }
1479 impl <'a> Drop for MyWrapper<'a> {
1480 fn drop(&mut self) {}
1486 In order to be consistent with Rust's lack of global type inference, type
1487 placeholders are disallowed by design in item signatures.
1489 Examples of this error include:
1491 ```compile_fail,E0121
1492 fn foo() -> _ { 5 } // error, explicitly write out the return type instead
1494 static BAR: _ = "test"; // error, explicitly write out the type instead
1499 You declared two fields of a struct with the same name. Erroneous code
1502 ```compile_fail,E0124
1505 field1: i32, // error: field is already declared
1509 Please verify that the field names have been correctly spelled. Example:
1520 It is not possible to define `main` with generic parameters.
1521 When `main` is present, it must take no arguments and return `()`.
1522 Erroneous code example:
1524 ```compile_fail,E0131
1525 fn main<T>() { // error: main function is not allowed to have generic parameters
1531 A function with the `start` attribute was declared with type parameters.
1533 Erroneous code example:
1535 ```compile_fail,E0132
1542 It is not possible to declare type parameters on a function that has the `start`
1543 attribute. Such a function must have the following type signature (for more
1544 information, view [the unstable book][1]):
1546 [1]: https://doc.rust-lang.org/unstable-book/language-features/lang-items.html#writing-an-executable-without-stdlib
1550 fn(isize, *const *const u8) -> isize;
1559 fn my_start(argc: isize, argv: *const *const u8) -> isize {
1566 This error means that an attempt was made to match a struct type enum
1567 variant as a non-struct type:
1569 ```compile_fail,E0164
1570 enum Foo { B { i: u32 } }
1572 fn bar(foo: Foo) -> u32 {
1574 Foo::B(i) => i, // error E0164
1579 Try using `{}` instead:
1582 enum Foo { B { i: u32 } }
1584 fn bar(foo: Foo) -> u32 {
1593 Explicitly implementing both Drop and Copy for a type is currently disallowed.
1594 This feature can make some sense in theory, but the current implementation is
1595 incorrect and can lead to memory unsafety (see [issue #20126][iss20126]), so
1596 it has been disabled for now.
1598 [iss20126]: https://github.com/rust-lang/rust/issues/20126
1602 An associated function for a trait was defined to be static, but an
1603 implementation of the trait declared the same function to be a method (i.e., to
1604 take a `self` parameter).
1606 Here's an example of this error:
1608 ```compile_fail,E0185
1616 // error, method `foo` has a `&self` declaration in the impl, but not in
1624 An associated function for a trait was defined to be a method (i.e., to take a
1625 `self` parameter), but an implementation of the trait declared the same function
1628 Here's an example of this error:
1630 ```compile_fail,E0186
1638 // error, method `foo` has a `&self` declaration in the trait, but not in
1646 Trait objects need to have all associated types specified. Erroneous code
1649 ```compile_fail,E0191
1654 type Foo = Trait; // error: the value of the associated type `Bar` (from
1655 // the trait `Trait`) must be specified
1658 Please verify you specified all associated types of the trait and that you
1659 used the right trait. Example:
1666 type Foo = Trait<Bar=i32>; // ok!
1671 Negative impls are only allowed for auto traits. For more
1672 information see the [opt-in builtin traits RFC][RFC 19].
1674 [RFC 19]: https://github.com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md
1678 #### Note: this error code is no longer emitted by the compiler.
1680 `where` clauses must use generic type parameters: it does not make sense to use
1681 them otherwise. An example causing this error:
1688 #[derive(Copy,Clone)]
1693 impl Foo for Wrapper<u32> where Wrapper<u32>: Clone {
1698 This use of a `where` clause is strange - a more common usage would look
1699 something like the following:
1706 #[derive(Copy,Clone)]
1710 impl <T> Foo for Wrapper<T> where Wrapper<T>: Clone {
1715 Here, we're saying that the implementation exists on Wrapper only when the
1716 wrapped type `T` implements `Clone`. The `where` clause is important because
1717 some types will not implement `Clone`, and thus will not get this method.
1719 In our erroneous example, however, we're referencing a single concrete type.
1720 Since we know for certain that `Wrapper<u32>` implements `Clone`, there's no
1721 reason to also specify it in a `where` clause.
1725 A type parameter was declared which shadows an existing one. An example of this
1728 ```compile_fail,E0194
1730 fn do_something(&self) -> T;
1731 fn do_something_else<T: Clone>(&self, bar: T);
1735 In this example, the trait `Foo` and the trait method `do_something_else` both
1736 define a type parameter `T`. This is not allowed: if the method wishes to
1737 define a type parameter, it must use a different name for it.
1741 Your method's lifetime parameters do not match the trait declaration.
1742 Erroneous code example:
1744 ```compile_fail,E0195
1746 fn bar<'a,'b:'a>(x: &'a str, y: &'b str);
1751 impl Trait for Foo {
1752 fn bar<'a,'b>(x: &'a str, y: &'b str) {
1753 // error: lifetime parameters or bounds on method `bar`
1754 // do not match the trait declaration
1759 The lifetime constraint `'b` for bar() implementation does not match the
1760 trait declaration. Ensure lifetime declarations match exactly in both trait
1761 declaration and implementation. Example:
1765 fn t<'a,'b:'a>(x: &'a str, y: &'b str);
1770 impl Trait for Foo {
1771 fn t<'a,'b:'a>(x: &'a str, y: &'b str) { // ok!
1778 Safe traits should not have unsafe implementations, therefore marking an
1779 implementation for a safe trait unsafe will cause a compiler error. Removing
1780 the unsafe marker on the trait noted in the error will resolve this problem.
1782 ```compile_fail,E0199
1787 // this won't compile because Bar is safe
1788 unsafe impl Bar for Foo { }
1789 // this will compile
1790 impl Bar for Foo { }
1795 Unsafe traits must have unsafe implementations. This error occurs when an
1796 implementation for an unsafe trait isn't marked as unsafe. This may be resolved
1797 by marking the unsafe implementation as unsafe.
1799 ```compile_fail,E0200
1802 unsafe trait Bar { }
1804 // this won't compile because Bar is unsafe and impl isn't unsafe
1805 impl Bar for Foo { }
1806 // this will compile
1807 unsafe impl Bar for Foo { }
1812 It is an error to define two associated items (like methods, associated types,
1813 associated functions, etc.) with the same identifier.
1817 ```compile_fail,E0201
1821 fn bar(&self) -> bool { self.0 > 5 }
1822 fn bar() {} // error: duplicate associated function
1827 fn baz(&self) -> bool;
1833 fn baz(&self) -> bool { true }
1835 // error: duplicate method
1836 fn baz(&self) -> bool { self.0 > 5 }
1838 // error: duplicate associated type
1843 Note, however, that items with the same name are allowed for inherent `impl`
1844 blocks that don't overlap:
1850 fn bar(&self) -> bool { self.0 > 5 }
1854 fn bar(&self) -> bool { self.0 }
1860 Inherent associated types were part of [RFC 195] but are not yet implemented.
1861 See [the tracking issue][iss8995] for the status of this implementation.
1863 [RFC 195]: https://github.com/rust-lang/rfcs/blob/master/text/0195-associated-items.md
1864 [iss8995]: https://github.com/rust-lang/rust/issues/8995
1868 An attempt to implement the `Copy` trait for a struct failed because one of the
1869 fields does not implement `Copy`. To fix this, you must implement `Copy` for the
1870 mentioned field. Note that this may not be possible, as in the example of
1872 ```compile_fail,E0204
1877 impl Copy for Foo { }
1880 This fails because `Vec<T>` does not implement `Copy` for any `T`.
1882 Here's another example that will fail:
1884 ```compile_fail,E0204
1891 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
1892 differs from the behavior for `&T`, which is always `Copy`).
1897 An attempt to implement the `Copy` trait for an enum failed because one of the
1898 variants does not implement `Copy`. To fix this, you must implement `Copy` for
1899 the mentioned variant. Note that this may not be possible, as in the example of
1901 ```compile_fail,E0205
1907 impl Copy for Foo { }
1910 This fails because `Vec<T>` does not implement `Copy` for any `T`.
1912 Here's another example that will fail:
1914 ```compile_fail,E0205
1922 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
1923 differs from the behavior for `&T`, which is always `Copy`).
