1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 #![allow(non_snake_case)]
13 register_long_diagnostics! {
16 A pattern used to match against an enum variant must provide a sub-pattern for
17 each field of the enum variant. This error indicates that a pattern attempted to
18 extract an incorrect number of fields from a variant.
22 Apple(String, String),
27 Here the `Apple` variant has two fields, and should be matched against like so:
31 Apple(String, String),
35 let x = Fruit::Apple(String::new(), String::new());
39 Fruit::Apple(a, b) => {},
44 Matching with the wrong number of fields has no sensible interpretation:
48 Apple(String, String),
52 let x = Fruit::Apple(String::new(), String::new());
56 Fruit::Apple(a) => {},
57 Fruit::Apple(a, b, c) => {},
61 Check how many fields the enum was declared with and ensure that your pattern
66 Each field of a struct can only be bound once in a pattern. Erroneous code
76 let x = Foo { a:1, b:2 };
78 let Foo { a: x, a: y } = x;
79 // error: field `a` bound multiple times in the pattern
83 Each occurrence of a field name binds the value of that field, so to fix this
84 error you will have to remove or alter the duplicate uses of the field name.
85 Perhaps you misspelled another field name? Example:
94 let x = Foo { a:1, b:2 };
96 let Foo { a: x, b: y } = x; // ok!
102 This error indicates that a struct pattern attempted to extract a non-existent
103 field from a struct. Struct fields are identified by the name used before the
104 colon `:` so struct patterns should resemble the declaration of the struct type
114 let thing = Thing { x: 1, y: 2 };
117 Thing { x: xfield, y: yfield } => {}
121 If you are using shorthand field patterns but want to refer to the struct field
122 by a different name, you should rename it explicitly.
126 ```compile_fail,E0026
132 let thing = Thing { x: 0, y: 0 };
147 let thing = Thing { x: 0, y: 0 };
150 Thing { x, y: z } => {}
156 This error indicates that a pattern for a struct fails to specify a sub-pattern
157 for every one of the struct's fields. Ensure that each field from the struct's
158 definition is mentioned in the pattern, or use `..` to ignore unwanted fields.
162 ```compile_fail,E0027
168 let d = Dog { name: "Rusty".to_string(), age: 8 };
170 // This is incorrect.
176 This is correct (explicit):
184 let d = Dog { name: "Rusty".to_string(), age: 8 };
187 Dog { name: ref n, age: x } => {}
190 // This is also correct (ignore unused fields).
192 Dog { age: x, .. } => {}
198 In a match expression, only numbers and characters can be matched against a
199 range. This is because the compiler checks that the range is non-empty at
200 compile-time, and is unable to evaluate arbitrary comparison functions. If you
201 want to capture values of an orderable type between two end-points, you can use
204 ```compile_fail,E0029
205 let string = "salutations !";
207 // The ordering relation for strings can't be evaluated at compile time,
208 // so this doesn't work:
210 "hello" ... "world" => {}
214 // This is a more general version, using a guard:
216 s if s >= "hello" && s <= "world" => {}
223 This error indicates that a pointer to a trait type cannot be implicitly
224 dereferenced by a pattern. Every trait defines a type, but because the
225 size of trait implementors isn't fixed, this type has no compile-time size.
226 Therefore, all accesses to trait types must be through pointers. If you
227 encounter this error you should try to avoid dereferencing the pointer.
229 ```compile_fail,E0033
230 # trait SomeTrait { fn method_one(&self){} fn method_two(&self){} }
231 # impl<T> SomeTrait for T {}
232 let trait_obj: &SomeTrait = &"some_value";
234 // This tries to implicitly dereference to create an unsized local variable.
235 let &invalid = trait_obj;
237 // You can call methods without binding to the value being pointed at.
238 trait_obj.method_one();
239 trait_obj.method_two();
242 You can read more about trait objects in the [Trait Objects] section of the
245 [Trait Objects]: https://doc.rust-lang.org/reference/types.html#trait-objects
249 The compiler doesn't know what method to call because more than one method
250 has the same prototype. Erroneous code example:
252 ```compile_fail,E0034
263 impl Trait1 for Test { fn foo() {} }
264 impl Trait2 for Test { fn foo() {} }
267 Test::foo() // error, which foo() to call?
271 To avoid this error, you have to keep only one of them and remove the others.
272 So let's take our example and fix it:
281 impl Trait1 for Test { fn foo() {} }
284 Test::foo() // and now that's good!
288 However, a better solution would be using fully explicit naming of type and
302 impl Trait1 for Test { fn foo() {} }
303 impl Trait2 for Test { fn foo() {} }
306 <Test as Trait1>::foo()
323 impl F for X { fn m(&self) { println!("I am F"); } }
324 impl G for X { fn m(&self) { println!("I am G"); } }
329 F::m(&f); // it displays "I am F"
330 G::m(&f); // it displays "I am G"
336 You tried to give a type parameter where it wasn't needed. Erroneous code
339 ```compile_fail,E0035
349 x.method::<i32>(); // Error: Test::method doesn't need type parameter!
353 To fix this error, just remove the type parameter:
365 x.method(); // OK, we're good!
371 This error occurrs when you pass too many or not enough type parameters to
372 a method. Erroneous code example:
374 ```compile_fail,E0036
378 fn method<T>(&self, v: &[T]) -> usize {
387 x.method::<i32, i32>(v); // error: only one type parameter is expected!
391 To fix it, just specify a correct number of type parameters:
397 fn method<T>(&self, v: &[T]) -> usize {
406 x.method::<i32>(v); // OK, we're good!
410 Please note on the last example that we could have called `method` like this:
414 # impl Test { fn method<T>(&self, v: &[T]) -> usize { v.len() } }
422 It is not allowed to manually call destructors in Rust. It is also not
423 necessary to do this since `drop` is called automatically whenever a value goes
426 Here's an example of this error:
428 ```compile_fail,E0040
440 let mut x = Foo { x: -7 };
441 x.drop(); // error: explicit use of destructor method
447 You can't use type parameters on foreign items. Example of erroneous code:
449 ```compile_fail,E0044
450 extern { fn some_func<T>(x: T); }
453 To fix this, replace the type parameter with the specializations that you
457 extern { fn some_func_i32(x: i32); }
458 extern { fn some_func_i64(x: i64); }
463 Rust only supports variadic parameters for interoperability with C code in its
464 FFI. As such, variadic parameters can only be used with functions which are
465 using the C ABI. Examples of erroneous code:
468 #![feature(unboxed_closures)]
470 extern "rust-call" { fn foo(x: u8, ...); }
474 fn foo(x: u8, ...) {}
477 To fix such code, put them in an extern "C" block:
487 Items are missing in a trait implementation. Erroneous code example:
489 ```compile_fail,E0046
497 // error: not all trait items implemented, missing: `foo`
500 When trying to make some type implement a trait `Foo`, you must, at minimum,
501 provide implementations for all of `Foo`'s required methods (meaning the
502 methods that do not have default implementations), as well as any required
503 trait items like associated types or constants. Example:
519 This error indicates that an attempted implementation of a trait method
520 has the wrong number of type parameters.
522 For example, the trait below has a method `foo` with a type parameter `T`,
523 but the implementation of `foo` for the type `Bar` is missing this parameter:
525 ```compile_fail,E0049
527 fn foo<T: Default>(x: T) -> Self;
532 // error: method `foo` has 0 type parameters but its trait declaration has 1
535 fn foo(x: bool) -> Self { Bar }
541 This error indicates that an attempted implementation of a trait method
542 has the wrong number of function parameters.
544 For example, the trait below has a method `foo` with two function parameters
545 (`&self` and `u8`), but the implementation of `foo` for the type `Bar` omits
548 ```compile_fail,E0050
550 fn foo(&self, x: u8) -> bool;
555 // error: method `foo` has 1 parameter but the declaration in trait `Foo::foo`
558 fn foo(&self) -> bool { true }
564 The parameters of any trait method must match between a trait implementation
565 and the trait definition.
567 Here are a couple examples of this error:
569 ```compile_fail,E0053
578 // error, expected u16, found i16
581 // error, types differ in mutability
582 fn bar(&mut self) { }
588 It is not allowed to cast to a bool. If you are trying to cast a numeric type
589 to a bool, you can compare it with zero instead:
591 ```compile_fail,E0054
594 // Not allowed, won't compile
595 let x_is_nonzero = x as bool;
602 let x_is_nonzero = x != 0;
607 During a method call, a value is automatically dereferenced as many times as
608 needed to make the value's type match the method's receiver. The catch is that
609 the compiler will only attempt to dereference a number of times up to the
610 recursion limit (which can be set via the `recursion_limit` attribute).
612 For a somewhat artificial example:
614 ```compile_fail,E0055
615 #![recursion_limit="2"]
627 // error, reached the recursion limit while auto-dereferencing &&Foo
632 One fix may be to increase the recursion limit. Note that it is possible to
633 create an infinite recursion of dereferencing, in which case the only fix is to
634 somehow break the recursion.
638 When invoking closures or other implementations of the function traits `Fn`,
639 `FnMut` or `FnOnce` using call notation, the number of parameters passed to the
640 function must match its definition.
642 An example using a closure:
644 ```compile_fail,E0057
646 let a = f(); // invalid, too few parameters
647 let b = f(4); // this works!
648 let c = f(2, 3); // invalid, too many parameters
651 A generic function must be treated similarly:
654 fn foo<F: Fn()>(f: F) {
655 f(); // this is valid, but f(3) would not work
661 The built-in function traits are generic over a tuple of the function arguments.
662 If one uses angle-bracket notation (`Fn<(T,), Output=U>`) instead of parentheses
663 (`Fn(T) -> U`) to denote the function trait, the type parameter should be a
664 tuple. Otherwise function call notation cannot be used and the trait will not be
665 implemented by closures.
667 The most likely source of this error is using angle-bracket notation without
668 wrapping the function argument type into a tuple, for example:
670 ```compile_fail,E0059
671 #![feature(unboxed_closures)]
673 fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) }
676 It can be fixed by adjusting the trait bound like this:
679 #![feature(unboxed_closures)]
681 fn foo<F: Fn<(i32,)>>(f: F) -> F::Output { f(3) }
684 Note that `(T,)` always denotes the type of a 1-tuple containing an element of
685 type `T`. The comma is necessary for syntactic disambiguation.
689 External C functions are allowed to be variadic. However, a variadic function
690 takes a minimum number of arguments. For example, consider C's variadic `printf`
694 use std::os::raw::{c_char, c_int};
697 fn printf(_: *const c_char, ...) -> c_int;
701 Using this declaration, it must be called with at least one argument, so
702 simply calling `printf()` is invalid. But the following uses are allowed:
705 # #![feature(static_nobundle)]
706 # use std::os::raw::{c_char, c_int};
707 # #[cfg_attr(all(windows, target_env = "msvc"),
708 # link(name = "legacy_stdio_definitions", kind = "static-nobundle"))]
709 # extern "C" { fn printf(_: *const c_char, ...) -> c_int; }
712 use std::ffi::CString;
714 let fmt = CString::new("test\n").unwrap();
715 printf(fmt.as_ptr());
717 let fmt = CString::new("number = %d\n").unwrap();
718 printf(fmt.as_ptr(), 3);
720 let fmt = CString::new("%d, %d\n").unwrap();
721 printf(fmt.as_ptr(), 10, 5);
726 // ^ Note: On MSVC 2015, the `printf` function is "inlined" in the C code, and
727 // the C runtime does not contain the `printf` definition. This leads to linker
728 // error from the doc test (issue #42830).
