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.
230 let trait_obj: &SomeTrait = ...;
232 // This tries to implicitly dereference to create an unsized local variable.
233 let &invalid = trait_obj;
235 // You can call methods without binding to the value being pointed at.
236 trait_obj.method_one();
237 trait_obj.method_two();
240 You can read more about trait objects in the Trait Object section of the
243 https://doc.rust-lang.org/reference.html#trait-objects
247 The compiler doesn't know what method to call because more than one method
248 has the same prototype. Erroneous code example:
250 ```compile_fail,E0034
261 impl Trait1 for Test { fn foo() {} }
262 impl Trait2 for Test { fn foo() {} }
265 Test::foo() // error, which foo() to call?
269 To avoid this error, you have to keep only one of them and remove the others.
270 So let's take our example and fix it:
279 impl Trait1 for Test { fn foo() {} }
282 Test::foo() // and now that's good!
286 However, a better solution would be using fully explicit naming of type and
300 impl Trait1 for Test { fn foo() {} }
301 impl Trait2 for Test { fn foo() {} }
304 <Test as Trait1>::foo()
321 impl F for X { fn m(&self) { println!("I am F"); } }
322 impl G for X { fn m(&self) { println!("I am G"); } }
327 F::m(&f); // it displays "I am F"
328 G::m(&f); // it displays "I am G"
334 You tried to give a type parameter where it wasn't needed. Erroneous code
337 ```compile_fail,E0035
347 x.method::<i32>(); // Error: Test::method doesn't need type parameter!
351 To fix this error, just remove the type parameter:
363 x.method(); // OK, we're good!
369 This error occurrs when you pass too many or not enough type parameters to
370 a method. Erroneous code example:
372 ```compile_fail,E0036
376 fn method<T>(&self, v: &[T]) -> usize {
385 x.method::<i32, i32>(v); // error: only one type parameter is expected!
389 To fix it, just specify a correct number of type parameters:
395 fn method<T>(&self, v: &[T]) -> usize {
404 x.method::<i32>(v); // OK, we're good!
408 Please note on the last example that we could have called `method` like this:
416 It is not allowed to manually call destructors in Rust. It is also not
417 necessary to do this since `drop` is called automatically whenever a value goes
420 Here's an example of this error:
422 ```compile_fail,E0040
434 let mut x = Foo { x: -7 };
435 x.drop(); // error: explicit use of destructor method
441 You can't use type parameters on foreign items. Example of erroneous code:
443 ```compile_fail,E0044
444 extern { fn some_func<T>(x: T); }
447 To fix this, replace the type parameter with the specializations that you
451 extern { fn some_func_i32(x: i32); }
452 extern { fn some_func_i64(x: i64); }
457 Rust only supports variadic parameters for interoperability with C code in its
458 FFI. As such, variadic parameters can only be used with functions which are
459 using the C ABI. Examples of erroneous code:
462 #![feature(unboxed_closures)]
464 extern "rust-call" { fn foo(x: u8, ...); }
468 fn foo(x: u8, ...) {}
471 To fix such code, put them in an extern "C" block:
481 Items are missing in a trait implementation. Erroneous code example:
483 ```compile_fail,E0046
491 // error: not all trait items implemented, missing: `foo`
494 When trying to make some type implement a trait `Foo`, you must, at minimum,
495 provide implementations for all of `Foo`'s required methods (meaning the
496 methods that do not have default implementations), as well as any required
497 trait items like associated types or constants. Example:
513 This error indicates that an attempted implementation of a trait method
514 has the wrong number of type parameters.
516 For example, the trait below has a method `foo` with a type parameter `T`,
517 but the implementation of `foo` for the type `Bar` is missing this parameter:
519 ```compile_fail,E0049
521 fn foo<T: Default>(x: T) -> Self;
526 // error: method `foo` has 0 type parameters but its trait declaration has 1
529 fn foo(x: bool) -> Self { Bar }
535 This error indicates that an attempted implementation of a trait method
536 has the wrong number of function parameters.
538 For example, the trait below has a method `foo` with two function parameters
539 (`&self` and `u8`), but the implementation of `foo` for the type `Bar` omits
542 ```compile_fail,E0050
544 fn foo(&self, x: u8) -> bool;
549 // error: method `foo` has 1 parameter but the declaration in trait `Foo::foo`
552 fn foo(&self) -> bool { true }
558 The parameters of any trait method must match between a trait implementation
559 and the trait definition.