1928 You can only implement `Copy` for a struct or enum. Both of the following
1929 examples will fail, because neither `[u8; 256]` nor `&'static mut Bar`
1930 (mutable reference to `Bar`) is a struct or enum:
1932 ```compile_fail,E0206
1933 type Foo = [u8; 256];
1934 impl Copy for Foo { } // error
1936 #[derive(Copy, Clone)]
1938 impl Copy for &'static mut Bar { } // error
1943 Any type parameter or lifetime parameter of an `impl` must meet at least one of
1944 the following criteria:
1946 - it appears in the _implementing type_ of the impl, e.g. `impl<T> Foo<T>`
1947 - for a trait impl, it appears in the _implemented trait_, e.g.
1948 `impl<T> SomeTrait<T> for Foo`
1949 - it is bound as an associated type, e.g. `impl<T, U> SomeTrait for T
1950 where T: AnotherTrait<AssocType=U>`
1954 Suppose we have a struct `Foo` and we would like to define some methods for it.
1955 The following definition leads to a compiler error:
1957 ```compile_fail,E0207
1960 impl<T: Default> Foo {
1961 // error: the type parameter `T` is not constrained by the impl trait, self
1962 // type, or predicates [E0207]
1963 fn get(&self) -> T {
1964 <T as Default>::default()
1969 The problem is that the parameter `T` does not appear in the implementing type
1970 (`Foo`) of the impl. In this case, we can fix the error by moving the type
1971 parameter from the `impl` to the method `get`:
1977 // Move the type parameter from the impl to the method
1979 fn get<T: Default>(&self) -> T {
1980 <T as Default>::default()
1987 As another example, suppose we have a `Maker` trait and want to establish a
1988 type `FooMaker` that makes `Foo`s:
1990 ```compile_fail,E0207
1993 fn make(&mut self) -> Self::Item;
2002 impl<T: Default> Maker for FooMaker {
2003 // error: the type parameter `T` is not constrained by the impl trait, self
2004 // type, or predicates [E0207]
2007 fn make(&mut self) -> Foo<T> {
2008 Foo { foo: <T as Default>::default() }
2013 This fails to compile because `T` does not appear in the trait or in the
2016 One way to work around this is to introduce a phantom type parameter into
2017 `FooMaker`, like so:
2020 use std::marker::PhantomData;
2024 fn make(&mut self) -> Self::Item;
2031 // Add a type parameter to `FooMaker`
2032 struct FooMaker<T> {
2033 phantom: PhantomData<T>,
2036 impl<T: Default> Maker for FooMaker<T> {
2039 fn make(&mut self) -> Foo<T> {
2041 foo: <T as Default>::default(),
2047 Another way is to do away with the associated type in `Maker` and use an input
2048 type parameter instead:
2051 // Use a type parameter instead of an associated type here
2053 fn make(&mut self) -> Item;
2062 impl<T: Default> Maker<Foo<T>> for FooMaker {
2063 fn make(&mut self) -> Foo<T> {
2064 Foo { foo: <T as Default>::default() }
2069 ### Additional information
2071 For more information, please see [RFC 447].
2073 [RFC 447]: https://github.com/rust-lang/rfcs/blob/master/text/0447-no-unused-impl-parameters.md
2077 This error indicates a violation of one of Rust's orphan rules for trait
2078 implementations. The rule concerns the use of type parameters in an
2079 implementation of a foreign trait (a trait defined in another crate), and
2080 states that type parameters must be "covered" by a local type. To understand
2081 what this means, it is perhaps easiest to consider a few examples.
2083 If `ForeignTrait` is a trait defined in some external crate `foo`, then the
2084 following trait `impl` is an error:
2086 ```compile_fail,E0210
2087 # #[cfg(for_demonstration_only)]
2089 # #[cfg(for_demonstration_only)]
2090 use foo::ForeignTrait;
2091 # use std::panic::UnwindSafe as ForeignTrait;
2093 impl<T> ForeignTrait for T { } // error
2097 To work around this, it can be covered with a local type, `MyType`:
2100 # use std::panic::UnwindSafe as ForeignTrait;
2101 struct MyType<T>(T);
2102 impl<T> ForeignTrait for MyType<T> { } // Ok
2105 Please note that a type alias is not sufficient.
2107 For another example of an error, suppose there's another trait defined in `foo`
2108 named `ForeignTrait2` that takes two type parameters. Then this `impl` results
2109 in the same rule violation:
2111 ```ignore (cannot-doctest-multicrate-project)
2113 impl<T> ForeignTrait2<T, MyType<T>> for MyType2 { } // error
2116 The reason for this is that there are two appearances of type parameter `T` in
2117 the `impl` header, both as parameters for `ForeignTrait2`. The first appearance
2118 is uncovered, and so runs afoul of the orphan rule.
2120 Consider one more example:
2122 ```ignore (cannot-doctest-multicrate-project)
2123 impl<T> ForeignTrait2<MyType<T>, T> for MyType2 { } // Ok
2126 This only differs from the previous `impl` in that the parameters `T` and
2127 `MyType<T>` for `ForeignTrait2` have been swapped. This example does *not*
2128 violate the orphan rule; it is permitted.
2130 To see why that last example was allowed, you need to understand the general
2131 rule. Unfortunately this rule is a bit tricky to state. Consider an `impl`:
2133 ```ignore (only-for-syntax-highlight)
2134 impl<P1, ..., Pm> ForeignTrait<T1, ..., Tn> for T0 { ... }
2137 where `P1, ..., Pm` are the type parameters of the `impl` and `T0, ..., Tn`
2138 are types. One of the types `T0, ..., Tn` must be a local type (this is another
2139 orphan rule, see the explanation for E0117). Let `i` be the smallest integer
2140 such that `Ti` is a local type. Then no type parameter can appear in any of the
2143 For information on the design of the orphan rules, see [RFC 1023].
2145 [RFC 1023]: https://github.com/rust-lang/rfcs/blob/master/text/1023-rebalancing-coherence.md
2150 You used a function or type which doesn't fit the requirements for where it was
2151 used. Erroneous code examples:
2154 #![feature(intrinsics)]
2156 extern "rust-intrinsic" {
2157 fn size_of<T>(); // error: intrinsic has wrong type
2162 fn main() -> i32 { 0 }
2163 // error: main function expects type: `fn() {main}`: expected (), found i32
2170 // error: mismatched types in range: expected u8, found i8
2180 fn x(self: Rc<Foo>) {}
2181 // error: mismatched self type: expected `Foo`: expected struct
2182 // `Foo`, found struct `alloc::rc::Rc`
2186 For the first code example, please check the function definition. Example:
2189 #![feature(intrinsics)]
2191 extern "rust-intrinsic" {
2192 fn size_of<T>() -> usize; // ok!
2196 The second case example is a bit particular : the main function must always
2197 have this definition:
2203 They never take parameters and never return types.
2205 For the third example, when you match, all patterns must have the same type
2206 as the type you're matching on. Example:
2212 0u8..=3u8 => (), // ok!
2217 And finally, for the last example, only `Box<Self>`, `&Self`, `Self`,
2218 or `&mut Self` work as explicit self parameters. Example:
2224 fn x(self: Box<Foo>) {} // ok!
2231 You used an associated type which isn't defined in the trait.
2232 Erroneous code example:
2234 ```compile_fail,E0220
2239 type Foo = T1<F=i32>; // error: associated type `F` not found for `T1`
2246 // error: Baz is used but not declared
2247 fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
2251 Make sure that you have defined the associated type in the trait body.
2252 Also, verify that you used the right trait or you didn't misspell the
2253 associated type name. Example:
2260 type Foo = T1<Bar=i32>; // ok!
2266 type Baz; // we declare `Baz` in our trait.
2268 // and now we can use it here:
2269 fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
2275 An attempt was made to retrieve an associated type, but the type was ambiguous.
2278 ```compile_fail,E0221
2294 In this example, `Foo` defines an associated type `A`. `Bar` inherits that type
2295 from `Foo`, and defines another associated type of the same name. As a result,
2296 when we attempt to use `Self::A`, it's ambiguous whether we mean the `A` defined
2297 by `Foo` or the one defined by `Bar`.