729 // This can be fixed by linking to the static library
730 // `legacy_stdio_definitions.lib` (see https://stackoverflow.com/a/36504365/).
731 // If this compatibility library is removed in the future, consider changing
732 // `printf` in this example to another well-known variadic function.
735 The number of arguments passed to a function must match the number of arguments
736 specified in the function signature.
738 For example, a function like:
741 fn f(a: u16, b: &str) {}
744 Must always be called with exactly two arguments, e.g. `f(2, "test")`.
746 Note that Rust does not have a notion of optional function arguments or
747 variadic functions (except for its C-FFI).
751 This error indicates that during an attempt to build a struct or struct-like
752 enum variant, one of the fields was specified more than once. Erroneous code
755 ```compile_fail,E0062
763 x: 0, // error: field `x` specified more than once
768 Each field should be specified exactly one time. Example:
776 let x = Foo { x: 0 }; // ok!
782 This error indicates that during an attempt to build a struct or struct-like
783 enum variant, one of the fields was not provided. Erroneous code example:
785 ```compile_fail,E0063
792 let x = Foo { x: 0 }; // error: missing field: `y`
796 Each field should be specified exactly once. Example:
805 let x = Foo { x: 0, y: 0 }; // ok!
811 Box placement expressions (like C++'s "placement new") do not yet support any
812 place expression except the exchange heap (i.e. `std::boxed::HEAP`).
813 Furthermore, the syntax is changing to use `in` instead of `box`. See [RFC 470]
814 and [RFC 809] for more details.
816 [RFC 470]: https://github.com/rust-lang/rfcs/pull/470
817 [RFC 809]: https://github.com/rust-lang/rfcs/blob/master/text/0809-box-and-in-for-stdlib.md
821 The left-hand side of a compound assignment expression must be an lvalue
822 expression. An lvalue expression represents a memory location and includes
823 item paths (ie, namespaced variables), dereferences, indexing expressions,
824 and field references.
826 Let's start with some erroneous code examples:
828 ```compile_fail,E0067
829 use std::collections::LinkedList;
831 // Bad: assignment to non-lvalue expression
832 LinkedList::new() += 1;
836 fn some_func(i: &mut i32) {
837 i += 12; // Error : '+=' operation cannot be applied on a reference !
841 And now some working examples:
850 fn some_func(i: &mut i32) {
857 The compiler found a function whose body contains a `return;` statement but
858 whose return type is not `()`. An example of this is:
860 ```compile_fail,E0069
867 Since `return;` is just like `return ();`, there is a mismatch between the
868 function's return type and the value being returned.
872 The left-hand side of an assignment operator must be an lvalue expression. An
873 lvalue expression represents a memory location and can be a variable (with
874 optional namespacing), a dereference, an indexing expression or a field
877 More details can be found in the [Expressions] section of the Reference.
879 [Expressions]: https://doc.rust-lang.org/reference/expressions.html#lvalues-rvalues-and-temporaries
881 Now, we can go further. Here are some erroneous code examples:
883 ```compile_fail,E0070
889 const SOME_CONST : i32 = 12;
891 fn some_other_func() {}
894 SOME_CONST = 14; // error : a constant value cannot be changed!
895 1 = 3; // error : 1 isn't a valid lvalue!
896 some_other_func() = 4; // error : we can't assign value to a function!
897 SomeStruct.x = 12; // error : SomeStruct a structure name but it is used
902 And now let's give working examples:
909 let mut s = SomeStruct {x: 0, y: 0};
911 s.x = 3; // that's good !
915 fn some_func(x: &mut i32) {
916 *x = 12; // that's good !
922 You tried to use structure-literal syntax to create an item that is
923 not a structure or enum variant.
925 Example of erroneous code:
927 ```compile_fail,E0071
929 let t = U32 { value: 4 }; // error: expected struct, variant or union type,
930 // found builtin type `u32`
933 To fix this, ensure that the name was correctly spelled, and that
934 the correct form of initializer was used.
936 For example, the code above can be fixed to:
944 let u = Foo::FirstValue(0i32);
952 #### Note: this error code is no longer emitted by the compiler.
954 You cannot define a struct (or enum) `Foo` that requires an instance of `Foo`
955 in order to make a new `Foo` value. This is because there would be no way a
956 first instance of `Foo` could be made to initialize another instance!
958 Here's an example of a struct that has this problem:
961 struct Foo { x: Box<Foo> } // error
964 One fix is to use `Option`, like so:
967 struct Foo { x: Option<Box<Foo>> }
970 Now it's possible to create at least one instance of `Foo`: `Foo { x: None }`.
974 #### Note: this error code is no longer emitted by the compiler.
976 When using the `#[simd]` attribute on a tuple struct, the components of the
977 tuple struct must all be of a concrete, nongeneric type so the compiler can
978 reason about how to use SIMD with them. This error will occur if the types
981 This will cause an error:
984 #![feature(repr_simd)]
987 struct Bad<T>(T, T, T);
993 #![feature(repr_simd)]
996 struct Good(u32, u32, u32);
1001 The `#[simd]` attribute can only be applied to non empty tuple structs, because
1002 it doesn't make sense to try to use SIMD operations when there are no values to
1005 This will cause an error:
1007 ```compile_fail,E0075
1008 #![feature(repr_simd)]
1017 #![feature(repr_simd)]
1025 When using the `#[simd]` attribute to automatically use SIMD operations in tuple
1026 struct, the types in the struct must all be of the same type, or the compiler
1027 will trigger this error.
1029 This will cause an error:
1031 ```compile_fail,E0076
1032 #![feature(repr_simd)]
1035 struct Bad(u16, u32, u32);
1041 #![feature(repr_simd)]
1044 struct Good(u32, u32, u32);
1049 When using the `#[simd]` attribute on a tuple struct, the elements in the tuple
1050 must be machine types so SIMD operations can be applied to them.
1052 This will cause an error:
1054 ```compile_fail,E0077
1055 #![feature(repr_simd)]
1064 #![feature(repr_simd)]
1067 struct Good(u32, u32, u32);
1072 Enum discriminants are used to differentiate enum variants stored in memory.
1073 This error indicates that the same value was used for two or more variants,
1074 making them impossible to tell apart.
1076 ```compile_fail,E0081
1094 Note that variants without a manually specified discriminant are numbered from
1095 top to bottom starting from 0, so clashes can occur with seemingly unrelated
1098 ```compile_fail,E0081
1105 Here `X` will have already been specified the discriminant 0 by the time `Y` is
1106 encountered, so a conflict occurs.
1110 #### Note: this error code is no longer emitted by the compiler.
1112 When you specify enum discriminants with `=`, the compiler expects `isize`
1113 values by default. Or you can add the `repr` attibute to the enum declaration
1114 for an explicit choice of the discriminant type. In either cases, the
1115 discriminant values must fall within a valid range for the expected type;
1116 otherwise this error is raised. For example:
1119 # #![deny(overflowing_literals)]
1127 Here, 1024 lies outside the valid range for `u8`, so the discriminant for `A` is
1128 invalid. Here is another, more subtle example which depends on target word size:
1130 ```compile_fail,E0080
1132 enum DependsOnPointerSize {
1137 Here, `1 << 32` is interpreted as an `isize` value. So it is invalid for 32 bit
1138 target (`target_pointer_width = "32"`) but valid for 64 bit target.
1140 You may want to change representation types to fix this, or else change invalid
1141 discriminant values so that they fit within the existing type.
1145 An unsupported representation was attempted on a zero-variant enum.
1147 Erroneous code example:
1149 ```compile_fail,E0084
1151 enum NightsWatch {} // error: unsupported representation for zero-variant enum
1154 It is impossible to define an integer type to be used to represent zero-variant
1155 enum values because there are no zero-variant enum values. There is no way to
1156 construct an instance of the following type using only safe code. So you have
1157 two solutions. Either you add variants in your enum:
1167 or you remove the integer represention of your enum:
1175 Too many type parameters were supplied for a function. For example:
1177 ```compile_fail,E0087
1181 foo::<f64, bool>(); // error, expected 1 parameter, found 2 parameters
1185 The number of supplied parameters must exactly match the number of defined type
1190 You gave too many lifetime parameters. Erroneous code example:
1192 ```compile_fail,E0088
1196 f::<'static>() // error: too many lifetime parameters provided
1200 Please check you give the right number of lifetime parameters. Example:
1210 It's also important to note that the Rust compiler can generally
1211 determine the lifetime by itself. Example:
1219 // it can be written like this
1220 fn get_value<'a>(&'a self) -> &'a str { &self.value }
1221 // but the compiler works fine with this too:
1222 fn without_lifetime(&self) -> &str { &self.value }
1226 let f = Foo { value: "hello".to_owned() };
1228 println!("{}", f.get_value());
1229 println!("{}", f.without_lifetime());
1235 Not enough type parameters were supplied for a function. For example:
1237 ```compile_fail,E0089
1241 foo::<f64>(); // error, expected 2 parameters, found 1 parameter
1245 Note that if a function takes multiple type parameters but you want the compiler
1246 to infer some of them, you can use type placeholders:
1248 ```compile_fail,E0089
1249 fn foo<T, U>(x: T) {}
1253 foo::<f64>(x); // error, expected 2 parameters, found 1 parameter
1254 foo::<_, f64>(x); // same as `foo::<bool, f64>(x)`
1260 You gave too few lifetime parameters. Example:
1262 ```compile_fail,E0090
1263 fn foo<'a: 'b, 'b: 'a>() {}
1266 foo::<'static>(); // error, expected 2 lifetime parameters
1270 Please check you give the right number of lifetime parameters. Example:
1273 fn foo<'a: 'b, 'b: 'a>() {}
1276 foo::<'static, 'static>();
1282 You gave an unnecessary type parameter in a type alias. Erroneous code
1285 ```compile_fail,E0091
1286 type Foo<T> = u32; // error: type parameter `T` is unused
1288 type Foo<A,B> = Box<A>; // error: type parameter `B` is unused
1291 Please check you didn't write too many type parameters. Example:
1294 type Foo = u32; // ok!
1295 type Foo2<A> = Box<A>; // ok!
1300 You tried to declare an undefined atomic operation function.
1301 Erroneous code example:
1303 ```compile_fail,E0092
1304 #![feature(intrinsics)]
1306 extern "rust-intrinsic" {
1307 fn atomic_foo(); // error: unrecognized atomic operation
1312 Please check you didn't make a mistake in the function's name. All intrinsic
1313 functions are defined in librustc_trans/trans/intrinsic.rs and in
1314 libcore/intrinsics.rs in the Rust source code. Example:
1317 #![feature(intrinsics)]
1319 extern "rust-intrinsic" {
1320 fn atomic_fence(); // ok!
1326 You declared an unknown intrinsic function. Erroneous code example:
1328 ```compile_fail,E0093
1329 #![feature(intrinsics)]
1331 extern "rust-intrinsic" {
1332 fn foo(); // error: unrecognized intrinsic function: `foo`
1342 Please check you didn't make a mistake in the function's name. All intrinsic
1343 functions are defined in librustc_trans/trans/intrinsic.rs and in
1344 libcore/intrinsics.rs in the Rust source code. Example:
1347 #![feature(intrinsics)]
1349 extern "rust-intrinsic" {
1350 fn atomic_fence(); // ok!