561 Here are a couple examples of this error:
563 ```compile_fail,E0053
572 // error, expected u16, found i16
575 // error, types differ in mutability
576 fn bar(&mut self) { }
582 It is not allowed to cast to a bool. If you are trying to cast a numeric type
583 to a bool, you can compare it with zero instead:
585 ```compile_fail,E0054
588 // Not allowed, won't compile
589 let x_is_nonzero = x as bool;
596 let x_is_nonzero = x != 0;
601 During a method call, a value is automatically dereferenced as many times as
602 needed to make the value's type match the method's receiver. The catch is that
603 the compiler will only attempt to dereference a number of times up to the
604 recursion limit (which can be set via the `recursion_limit` attribute).
606 For a somewhat artificial example:
608 ```compile_fail,E0055
609 #![recursion_limit="2"]
621 // error, reached the recursion limit while auto-dereferencing &&Foo
626 One fix may be to increase the recursion limit. Note that it is possible to
627 create an infinite recursion of dereferencing, in which case the only fix is to
628 somehow break the recursion.
632 When invoking closures or other implementations of the function traits `Fn`,
633 `FnMut` or `FnOnce` using call notation, the number of parameters passed to the
634 function must match its definition.
636 An example using a closure:
638 ```compile_fail,E0057
640 let a = f(); // invalid, too few parameters
641 let b = f(4); // this works!
642 let c = f(2, 3); // invalid, too many parameters
645 A generic function must be treated similarly:
648 fn foo<F: Fn()>(f: F) {
649 f(); // this is valid, but f(3) would not work
655 The built-in function traits are generic over a tuple of the function arguments.
656 If one uses angle-bracket notation (`Fn<(T,), Output=U>`) instead of parentheses
657 (`Fn(T) -> U`) to denote the function trait, the type parameter should be a
658 tuple. Otherwise function call notation cannot be used and the trait will not be
659 implemented by closures.
661 The most likely source of this error is using angle-bracket notation without
662 wrapping the function argument type into a tuple, for example:
664 ```compile_fail,E0059
665 #![feature(unboxed_closures)]
667 fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) }
670 It can be fixed by adjusting the trait bound like this:
673 #![feature(unboxed_closures)]
675 fn foo<F: Fn<(i32,)>>(f: F) -> F::Output { f(3) }
678 Note that `(T,)` always denotes the type of a 1-tuple containing an element of
679 type `T`. The comma is necessary for syntactic disambiguation.
683 External C functions are allowed to be variadic. However, a variadic function
684 takes a minimum number of arguments. For example, consider C's variadic `printf`
689 use libc::{ c_char, c_int };
692 fn printf(_: *const c_char, ...) -> c_int;
696 Using this declaration, it must be called with at least one argument, so
697 simply calling `printf()` is invalid. But the following uses are allowed:
701 use std::ffi::CString;
703 printf(CString::new("test\n").unwrap().as_ptr());
704 printf(CString::new("number = %d\n").unwrap().as_ptr(), 3);
705 printf(CString::new("%d, %d\n").unwrap().as_ptr(), 10, 5);
711 The number of arguments passed to a function must match the number of arguments
712 specified in the function signature.
714 For example, a function like:
717 fn f(a: u16, b: &str) {}
720 Must always be called with exactly two arguments, e.g. `f(2, "test")`.
722 Note that Rust does not have a notion of optional function arguments or
723 variadic functions (except for its C-FFI).
727 This error indicates that during an attempt to build a struct or struct-like
728 enum variant, one of the fields was specified more than once. Erroneous code
731 ```compile_fail,E0062
739 x: 0, // error: field `x` specified more than once
744 Each field should be specified exactly one time. Example:
752 let x = Foo { x: 0 }; // ok!
758 This error indicates that during an attempt to build a struct or struct-like
759 enum variant, one of the fields was not provided. Erroneous code example:
761 ```compile_fail,E0063
768 let x = Foo { x: 0 }; // error: missing field: `y`
772 Each field should be specified exactly once. Example:
781 let x = Foo { x: 0, y: 0 }; // ok!
787 Box placement expressions (like C++'s "placement new") do not yet support any
788 place expression except the exchange heap (i.e. `std::boxed::HEAP`).
789 Furthermore, the syntax is changing to use `in` instead of `box`. See [RFC 470]
790 and [RFC 809] for more details.
792 [RFC 470]: https://github.com/rust-lang/rfcs/pull/470
793 [RFC 809]: https://github.com/rust-lang/rfcs/blob/master/text/0809-box-and-in-for-stdlib.md
797 The left-hand side of a compound assignment expression must be an lvalue
798 expression. An lvalue expression represents a memory location and includes
799 item paths (ie, namespaced variables), dereferences, indexing expressions,
800 and field references.
802 Let's start with some erroneous code examples:
804 ```compile_fail,E0067
805 use std::collections::LinkedList;
807 // Bad: assignment to non-lvalue expression
808 LinkedList::new() += 1;
812 fn some_func(i: &mut i32) {
813 i += 12; // Error : '+=' operation cannot be applied on a reference !