2299 There are two options to work around this issue. The first is simply to rename
2300 one of the types. Alternatively, one can specify the intended type using the
2314 let _: <Self as Bar>::A;
2321 An attempt was made to retrieve an associated type, but the type was ambiguous.
2324 ```compile_fail,E0223
2325 trait MyTrait {type X; }
2328 let foo: MyTrait::X;
2332 The problem here is that we're attempting to take the type of X from MyTrait.
2333 Unfortunately, the type of X is not defined, because it's only made concrete in
2334 implementations of the trait. A working version of this code might look like:
2337 trait MyTrait {type X; }
2340 impl MyTrait for MyStruct {
2345 let foo: <MyStruct as MyTrait>::X;
2349 This syntax specifies that we want the X type from MyTrait, as made concrete in
2350 MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct
2351 might implement two different traits with identically-named associated types.
2352 This syntax allows disambiguation between the two.
2356 You attempted to use multiple types as bounds for a closure or trait object.
2357 Rust does not currently support this. A simple example that causes this error:
2359 ```compile_fail,E0225
2361 let _: Box<dyn std::io::Read + std::io::Write>;
2365 Auto traits such as Send and Sync are an exception to this rule:
2366 It's possible to have bounds of one non-builtin trait, plus any number of
2367 auto traits. For example, the following compiles correctly:
2371 let _: Box<dyn std::io::Read + Send + Sync>;
2377 An associated type binding was done outside of the type parameter declaration
2378 and `where` clause. Erroneous code example:
2380 ```compile_fail,E0229
2383 fn boo(&self) -> <Self as Foo>::A;
2388 impl Foo for isize {
2390 fn boo(&self) -> usize { 42 }
2393 fn baz<I>(x: &<I as Foo<A=Bar>>::A) {}
2394 // error: associated type bindings are not allowed here
2397 To solve this error, please move the type bindings in the type parameter
2402 # trait Foo { type A; }
2403 fn baz<I: Foo<A=Bar>>(x: &<I as Foo>::A) {} // ok!
2406 Or in the `where` clause:
2410 # trait Foo { type A; }
2411 fn baz<I>(x: &<I as Foo>::A) where I: Foo<A=Bar> {}
2416 #### Note: this error code is no longer emitted by the compiler.
2418 This error indicates that not enough type parameters were found in a type or
2421 For example, the `Foo` struct below is defined to be generic in `T`, but the
2422 type parameter is missing in the definition of `Bar`:
2424 ```compile_fail,E0107
2425 struct Foo<T> { x: T }
2427 struct Bar { x: Foo }
2432 #### Note: this error code is no longer emitted by the compiler.
2434 This error indicates that too many type parameters were found in a type or
2437 For example, the `Foo` struct below has no type parameters, but is supplied
2438 with two in the definition of `Bar`:
2440 ```compile_fail,E0107
2441 struct Foo { x: bool }
2443 struct Bar<S, T> { x: Foo<S, T> }
2448 A cross-crate opt-out trait was implemented on something which wasn't a struct
2449 or enum type. Erroneous code example:
2451 ```compile_fail,E0321
2452 #![feature(optin_builtin_traits)]
2456 impl !Sync for Foo {}
2458 unsafe impl Send for &'static Foo {}
2459 // error: cross-crate traits with a default impl, like `core::marker::Send`,
2460 // can only be implemented for a struct/enum type, not
2464 Only structs and enums are permitted to impl Send, Sync, and other opt-out
2465 trait, and the struct or enum must be local to the current crate. So, for
2466 example, `unsafe impl Send for Rc<Foo>` is not allowed.
2470 The `Sized` trait is a special trait built-in to the compiler for types with a
2471 constant size known at compile-time. This trait is automatically implemented
2472 for types as needed by the compiler, and it is currently disallowed to
2473 explicitly implement it for a type.
2477 An associated const was implemented when another trait item was expected.
2478 Erroneous code example:
2480 ```compile_fail,E0323
2489 // error: item `N` is an associated const, which doesn't match its
2490 // trait `<Bar as Foo>`
2494 Please verify that the associated const wasn't misspelled and the correct trait
2495 was implemented. Example:
2505 type N = u32; // ok!
2519 const N : u32 = 0; // ok!
2525 A method was implemented when another trait item was expected. Erroneous
2528 ```compile_fail,E0324
2539 // error: item `N` is an associated method, which doesn't match its
2540 // trait `<Bar as Foo>`
2544 To fix this error, please verify that the method name wasn't misspelled and
2545 verify that you are indeed implementing the correct trait items. Example:
2565 An associated type was implemented when another trait item was expected.
2566 Erroneous code example:
2568 ```compile_fail,E0325
2577 // error: item `N` is an associated type, which doesn't match its
2578 // trait `<Bar as Foo>`
2582 Please verify that the associated type name wasn't misspelled and your
2583 implementation corresponds to the trait definition. Example:
2593 type N = u32; // ok!
2607 const N : u32 = 0; // ok!
2613 The types of any associated constants in a trait implementation must match the
2614 types in the trait definition. This error indicates that there was a mismatch.
2616 Here's an example of this error:
2618 ```compile_fail,E0326
2626 const BAR: u32 = 5; // error, expected bool, found u32
2632 The Unsize trait should not be implemented directly. All implementations of
2633 Unsize are provided automatically by the compiler.
2635 Erroneous code example:
2637 ```compile_fail,E0328
2640 use std::marker::Unsize;
2644 impl<T> Unsize<T> for MyType {}
2647 If you are defining your own smart pointer type and would like to enable
2648 conversion from a sized to an unsized type with the
2649 [DST coercion system][RFC 982], use [`CoerceUnsized`] instead.
2652 #![feature(coerce_unsized)]
2654 use std::ops::CoerceUnsized;
2656 pub struct MyType<T: ?Sized> {
2657 field_with_unsized_type: T,
2660 impl<T, U> CoerceUnsized<MyType<U>> for MyType<T>
2661 where T: CoerceUnsized<U> {}
2664 [RFC 982]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md
2665 [`CoerceUnsized`]: https://doc.rust-lang.org/std/ops/trait.CoerceUnsized.html
2669 // Associated consts can now be accessed through generic type parameters, and
2670 // this error is no longer emitted.
2672 // FIXME: consider whether to leave it in the error index, or remove it entirely
2673 // as associated consts is not stabilized yet.
2676 An attempt was made to access an associated constant through either a generic
2677 type parameter or `Self`. This is not supported yet. An example causing this
2678 error is shown below:
2687 impl Foo for MyStruct {
2688 const BAR: f64 = 0f64;
2691 fn get_bar_bad<F: Foo>(t: F) -> f64 {
2696 Currently, the value of `BAR` for a particular type can only be accessed
2697 through a concrete type, as shown below:
2706 fn get_bar_good() -> f64 {
2707 <MyStruct as Foo>::BAR
2714 An attempt was made to implement `Drop` on a concrete specialization of a
2715 generic type. An example is shown below:
2717 ```compile_fail,E0366
2722 impl Drop for Foo<u32> {
2723 fn drop(&mut self) {}
2727 This code is not legal: it is not possible to specialize `Drop` to a subset of
2728 implementations of a generic type. One workaround for this is to wrap the
2729 generic type, as shown below:
2741 fn drop(&mut self) {}
2747 An attempt was made to implement `Drop` on a specialization of a generic type.
2748 An example is shown below:
2750 ```compile_fail,E0367
2753 struct MyStruct<T> {
2757 impl<T: Foo> Drop for MyStruct<T> {
2758 fn drop(&mut self) {}
2762 This code is not legal: it is not possible to specialize `Drop` to a subset of
2763 implementations of a generic type. In order for this code to work, `MyStruct`
2764 must also require that `T` implements `Foo`. Alternatively, another option is
2765 to wrap the generic type in another that specializes appropriately:
2770 struct MyStruct<T> {
2774 struct MyStructWrapper<T: Foo> {
2778 impl <T: Foo> Drop for MyStructWrapper<T> {
2779 fn drop(&mut self) {}
2785 This error indicates that a binary assignment operator like `+=` or `^=` was
2786 applied to a type that doesn't support it. For example:
2788 ```compile_fail,E0368
2789 let mut x = 12f32; // error: binary operation `<<` cannot be applied to
2795 To fix this error, please check that this type implements this binary
2799 let mut x = 12u32; // the `u32` type does implement the `ShlAssign` trait
2804 It is also possible to overload most operators for your own type by
2805 implementing the `[OP]Assign` traits from `std::ops`.