1362 You gave an invalid number of type parameters to an intrinsic function.
1363 Erroneous code example:
1365 ```compile_fail,E0094
1366 #![feature(intrinsics)]
1368 extern "rust-intrinsic" {
1369 fn size_of<T, U>() -> usize; // error: intrinsic has wrong number
1370 // of type parameters
1374 Please check that you provided the right number of type parameters
1375 and verify with the function declaration in the Rust source code.
1379 #![feature(intrinsics)]
1381 extern "rust-intrinsic" {
1382 fn size_of<T>() -> usize; // ok!
1388 This error means that an incorrect number of lifetime parameters were provided
1389 for a type (like a struct or enum) or trait:
1391 ```compile_fail,E0107
1392 struct Foo<'a, 'b>(&'a str, &'b str);
1393 enum Bar { A, B, C }
1396 foo: Foo<'a>, // error: expected 2, found 1
1397 bar: Bar<'a>, // error: expected 0, found 1
1403 You tried to give a type parameter to a type which doesn't need it. Erroneous
1406 ```compile_fail,E0109
1407 type X = u32<i32>; // error: type parameters are not allowed on this type
1410 Please check that you used the correct type and recheck its definition. Perhaps
1411 it doesn't need the type parameter.
1416 type X = u32; // this compiles
1419 Note that type parameters for enum-variant constructors go after the variant,
1420 not after the enum (`Option::None::<u32>`, not `Option::<u32>::None`).
1424 You tried to give a lifetime parameter to a type which doesn't need it.
1425 Erroneous code example:
1427 ```compile_fail,E0110
1428 type X = u32<'static>; // error: lifetime parameters are not allowed on
1432 Please check that the correct type was used and recheck its definition; perhaps
1433 it doesn't need the lifetime parameter. Example:
1436 type X = u32; // ok!
1441 You can only define an inherent implementation for a type in the same crate
1442 where the type was defined. For example, an `impl` block as below is not allowed
1443 since `Vec` is defined in the standard library:
1445 ```compile_fail,E0116
1446 impl Vec<u8> { } // error
1449 To fix this problem, you can do either of these things:
1451 - define a trait that has the desired associated functions/types/constants and
1452 implement the trait for the type in question
1453 - define a new type wrapping the type and define an implementation on the new
1456 Note that using the `type` keyword does not work here because `type` only
1457 introduces a type alias:
1459 ```compile_fail,E0116
1460 type Bytes = Vec<u8>;
1462 impl Bytes { } // error, same as above
1467 This error indicates a violation of one of Rust's orphan rules for trait
1468 implementations. The rule prohibits any implementation of a foreign trait (a
1469 trait defined in another crate) where
1471 - the type that is implementing the trait is foreign
1472 - all of the parameters being passed to the trait (if there are any) are also
1475 Here's one example of this error:
1477 ```compile_fail,E0117
1478 impl Drop for u32 {}
1481 To avoid this kind of error, ensure that at least one local type is referenced
1485 pub struct Foo; // you define your type in your crate
1487 impl Drop for Foo { // and you can implement the trait on it!
1488 // code of trait implementation here
1489 # fn drop(&mut self) { }
1492 impl From<Foo> for i32 { // or you use a type from your crate as
1494 fn from(i: Foo) -> i32 {
1500 Alternatively, define a trait locally and implement that instead:
1504 fn get(&self) -> usize;
1508 fn get(&self) -> usize { 0 }
1512 For information on the design of the orphan rules, see [RFC 1023].
1514 [RFC 1023]: https://github.com/rust-lang/rfcs/blob/master/text/1023-rebalancing-coherence.md
1518 You're trying to write an inherent implementation for something which isn't a
1519 struct nor an enum. Erroneous code example:
1521 ```compile_fail,E0118
1522 impl (u8, u8) { // error: no base type found for inherent implementation
1523 fn get_state(&self) -> String {
1529 To fix this error, please implement a trait on the type or wrap it in a struct.
1533 // we create a trait here
1534 trait LiveLongAndProsper {
1535 fn get_state(&self) -> String;
1538 // and now you can implement it on (u8, u8)
1539 impl LiveLongAndProsper for (u8, u8) {
1540 fn get_state(&self) -> String {
1541 "He's dead, Jim!".to_owned()
1546 Alternatively, you can create a newtype. A newtype is a wrapping tuple-struct.
1547 For example, `NewType` is a newtype over `Foo` in `struct NewType(Foo)`.
1551 struct TypeWrapper((u8, u8));
1554 fn get_state(&self) -> String {
1555 "Fascinating!".to_owned()
1562 An attempt was made to implement Drop on a trait, which is not allowed: only
1563 structs and enums can implement Drop. An example causing this error:
1565 ```compile_fail,E0120
1568 impl Drop for MyTrait {
1569 fn drop(&mut self) {}
1573 A workaround for this problem is to wrap the trait up in a struct, and implement
1574 Drop on that. An example is shown below:
1578 struct MyWrapper<T: MyTrait> { foo: T }
1580 impl <T: MyTrait> Drop for MyWrapper<T> {
1581 fn drop(&mut self) {}
1586 Alternatively, wrapping trait objects requires something like the following:
1591 //or Box<MyTrait>, if you wanted an owned trait object
1592 struct MyWrapper<'a> { foo: &'a MyTrait }
1594 impl <'a> Drop for MyWrapper<'a> {
1595 fn drop(&mut self) {}
1601 In order to be consistent with Rust's lack of global type inference, type
1602 placeholders are disallowed by design in item signatures.
1604 Examples of this error include:
1606 ```compile_fail,E0121
1607 fn foo() -> _ { 5 } // error, explicitly write out the return type instead
1609 static BAR: _ = "test"; // error, explicitly write out the type instead
1614 An attempt was made to add a generic constraint to a type alias. While Rust will
1615 allow this with a warning, it will not currently enforce the constraint.
1616 Consider the example below:
1621 type MyType<R: Foo> = (R, ());
1628 We're able to declare a variable of type `MyType<u32>`, despite the fact that
1629 `u32` does not implement `Foo`. As a result, one should avoid using generic
1630 constraints in concert with type aliases.
1634 You declared two fields of a struct with the same name. Erroneous code
1637 ```compile_fail,E0124
1640 field1: i32, // error: field is already declared
1644 Please verify that the field names have been correctly spelled. Example:
1655 It is not possible to define `main` with type parameters, or even with function
1656 parameters. When `main` is present, it must take no arguments and return `()`.
1657 Erroneous code example:
1659 ```compile_fail,E0131
1660 fn main<T>() { // error: main function is not allowed to have type parameters
1666 A function with the `start` attribute was declared with type parameters.
1668 Erroneous code example:
1670 ```compile_fail,E0132
1677 It is not possible to declare type parameters on a function that has the `start`
1678 attribute. Such a function must have the following type signature (for more
1679 information: http://doc.rust-lang.org/stable/book/first-edition/no-stdlib.html):
1683 fn(isize, *const *const u8) -> isize;
1692 fn my_start(argc: isize, argv: *const *const u8) -> isize {
1699 This error means that an attempt was made to match a struct type enum
1700 variant as a non-struct type:
1702 ```compile_fail,E0164
1703 enum Foo { B { i: u32 } }
1705 fn bar(foo: Foo) -> u32 {
1707 Foo::B(i) => i, // error E0164
1712 Try using `{}` instead:
1715 enum Foo { B { i: u32 } }
1717 fn bar(foo: Foo) -> u32 {
1726 You bound an associated type in an expression path which is not
1729 Erroneous code example:
1731 ```compile_fail,E0182
1737 impl Foo for isize {
1739 fn bar() -> isize { 42 }
1742 // error: unexpected binding of associated item in expression path
1743 let x: isize = Foo::<A=usize>::bar();
1746 To give a concrete type when using the Universal Function Call Syntax,
1747 use "Type as Trait". Example:
1755 impl Foo for isize {
1757 fn bar() -> isize { 42 }
1760 let x: isize = <isize as Foo>::bar(); // ok!
1765 Explicitly implementing both Drop and Copy for a type is currently disallowed.
1766 This feature can make some sense in theory, but the current implementation is
1767 incorrect and can lead to memory unsafety (see [issue #20126][iss20126]), so
1768 it has been disabled for now.
1770 [iss20126]: https://github.com/rust-lang/rust/issues/20126
1774 An associated function for a trait was defined to be static, but an
1775 implementation of the trait declared the same function to be a method (i.e. to
1776 take a `self` parameter).
1778 Here's an example of this error:
1780 ```compile_fail,E0185
1788 // error, method `foo` has a `&self` declaration in the impl, but not in
1796 An associated function for a trait was defined to be a method (i.e. to take a
1797 `self` parameter), but an implementation of the trait declared the same function
1800 Here's an example of this error:
1802 ```compile_fail,E0186
1810 // error, method `foo` has a `&self` declaration in the trait, but not in
1818 Trait objects need to have all associated types specified. Erroneous code
1821 ```compile_fail,E0191
1826 type Foo = Trait; // error: the value of the associated type `Bar` (from
1827 // the trait `Trait`) must be specified
1830 Please verify you specified all associated types of the trait and that you
1831 used the right trait. Example:
1838 type Foo = Trait<Bar=i32>; // ok!
1843 Negative impls are only allowed for traits with default impls. For more
1844 information see the [opt-in builtin traits RFC][RFC 19].
1846 [RFC 19]: https://github.com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md
1850 #### Note: this error code is no longer emitted by the compiler.
1852 `where` clauses must use generic type parameters: it does not make sense to use
1853 them otherwise. An example causing this error:
1860 #[derive(Copy,Clone)]
1865 impl Foo for Wrapper<u32> where Wrapper<u32>: Clone {
1870 This use of a `where` clause is strange - a more common usage would look
1871 something like the following:
1878 #[derive(Copy,Clone)]
1882 impl <T> Foo for Wrapper<T> where Wrapper<T>: Clone {
1887 Here, we're saying that the implementation exists on Wrapper only when the
1888 wrapped type `T` implements `Clone`. The `where` clause is important because
1889 some types will not implement `Clone`, and thus will not get this method.
1891 In our erroneous example, however, we're referencing a single concrete type.
1892 Since we know for certain that `Wrapper<u32>` implements `Clone`, there's no
1893 reason to also specify it in a `where` clause.
1897 A type parameter was declared which shadows an existing one. An example of this
1900 ```compile_fail,E0194
1902 fn do_something(&self) -> T;
1903 fn do_something_else<T: Clone>(&self, bar: T);
1907 In this example, the trait `Foo` and the trait method `do_something_else` both
1908 define a type parameter `T`. This is not allowed: if the method wishes to
1909 define a type parameter, it must use a different name for it.
1913 Your method's lifetime parameters do not match the trait declaration.
1914 Erroneous code example:
1916 ```compile_fail,E0195
1918 fn bar<'a,'b:'a>(x: &'a str, y: &'b str);
1923 impl Trait for Foo {
1924 fn bar<'a,'b>(x: &'a str, y: &'b str) {
1925 // error: lifetime parameters or bounds on method `bar`
1926 // do not match the trait declaration
1931 The lifetime constraint `'b` for bar() implementation does not match the
1932 trait declaration. Ensure lifetime declarations match exactly in both trait
1933 declaration and implementation. Example:
1937 fn t<'a,'b:'a>(x: &'a str, y: &'b str);
1942 impl Trait for Foo {
1943 fn t<'a,'b:'a>(x: &'a str, y: &'b str) { // ok!