817 And now some working examples:
826 fn some_func(i: &mut i32) {
833 The compiler found a function whose body contains a `return;` statement but
834 whose return type is not `()`. An example of this is:
836 ```compile_fail,E0069
843 Since `return;` is just like `return ();`, there is a mismatch between the
844 function's return type and the value being returned.
848 The left-hand side of an assignment operator must be an lvalue expression. An
849 lvalue expression represents a memory location and can be a variable (with
850 optional namespacing), a dereference, an indexing expression or a field
853 More details can be found here:
854 https://doc.rust-lang.org/reference.html#lvalues-rvalues-and-temporaries
856 Now, we can go further. Here are some erroneous code examples:
858 ```compile_fail,E0070
864 const SOME_CONST : i32 = 12;
866 fn some_other_func() {}
869 SOME_CONST = 14; // error : a constant value cannot be changed!
870 1 = 3; // error : 1 isn't a valid lvalue!
871 some_other_func() = 4; // error : we can't assign value to a function!
872 SomeStruct.x = 12; // error : SomeStruct a structure name but it is used
877 And now let's give working examples:
884 let mut s = SomeStruct {x: 0, y: 0};
886 s.x = 3; // that's good !
890 fn some_func(x: &mut i32) {
891 *x = 12; // that's good !
897 You tried to use structure-literal syntax to create an item that is
898 not a structure or enum variant.
900 Example of erroneous code:
902 ```compile_fail,E0071
904 let t = U32 { value: 4 }; // error: expected struct, variant or union type,
905 // found builtin type `u32`
908 To fix this, ensure that the name was correctly spelled, and that
909 the correct form of initializer was used.
911 For example, the code above can be fixed to:
919 let u = Foo::FirstValue(0i32);
927 You cannot define a struct (or enum) `Foo` that requires an instance of `Foo`
928 in order to make a new `Foo` value. This is because there would be no way a
929 first instance of `Foo` could be made to initialize another instance!
931 Here's an example of a struct that has this problem:
934 struct Foo { x: Box<Foo> } // error
937 One fix is to use `Option`, like so:
940 struct Foo { x: Option<Box<Foo>> }
943 Now it's possible to create at least one instance of `Foo`: `Foo { x: None }`.
947 When using the `#[simd]` attribute on a tuple struct, the components of the
948 tuple struct must all be of a concrete, nongeneric type so the compiler can
949 reason about how to use SIMD with them. This error will occur if the types
952 This will cause an error:
955 #![feature(repr_simd)]
958 struct Bad<T>(T, T, T);
964 #![feature(repr_simd)]
967 struct Good(u32, u32, u32);
972 The `#[simd]` attribute can only be applied to non empty tuple structs, because
973 it doesn't make sense to try to use SIMD operations when there are no values to
976 This will cause an error:
978 ```compile_fail,E0075
979 #![feature(repr_simd)]
988 #![feature(repr_simd)]
996 When using the `#[simd]` attribute to automatically use SIMD operations in tuple
997 struct, the types in the struct must all be of the same type, or the compiler
998 will trigger this error.
1000 This will cause an error:
1002 ```compile_fail,E0076
1003 #![feature(repr_simd)]
1006 struct Bad(u16, u32, u32);
1012 #![feature(repr_simd)]
1015 struct Good(u32, u32, u32);
1020 When using the `#[simd]` attribute on a tuple struct, the elements in the tuple
1021 must be machine types so SIMD operations can be applied to them.
1023 This will cause an error:
1025 ```compile_fail,E0077
1026 #![feature(repr_simd)]
1035 #![feature(repr_simd)]
1038 struct Good(u32, u32, u32);
1043 Enum discriminants are used to differentiate enum variants stored in memory.
1044 This error indicates that the same value was used for two or more variants,
1045 making them impossible to tell apart.
1047 ```compile_fail,E0081
1065 Note that variants without a manually specified discriminant are numbered from
1066 top to bottom starting from 0, so clashes can occur with seemingly unrelated
1069 ```compile_fail,E0081
1076 Here `X` will have already been specified the discriminant 0 by the time `Y` is
1077 encountered, so a conflict occurs.
1081 When you specify enum discriminants with `=`, the compiler expects `isize`
1082 values by default. Or you can add the `repr` attibute to the enum declaration
1083 for an explicit choice of the discriminant type. In either cases, the
1084 discriminant values must fall within a valid range for the expected type;
1085 otherwise this error is raised. For example:
1095 Here, 1024 lies outside the valid range for `u8`, so the discriminant for `A` is
1096 invalid. Here is another, more subtle example which depends on target word size:
1099 enum DependsOnPointerSize {
1104 Here, `1 << 32` is interpreted as an `isize` value. So it is invalid for 32 bit
1105 target (`target_pointer_width = "32"`) but valid for 64 bit target.