2807 Another problem you might be facing is this: suppose you've overloaded the `+`
2808 operator for some type `Foo` by implementing the `std::ops::Add` trait for
2809 `Foo`, but you find that using `+=` does not work, as in this example:
2811 ```compile_fail,E0368
2819 fn add(self, rhs: Foo) -> Foo {
2825 let mut x: Foo = Foo(5);
2826 x += Foo(7); // error, `+= cannot be applied to the type `Foo`
2830 This is because `AddAssign` is not automatically implemented, so you need to
2831 manually implement it for your type.
2835 A binary operation was attempted on a type which doesn't support it.
2836 Erroneous code example:
2838 ```compile_fail,E0369
2839 let x = 12f32; // error: binary operation `<<` cannot be applied to
2845 To fix this error, please check that this type implements this binary
2849 let x = 12u32; // the `u32` type does implement it:
2850 // https://doc.rust-lang.org/stable/std/ops/trait.Shl.html
2855 It is also possible to overload most operators for your own type by
2856 implementing traits from `std::ops`.
2858 String concatenation appends the string on the right to the string on the
2859 left and may require reallocation. This requires ownership of the string
2860 on the left. If something should be added to a string literal, move the
2861 literal to the heap by allocating it with `to_owned()` like in
2862 `"Your text".to_owned()`.
2867 The maximum value of an enum was reached, so it cannot be automatically
2868 set in the next enum value. Erroneous code example:
2870 ```compile_fail,E0370
2873 X = 0x7fffffffffffffff,
2874 Y, // error: enum discriminant overflowed on value after
2875 // 9223372036854775807: i64; set explicitly via
2876 // Y = -9223372036854775808 if that is desired outcome
2880 To fix this, please set manually the next enum value or put the enum variant
2881 with the maximum value at the end of the enum. Examples:
2886 X = 0x7fffffffffffffff,
2897 X = 0x7fffffffffffffff,
2903 When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
2904 definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
2905 `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
2906 definition, so it is not useful to do this.
2910 ```compile_fail,E0371
2911 trait Foo { fn foo(&self) { } }
2915 impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
2916 impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
2917 impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
2918 impl Baz for Bar { } // Note: This is OK
2923 A struct without a field containing an unsized type cannot implement
2924 `CoerceUnsized`. An [unsized type][1] is any type that the compiler
2925 doesn't know the length or alignment of at compile time. Any struct
2926 containing an unsized type is also unsized.
2928 [1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait
2930 Example of erroneous code:
2932 ```compile_fail,E0374
2933 #![feature(coerce_unsized)]
2934 use std::ops::CoerceUnsized;
2936 struct Foo<T: ?Sized> {
2940 // error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
2941 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
2942 where T: CoerceUnsized<U> {}
2945 `CoerceUnsized` is used to coerce one struct containing an unsized type
2946 into another struct containing a different unsized type. If the struct
2947 doesn't have any fields of unsized types then you don't need explicit
2948 coercion to get the types you want. To fix this you can either
2949 not try to implement `CoerceUnsized` or you can add a field that is
2950 unsized to the struct.
2955 #![feature(coerce_unsized)]
2956 use std::ops::CoerceUnsized;
2958 // We don't need to impl `CoerceUnsized` here.
2963 // We add the unsized type field to the struct.
2964 struct Bar<T: ?Sized> {
2969 // The struct has an unsized field so we can implement
2970 // `CoerceUnsized` for it.
2971 impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
2972 where T: CoerceUnsized<U> {}
2975 Note that `CoerceUnsized` is mainly used by smart pointers like `Box`, `Rc`
2976 and `Arc` to be able to mark that they can coerce unsized types that they
2981 A struct with more than one field containing an unsized type cannot implement
2982 `CoerceUnsized`. This only occurs when you are trying to coerce one of the
2983 types in your struct to another type in the struct. In this case we try to
2984 impl `CoerceUnsized` from `T` to `U` which are both types that the struct
2985 takes. An [unsized type][1] is any type that the compiler doesn't know the
2986 length or alignment of at compile time. Any struct containing an unsized type
2989 Example of erroneous code:
2991 ```compile_fail,E0375
2992 #![feature(coerce_unsized)]
2993 use std::ops::CoerceUnsized;
2995 struct Foo<T: ?Sized, U: ?Sized> {
3001 // error: Struct `Foo` has more than one unsized field.
3002 impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
3005 `CoerceUnsized` only allows for coercion from a structure with a single
3006 unsized type field to another struct with a single unsized type field.
3007 In fact Rust only allows for a struct to have one unsized type in a struct
3008 and that unsized type must be the last field in the struct. So having two
3009 unsized types in a single struct is not allowed by the compiler. To fix this
3010 use only one field containing an unsized type in the struct and then use
3011 multiple structs to manage each unsized type field you need.
3016 #![feature(coerce_unsized)]
3017 use std::ops::CoerceUnsized;
3019 struct Foo<T: ?Sized> {
3024 impl <T, U> CoerceUnsized<Foo<U>> for Foo<T>
3025 where T: CoerceUnsized<U> {}
3027 fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
3028 Foo { a: 12i32, b: t } // we use coercion to get the `Foo<U>` type we need
3032 [1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait
3036 The type you are trying to impl `CoerceUnsized` for is not a struct.
3037 `CoerceUnsized` can only be implemented for a struct. Unsized types are
3038 already able to be coerced without an implementation of `CoerceUnsized`
3039 whereas a struct containing an unsized type needs to know the unsized type
3040 field it's containing is able to be coerced. An [unsized type][1]
3041 is any type that the compiler doesn't know the length or alignment of at
3042 compile time. Any struct containing an unsized type is also unsized.
3044 [1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait
3046 Example of erroneous code:
3048 ```compile_fail,E0376
3049 #![feature(coerce_unsized)]
3050 use std::ops::CoerceUnsized;
3052 struct Foo<T: ?Sized> {
3056 // error: The type `U` is not a struct
3057 impl<T, U> CoerceUnsized<U> for Foo<T> {}
3060 The `CoerceUnsized` trait takes a struct type. Make sure the type you are
3061 providing to `CoerceUnsized` is a struct with only the last field containing an
3067 #![feature(coerce_unsized)]
3068 use std::ops::CoerceUnsized;
3074 // The `Foo<U>` is a struct so `CoerceUnsized` can be implemented
3075 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}
3078 Note that in Rust, structs can only contain an unsized type if the field
3079 containing the unsized type is the last and only unsized type field in the
3084 The `DispatchFromDyn` trait currently can only be implemented for
3085 builtin pointer types and structs that are newtype wrappers around them
3086 — that is, the struct must have only one field (except for`PhantomData`),
3087 and that field must itself implement `DispatchFromDyn`.
3092 #![feature(dispatch_from_dyn, unsize)]
3095 ops::DispatchFromDyn,
3098 struct Ptr<T: ?Sized>(*const T);
3100 impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Ptr<U>> for Ptr<T>
3107 #![feature(dispatch_from_dyn)]
3109 ops::DispatchFromDyn,
3110 marker::PhantomData,
3115 _phantom: PhantomData<()>,
3118 impl<T, U> DispatchFromDyn<Wrapper<U>> for Wrapper<T>
3120 T: DispatchFromDyn<U>,
3124 Example of illegal `DispatchFromDyn` implementation
3125 (illegal because of extra field)
3127 ```compile-fail,E0378
3128 #![feature(dispatch_from_dyn)]
3129 use std::ops::DispatchFromDyn;
3131 struct WrapperExtraField<T> {
3136 impl<T, U> DispatchFromDyn<WrapperExtraField<U>> for WrapperExtraField<T>
3138 T: DispatchFromDyn<U>,
3144 You tried to implement methods for a primitive type. Erroneous code example:
3146 ```compile_fail,E0390
3152 // error: only a single inherent implementation marked with
3153 // `#[lang = "mut_ptr"]` is allowed for the `*mut T` primitive
3156 This isn't allowed, but using a trait to implement a method is a good solution.
3168 impl Bar for *mut Foo {
3175 This error indicates that a type or lifetime parameter has been declared
3176 but not actually used. Here is an example that demonstrates the error:
3178 ```compile_fail,E0392
3184 If the type parameter was included by mistake, this error can be fixed
3185 by simply removing the type parameter, as shown below:
3193 Alternatively, if the type parameter was intentionally inserted, it must be
3194 used. A simple fix is shown below:
3202 This error may also commonly be found when working with unsafe code. For
3203 example, when using raw pointers one may wish to specify the lifetime for
3204 which the pointed-at data is valid. An initial attempt (below) causes this
3207 ```compile_fail,E0392
3213 We want to express the constraint that Foo should not outlive `'a`, because
3214 the data pointed to by `T` is only valid for that lifetime. The problem is
3215 that there are no actual uses of `'a`. It's possible to work around this
3216 by adding a PhantomData type to the struct, using it to tell the compiler
3217 to act as if the struct contained a borrowed reference `&'a T`:
3220 use std::marker::PhantomData;
3222 struct Foo<'a, T: 'a> {
3224 phantom: PhantomData<&'a T>
3228 [PhantomData] can also be used to express information about unused type
3231 [PhantomData]: https://doc.rust-lang.org/std/marker/struct.PhantomData.html
3235 A type parameter which references `Self` in its default value was not specified.
3236 Example of erroneous code:
3238 ```compile_fail,E0393
3241 fn together_we_will_rule_the_galaxy(son: &A) {}
3242 // error: the type parameter `T` must be explicitly specified in an
3243 // object type because its default value `Self` references the
3247 A trait object is defined over a single, fully-defined trait. With a regular
3248 default parameter, this parameter can just be substituted in. However, if the
3249 default parameter is `Self`, the trait changes for each concrete type; i.e.