1950 Inherent implementations (one that do not implement a trait but provide
1951 methods associated with a type) are always safe because they are not
1952 implementing an unsafe trait. Removing the `unsafe` keyword from the inherent
1953 implementation will resolve this error.
1955 ```compile_fail,E0197
1958 // this will cause this error
1960 // converting it to this will fix it
1966 A negative implementation is one that excludes a type from implementing a
1967 particular trait. Not being able to use a trait is always a safe operation,
1968 so negative implementations are always safe and never need to be marked as
1972 #![feature(optin_builtin_traits)]
1976 // unsafe is unnecessary
1977 unsafe impl !Clone for Foo { }
1983 #![feature(optin_builtin_traits)]
1989 impl Enterprise for .. { }
1991 impl !Enterprise for Foo { }
1994 Please note that negative impls are only allowed for traits with default impls.
1998 Safe traits should not have unsafe implementations, therefore marking an
1999 implementation for a safe trait unsafe will cause a compiler error. Removing
2000 the unsafe marker on the trait noted in the error will resolve this problem.
2002 ```compile_fail,E0199
2007 // this won't compile because Bar is safe
2008 unsafe impl Bar for Foo { }
2009 // this will compile
2010 impl Bar for Foo { }
2015 Unsafe traits must have unsafe implementations. This error occurs when an
2016 implementation for an unsafe trait isn't marked as unsafe. This may be resolved
2017 by marking the unsafe implementation as unsafe.
2019 ```compile_fail,E0200
2022 unsafe trait Bar { }
2024 // this won't compile because Bar is unsafe and impl isn't unsafe
2025 impl Bar for Foo { }
2026 // this will compile
2027 unsafe impl Bar for Foo { }
2032 It is an error to define two associated items (like methods, associated types,
2033 associated functions, etc.) with the same identifier.
2037 ```compile_fail,E0201
2041 fn bar(&self) -> bool { self.0 > 5 }
2042 fn bar() {} // error: duplicate associated function
2047 fn baz(&self) -> bool;
2053 fn baz(&self) -> bool { true }
2055 // error: duplicate method
2056 fn baz(&self) -> bool { self.0 > 5 }
2058 // error: duplicate associated type
2063 Note, however, that items with the same name are allowed for inherent `impl`
2064 blocks that don't overlap:
2070 fn bar(&self) -> bool { self.0 > 5 }
2074 fn bar(&self) -> bool { self.0 }
2080 Inherent associated types were part of [RFC 195] but are not yet implemented.
2081 See [the tracking issue][iss8995] for the status of this implementation.
2083 [RFC 195]: https://github.com/rust-lang/rfcs/blob/master/text/0195-associated-items.md
2084 [iss8995]: https://github.com/rust-lang/rust/issues/8995
2088 An attempt to implement the `Copy` trait for a struct failed because one of the
2089 fields does not implement `Copy`. To fix this, you must implement `Copy` for the
2090 mentioned field. Note that this may not be possible, as in the example of
2092 ```compile_fail,E0204
2097 impl Copy for Foo { }
2100 This fails because `Vec<T>` does not implement `Copy` for any `T`.
2102 Here's another example that will fail:
2104 ```compile_fail,E0204
2111 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
2112 differs from the behavior for `&T`, which is always `Copy`).
2117 An attempt to implement the `Copy` trait for an enum failed because one of the
2118 variants does not implement `Copy`. To fix this, you must implement `Copy` for
2119 the mentioned variant. Note that this may not be possible, as in the example of
2121 ```compile_fail,E0205
2127 impl Copy for Foo { }
2130 This fails because `Vec<T>` does not implement `Copy` for any `T`.
2132 Here's another example that will fail:
2134 ```compile_fail,E0205
2142 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
2143 differs from the behavior for `&T`, which is always `Copy`).
2148 You can only implement `Copy` for a struct or enum. Both of the following
2149 examples will fail, because neither `i32` (primitive type) nor `&'static Bar`
2150 (reference to `Bar`) is a struct or enum:
2152 ```compile_fail,E0206
2154 impl Copy for Foo { } // error
2156 #[derive(Copy, Clone)]
2158 impl Copy for &'static Bar { } // error
2163 Any type parameter or lifetime parameter of an `impl` must meet at least one of
2164 the following criteria:
2166 - it appears in the self type of the impl
2167 - for a trait impl, it appears in the trait reference
2168 - it is bound as an associated type
2172 Suppose we have a struct `Foo` and we would like to define some methods for it.
2173 The following definition leads to a compiler error:
2175 ```compile_fail,E0207
2178 impl<T: Default> Foo {
2179 // error: the type parameter `T` is not constrained by the impl trait, self
2180 // type, or predicates [E0207]
2181 fn get(&self) -> T {
2182 <T as Default>::default()
2187 The problem is that the parameter `T` does not appear in the self type (`Foo`)
2188 of the impl. In this case, we can fix the error by moving the type parameter
2189 from the `impl` to the method `get`:
2195 // Move the type parameter from the impl to the method
2197 fn get<T: Default>(&self) -> T {
2198 <T as Default>::default()
2205 As another example, suppose we have a `Maker` trait and want to establish a
2206 type `FooMaker` that makes `Foo`s:
2208 ```compile_fail,E0207
2211 fn make(&mut self) -> Self::Item;
2220 impl<T: Default> Maker for FooMaker {
2221 // error: the type parameter `T` is not constrained by the impl trait, self
2222 // type, or predicates [E0207]
2225 fn make(&mut self) -> Foo<T> {
2226 Foo { foo: <T as Default>::default() }
2231 This fails to compile because `T` does not appear in the trait or in the
2234 One way to work around this is to introduce a phantom type parameter into
2235 `FooMaker`, like so:
2238 use std::marker::PhantomData;
2242 fn make(&mut self) -> Self::Item;
2249 // Add a type parameter to `FooMaker`
2250 struct FooMaker<T> {
2251 phantom: PhantomData<T>,
2254 impl<T: Default> Maker for FooMaker<T> {
2257 fn make(&mut self) -> Foo<T> {
2259 foo: <T as Default>::default(),
2265 Another way is to do away with the associated type in `Maker` and use an input
2266 type parameter instead:
2269 // Use a type parameter instead of an associated type here
2271 fn make(&mut self) -> Item;
2280 impl<T: Default> Maker<Foo<T>> for FooMaker {
2281 fn make(&mut self) -> Foo<T> {
2282 Foo { foo: <T as Default>::default() }
2287 ### Additional information
2289 For more information, please see [RFC 447].
2291 [RFC 447]: https://github.com/rust-lang/rfcs/blob/master/text/0447-no-unused-impl-parameters.md
2295 This error indicates a violation of one of Rust's orphan rules for trait
2296 implementations. The rule concerns the use of type parameters in an
2297 implementation of a foreign trait (a trait defined in another crate), and
2298 states that type parameters must be "covered" by a local type. To understand
2299 what this means, it is perhaps easiest to consider a few examples.
2301 If `ForeignTrait` is a trait defined in some external crate `foo`, then the
2302 following trait `impl` is an error:
2304 ```compile_fail,E0210
2305 # #[cfg(for_demonstration_only)]
2307 # #[cfg(for_demonstration_only)]
2308 use foo::ForeignTrait;
2309 # use std::panic::UnwindSafe as ForeignTrait;
2311 impl<T> ForeignTrait for T { } // error
2315 To work around this, it can be covered with a local type, `MyType`:
2318 # use std::panic::UnwindSafe as ForeignTrait;
2319 struct MyType<T>(T);
2320 impl<T> ForeignTrait for MyType<T> { } // Ok
2323 Please note that a type alias is not sufficient.
2325 For another example of an error, suppose there's another trait defined in `foo`
2326 named `ForeignTrait2` that takes two type parameters. Then this `impl` results
2327 in the same rule violation:
2329 ```ignore (cannot-doctest-multicrate-project)
2331 impl<T> ForeignTrait2<T, MyType<T>> for MyType2 { } // error
2334 The reason for this is that there are two appearances of type parameter `T` in
2335 the `impl` header, both as parameters for `ForeignTrait2`. The first appearance
2336 is uncovered, and so runs afoul of the orphan rule.
2338 Consider one more example:
2340 ```ignore (cannot-doctest-multicrate-project)
2341 impl<T> ForeignTrait2<MyType<T>, T> for MyType2 { } // Ok
2344 This only differs from the previous `impl` in that the parameters `T` and
2345 `MyType<T>` for `ForeignTrait2` have been swapped. This example does *not*
2346 violate the orphan rule; it is permitted.
2348 To see why that last example was allowed, you need to understand the general
2349 rule. Unfortunately this rule is a bit tricky to state. Consider an `impl`:
2351 ```ignore (only-for-syntax-highlight)
2352 impl<P1, ..., Pm> ForeignTrait<T1, ..., Tn> for T0 { ... }
2355 where `P1, ..., Pm` are the type parameters of the `impl` and `T0, ..., Tn`
2356 are types. One of the types `T0, ..., Tn` must be a local type (this is another
2357 orphan rule, see the explanation for E0117). Let `i` be the smallest integer
2358 such that `Ti` is a local type. Then no type parameter can appear in any of the
2361 For information on the design of the orphan rules, see [RFC 1023].
2363 [RFC 1023]: https://github.com/rust-lang/rfcs/blob/master/text/1023-rebalancing-coherence.md
2368 You used a function or type which doesn't fit the requirements for where it was
2369 used. Erroneous code examples:
2372 #![feature(intrinsics)]
2374 extern "rust-intrinsic" {
2375 fn size_of<T>(); // error: intrinsic has wrong type
2380 fn main() -> i32 { 0 }
2381 // error: main function expects type: `fn() {main}`: expected (), found i32
2388 // error: mismatched types in range: expected u8, found i8
2398 fn x(self: Rc<Foo>) {}
2399 // error: mismatched self type: expected `Foo`: expected struct
2400 // `Foo`, found struct `alloc::rc::Rc`
2404 For the first code example, please check the function definition. Example:
2407 #![feature(intrinsics)]
2409 extern "rust-intrinsic" {
2410 fn size_of<T>() -> usize; // ok!
2414 The second case example is a bit particular : the main function must always
2415 have this definition:
2421 They never take parameters and never return types.
2423 For the third example, when you match, all patterns must have the same type
2424 as the type you're matching on. Example:
2430 0u8...3u8 => (), // ok!
2435 And finally, for the last example, only `Box<Self>`, `&Self`, `Self`,
2436 or `&mut Self` work as explicit self parameters. Example:
2442 fn x(self: Box<Foo>) {} // ok!
2449 A generic type was described using parentheses rather than angle brackets. For
2452 ```compile_fail,E0214
2454 let v: Vec(&str) = vec!["foo"];
2458 This is not currently supported: `v` should be defined as `Vec<&str>`.
2459 Parentheses are currently only used with generic types when defining parameters
2460 for `Fn`-family traits.
2464 You used an associated type which isn't defined in the trait.
2465 Erroneous code example:
2467 ```compile_fail,E0220
2472 type Foo = T1<F=i32>; // error: associated type `F` not found for `T1`
2479 // error: Baz is used but not declared
2480 fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
2484 Make sure that you have defined the associated type in the trait body.
2485 Also, verify that you used the right trait or you didn't misspell the
2486 associated type name. Example:
2493 type Foo = T1<Bar=i32>; // ok!
2499 type Baz; // we declare `Baz` in our trait.