1107 You may want to change representation types to fix this, or else change invalid
1108 discriminant values so that they fit within the existing type.
1112 An unsupported representation was attempted on a zero-variant enum.
1114 Erroneous code example:
1116 ```compile_fail,E0084
1118 enum NightsWatch {} // error: unsupported representation for zero-variant enum
1121 It is impossible to define an integer type to be used to represent zero-variant
1122 enum values because there are no zero-variant enum values. There is no way to
1123 construct an instance of the following type using only safe code. So you have
1124 two solutions. Either you add variants in your enum:
1134 or you remove the integer represention of your enum:
1142 Too many type parameters were supplied for a function. For example:
1144 ```compile_fail,E0087
1148 foo::<f64, bool>(); // error, expected 1 parameter, found 2 parameters
1152 The number of supplied parameters must exactly match the number of defined type
1157 You gave too many lifetime parameters. Erroneous code example:
1159 ```compile_fail,E0088
1163 f::<'static>() // error: too many lifetime parameters provided
1167 Please check you give the right number of lifetime parameters. Example:
1177 It's also important to note that the Rust compiler can generally
1178 determine the lifetime by itself. Example:
1186 // it can be written like this
1187 fn get_value<'a>(&'a self) -> &'a str { &self.value }
1188 // but the compiler works fine with this too:
1189 fn without_lifetime(&self) -> &str { &self.value }
1193 let f = Foo { value: "hello".to_owned() };
1195 println!("{}", f.get_value());
1196 println!("{}", f.without_lifetime());
1202 Not enough type parameters were supplied for a function. For example:
1204 ```compile_fail,E0089
1208 foo::<f64>(); // error, expected 2 parameters, found 1 parameter
1212 Note that if a function takes multiple type parameters but you want the compiler
1213 to infer some of them, you can use type placeholders:
1215 ```compile_fail,E0089
1216 fn foo<T, U>(x: T) {}
1220 foo::<f64>(x); // error, expected 2 parameters, found 1 parameter
1221 foo::<_, f64>(x); // same as `foo::<bool, f64>(x)`
1227 You gave too few lifetime parameters. Example:
1229 ```compile_fail,E0090
1230 fn foo<'a: 'b, 'b: 'a>() {}
1233 foo::<'static>(); // error, expected 2 lifetime parameters
1237 Please check you give the right number of lifetime parameters. Example:
1240 fn foo<'a: 'b, 'b: 'a>() {}
1243 foo::<'static, 'static>();
1249 You gave an unnecessary type parameter in a type alias. Erroneous code
1252 ```compile_fail,E0091
1253 type Foo<T> = u32; // error: type parameter `T` is unused
1255 type Foo<A,B> = Box<A>; // error: type parameter `B` is unused
1258 Please check you didn't write too many type parameters. Example:
1261 type Foo = u32; // ok!
1262 type Foo2<A> = Box<A>; // ok!
1267 You tried to declare an undefined atomic operation function.
1268 Erroneous code example:
1270 ```compile_fail,E0092
1271 #![feature(intrinsics)]
1273 extern "rust-intrinsic" {
1274 fn atomic_foo(); // error: unrecognized atomic operation
1279 Please check you didn't make a mistake in the function's name. All intrinsic
1280 functions are defined in librustc_trans/trans/intrinsic.rs and in
1281 libcore/intrinsics.rs in the Rust source code. Example:
1284 #![feature(intrinsics)]
1286 extern "rust-intrinsic" {
1287 fn atomic_fence(); // ok!
1293 You declared an unknown intrinsic function. Erroneous code example:
1295 ```compile_fail,E0093
1296 #![feature(intrinsics)]
1298 extern "rust-intrinsic" {
1299 fn foo(); // error: unrecognized intrinsic function: `foo`
1309 Please check you didn't make a mistake in the function's name. All intrinsic
1310 functions are defined in librustc_trans/trans/intrinsic.rs and in
1311 libcore/intrinsics.rs in the Rust source code. Example:
1314 #![feature(intrinsics)]
1316 extern "rust-intrinsic" {
1317 fn atomic_fence(); // ok!
1329 You gave an invalid number of type parameters to an intrinsic function.
1330 Erroneous code example:
1332 ```compile_fail,E0094
1333 #![feature(intrinsics)]
1335 extern "rust-intrinsic" {
1336 fn size_of<T, U>() -> usize; // error: intrinsic has wrong number
1337 // of type parameters
1341 Please check that you provided the right number of type parameters
1342 and verify with the function declaration in the Rust source code.