3250 `i32` will be expected to implement `A<i32>`, `bool` will be expected to
3251 implement `A<bool>`, etc... These types will not share an implementation of a
3252 fully-defined trait; instead they share implementations of a trait with
3253 different parameters substituted in for each implementation. This is
3254 irreconcilable with what we need to make a trait object work, and is thus
3255 disallowed. Making the trait concrete by explicitly specifying the value of the
3256 defaulted parameter will fix this issue. Fixed example:
3261 fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!
3266 You implemented a trait, overriding one or more of its associated types but did
3267 not reimplement its default methods.
3269 Example of erroneous code:
3271 ```compile_fail,E0399
3272 #![feature(associated_type_defaults)]
3280 // error - the following trait items need to be reimplemented as
3281 // `Assoc` was overridden: `bar`
3286 To fix this, add an implementation for each default method from the trait:
3289 #![feature(associated_type_defaults)]
3298 fn bar(&self) {} // ok!
3304 The functional record update syntax is only allowed for structs. (Struct-like
3305 enum variants don't qualify, for example.)
3307 Erroneous code example:
3309 ```compile_fail,E0436
3310 enum PublicationFrequency {
3312 SemiMonthly { days: (u8, u8), annual_special: bool },
3315 fn one_up_competitor(competitor_frequency: PublicationFrequency)
3316 -> PublicationFrequency {
3317 match competitor_frequency {
3318 PublicationFrequency::Weekly => PublicationFrequency::SemiMonthly {
3319 days: (1, 15), annual_special: false
3321 c @ PublicationFrequency::SemiMonthly{ .. } =>
3322 PublicationFrequency::SemiMonthly {
3323 annual_special: true, ..c // error: functional record update
3324 // syntax requires a struct
3330 Rewrite the expression without functional record update syntax:
3333 enum PublicationFrequency {
3335 SemiMonthly { days: (u8, u8), annual_special: bool },
3338 fn one_up_competitor(competitor_frequency: PublicationFrequency)
3339 -> PublicationFrequency {
3340 match competitor_frequency {
3341 PublicationFrequency::Weekly => PublicationFrequency::SemiMonthly {
3342 days: (1, 15), annual_special: false
3344 PublicationFrequency::SemiMonthly{ days, .. } =>
3345 PublicationFrequency::SemiMonthly {
3346 days, annual_special: true // ok!
3354 The length of the platform-intrinsic function `simd_shuffle`
3355 wasn't specified. Erroneous code example:
3357 ```compile_fail,E0439
3358 #![feature(platform_intrinsics)]
3360 extern "platform-intrinsic" {
3361 fn simd_shuffle<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3362 // error: invalid `simd_shuffle`, needs length: `simd_shuffle`
3366 The `simd_shuffle` function needs the length of the array passed as
3367 last parameter in its name. Example:
3370 #![feature(platform_intrinsics)]
3372 extern "platform-intrinsic" {
3373 fn simd_shuffle8<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3379 The `typeof` keyword is currently reserved but unimplemented.
3380 Erroneous code example:
3382 ```compile_fail,E0516
3384 let x: typeof(92) = 92;
3388 Try using type inference instead. Example:
3398 A non-default implementation was already made on this type so it cannot be
3399 specialized further. Erroneous code example:
3401 ```compile_fail,E0520
3402 #![feature(specialization)]
3409 impl<T> SpaceLlama for T {
3410 default fn fly(&self) {}
3414 // applies to all `Clone` T and overrides the previous impl
3415 impl<T: Clone> SpaceLlama for T {
3419 // since `i32` is clone, this conflicts with the previous implementation
3420 impl SpaceLlama for i32 {
3421 default fn fly(&self) {}
3422 // error: item `fly` is provided by an `impl` that specializes
3423 // another, but the item in the parent `impl` is not marked
3424 // `default` and so it cannot be specialized.
3428 Specialization only allows you to override `default` functions in
3431 To fix this error, you need to mark all the parent implementations as default.
3435 #![feature(specialization)]
3442 impl<T> SpaceLlama for T {
3443 default fn fly(&self) {} // This is a parent implementation.
3446 // applies to all `Clone` T; overrides the previous impl
3447 impl<T: Clone> SpaceLlama for T {
3448 default fn fly(&self) {} // This is a parent implementation but was
3449 // previously not a default one, causing the error
3452 // applies to i32, overrides the previous two impls
3453 impl SpaceLlama for i32 {
3454 fn fly(&self) {} // And now that's ok!
3460 The number of elements in an array or slice pattern differed from the number of
3461 elements in the array being matched.
3463 Example of erroneous code:
3465 ```compile_fail,E0527
3466 let r = &[1, 2, 3, 4];
3468 &[a, b] => { // error: pattern requires 2 elements but array
3470 println!("a={}, b={}", a, b);
3475 Ensure that the pattern is consistent with the size of the matched
3476 array. Additional elements can be matched with `..`:
3479 #![feature(slice_patterns)]
3481 let r = &[1, 2, 3, 4];
3483 &[a, b, ..] => { // ok!
3484 println!("a={}, b={}", a, b);
3491 An array or slice pattern required more elements than were present in the
3494 Example of erroneous code:
3496 ```compile_fail,E0528
3497 #![feature(slice_patterns)]
3501 &[a, b, c, rest..] => { // error: pattern requires at least 3
3502 // elements but array has 2
3503 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3508 Ensure that the matched array has at least as many elements as the pattern
3509 requires. You can match an arbitrary number of remaining elements with `..`:
3512 #![feature(slice_patterns)]
3514 let r = &[1, 2, 3, 4, 5];
3516 &[a, b, c, rest..] => { // ok!
3517 // prints `a=1, b=2, c=3 rest=[4, 5]`
3518 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3525 An array or slice pattern was matched against some other type.
3527 Example of erroneous code:
3529 ```compile_fail,E0529
3532 [a, b] => { // error: expected an array or slice, found `f32`
3533 println!("a={}, b={}", a, b);
3538 Ensure that the pattern and the expression being matched on are of consistent
3545 println!("a={}, b={}", a, b);
3552 The `inline` attribute was malformed.
3554 Erroneous code example:
3556 ```ignore (compile_fail not working here; see Issue #43707)
3557 #[inline()] // error: expected one argument
3558 pub fn something() {}
3563 The parenthesized `inline` attribute requires the parameter to be specified:
3577 Alternatively, a paren-less version of the attribute may be used to hint the
3578 compiler about inlining opportunity:
3585 For more information about the inline attribute, read:
3586 https://doc.rust-lang.org/reference.html#inline-attributes
3590 An unknown argument was given to the `inline` attribute.
3592 Erroneous code example:
3594 ```ignore (compile_fail not working here; see Issue #43707)
3595 #[inline(unknown)] // error: invalid argument
3596 pub fn something() {}
3601 The `inline` attribute only supports two arguments:
3606 All other arguments given to the `inline` attribute will return this error.
3610 #[inline(never)] // ok!