2501 // and now we can use it here:
2502 fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
2508 An attempt was made to retrieve an associated type, but the type was ambiguous.
2511 ```compile_fail,E0221
2527 In this example, `Foo` defines an associated type `A`. `Bar` inherits that type
2528 from `Foo`, and defines another associated type of the same name. As a result,
2529 when we attempt to use `Self::A`, it's ambiguous whether we mean the `A` defined
2530 by `Foo` or the one defined by `Bar`.
2532 There are two options to work around this issue. The first is simply to rename
2533 one of the types. Alternatively, one can specify the intended type using the
2547 let _: <Self as Bar>::A;
2554 An attempt was made to retrieve an associated type, but the type was ambiguous.
2557 ```compile_fail,E0223
2558 trait MyTrait {type X; }
2561 let foo: MyTrait::X;
2565 The problem here is that we're attempting to take the type of X from MyTrait.
2566 Unfortunately, the type of X is not defined, because it's only made concrete in
2567 implementations of the trait. A working version of this code might look like:
2570 trait MyTrait {type X; }
2573 impl MyTrait for MyStruct {
2578 let foo: <MyStruct as MyTrait>::X;
2582 This syntax specifies that we want the X type from MyTrait, as made concrete in
2583 MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct
2584 might implement two different traits with identically-named associated types.
2585 This syntax allows disambiguation between the two.
2589 You attempted to use multiple types as bounds for a closure or trait object.
2590 Rust does not currently support this. A simple example that causes this error:
2592 ```compile_fail,E0225
2594 let _: Box<std::io::Read + std::io::Write>;
2598 Send and Sync are an exception to this rule: it's possible to have bounds of
2599 one non-builtin trait, plus either or both of Send and Sync. For example, the
2600 following compiles correctly:
2604 let _: Box<std::io::Read + Send + Sync>;
2610 An associated type binding was done outside of the type parameter declaration
2611 and `where` clause. Erroneous code example:
2613 ```compile_fail,E0229
2616 fn boo(&self) -> <Self as Foo>::A;
2621 impl Foo for isize {
2623 fn boo(&self) -> usize { 42 }
2626 fn baz<I>(x: &<I as Foo<A=Bar>>::A) {}
2627 // error: associated type bindings are not allowed here
2630 To solve this error, please move the type bindings in the type parameter
2635 # trait Foo { type A; }
2636 fn baz<I: Foo<A=Bar>>(x: &<I as Foo>::A) {} // ok!
2639 Or in the `where` clause:
2643 # trait Foo { type A; }
2644 fn baz<I>(x: &<I as Foo>::A) where I: Foo<A=Bar> {}
2649 The trait has more type parameters specified than appear in its definition.
2651 Erroneous example code:
2653 ```compile_fail,E0230
2654 #![feature(on_unimplemented)]
2655 #[rustc_on_unimplemented = "Trait error on `{Self}` with `<{A},{B},{C}>`"]
2656 // error: there is no type parameter C on trait TraitWithThreeParams
2657 trait TraitWithThreeParams<A,B>
2661 Include the correct number of type parameters and the compilation should
2665 #![feature(on_unimplemented)]
2666 #[rustc_on_unimplemented = "Trait error on `{Self}` with `<{A},{B},{C}>`"]
2667 trait TraitWithThreeParams<A,B,C> // ok!
2673 The attribute must have a value. Erroneous code example:
2675 ```compile_fail,E0232
2676 #![feature(on_unimplemented)]
2678 #[rustc_on_unimplemented] // error: this attribute must have a value
2682 Please supply the missing value of the attribute. Example:
2685 #![feature(on_unimplemented)]
2687 #[rustc_on_unimplemented = "foo"] // ok!
2693 This error indicates that not enough type parameters were found in a type or
2696 For example, the `Foo` struct below is defined to be generic in `T`, but the
2697 type parameter is missing in the definition of `Bar`:
2699 ```compile_fail,E0243
2700 struct Foo<T> { x: T }
2702 struct Bar { x: Foo }
2707 This error indicates that too many type parameters were found in a type or
2710 For example, the `Foo` struct below has no type parameters, but is supplied
2711 with two in the definition of `Bar`:
2713 ```compile_fail,E0244
2714 struct Foo { x: bool }
2716 struct Bar<S, T> { x: Foo<S, T> }
2721 If an impl has a generic parameter with the `#[may_dangle]` attribute, then
2722 that impl must be declared as an `unsafe impl. For example:
2724 ```compile_fail,E0569
2725 #![feature(generic_param_attrs)]
2726 #![feature(dropck_eyepatch)]
2729 impl<#[may_dangle] X> Drop for Foo<X> {
2730 fn drop(&mut self) { }
2734 In this example, we are asserting that the destructor for `Foo` will not
2735 access any data of type `X`, and require this assertion to be true for
2736 overall safety in our program. The compiler does not currently attempt to
2737 verify this assertion; therefore we must tag this `impl` as unsafe.
2741 Default impls for a trait must be located in the same crate where the trait was
2742 defined. For more information see the [opt-in builtin traits RFC][RFC 19].
2744 [RFC 19]: https://github.com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md
2748 A cross-crate opt-out trait was implemented on something which wasn't a struct
2749 or enum type. Erroneous code example:
2751 ```compile_fail,E0321
2752 #![feature(optin_builtin_traits)]
2756 impl !Sync for Foo {}
2758 unsafe impl Send for &'static Foo {}
2759 // error: cross-crate traits with a default impl, like `core::marker::Send`,
2760 // can only be implemented for a struct/enum type, not
2764 Only structs and enums are permitted to impl Send, Sync, and other opt-out
2765 trait, and the struct or enum must be local to the current crate. So, for
2766 example, `unsafe impl Send for Rc<Foo>` is not allowed.
2770 The `Sized` trait is a special trait built-in to the compiler for types with a
2771 constant size known at compile-time. This trait is automatically implemented
2772 for types as needed by the compiler, and it is currently disallowed to
2773 explicitly implement it for a type.
2777 An associated const was implemented when another trait item was expected.
2778 Erroneous code example:
2780 ```compile_fail,E0323
2789 // error: item `N` is an associated const, which doesn't match its
2790 // trait `<Bar as Foo>`
2794 Please verify that the associated const wasn't misspelled and the correct trait
2795 was implemented. Example:
2805 type N = u32; // ok!
2819 const N : u32 = 0; // ok!
2825 A method was implemented when another trait item was expected. Erroneous
2828 ```compile_fail,E0324
2839 // error: item `N` is an associated method, which doesn't match its
2840 // trait `<Bar as Foo>`
2844 To fix this error, please verify that the method name wasn't misspelled and
2845 verify that you are indeed implementing the correct trait items. Example:
2865 An associated type was implemented when another trait item was expected.
2866 Erroneous code example:
2868 ```compile_fail,E0325
2877 // error: item `N` is an associated type, which doesn't match its
2878 // trait `<Bar as Foo>`
2882 Please verify that the associated type name wasn't misspelled and your
2883 implementation corresponds to the trait definition. Example:
2893 type N = u32; // ok!
2907 const N : u32 = 0; // ok!
2913 The types of any associated constants in a trait implementation must match the
2914 types in the trait definition. This error indicates that there was a mismatch.
2916 Here's an example of this error:
2918 ```compile_fail,E0326
2926 const BAR: u32 = 5; // error, expected bool, found u32
2932 The Unsize trait should not be implemented directly. All implementations of
2933 Unsize are provided automatically by the compiler.
2935 Erroneous code example:
2937 ```compile_fail,E0328
2940 use std::marker::Unsize;
2944 impl<T> Unsize<T> for MyType {}
2947 If you are defining your own smart pointer type and would like to enable
2948 conversion from a sized to an unsized type with the
2949 [DST coercion system][RFC 982], use [`CoerceUnsized`] instead.
2952 #![feature(coerce_unsized)]
2954 use std::ops::CoerceUnsized;
2956 pub struct MyType<T: ?Sized> {
2957 field_with_unsized_type: T,
2960 impl<T, U> CoerceUnsized<MyType<U>> for MyType<T>
2961 where T: CoerceUnsized<U> {}
2964 [RFC 982]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md
2965 [`CoerceUnsized`]: https://doc.rust-lang.org/std/ops/trait.CoerceUnsized.html
2969 // Associated consts can now be accessed through generic type parameters, and
2970 // this error is no longer emitted.
2972 // FIXME: consider whether to leave it in the error index, or remove it entirely
2973 // as associated consts is not stabilized yet.
2976 An attempt was made to access an associated constant through either a generic
2977 type parameter or `Self`. This is not supported yet. An example causing this
2978 error is shown below:
2987 impl Foo for MyStruct {
2988 const BAR: f64 = 0f64;
2991 fn get_bar_bad<F: Foo>(t: F) -> f64 {
2996 Currently, the value of `BAR` for a particular type can only be accessed
2997 through a concrete type, as shown below:
3006 fn get_bar_good() -> f64 {
3007 <MyStruct as Foo>::BAR
3014 An attempt was made to implement `Drop` on a concrete specialization of a
3015 generic type. An example is shown below:
3017 ```compile_fail,E0366
3022 impl Drop for Foo<u32> {
3023 fn drop(&mut self) {}
3027 This code is not legal: it is not possible to specialize `Drop` to a subset of
3028 implementations of a generic type. One workaround for this is to wrap the
3029 generic type, as shown below:
3041 fn drop(&mut self) {}
3047 An attempt was made to implement `Drop` on a specialization of a generic type.
3048 An example is shown below:
3050 ```compile_fail,E0367
3053 struct MyStruct<T> {
3057 impl<T: Foo> Drop for MyStruct<T> {
3058 fn drop(&mut self) {}
3062 This code is not legal: it is not possible to specialize `Drop` to a subset of
3063 implementations of a generic type. In order for this code to work, `MyStruct`
3064 must also require that `T` implements `Foo`. Alternatively, another option is
3065 to wrap the generic type in another that specializes appropriately:
3070 struct MyStruct<T> {
3074 struct MyStructWrapper<T: Foo> {
3078 impl <T: Foo> Drop for MyStructWrapper<T> {
3079 fn drop(&mut self) {}
3085 This error indicates that a binary assignment operator like `+=` or `^=` was
3086 applied to a type that doesn't support it. For example:
3088 ```compile_fail,E0368
3089 let mut x = 12f32; // error: binary operation `<<` cannot be applied to
3095 To fix this error, please check that this type implements this binary
3099 let mut x = 12u32; // the `u32` type does implement the `ShlAssign` trait
3104 It is also possible to overload most operators for your own type by
3105 implementing the `[OP]Assign` traits from `std::ops`.
3107 Another problem you might be facing is this: suppose you've overloaded the `+`
3108 operator for some type `Foo` by implementing the `std::ops::Add` trait for
3109 `Foo`, but you find that using `+=` does not work, as in this example:
3111 ```compile_fail,E0368
3119 fn add(self, rhs: Foo) -> Foo {
3125 let mut x: Foo = Foo(5);
3126 x += Foo(7); // error, `+= cannot be applied to the type `Foo`
3130 This is because `AddAssign` is not automatically implemented, so you need to
3131 manually implement it for your type.
3135 A binary operation was attempted on a type which doesn't support it.
3136 Erroneous code example:
3138 ```compile_fail,E0369
3139 let x = 12f32; // error: binary operation `<<` cannot be applied to
3145 To fix this error, please check that this type implements this binary
3149 let x = 12u32; // the `u32` type does implement it:
3150 // https://doc.rust-lang.org/stable/std/ops/trait.Shl.html
3155 It is also possible to overload most operators for your own type by
3156 implementing traits from `std::ops`.