1346 #![feature(intrinsics)]
1348 extern "rust-intrinsic" {
1349 fn size_of<T>() -> usize; // ok!
1355 This error means that an incorrect number of lifetime parameters were provided
1356 for a type (like a struct or enum) or trait:
1358 ```compile_fail,E0107
1359 struct Foo<'a, 'b>(&'a str, &'b str);
1360 enum Bar { A, B, C }
1363 foo: Foo<'a>, // error: expected 2, found 1
1364 bar: Bar<'a>, // error: expected 0, found 1
1370 You tried to give a type parameter to a type which doesn't need it. Erroneous
1373 ```compile_fail,E0109
1374 type X = u32<i32>; // error: type parameters are not allowed on this type
1377 Please check that you used the correct type and recheck its definition. Perhaps
1378 it doesn't need the type parameter.
1383 type X = u32; // this compiles
1386 Note that type parameters for enum-variant constructors go after the variant,
1387 not after the enum (`Option::None::<u32>`, not `Option::<u32>::None`).
1391 You tried to give a lifetime parameter to a type which doesn't need it.
1392 Erroneous code example:
1394 ```compile_fail,E0110
1395 type X = u32<'static>; // error: lifetime parameters are not allowed on
1399 Please check that the correct type was used and recheck its definition; perhaps
1400 it doesn't need the lifetime parameter. Example:
1403 type X = u32; // ok!
1408 You can only define an inherent implementation for a type in the same crate
1409 where the type was defined. For example, an `impl` block as below is not allowed
1410 since `Vec` is defined in the standard library:
1412 ```compile_fail,E0116
1413 impl Vec<u8> { } // error
1416 To fix this problem, you can do either of these things:
1418 - define a trait that has the desired associated functions/types/constants and
1419 implement the trait for the type in question
1420 - define a new type wrapping the type and define an implementation on the new
1423 Note that using the `type` keyword does not work here because `type` only
1424 introduces a type alias:
1426 ```compile_fail,E0116
1427 type Bytes = Vec<u8>;
1429 impl Bytes { } // error, same as above
1434 This error indicates a violation of one of Rust's orphan rules for trait
1435 implementations. The rule prohibits any implementation of a foreign trait (a
1436 trait defined in another crate) where
1438 - the type that is implementing the trait is foreign
1439 - all of the parameters being passed to the trait (if there are any) are also
1442 Here's one example of this error:
1444 ```compile_fail,E0117
1445 impl Drop for u32 {}
1448 To avoid this kind of error, ensure that at least one local type is referenced
1452 pub struct Foo; // you define your type in your crate
1454 impl Drop for Foo { // and you can implement the trait on it!
1455 // code of trait implementation here
1458 impl From<Foo> for i32 { // or you use a type from your crate as
1460 fn from(i: Foo) -> i32 {
1466 Alternatively, define a trait locally and implement that instead:
1470 fn get(&self) -> usize;
1474 fn get(&self) -> usize { 0 }
1478 For information on the design of the orphan rules, see [RFC 1023].
1480 [RFC 1023]: https://github.com/rust-lang/rfcs/blob/master/text/1023-rebalancing-coherence.md
1484 You're trying to write an inherent implementation for something which isn't a
1485 struct nor an enum. Erroneous code example:
1487 ```compile_fail,E0118
1488 impl (u8, u8) { // error: no base type found for inherent implementation
1489 fn get_state(&self) -> String {
1495 To fix this error, please implement a trait on the type or wrap it in a struct.
1499 // we create a trait here
1500 trait LiveLongAndProsper {
1501 fn get_state(&self) -> String;
1504 // and now you can implement it on (u8, u8)
1505 impl LiveLongAndProsper for (u8, u8) {
1506 fn get_state(&self) -> String {
1507 "He's dead, Jim!".to_owned()
1512 Alternatively, you can create a newtype. A newtype is a wrapping tuple-struct.
1513 For example, `NewType` is a newtype over `Foo` in `struct NewType(Foo)`.
1517 struct TypeWrapper((u8, u8));
1520 fn get_state(&self) -> String {
1521 "Fascinating!".to_owned()