3611 pub fn something() {}
3616 For more information about the inline attribute, https:
3617 read://doc.rust-lang.org/reference.html#inline-attributes
3621 An unknown field was specified into an enum's structure variant.
3623 Erroneous code example:
3625 ```compile_fail,E0559
3630 let s = Field::Fool { joke: 0 };
3631 // error: struct variant `Field::Fool` has no field named `joke`
3634 Verify you didn't misspell the field's name or that the field exists. Example:
3641 let s = Field::Fool { joke: 0 }; // ok!
3646 An unknown field was specified into a structure.
3648 Erroneous code example:
3650 ```compile_fail,E0560
3655 let s = Simba { mother: 1, father: 0 };
3656 // error: structure `Simba` has no field named `father`
3659 Verify you didn't misspell the field's name or that the field exists. Example:
3667 let s = Simba { mother: 1, father: 0 }; // ok!
3672 If an impl has a generic parameter with the `#[may_dangle]` attribute, then
3673 that impl must be declared as an `unsafe impl.
3675 Erroneous code example:
3677 ```compile_fail,E0569
3678 #![feature(dropck_eyepatch)]
3681 impl<#[may_dangle] X> Drop for Foo<X> {
3682 fn drop(&mut self) { }
3686 In this example, we are asserting that the destructor for `Foo` will not
3687 access any data of type `X`, and require this assertion to be true for
3688 overall safety in our program. The compiler does not currently attempt to
3689 verify this assertion; therefore we must tag this `impl` as unsafe.
3693 The requested ABI is unsupported by the current target.
3695 The rust compiler maintains for each target a blacklist of ABIs unsupported on
3696 that target. If an ABI is present in such a list this usually means that the
3697 target / ABI combination is currently unsupported by llvm.
3699 If necessary, you can circumvent this check using custom target specifications.
3703 A return statement was found outside of a function body.
3705 Erroneous code example:
3707 ```compile_fail,E0572
3708 const FOO: u32 = return 0; // error: return statement outside of function body
3713 To fix this issue, just remove the return keyword or move the expression into a
3719 fn some_fn() -> u32 {
3730 In a `fn` type, a lifetime appears only in the return type,
3731 and not in the arguments types.
3733 Erroneous code example:
3735 ```compile_fail,E0581
3737 // Here, `'a` appears only in the return type:
3738 let x: for<'a> fn() -> &'a i32;
3742 To fix this issue, either use the lifetime in the arguments, or use
3747 // Here, `'a` appears only in the return type:
3748 let x: for<'a> fn(&'a i32) -> &'a i32;
3749 let y: fn() -> &'static i32;
3753 Note: The examples above used to be (erroneously) accepted by the
3754 compiler, but this was since corrected. See [issue #33685] for more
3757 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
3761 A lifetime appears only in an associated-type binding,
3762 and not in the input types to the trait.
3764 Erroneous code example:
3766 ```compile_fail,E0582
3768 // No type can satisfy this requirement, since `'a` does not
3769 // appear in any of the input types (here, `i32`):
3770 where F: for<'a> Fn(i32) -> Option<&'a i32>
3777 To fix this issue, either use the lifetime in the inputs, or use
3781 fn bar<F, G>(t: F, u: G)
3782 where F: for<'a> Fn(&'a i32) -> Option<&'a i32>,
3783 G: Fn(i32) -> Option<&'static i32>,
3790 Note: The examples above used to be (erroneously) accepted by the
3791 compiler, but this was since corrected. See [issue #33685] for more
3794 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
3798 This error occurs when a method is used on a type which doesn't implement it:
3800 Erroneous code example:
3802 ```compile_fail,E0599
3806 x.chocolate(); // error: no method named `chocolate` found for type `Mouth`
3807 // in the current scope
3812 An unary operator was used on a type which doesn't implement it.
3814 Example of erroneous code:
3816 ```compile_fail,E0600
3822 !Question::Yes; // error: cannot apply unary operator `!` to type `Question`
3825 In this case, `Question` would need to implement the `std::ops::Not` trait in
3826 order to be able to use `!` on it. Let's implement it:
3836 // We implement the `Not` trait on the enum.
3837 impl Not for Question {
3840 fn not(self) -> bool {
3842 Question::Yes => false, // If the `Answer` is `Yes`, then it
3844 Question::No => true, // And here we do the opposite.
3849 assert_eq!(!Question::Yes, false);
3850 assert_eq!(!Question::No, true);
3855 An attempt to index into a type which doesn't implement the `std::ops::Index`
3856 trait was performed.
3858 Erroneous code example:
3860 ```compile_fail,E0608
3861 0u8[2]; // error: cannot index into a value of type `u8`
3864 To be able to index into a type it needs to implement the `std::ops::Index`
3868 let v: Vec<u8> = vec![0, 1, 2, 3];
3870 // The `Vec` type implements the `Index` trait so you can do:
3871 println!("{}", v[2]);
3876 A cast to `char` was attempted on a type other than `u8`.
3878 Erroneous code example:
3880 ```compile_fail,E0604
3881 0u32 as char; // error: only `u8` can be cast as `char`, not `u32`
3884 As the error message indicates, only `u8` can be cast into `char`. Example:
3887 let c = 86u8 as char; // ok!
3891 For more information about casts, take a look at the Type cast section in
3892 [The Reference Book][1].
3894 [1]: https://doc.rust-lang.org/reference/expressions/operator-expr.html#type-cast-expressions
3898 An invalid cast was attempted.
3900 Erroneous code examples:
3902 ```compile_fail,E0605
3904 x as Vec<u8>; // error: non-primitive cast: `u8` as `std::vec::Vec<u8>`
3908 let v = 0 as *const u8; // So here, `v` is a `*const u8`.
3909 v as &u8; // error: non-primitive cast: `*const u8` as `&u8`
3912 Only primitive types can be cast into each other. Examples:
3918 let v = 0 as *const u8;
3919 v as *const i8; // ok!
3922 For more information about casts, take a look at the Type cast section in
3923 [The Reference Book][1].
3925 [1]: https://doc.rust-lang.org/reference/expressions/operator-expr.html#type-cast-expressions
3929 An incompatible cast was attempted.
3931 Erroneous code example:
3933 ```compile_fail,E0606
3934 let x = &0u8; // Here, `x` is a `&u8`.
3935 let y: u32 = x as u32; // error: casting `&u8` as `u32` is invalid
3938 When casting, keep in mind that only primitive types can be cast into each
3943 let y: u32 = *x as u32; // We dereference it first and then cast it.
3946 For more information about casts, take a look at the Type cast section in
3947 [The Reference Book][1].
3949 [1]: https://doc.rust-lang.org/reference/expressions/operator-expr.html#type-cast-expressions
3953 A cast between a thin and a fat pointer was attempted.
3955 Erroneous code example:
3957 ```compile_fail,E0607
3958 let v = 0 as *const u8;
3962 First: what are thin and fat pointers?
3964 Thin pointers are "simple" pointers: they are purely a reference to a memory
3967 Fat pointers are pointers referencing Dynamically Sized Types (also called DST).
3968 DST don't have a statically known size, therefore they can only exist behind
3969 some kind of pointers that contain additional information. Slices and trait
3970 objects are DSTs. In the case of slices, the additional information the fat
3971 pointer holds is their size.
3973 To fix this error, don't try to cast directly between thin and fat pointers.
3975 For more information about casts, take a look at the Type cast section in
3976 [The Reference Book][1].
3978 [1]: https://doc.rust-lang.org/reference/expressions/operator-expr.html#type-cast-expressions
3982 Attempted to access a non-existent field in a struct.
3984 Erroneous code example:
3986 ```compile_fail,E0609
3987 struct StructWithFields {
3991 let s = StructWithFields { x: 0 };
3992 println!("{}", s.foo); // error: no field `foo` on type `StructWithFields`
3995 To fix this error, check that you didn't misspell the field's name or that the
3996 field actually exists. Example:
3999 struct StructWithFields {
4003 let s = StructWithFields { x: 0 };
4004 println!("{}", s.x); // ok!