3158 String concatenation appends the string on the right to the string on the
3159 left and may require reallocation. This requires ownership of the string
3160 on the left. If something should be added to a string literal, move the
3161 literal to the heap by allocating it with `to_owned()` like in
3162 `"Your text".to_owned()`.
3167 The maximum value of an enum was reached, so it cannot be automatically
3168 set in the next enum value. Erroneous code example:
3171 #[deny(overflowing_literals)]
3173 X = 0x7fffffffffffffff,
3174 Y, // error: enum discriminant overflowed on value after
3175 // 9223372036854775807: i64; set explicitly via
3176 // Y = -9223372036854775808 if that is desired outcome
3180 To fix this, please set manually the next enum value or put the enum variant
3181 with the maximum value at the end of the enum. Examples:
3185 X = 0x7fffffffffffffff,
3195 X = 0x7fffffffffffffff,
3201 When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
3202 definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
3203 `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
3204 definition, so it is not useful to do this.
3208 ```compile_fail,E0371
3209 trait Foo { fn foo(&self) { } }
3213 impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
3214 impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
3215 impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
3216 impl Baz for Bar { } // Note: This is OK
3221 A struct without a field containing an unsized type cannot implement
3223 [unsized type](https://doc.rust-lang.org/book/first-edition/unsized-types.html)
3224 is any type that the compiler doesn't know the length or alignment of at
3225 compile time. Any struct containing an unsized type is also unsized.
3227 Example of erroneous code:
3229 ```compile_fail,E0374
3230 #![feature(coerce_unsized)]
3231 use std::ops::CoerceUnsized;
3233 struct Foo<T: ?Sized> {
3237 // error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
3238 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
3239 where T: CoerceUnsized<U> {}
3242 `CoerceUnsized` is used to coerce one struct containing an unsized type
3243 into another struct containing a different unsized type. If the struct
3244 doesn't have any fields of unsized types then you don't need explicit
3245 coercion to get the types you want. To fix this you can either
3246 not try to implement `CoerceUnsized` or you can add a field that is
3247 unsized to the struct.
3252 #![feature(coerce_unsized)]
3253 use std::ops::CoerceUnsized;
3255 // We don't need to impl `CoerceUnsized` here.
3260 // We add the unsized type field to the struct.
3261 struct Bar<T: ?Sized> {
3266 // The struct has an unsized field so we can implement
3267 // `CoerceUnsized` for it.
3268 impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
3269 where T: CoerceUnsized<U> {}
3272 Note that `CoerceUnsized` is mainly used by smart pointers like `Box`, `Rc`
3273 and `Arc` to be able to mark that they can coerce unsized types that they
3278 A struct with more than one field containing an unsized type cannot implement
3279 `CoerceUnsized`. This only occurs when you are trying to coerce one of the
3280 types in your struct to another type in the struct. In this case we try to
3281 impl `CoerceUnsized` from `T` to `U` which are both types that the struct
3282 takes. An [unsized type] is any type that the compiler doesn't know the length
3283 or alignment of at compile time. Any struct containing an unsized type is also
3286 Example of erroneous code:
3288 ```compile_fail,E0375
3289 #![feature(coerce_unsized)]
3290 use std::ops::CoerceUnsized;
3292 struct Foo<T: ?Sized, U: ?Sized> {
3298 // error: Struct `Foo` has more than one unsized field.
3299 impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
3302 `CoerceUnsized` only allows for coercion from a structure with a single
3303 unsized type field to another struct with a single unsized type field.
3304 In fact Rust only allows for a struct to have one unsized type in a struct
3305 and that unsized type must be the last field in the struct. So having two
3306 unsized types in a single struct is not allowed by the compiler. To fix this
3307 use only one field containing an unsized type in the struct and then use
3308 multiple structs to manage each unsized type field you need.
3313 #![feature(coerce_unsized)]
3314 use std::ops::CoerceUnsized;
3316 struct Foo<T: ?Sized> {
3321 impl <T, U> CoerceUnsized<Foo<U>> for Foo<T>
3322 where T: CoerceUnsized<U> {}
3324 fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
3325 Foo { a: 12i32, b: t } // we use coercion to get the `Foo<U>` type we need
3329 [unsized type]: https://doc.rust-lang.org/book/first-edition/unsized-types.html
3333 The type you are trying to impl `CoerceUnsized` for is not a struct.
3334 `CoerceUnsized` can only be implemented for a struct. Unsized types are
3335 already able to be coerced without an implementation of `CoerceUnsized`
3336 whereas a struct containing an unsized type needs to know the unsized type
3337 field it's containing is able to be coerced. An
3338 [unsized type](https://doc.rust-lang.org/book/first-edition/unsized-types.html)
3339 is any type that the compiler doesn't know the length or alignment of at
3340 compile time. Any struct containing an unsized type is also unsized.
3342 Example of erroneous code:
3344 ```compile_fail,E0376
3345 #![feature(coerce_unsized)]
3346 use std::ops::CoerceUnsized;
3348 struct Foo<T: ?Sized> {
3352 // error: The type `U` is not a struct
3353 impl<T, U> CoerceUnsized<U> for Foo<T> {}
3356 The `CoerceUnsized` trait takes a struct type. Make sure the type you are
3357 providing to `CoerceUnsized` is a struct with only the last field containing an
3363 #![feature(coerce_unsized)]
3364 use std::ops::CoerceUnsized;
3370 // The `Foo<U>` is a struct so `CoerceUnsized` can be implemented
3371 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}
3374 Note that in Rust, structs can only contain an unsized type if the field
3375 containing the unsized type is the last and only unsized type field in the
3380 Default impls are only allowed for traits with no methods or associated items.
3381 For more information see the [opt-in builtin traits RFC][RFC 19].
3383 [RFC 19]: https://github.com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md
3387 You tried to implement methods for a primitive type. Erroneous code example:
3389 ```compile_fail,E0390
3395 // error: only a single inherent implementation marked with
3396 // `#[lang = "mut_ptr"]` is allowed for the `*mut T` primitive
3399 This isn't allowed, but using a trait to implement a method is a good solution.
3411 impl Bar for *mut Foo {
3418 This error indicates that a type or lifetime parameter has been declared
3419 but not actually used. Here is an example that demonstrates the error:
3421 ```compile_fail,E0392
3427 If the type parameter was included by mistake, this error can be fixed
3428 by simply removing the type parameter, as shown below:
3436 Alternatively, if the type parameter was intentionally inserted, it must be
3437 used. A simple fix is shown below:
3445 This error may also commonly be found when working with unsafe code. For
3446 example, when using raw pointers one may wish to specify the lifetime for
3447 which the pointed-at data is valid. An initial attempt (below) causes this
3450 ```compile_fail,E0392
3456 We want to express the constraint that Foo should not outlive `'a`, because
3457 the data pointed to by `T` is only valid for that lifetime. The problem is
3458 that there are no actual uses of `'a`. It's possible to work around this
3459 by adding a PhantomData type to the struct, using it to tell the compiler
3460 to act as if the struct contained a borrowed reference `&'a T`:
3463 use std::marker::PhantomData;
3465 struct Foo<'a, T: 'a> {
3467 phantom: PhantomData<&'a T>
3471 [PhantomData] can also be used to express information about unused type
3474 [PhantomData]: https://doc.rust-lang.org/std/marker/struct.PhantomData.html
3478 A type parameter which references `Self` in its default value was not specified.
3479 Example of erroneous code:
3481 ```compile_fail,E0393
3484 fn together_we_will_rule_the_galaxy(son: &A) {}
3485 // error: the type parameter `T` must be explicitly specified in an
3486 // object type because its default value `Self` references the
3490 A trait object is defined over a single, fully-defined trait. With a regular
3491 default parameter, this parameter can just be substituted in. However, if the
3492 default parameter is `Self`, the trait changes for each concrete type; i.e.
3493 `i32` will be expected to implement `A<i32>`, `bool` will be expected to
3494 implement `A<bool>`, etc... These types will not share an implementation of a
3495 fully-defined trait; instead they share implementations of a trait with
3496 different parameters substituted in for each implementation. This is
3497 irreconcilable with what we need to make a trait object work, and is thus
3498 disallowed. Making the trait concrete by explicitly specifying the value of the
3499 defaulted parameter will fix this issue. Fixed example:
3504 fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!
3509 You implemented a trait, overriding one or more of its associated types but did
3510 not reimplement its default methods.
3512 Example of erroneous code:
3514 ```compile_fail,E0399
3515 #![feature(associated_type_defaults)]
3523 // error - the following trait items need to be reimplemented as
3524 // `Assoc` was overridden: `bar`
3529 To fix this, add an implementation for each default method from the trait:
3532 #![feature(associated_type_defaults)]
3541 fn bar(&self) {} // ok!
3547 The length of the platform-intrinsic function `simd_shuffle`
3548 wasn't specified. Erroneous code example:
3550 ```compile_fail,E0439
3551 #![feature(platform_intrinsics)]
3553 extern "platform-intrinsic" {
3554 fn simd_shuffle<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3555 // error: invalid `simd_shuffle`, needs length: `simd_shuffle`
3559 The `simd_shuffle` function needs the length of the array passed as
3560 last parameter in its name. Example:
3563 #![feature(platform_intrinsics)]
3565 extern "platform-intrinsic" {
3566 fn simd_shuffle8<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3572 A platform-specific intrinsic function has the wrong number of type
3573 parameters. Erroneous code example:
3575 ```compile_fail,E0440
3576 #![feature(repr_simd)]
3577 #![feature(platform_intrinsics)]
3580 struct f64x2(f64, f64);
3582 extern "platform-intrinsic" {
3583 fn x86_mm_movemask_pd<T>(x: f64x2) -> i32;
3584 // error: platform-specific intrinsic has wrong number of type
3589 Please refer to the function declaration to see if it corresponds
3590 with yours. Example:
3593 #![feature(repr_simd)]
3594 #![feature(platform_intrinsics)]
3597 struct f64x2(f64, f64);
3599 extern "platform-intrinsic" {
3600 fn x86_mm_movemask_pd(x: f64x2) -> i32;
3606 An unknown platform-specific intrinsic function was used. Erroneous
3609 ```compile_fail,E0441
3610 #![feature(repr_simd)]
3611 #![feature(platform_intrinsics)]
3614 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3616 extern "platform-intrinsic" {
3617 fn x86_mm_adds_ep16(x: i16x8, y: i16x8) -> i16x8;
3618 // error: unrecognized platform-specific intrinsic function
3622 Please verify that the function name wasn't misspelled, and ensure
3623 that it is declared in the rust source code (in the file
3624 src/librustc_platform_intrinsics/x86.rs). Example:
3627 #![feature(repr_simd)]
3628 #![feature(platform_intrinsics)]
3631 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3633 extern "platform-intrinsic" {
3634 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3640 Intrinsic argument(s) and/or return value have the wrong type.