1528 There are conflicting trait implementations for the same type.
1529 Example of erroneous code:
1531 ```compile_fail,E0119
1533 fn get(&self) -> usize;
1536 impl<T> MyTrait for T {
1537 fn get(&self) -> usize { 0 }
1544 impl MyTrait for Foo { // error: conflicting implementations of trait
1545 // `MyTrait` for type `Foo`
1546 fn get(&self) -> usize { self.value }
1550 When looking for the implementation for the trait, the compiler finds
1551 both the `impl<T> MyTrait for T` where T is all types and the `impl
1552 MyTrait for Foo`. Since a trait cannot be implemented multiple times,
1553 this is an error. So, when you write:
1557 fn get(&self) -> usize;
1560 impl<T> MyTrait for T {
1561 fn get(&self) -> usize { 0 }
1565 This makes the trait implemented on all types in the scope. So if you
1566 try to implement it on another one after that, the implementations will
1571 fn get(&self) -> usize;
1574 impl<T> MyTrait for T {
1575 fn get(&self) -> usize { 0 }
1583 f.get(); // the trait is implemented so we can use it
1589 An attempt was made to implement Drop on a trait, which is not allowed: only
1590 structs and enums can implement Drop. An example causing this error:
1592 ```compile_fail,E0120
1595 impl Drop for MyTrait {
1596 fn drop(&mut self) {}
1600 A workaround for this problem is to wrap the trait up in a struct, and implement
1601 Drop on that. An example is shown below:
1605 struct MyWrapper<T: MyTrait> { foo: T }
1607 impl <T: MyTrait> Drop for MyWrapper<T> {
1608 fn drop(&mut self) {}
1613 Alternatively, wrapping trait objects requires something like the following:
1618 //or Box<MyTrait>, if you wanted an owned trait object
1619 struct MyWrapper<'a> { foo: &'a MyTrait }
1621 impl <'a> Drop for MyWrapper<'a> {
1622 fn drop(&mut self) {}
1628 In order to be consistent with Rust's lack of global type inference, type
1629 placeholders are disallowed by design in item signatures.
1631 Examples of this error include:
1633 ```compile_fail,E0121
1634 fn foo() -> _ { 5 } // error, explicitly write out the return type instead
1636 static BAR: _ = "test"; // error, explicitly write out the type instead
1641 An attempt was made to add a generic constraint to a type alias. While Rust will
1642 allow this with a warning, it will not currently enforce the constraint.
1643 Consider the example below:
1648 type MyType<R: Foo> = (R, ());
1655 We're able to declare a variable of type `MyType<u32>`, despite the fact that
1656 `u32` does not implement `Foo`. As a result, one should avoid using generic
1657 constraints in concert with type aliases.
1661 You declared two fields of a struct with the same name. Erroneous code
1664 ```compile_fail,E0124
1667 field1: i32, // error: field is already declared
1671 Please verify that the field names have been correctly spelled. Example:
1682 It is not possible to define `main` with type parameters, or even with function
1683 parameters. When `main` is present, it must take no arguments and return `()`.
1684 Erroneous code example:
1686 ```compile_fail,E0131
1687 fn main<T>() { // error: main function is not allowed to have type parameters
1693 A function with the `start` attribute was declared with type parameters.
1695 Erroneous code example:
1697 ```compile_fail,E0132
1704 It is not possible to declare type parameters on a function that has the `start`
1705 attribute. Such a function must have the following type signature (for more
1706 information: http://doc.rust-lang.org/stable/book/no-stdlib.html):
1709 fn(isize, *const *const u8) -> isize;
1718 fn my_start(argc: isize, argv: *const *const u8) -> isize {
1725 This error means that an attempt was made to match a struct type enum
1726 variant as a non-struct type:
1728 ```compile_fail,E0164
1729 enum Foo { B { i: u32 } }
1731 fn bar(foo: Foo) -> u32 {
1733 Foo::B(i) => i, // error E0164
1738 Try using `{}` instead:
1741 enum Foo { B { i: u32 } }
1743 fn bar(foo: Foo) -> u32 {
1752 You bound an associated type in an expression path which is not
1755 Erroneous code example:
1757 ```compile_fail,E0182
1763 impl Foo for isize {
1765 fn bar() -> isize { 42 }
1768 // error: unexpected binding of associated item in expression path
1769 let x: isize = Foo::<A=usize>::bar();
1772 To give a concrete type when using the Universal Function Call Syntax,
1773 use "Type as Trait". Example:
1781 impl Foo for isize {
1783 fn bar() -> isize { 42 }
1786 let x: isize = <isize as Foo>::bar(); // ok!
1791 Explicitly implementing both Drop and Copy for a type is currently disallowed.
1792 This feature can make some sense in theory, but the current implementation is
1793 incorrect and can lead to memory unsafety (see [issue #20126][iss20126]), so
1794 it has been disabled for now.
1796 [iss20126]: https://github.com/rust-lang/rust/issues/20126
1800 An associated function for a trait was defined to be static, but an
1801 implementation of the trait declared the same function to be a method (i.e. to
1802 take a `self` parameter).