4009 Attempted to access a field on a primitive type.
4011 Erroneous code example:
4013 ```compile_fail,E0610
4015 println!("{}", x.foo); // error: `{integer}` is a primitive type, therefore
4016 // doesn't have fields
4019 Primitive types are the most basic types available in Rust and don't have
4020 fields. To access data via named fields, struct types are used. Example:
4023 // We declare struct called `Foo` containing two fields:
4029 // We create an instance of this struct:
4030 let variable = Foo { x: 0, y: -12 };
4031 // And we can now access its fields:
4032 println!("x: {}, y: {}", variable.x, variable.y);
4035 For more information about primitives and structs, take a look at The Book:
4036 https://doc.rust-lang.org/book/ch03-02-data-types.html
4037 https://doc.rust-lang.org/book/ch05-00-structs.html
4041 Attempted to dereference a variable which cannot be dereferenced.
4043 Erroneous code example:
4045 ```compile_fail,E0614
4047 *y; // error: type `u32` cannot be dereferenced
4050 Only types implementing `std::ops::Deref` can be dereferenced (such as `&T`).
4056 // So here, `x` is a `&u32`, so we can dereference it:
4062 Attempted to access a method like a field.
4064 Erroneous code example:
4066 ```compile_fail,E0615
4075 let f = Foo { x: 0 };
4076 f.method; // error: attempted to take value of method `method` on type `Foo`
4079 If you want to use a method, add `()` after it:
4082 # struct Foo { x: u32 }
4083 # impl Foo { fn method(&self) {} }
4084 # let f = Foo { x: 0 };
4088 However, if you wanted to access a field of a struct check that the field name
4089 is spelled correctly. Example:
4092 # struct Foo { x: u32 }
4093 # impl Foo { fn method(&self) {} }
4094 # let f = Foo { x: 0 };
4095 println!("{}", f.x);
4100 Attempted to access a private field on a struct.
4102 Erroneous code example:
4104 ```compile_fail,E0616
4107 x: u32, // So `x` is private in here.
4111 pub fn new() -> Foo { Foo { x: 0 } }
4115 let f = some_module::Foo::new();
4116 println!("{}", f.x); // error: field `x` of struct `some_module::Foo` is private
4119 If you want to access this field, you have two options:
4121 1) Set the field public:
4126 pub x: u32, // `x` is now public.
4130 pub fn new() -> Foo { Foo { x: 0 } }
4134 let f = some_module::Foo::new();
4135 println!("{}", f.x); // ok!
4138 2) Add a getter function:
4143 x: u32, // So `x` is still private in here.
4147 pub fn new() -> Foo { Foo { x: 0 } }
4149 // We create the getter function here:
4150 pub fn get_x(&self) -> &u32 { &self.x }
4154 let f = some_module::Foo::new();
4155 println!("{}", f.get_x()); // ok!
4160 Attempted to pass an invalid type of variable into a variadic function.
4162 Erroneous code example:
4164 ```compile_fail,E0617
4166 fn printf(c: *const i8, ...);
4170 printf(::std::ptr::null(), 0f32);
4171 // error: can't pass an `f32` to variadic function, cast to `c_double`
4175 Certain Rust types must be cast before passing them to a variadic function,
4176 because of arcane ABI rules dictated by the C standard. To fix the error,
4177 cast the value to the type specified by the error message (which you may need
4178 to import from `std::os::raw`).
4182 Attempted to call something which isn't a function nor a method.
4184 Erroneous code examples:
4186 ```compile_fail,E0618
4191 X::Entry(); // error: expected function, found `X::Entry`
4195 x(); // error: expected function, found `i32`
4198 Only functions and methods can be called using `()`. Example:
4201 // We declare a function:
4202 fn i_am_a_function() {}
4210 #### Note: this error code is no longer emitted by the compiler.
4211 The type-checker needed to know the type of an expression, but that type had not
4214 Erroneous code example:
4220 // Here, the type of `v` is not (yet) known, so we
4221 // cannot resolve this method call:
4222 v.to_uppercase(); // error: the type of this value must be known in
4229 Type inference typically proceeds from the top of the function to the bottom,
4230 figuring out types as it goes. In some cases -- notably method calls and
4231 overloadable operators like `*` -- the type checker may not have enough
4232 information *yet* to make progress. This can be true even if the rest of the
4233 function provides enough context (because the type-checker hasn't looked that
4234 far ahead yet). In this case, type annotations can be used to help it along.
4236 To fix this error, just specify the type of the variable. Example:
4239 let mut x: Vec<String> = vec![]; // We precise the type of the vec elements.
4242 v.to_uppercase(); // Since rustc now knows the type of the vec elements,
4243 // we can use `v`'s methods.
4251 A cast to an unsized type was attempted.
4253 Erroneous code example:
4255 ```compile_fail,E0620
4256 let x = &[1_usize, 2] as [usize]; // error: cast to unsized type: `&[usize; 2]`
4260 In Rust, some types don't have a known size at compile-time. For example, in a
4261 slice type like `[u32]`, the number of elements is not known at compile-time and
4262 hence the overall size cannot be computed. As a result, such types can only be
4263 manipulated through a reference (e.g., `&T` or `&mut T`) or other pointer-type
4264 (e.g., `Box` or `Rc`). Try casting to a reference instead:
4267 let x = &[1_usize, 2] as &[usize]; // ok!
4272 An intrinsic was declared without being a function.
4274 Erroneous code example:
4276 ```compile_fail,E0622
4277 #![feature(intrinsics)]
4278 extern "rust-intrinsic" {
4279 pub static breakpoint : unsafe extern "rust-intrinsic" fn();
4280 // error: intrinsic must be a function
4283 fn main() { unsafe { breakpoint(); } }
4286 An intrinsic is a function available for use in a given programming language
4287 whose implementation is handled specially by the compiler. In order to fix this
4288 error, just declare a function.
4292 A private item was used outside of its scope.
4294 Erroneous code example:
4296 ```compile_fail,E0624
4305 let foo = inner::Foo;
4306 foo.method(); // error: method `method` is private
4309 Two possibilities are available to solve this issue:
4311 1. Only use the item in the scope it has been defined:
4321 pub fn call_method(foo: &Foo) { // We create a public function.
4322 foo.method(); // Which calls the item.
4326 let foo = inner::Foo;
4327 inner::call_method(&foo); // And since the function is public, we can call the
4328 // method through it.
4331 2. Make the item public:
4338 pub fn method(&self) {} // It's now public.
4342 let foo = inner::Foo;
4343 foo.method(); // Ok!
4348 This error indicates that the struct, enum or enum variant must be matched
4349 non-exhaustively as it has been marked as `non_exhaustive`.
4351 When applied within a crate, downstream users of the crate will need to use the
4352 `_` pattern when matching enums and use the `..` pattern when matching structs.
4353 Downstream crates cannot match against non-exhaustive enum variants.
4355 For example, in the below example, since the enum is marked as
4356 `non_exhaustive`, it is required that downstream crates match non-exhaustively
4359 ```rust,ignore (pseudo-Rust)
4360 use std::error::Error as StdError;
4362 #[non_exhaustive] pub enum Error {
4367 impl StdError for Error {
4368 fn description(&self) -> &str {
4369 // This will not error, despite being marked as non_exhaustive, as this
4370 // enum is defined within the current crate, it can be matched
4373 Message(ref s) => s,
4374 Other => "other or unknown error",
4380 An example of matching non-exhaustively on the above enum is provided below:
4382 ```rust,ignore (pseudo-Rust)
4385 // This will not error as the non_exhaustive Error enum has been matched with a
4388 Message(ref s) => ...,
4394 Similarly, for structs, match with `..` to avoid this error.
4398 This error indicates that the struct, enum or enum variant cannot be
4399 instantiated from outside of the defining crate as it has been marked
4400 as `non_exhaustive` and as such more fields/variants may be added in
4401 future that could cause adverse side effects for this code.
4403 It is recommended that you look for a `new` function or equivalent in the
4404 crate's documentation.
4408 This error indicates that there is a mismatch between generic parameters and
4409 impl Trait parameters in a trait declaration versus its impl.
4411 ```compile_fail,E0643
4413 fn foo(&self, _: &impl Iterator);
4416 fn foo<U: Iterator>(&self, _: &U) { } // error method `foo` has incompatible
4417 // signature for trait
4423 It is not possible to define `main` with a where clause.
4424 Erroneous code example:
4426 ```compile_fail,E0646
4427 fn main() where i32: Copy { // error: main function is not allowed to have
4434 It is not possible to define `start` with a where clause.
4435 Erroneous code example:
4437 ```compile_fail,E0647
4441 fn start(_: isize, _: *const *const u8) -> isize where (): Copy {
4442 //^ error: start function is not allowed to have a where clause
4449 `export_name` attributes may not contain null characters (`\0`).