3641 Erroneous code example:
3643 ```compile_fail,E0442
3644 #![feature(repr_simd)]
3645 #![feature(platform_intrinsics)]
3648 struct i8x16(i8, i8, i8, i8, i8, i8, i8, i8,
3649 i8, i8, i8, i8, i8, i8, i8, i8);
3651 struct i32x4(i32, i32, i32, i32);
3653 struct i64x2(i64, i64);
3655 extern "platform-intrinsic" {
3656 fn x86_mm_adds_epi16(x: i8x16, y: i32x4) -> i64x2;
3657 // error: intrinsic arguments/return value have wrong type
3661 To fix this error, please refer to the function declaration to give
3662 it the awaited types. Example:
3665 #![feature(repr_simd)]
3666 #![feature(platform_intrinsics)]
3669 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3671 extern "platform-intrinsic" {
3672 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3678 Intrinsic argument(s) and/or return value have the wrong type.
3679 Erroneous code example:
3681 ```compile_fail,E0443
3682 #![feature(repr_simd)]
3683 #![feature(platform_intrinsics)]
3686 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3688 struct i64x8(i64, i64, i64, i64, i64, i64, i64, i64);
3690 extern "platform-intrinsic" {
3691 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i64x8;
3692 // error: intrinsic argument/return value has wrong type
3696 To fix this error, please refer to the function declaration to give
3697 it the awaited types. Example:
3700 #![feature(repr_simd)]
3701 #![feature(platform_intrinsics)]
3704 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3706 extern "platform-intrinsic" {
3707 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3713 A platform-specific intrinsic function has wrong number of arguments.
3714 Erroneous code example:
3716 ```compile_fail,E0444
3717 #![feature(repr_simd)]
3718 #![feature(platform_intrinsics)]
3721 struct f64x2(f64, f64);
3723 extern "platform-intrinsic" {
3724 fn x86_mm_movemask_pd(x: f64x2, y: f64x2, z: f64x2) -> i32;
3725 // error: platform-specific intrinsic has invalid number of arguments
3729 Please refer to the function declaration to see if it corresponds
3730 with yours. Example:
3733 #![feature(repr_simd)]
3734 #![feature(platform_intrinsics)]
3737 struct f64x2(f64, f64);
3739 extern "platform-intrinsic" {
3740 fn x86_mm_movemask_pd(x: f64x2) -> i32; // ok!
3746 The `typeof` keyword is currently reserved but unimplemented.
3747 Erroneous code example:
3749 ```compile_fail,E0516
3751 let x: typeof(92) = 92;
3755 Try using type inference instead. Example:
3765 A non-default implementation was already made on this type so it cannot be
3766 specialized further. Erroneous code example:
3768 ```compile_fail,E0520
3769 #![feature(specialization)]
3776 impl<T> SpaceLlama for T {
3777 default fn fly(&self) {}
3781 // applies to all `Clone` T and overrides the previous impl
3782 impl<T: Clone> SpaceLlama for T {
3786 // since `i32` is clone, this conflicts with the previous implementation
3787 impl SpaceLlama for i32 {
3788 default fn fly(&self) {}
3789 // error: item `fly` is provided by an `impl` that specializes
3790 // another, but the item in the parent `impl` is not marked
3791 // `default` and so it cannot be specialized.
3795 Specialization only allows you to override `default` functions in
3798 To fix this error, you need to mark all the parent implementations as default.
3802 #![feature(specialization)]
3809 impl<T> SpaceLlama for T {
3810 default fn fly(&self) {} // This is a parent implementation.
3813 // applies to all `Clone` T; overrides the previous impl
3814 impl<T: Clone> SpaceLlama for T {
3815 default fn fly(&self) {} // This is a parent implementation but was
3816 // previously not a default one, causing the error
3819 // applies to i32, overrides the previous two impls
3820 impl SpaceLlama for i32 {
3821 fn fly(&self) {} // And now that's ok!
3827 The number of elements in an array or slice pattern differed from the number of
3828 elements in the array being matched.
3830 Example of erroneous code:
3832 ```compile_fail,E0527
3833 #![feature(slice_patterns)]
3835 let r = &[1, 2, 3, 4];
3837 &[a, b] => { // error: pattern requires 2 elements but array
3839 println!("a={}, b={}", a, b);
3844 Ensure that the pattern is consistent with the size of the matched
3845 array. Additional elements can be matched with `..`:
3848 #![feature(slice_patterns)]
3850 let r = &[1, 2, 3, 4];
3852 &[a, b, ..] => { // ok!
3853 println!("a={}, b={}", a, b);
3860 An array or slice pattern required more elements than were present in the
3863 Example of erroneous code:
3865 ```compile_fail,E0528
3866 #![feature(slice_patterns)]
3870 &[a, b, c, rest..] => { // error: pattern requires at least 3
3871 // elements but array has 2
3872 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3877 Ensure that the matched array has at least as many elements as the pattern
3878 requires. You can match an arbitrary number of remaining elements with `..`:
3881 #![feature(slice_patterns)]
3883 let r = &[1, 2, 3, 4, 5];
3885 &[a, b, c, rest..] => { // ok!
3886 // prints `a=1, b=2, c=3 rest=[4, 5]`
3887 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3894 An array or slice pattern was matched against some other type.
3896 Example of erroneous code:
3898 ```compile_fail,E0529
3899 #![feature(slice_patterns)]
3903 [a, b] => { // error: expected an array or slice, found `f32`
3904 println!("a={}, b={}", a, b);
3909 Ensure that the pattern and the expression being matched on are of consistent
3913 #![feature(slice_patterns)]
3918 println!("a={}, b={}", a, b);
3925 An unknown field was specified into an enum's structure variant.
3927 Erroneous code example:
3929 ```compile_fail,E0559
3934 let s = Field::Fool { joke: 0 };
3935 // error: struct variant `Field::Fool` has no field named `joke`
3938 Verify you didn't misspell the field's name or that the field exists. Example:
3945 let s = Field::Fool { joke: 0 }; // ok!
3950 An unknown field was specified into a structure.
3952 Erroneous code example:
3954 ```compile_fail,E0560
3959 let s = Simba { mother: 1, father: 0 };
3960 // error: structure `Simba` has no field named `father`
3963 Verify you didn't misspell the field's name or that the field exists. Example:
3971 let s = Simba { mother: 1, father: 0 }; // ok!
3976 Abstract return types (written `impl Trait` for some trait `Trait`) are only
3977 allowed as function return types.
3979 Erroneous code example:
3981 ```compile_fail,E0562
3982 #![feature(conservative_impl_trait)]
3985 let count_to_ten: impl Iterator<Item=usize> = 0..10;
3986 // error: `impl Trait` not allowed outside of function and inherent method
3988 for i in count_to_ten {
3994 Make sure `impl Trait` only appears in return-type position.
3997 #![feature(conservative_impl_trait)]
3999 fn count_to_n(n: usize) -> impl Iterator<Item=usize> {
4004 for i in count_to_n(10) { // ok!
4010 See [RFC 1522] for more details.
4012 [RFC 1522]: https://github.com/rust-lang/rfcs/blob/master/text/1522-conservative-impl-trait.md
4016 The requested ABI is unsupported by the current target.
4018 The rust compiler maintains for each target a blacklist of ABIs unsupported on
4019 that target. If an ABI is present in such a list this usually means that the
4020 target / ABI combination is currently unsupported by llvm.
4022 If necessary, you can circumvent this check using custom target specifications.
4026 A return statement was found outside of a function body.
4028 Erroneous code example:
4030 ```compile_fail,E0572
4031 const FOO: u32 = return 0; // error: return statement outside of function body
4036 To fix this issue, just remove the return keyword or move the expression into a
4042 fn some_fn() -> u32 {
4053 In a `fn` type, a lifetime appears only in the return type,
4054 and not in the arguments types.
4056 Erroneous code example:
4058 ```compile_fail,E0581
4060 // Here, `'a` appears only in the return type:
4061 let x: for<'a> fn() -> &'a i32;
4065 To fix this issue, either use the lifetime in the arguments, or use
4070 // Here, `'a` appears only in the return type:
4071 let x: for<'a> fn(&'a i32) -> &'a i32;
4072 let y: fn() -> &'static i32;
4076 Note: The examples above used to be (erroneously) accepted by the
4077 compiler, but this was since corrected. See [issue #33685] for more
4080 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
4084 A lifetime appears only in an associated-type binding,
4085 and not in the input types to the trait.
4087 Erroneous code example:
4089 ```compile_fail,E0582
4091 // No type can satisfy this requirement, since `'a` does not
4092 // appear in any of the input types (here, `i32`):
4093 where F: for<'a> Fn(i32) -> Option<&'a i32>
4100 To fix this issue, either use the lifetime in the inputs, or use
4104 fn bar<F, G>(t: F, u: G)
4105 where F: for<'a> Fn(&'a i32) -> Option<&'a i32>,
4106 G: Fn(i32) -> Option<&'static i32>,
4113 Note: The examples above used to be (erroneously) accepted by the
4114 compiler, but this was since corrected. See [issue #33685] for more
4117 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
4121 ```compile_fail,E0599
4125 x.chocolate(); // error: no method named `chocolate` found for type `Mouth`
4126 // in the current scope
4131 An unary operator was used on a type which doesn't implement it.
4133 Example of erroneous code:
4135 ```compile_fail,E0600
4141 !Question::Yes; // error: cannot apply unary operator `!` to type `Question`
4144 In this case, `Question` would need to implement the `std::ops::Not` trait in
4145 order to be able to use `!` on it. Let's implement it:
4155 // We implement the `Not` trait on the enum.
4156 impl Not for Question {
4159 fn not(self) -> bool {
4161 Question::Yes => false, // If the `Answer` is `Yes`, then it
4163 Question::No => true, // And here we do the opposite.
4168 assert_eq!(!Question::Yes, false);
4169 assert_eq!(!Question::No, true);
4174 An attempt to index into a type which doesn't implement the `std::ops::Index`
4175 trait was performed.
4177 Erroneous code example:
4179 ```compile_fail,E0608
4180 0u8[2]; // error: cannot index into a value of type `u8`
4183 To be able to index into a type it needs to implement the `std::ops::Index`
4187 let v: Vec<u8> = vec![0, 1, 2, 3];
4189 // The `Vec` type implements the `Index` trait so you can do:
4190 println!("{}", v[2]);
4195 A cast to `char` was attempted on a type other than `u8`.
4197 Erroneous code example:
4199 ```compile_fail,E0604
4200 0u32 as char; // error: only `u8` can be cast as `char`, not `u32`
4203 As the error message indicates, only `u8` can be cast into `char`. Example:
4206 let c = 86u8 as char; // ok!
4210 For more information about casts, take a look at The Book:
4211 https://doc.rust-lang.org/book/first-edition/casting-between-types.html
4215 An invalid cast was attempted.
4217 Erroneous code examples:
4219 ```compile_fail,E0605
4221 x as Vec<u8>; // error: non-primitive cast: `u8` as `std::vec::Vec<u8>`
4225 let v = 0 as *const u8; // So here, `v` is a `*const u8`.
4226 v as &u8; // error: non-primitive cast: `*const u8` as `&u8`
4229 Only primitive types can be cast into each other. Examples:
4235 let v = 0 as *const u8;
4236 v as *const i8; // ok!
4239 For more information about casts, take a look at The Book:
4240 https://doc.rust-lang.org/book/first-edition/casting-between-types.html
4244 An incompatible cast was attempted.
4246 Erroneous code example:
4248 ```compile_fail,E0606
4249 let x = &0u8; // Here, `x` is a `&u8`.
4250 let y: u32 = x as u32; // error: casting `&u8` as `u32` is invalid
4253 When casting, keep in mind that only primitive types can be cast into each
4258 let y: u32 = *x as u32; // We dereference it first and then cast it.