1804 Here's an example of this error:
1806 ```compile_fail,E0185
1814 // error, method `foo` has a `&self` declaration in the impl, but not in
1822 An associated function for a trait was defined to be a method (i.e. to take a
1823 `self` parameter), but an implementation of the trait declared the same function
1826 Here's an example of this error:
1828 ```compile_fail,E0186
1836 // error, method `foo` has a `&self` declaration in the trait, but not in
1844 Trait objects need to have all associated types specified. Erroneous code
1847 ```compile_fail,E0191
1852 type Foo = Trait; // error: the value of the associated type `Bar` (from
1853 // the trait `Trait`) must be specified
1856 Please verify you specified all associated types of the trait and that you
1857 used the right trait. Example:
1864 type Foo = Trait<Bar=i32>; // ok!
1869 Negative impls are only allowed for traits with default impls. For more
1870 information see the [opt-in builtin traits RFC][RFC 19].
1872 [RFC 19]: https://github.com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md
1876 `where` clauses must use generic type parameters: it does not make sense to use
1877 them otherwise. An example causing this error:
1884 #[derive(Copy,Clone)]
1889 impl Foo for Wrapper<u32> where Wrapper<u32>: Clone {
1894 This use of a `where` clause is strange - a more common usage would look
1895 something like the following:
1902 #[derive(Copy,Clone)]
1906 impl <T> Foo for Wrapper<T> where Wrapper<T>: Clone {
1911 Here, we're saying that the implementation exists on Wrapper only when the
1912 wrapped type `T` implements `Clone`. The `where` clause is important because
1913 some types will not implement `Clone`, and thus will not get this method.
1915 In our erroneous example, however, we're referencing a single concrete type.
1916 Since we know for certain that `Wrapper<u32>` implements `Clone`, there's no
1917 reason to also specify it in a `where` clause.
1921 A type parameter was declared which shadows an existing one. An example of this
1924 ```compile_fail,E0194
1926 fn do_something(&self) -> T;
1927 fn do_something_else<T: Clone>(&self, bar: T);
1931 In this example, the trait `Foo` and the trait method `do_something_else` both
1932 define a type parameter `T`. This is not allowed: if the method wishes to
1933 define a type parameter, it must use a different name for it.
1937 Your method's lifetime parameters do not match the trait declaration.
1938 Erroneous code example:
1940 ```compile_fail,E0195
1942 fn bar<'a,'b:'a>(x: &'a str, y: &'b str);
1947 impl Trait for Foo {
1948 fn bar<'a,'b>(x: &'a str, y: &'b str) {
1949 // error: lifetime parameters or bounds on method `bar`
1950 // do not match the trait declaration
1955 The lifetime constraint `'b` for bar() implementation does not match the
1956 trait declaration. Ensure lifetime declarations match exactly in both trait
1957 declaration and implementation. Example:
1961 fn t<'a,'b:'a>(x: &'a str, y: &'b str);
1966 impl Trait for Foo {
1967 fn t<'a,'b:'a>(x: &'a str, y: &'b str) { // ok!
1974 Inherent implementations (one that do not implement a trait but provide
1975 methods associated with a type) are always safe because they are not
1976 implementing an unsafe trait. Removing the `unsafe` keyword from the inherent
1977 implementation will resolve this error.
1979 ```compile_fail,E0197
1982 // this will cause this error
1984 // converting it to this will fix it
1990 A negative implementation is one that excludes a type from implementing a
1991 particular trait. Not being able to use a trait is always a safe operation,
1992 so negative implementations are always safe and never need to be marked as
1996 #![feature(optin_builtin_traits)]
2000 // unsafe is unnecessary
2001 unsafe impl !Clone for Foo { }
2007 #![feature(optin_builtin_traits)]
2013 impl Enterprise for .. { }
2015 impl !Enterprise for Foo { }
2018 Please note that negative impls are only allowed for traits with default impls.
2022 Safe traits should not have unsafe implementations, therefore marking an
2023 implementation for a safe trait unsafe will cause a compiler error. Removing
2024 the unsafe marker on the trait noted in the error will resolve this problem.
2026 ```compile_fail,E0199
2031 // this won't compile because Bar is safe
2032 unsafe impl Bar for Foo { }
2033 // this will compile
2034 impl Bar for Foo { }
2039 Unsafe traits must have unsafe implementations. This error occurs when an
2040 implementation for an unsafe trait isn't marked as unsafe. This may be resolved
2041 by marking the unsafe implementation as unsafe.
2043 ```compile_fail,E0200
2046 unsafe trait Bar { }
2048 // this won't compile because Bar is unsafe and impl isn't unsafe
2049 impl Bar for Foo { }
2050 // this will compile
2051 unsafe impl Bar for Foo { }
2056 It is an error to define two associated items (like methods, associated types,
2057 associated functions, etc.) with the same identifier.