4451 ```compile_fail,E0648
4452 #[export_name="\0foo"] // error: `export_name` may not contain null characters
4458 This error indicates that the numeric value for the method being passed exists
4459 but the type of the numeric value or binding could not be identified.
4461 The error happens on numeric literals:
4463 ```compile_fail,E0689
4467 and on numeric bindings without an identified concrete type:
4469 ```compile_fail,E0689
4471 x.neg(); // same error as above
4474 Because of this, you must give the numeric literal or binding a type:
4479 let _ = 2.0_f32.neg();
4482 let _ = (2.0 as f32).neg();
4487 A struct with the representation hint `repr(transparent)` had zero or more than
4488 on fields that were not guaranteed to be zero-sized.
4490 Erroneous code example:
4492 ```compile_fail,E0690
4493 #[repr(transparent)]
4494 struct LengthWithUnit<U> { // error: transparent struct needs exactly one
4495 value: f32, // non-zero-sized field, but has 2
4500 Because transparent structs are represented exactly like one of their fields at
4501 run time, said field must be uniquely determined. If there is no field, or if
4502 there are multiple fields, it is not clear how the struct should be represented.
4503 Note that fields of zero-typed types (e.g., `PhantomData`) can also exist
4504 alongside the field that contains the actual data, they do not count for this
4505 error. When generic types are involved (as in the above example), an error is
4506 reported because the type parameter could be non-zero-sized.
4508 To combine `repr(transparent)` with type parameters, `PhantomData` may be
4512 use std::marker::PhantomData;
4514 #[repr(transparent)]
4515 struct LengthWithUnit<U> {
4517 unit: PhantomData<U>,
4523 A struct with the `repr(transparent)` representation hint contains a zero-sized
4524 field that requires non-trivial alignment.
4526 Erroneous code example:
4528 ```compile_fail,E0691
4529 #![feature(repr_align)]
4532 struct ForceAlign32;
4534 #[repr(transparent)]
4535 struct Wrapper(f32, ForceAlign32); // error: zero-sized field in transparent
4536 // struct has alignment larger than 1
4539 A transparent struct is supposed to be represented exactly like the piece of
4540 data it contains. Zero-sized fields with different alignment requirements
4541 potentially conflict with this property. In the example above, `Wrapper` would
4542 have to be aligned to 32 bytes even though `f32` has a smaller alignment
4545 Consider removing the over-aligned zero-sized field:
4548 #[repr(transparent)]
4549 struct Wrapper(f32);
4552 Alternatively, `PhantomData<T>` has alignment 1 for all `T`, so you can use it
4553 if you need to keep the field for some reason:
4556 #![feature(repr_align)]
4558 use std::marker::PhantomData;
4561 struct ForceAlign32;
4563 #[repr(transparent)]
4564 struct Wrapper(f32, PhantomData<ForceAlign32>);
4567 Note that empty arrays `[T; 0]` have the same alignment requirement as the
4568 element type `T`. Also note that the error is conservatively reported even when
4569 the alignment of the zero-sized type is less than or equal to the data field's
4575 A method was called on a raw pointer whose inner type wasn't completely known.
4577 For example, you may have done something like:
4580 # #![deny(warnings)]
4582 let bar = foo as *const _;
4588 Here, the type of `bar` isn't known; it could be a pointer to anything. Instead,
4589 specify a type for the pointer (preferably something that makes sense for the
4590 thing you're pointing to):
4594 let bar = foo as *const i32;
4600 Even though `is_null()` exists as a method on any raw pointer, Rust shows this
4601 error because Rust allows for `self` to have arbitrary types (behind the
4602 arbitrary_self_types feature flag).
4604 This means that someone can specify such a function:
4606 ```ignore (cannot-doctest-feature-doesnt-exist-yet)
4608 fn is_null(self: *const Self) -> bool {
4609 // do something else
4614 and now when you call `.is_null()` on a raw pointer to `Foo`, there's ambiguity.
4616 Given that we don't know what type the pointer is, and there's potential
4617 ambiguity for some types, we disallow calling methods on raw pointers when
4618 the type is unknown.
4622 A `#[marker]` trait contained an associated item.
4624 The items of marker traits cannot be overridden, so there's no need to have them
4625 when they cannot be changed per-type anyway. If you wanted them for ergonomic
4626 reasons, consider making an extension trait instead.
4630 An `impl` for a `#[marker]` trait tried to override an associated item.
4632 Because marker traits are allowed to have multiple implementations for the same
4633 type, it's not allowed to override anything in those implementations, as it
4634 would be ambiguous which override should actually be used.
4639 An `impl Trait` type expands to a recursive type.
4641 An `impl Trait` type must be expandable to a concrete type that contains no
4642 `impl Trait` types. For example the following example tries to create an
4643 `impl Trait` type `T` that is equal to `[T, T]`:
4645 ```compile_fail,E0720
4646 fn make_recursive_type() -> impl Sized {
4647 [make_recursive_type(), make_recursive_type()]
4654 register_diagnostics! {
4655 // E0035, merged into E0087/E0089
4656 // E0036, merged into E0087/E0089
4662 // E0122, // bounds in type aliases are ignored, turned into proper lint
4667 // E0159, // use of trait `{}` as struct constructor
4668 // E0163, // merged into E0071
4671 // E0172, // non-trait found in a type sum, moved to resolve
4672 // E0173, // manual implementations of unboxed closure traits are experimental
4674 // E0182, // merged into E0229
4676 // E0187, // can't infer the kind of the closure
4677 // E0188, // can not cast an immutable reference to a mutable pointer
4678 // E0189, // deprecated: can only cast a boxed pointer to a boxed object
4679 // E0190, // deprecated: can only cast a &-pointer to an &-object
4680 // E0196, // cannot determine a type for this closure
4681 E0203, // type parameter has more than one relaxed default bound,
4682 // and only one is supported
4684 // E0209, // builtin traits can only be implemented on structs or enums
4685 E0212, // cannot extract an associated type from a higher-ranked trait bound
4686 // E0213, // associated types are not accepted in this context
4687 // E0215, // angle-bracket notation is not stable with `Fn`
4688 // E0216, // parenthetical notation is only stable with `Fn`
4689 // E0217, // ambiguous associated type, defined in multiple supertraits
4690 // E0218, // no associated type defined
4691 // E0219, // associated type defined in higher-ranked supertrait
4692 // E0222, // Error code E0045 (variadic function must have C or cdecl calling
4693 // convention) duplicate
4694 E0224, // at least one non-builtin train is required for an object type
4695 E0227, // ambiguous lifetime bound, explicit lifetime bound required
4696 E0228, // explicit lifetime bound required
4699 // E0235, // structure constructor specifies a structure of type but
4700 // E0236, // no lang item for range syntax
4701 // E0237, // no lang item for range syntax
4702 // E0238, // parenthesized parameters may only be used with a trait
4703 // E0239, // `next` method of `Iterator` trait has unexpected type
4707 // E0245, // not a trait
4708 // E0246, // invalid recursive type
4710 // E0248, // value used as a type, now reported earlier during resolution as E0412
4712 E0307, // invalid method `self` type
4713 // E0319, // trait impls for defaulted traits allowed just for structs/enums
4714 // E0372, // coherence not object safe
4715 E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
4716 // between structures with the same definition
4717 // E0558, // replaced with a generic attribute input check
4718 E0533, // `{}` does not name a unit variant, unit struct or a constant
4719 // E0563, // cannot determine a type for this `impl Trait`: {} // removed in 6383de15
4720 E0564, // only named lifetimes are allowed in `impl Trait`,
4721 // but `{}` was found in the type `{}`
4722 E0587, // type has conflicting packed and align representation hints
4723 E0588, // packed type cannot transitively contain a `[repr(align)]` type
4724 E0592, // duplicate definitions with name `{}`
4725 // E0611, // merged into E0616
4726 // E0612, // merged into E0609
4727 // E0613, // Removed (merged with E0609)
4728 E0627, // yield statement outside of generator literal
4729 E0632, // cannot provide explicit type parameters when `impl Trait` is used in
4730 // argument position.
4731 E0634, // type has conflicting packed representaton hints
4732 E0640, // infer outlives requirements
4733 E0641, // cannot cast to/from a pointer with an unknown kind
4734 E0645, // trait aliases not finished
4735 E0719, // duplicate values for associated type binding
4736 E0722, // Malformed #[optimize] attribute
4737 E0724, // `#[ffi_returns_twice]` is only allowed in foreign functions