4261 For more information about casts, take a look at The Book:
4262 https://doc.rust-lang.org/book/first-edition/casting-between-types.html
4266 A cast between a thin and a fat pointer was attempted.
4268 Erroneous code example:
4270 ```compile_fail,E0607
4271 let v = 0 as *const u8;
4275 First: what are thin and fat pointers?
4277 Thin pointers are "simple" pointers: they are purely a reference to a memory
4280 Fat pointers are pointers referencing Dynamically Sized Types (also called DST).
4281 DST don't have a statically known size, therefore they can only exist behind
4282 some kind of pointers that contain additional information. Slices and trait
4283 objects are DSTs. In the case of slices, the additional information the fat
4284 pointer holds is their size.
4286 To fix this error, don't try to cast directly between thin and fat pointers.
4288 For more information about casts, take a look at The Book:
4289 https://doc.rust-lang.org/book/first-edition/casting-between-types.html
4293 Attempted to access a non-existent field in a struct.
4295 Erroneous code example:
4297 ```compile_fail,E0609
4298 struct StructWithFields {
4302 let s = StructWithFields { x: 0 };
4303 println!("{}", s.foo); // error: no field `foo` on type `StructWithFields`
4306 To fix this error, check that you didn't misspell the field's name or that the
4307 field actually exists. Example:
4310 struct StructWithFields {
4314 let s = StructWithFields { x: 0 };
4315 println!("{}", s.x); // ok!
4320 Attempted to access a field on a primitive type.
4322 Erroneous code example:
4324 ```compile_fail,E0610
4326 println!("{}", x.foo); // error: `{integer}` is a primitive type, therefore
4327 // doesn't have fields
4330 Primitive types are the most basic types available in Rust and don't have
4331 fields. To access data via named fields, struct types are used. Example:
4334 // We declare struct called `Foo` containing two fields:
4340 // We create an instance of this struct:
4341 let variable = Foo { x: 0, y: -12 };
4342 // And we can now access its fields:
4343 println!("x: {}, y: {}", variable.x, variable.y);
4346 For more information about primitives and structs, take a look at The Book:
4347 https://doc.rust-lang.org/book/first-edition/primitive-types.html
4348 https://doc.rust-lang.org/book/first-edition/structs.html
4352 Attempted to access a private field on a tuple-struct.
4354 Erroneous code example:
4356 ```compile_fail,E0611
4358 pub struct Foo(u32);
4361 pub fn new() -> Foo { Foo(0) }
4365 let y = some_module::Foo::new();
4366 println!("{}", y.0); // error: field `0` of tuple-struct `some_module::Foo`
4370 Since the field is private, you have two solutions:
4372 1) Make the field public:
4376 pub struct Foo(pub u32); // The field is now public.
4379 pub fn new() -> Foo { Foo(0) }
4383 let y = some_module::Foo::new();
4384 println!("{}", y.0); // So we can access it directly.
4387 2) Add a getter function to keep the field private but allow for accessing its
4392 pub struct Foo(u32);
4395 pub fn new() -> Foo { Foo(0) }
4397 // We add the getter function.
4398 pub fn get(&self) -> &u32 { &self.0 }
4402 let y = some_module::Foo::new();
4403 println!("{}", y.get()); // So we can get the value through the function.
4408 Attempted out-of-bounds tuple index.
4410 Erroneous code example:
4412 ```compile_fail,E0612
4416 println!("{}", y.1); // error: attempted out-of-bounds tuple index `1`
4420 If a tuple/tuple-struct type has n fields, you can only try to access these n
4421 fields from 0 to (n - 1). So in this case, you can only index `0`. Example:
4427 println!("{}", y.0); // ok!
4432 Attempted to dereference a variable which cannot be dereferenced.
4434 Erroneous code example:
4436 ```compile_fail,E0614
4438 *y; // error: type `u32` cannot be dereferenced
4441 Only types implementing `std::ops::Deref` can be dereferenced (such as `&T`).
4447 // So here, `x` is a `&u32`, so we can dereference it:
4453 Attempted to access a method like a field.
4455 Erroneous code example:
4457 ```compile_fail,E0615
4466 let f = Foo { x: 0 };
4467 f.method; // error: attempted to take value of method `method` on type `Foo`
4470 If you want to use a method, add `()` after it:
4473 # struct Foo { x: u32 }
4474 # impl Foo { fn method(&self) {} }
4475 # let f = Foo { x: 0 };
4479 However, if you wanted to access a field of a struct check that the field name
4480 is spelled correctly. Example:
4483 # struct Foo { x: u32 }
4484 # impl Foo { fn method(&self) {} }
4485 # let f = Foo { x: 0 };
4486 println!("{}", f.x);
4491 Attempted to access a private field on a struct.
4493 Erroneous code example:
4495 ```compile_fail,E0616
4498 x: u32, // So `x` is private in here.
4502 pub fn new() -> Foo { Foo { x: 0 } }
4506 let f = some_module::Foo::new();
4507 println!("{}", f.x); // error: field `x` of struct `some_module::Foo` is private
4510 If you want to access this field, you have two options:
4512 1) Set the field public:
4517 pub x: u32, // `x` is now public.
4521 pub fn new() -> Foo { Foo { x: 0 } }
4525 let f = some_module::Foo::new();
4526 println!("{}", f.x); // ok!
4529 2) Add a getter function:
4534 x: u32, // So `x` is still private in here.
4538 pub fn new() -> Foo { Foo { x: 0 } }
4540 // We create the getter function here:
4541 pub fn get_x(&self) -> &u32 { &self.x }
4545 let f = some_module::Foo::new();
4546 println!("{}", f.get_x()); // ok!
4551 Attempted to pass an invalid type of variable into a variadic function.
4553 Erroneous code example:
4555 ```compile_fail,E0617
4557 fn printf(c: *const i8, ...);
4561 printf(::std::ptr::null(), 0f32);
4562 // error: can't pass an `f32` to variadic function, cast to `c_double`
4566 Certain Rust types must be cast before passing them to a variadic function,
4567 because of arcane ABI rules dictated by the C standard. To fix the error,
4568 cast the value to the type specified by the error message (which you may need
4569 to import from `std::os::raw`).
4573 Attempted to call something which isn't a function nor a method.
4575 Erroneous code examples:
4577 ```compile_fail,E0618
4582 X::Entry(); // error: expected function, found `X::Entry`
4586 x(); // error: expected function, found `i32`
4589 Only functions and methods can be called using `()`. Example:
4592 // We declare a function:
4593 fn i_am_a_function() {}
4601 The type-checker needed to know the type of an expression, but that type had not
4604 Erroneous code example:
4606 ```compile_fail,E0619
4610 // Here, the type of `v` is not (yet) known, so we
4611 // cannot resolve this method call:
4612 v.to_uppercase(); // error: the type of this value must be known in
4619 Type inference typically proceeds from the top of the function to the bottom,
4620 figuring out types as it goes. In some cases -- notably method calls and
4621 overloadable operators like `*` -- the type checker may not have enough
4622 information *yet* to make progress. This can be true even if the rest of the
4623 function provides enough context (because the type-checker hasn't looked that
4624 far ahead yet). In this case, type annotations can be used to help it along.
4626 To fix this error, just specify the type of the variable. Example:
4629 let mut x: Vec<String> = vec![]; // We precise the type of the vec elements.
4632 v.to_uppercase(); // Since rustc now knows the type of the vec elements,
4633 // we can use `v`'s methods.
4641 A cast to an unsized type was attempted.
4643 Erroneous code example:
4645 ```compile_fail,E0620
4646 let x = &[1_usize, 2] as [usize]; // error: cast to unsized type: `&[usize; 2]`
4650 In Rust, some types don't have a known size at compile-time. For example, in a
4651 slice type like `[u32]`, the number of elements is not known at compile-time and
4652 hence the overall size cannot be computed. As a result, such types can only be
4653 manipulated through a reference (e.g., `&T` or `&mut T`) or other pointer-type
4654 (e.g., `Box` or `Rc`). Try casting to a reference instead:
4657 let x = &[1_usize, 2] as &[usize]; // ok!
4662 An intrinsic was declared without being a function.
4664 Erroneous code example:
4666 ```compile_fail,E0622
4667 #![feature(intrinsics)]
4668 extern "rust-intrinsic" {
4669 pub static breakpoint : unsafe extern "rust-intrinsic" fn();
4670 // error: intrinsic must be a function
4673 fn main() { unsafe { breakpoint(); } }
4676 An intrinsic is a function available for use in a given programming language
4677 whose implementation is handled specially by the compiler. In order to fix this
4678 error, just declare a function.
4683 register_diagnostics! {
4693 // E0159, // use of trait `{}` as struct constructor
4694 // E0163, // merged into E0071
4697 // E0172, // non-trait found in a type sum, moved to resolve
4698 // E0173, // manual implementations of unboxed closure traits are experimental
4701 // E0187, // can't infer the kind of the closure
4702 // E0188, // can not cast an immutable reference to a mutable pointer
4703 // E0189, // deprecated: can only cast a boxed pointer to a boxed object
4704 // E0190, // deprecated: can only cast a &-pointer to an &-object
4705 // E0196, // cannot determine a type for this closure
4706 E0203, // type parameter has more than one relaxed default bound,
4707 // and only one is supported
4709 // E0209, // builtin traits can only be implemented on structs or enums
4710 E0212, // cannot extract an associated type from a higher-ranked trait bound
4711 // E0213, // associated types are not accepted in this context
4712 // E0215, // angle-bracket notation is not stable with `Fn`
4713 // E0216, // parenthetical notation is only stable with `Fn`
4714 // E0217, // ambiguous associated type, defined in multiple supertraits
4715 // E0218, // no associated type defined
4716 // E0219, // associated type defined in higher-ranked supertrait
4717 // E0222, // Error code E0045 (variadic function must have C or cdecl calling
4718 // convention) duplicate
4719 E0224, // at least one non-builtin train is required for an object type
4720 E0227, // ambiguous lifetime bound, explicit lifetime bound required
4721 E0228, // explicit lifetime bound required
4722 E0231, // only named substitution parameters are allowed
4725 // E0235, // structure constructor specifies a structure of type but
4726 // E0236, // no lang item for range syntax
4727 // E0237, // no lang item for range syntax
4728 // E0238, // parenthesized parameters may only be used with a trait
4729 // E0239, // `next` method of `Iterator` trait has unexpected type
4733 E0245, // not a trait
4734 // E0246, // invalid recursive type
4736 // E0248, // value used as a type, now reported earlier during resolution as E0412
4738 // E0319, // trait impls for defaulted traits allowed just for structs/enums
4739 // E0372, // coherence not object safe
4740 E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
4741 // between structures with the same definition
4742 E0436, // functional record update requires a struct
4743 E0521, // redundant default implementations of trait
4744 E0533, // `{}` does not name a unit variant, unit struct or a constant
4745 E0563, // cannot determine a type for this `impl Trait`: {}
4746 E0564, // only named lifetimes are allowed in `impl Trait`,
4747 // but `{}` was found in the type `{}`
4748 E0567, // auto traits can not have type parameters
4749 E0568, // auto-traits can not have predicates,
4750 E0588, // packed struct cannot transitively contain a `[repr(align)]` struct
4751 E0592, // duplicate definitions with name `{}`
4752 // E0613, // Removed (merged with E0609)