2061 ```compile_fail,E0201
2065 fn bar(&self) -> bool { self.0 > 5 }
2066 fn bar() {} // error: duplicate associated function
2071 fn baz(&self) -> bool;
2077 fn baz(&self) -> bool { true }
2079 // error: duplicate method
2080 fn baz(&self) -> bool { self.0 > 5 }
2082 // error: duplicate associated type
2087 Note, however, that items with the same name are allowed for inherent `impl`
2088 blocks that don't overlap:
2094 fn bar(&self) -> bool { self.0 > 5 }
2098 fn bar(&self) -> bool { self.0 }
2104 Inherent associated types were part of [RFC 195] but are not yet implemented.
2105 See [the tracking issue][iss8995] for the status of this implementation.
2107 [RFC 195]: https://github.com/rust-lang/rfcs/blob/master/text/0195-associated-items.md
2108 [iss8995]: https://github.com/rust-lang/rust/issues/8995
2112 An attempt to implement the `Copy` trait for a struct failed because one of the
2113 fields does not implement `Copy`. To fix this, you must implement `Copy` for the
2114 mentioned field. Note that this may not be possible, as in the example of
2116 ```compile_fail,E0204
2121 impl Copy for Foo { }
2124 This fails because `Vec<T>` does not implement `Copy` for any `T`.
2126 Here's another example that will fail:
2128 ```compile_fail,E0204
2135 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
2136 differs from the behavior for `&T`, which is always `Copy`).
2141 An attempt to implement the `Copy` trait for an enum failed because one of the
2142 variants does not implement `Copy`. To fix this, you must implement `Copy` for
2143 the mentioned variant. Note that this may not be possible, as in the example of
2145 ```compile_fail,E0205
2151 impl Copy for Foo { }
2154 This fails because `Vec<T>` does not implement `Copy` for any `T`.
2156 Here's another example that will fail:
2158 ```compile_fail,E0205
2166 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
2167 differs from the behavior for `&T`, which is always `Copy`).
2172 You can only implement `Copy` for a struct or enum. Both of the following
2173 examples will fail, because neither `i32` (primitive type) nor `&'static Bar`
2174 (reference to `Bar`) is a struct or enum:
2176 ```compile_fail,E0206
2178 impl Copy for Foo { } // error
2180 #[derive(Copy, Clone)]
2182 impl Copy for &'static Bar { } // error
2187 Any type parameter or lifetime parameter of an `impl` must meet at least one of
2188 the following criteria:
2190 - it appears in the self type of the impl
2191 - for a trait impl, it appears in the trait reference
2192 - it is bound as an associated type
2196 Suppose we have a struct `Foo` and we would like to define some methods for it.
2197 The following definition leads to a compiler error:
2199 ```compile_fail,E0207
2202 impl<T: Default> Foo {
2203 // error: the type parameter `T` is not constrained by the impl trait, self
2204 // type, or predicates [E0207]
2205 fn get(&self) -> T {
2206 <T as Default>::default()
2211 The problem is that the parameter `T` does not appear in the self type (`Foo`)
2212 of the impl. In this case, we can fix the error by moving the type parameter
2213 from the `impl` to the method `get`:
2219 // Move the type parameter from the impl to the method
2221 fn get<T: Default>(&self) -> T {
2222 <T as Default>::default()
2229 As another example, suppose we have a `Maker` trait and want to establish a
2230 type `FooMaker` that makes `Foo`s:
2232 ```compile_fail,E0207
2235 fn make(&mut self) -> Self::Item;
2244 impl<T: Default> Maker for FooMaker {
2245 // error: the type parameter `T` is not constrained by the impl trait, self
2246 // type, or predicates [E0207]
2249 fn make(&mut self) -> Foo<T> {
2250 Foo { foo: <T as Default>::default() }
2255 This fails to compile because `T` does not appear in the trait or in the
2258 One way to work around this is to introduce a phantom type parameter into
2259 `FooMaker`, like so:
2262 use std::marker::PhantomData;
2266 fn make(&mut self) -> Self::Item;
2273 // Add a type parameter to `FooMaker`
2274 struct FooMaker<T> {
2275 phantom: PhantomData<T>,
2278 impl<T: Default> Maker for FooMaker<T> {
2281 fn make(&mut self) -> Foo<T> {
2283 foo: <T as Default>::default(),
2289 Another way is to do away with the associated type in `Maker` and use an input
2290 type parameter instead:
2293 // Use a type parameter instead of an associated type here
2295 fn make(&mut self) -> Item;
2304 impl<T: Default> Maker<Foo<T>> for FooMaker {
2305 fn make(&mut self) -> Foo<T> {
2306 Foo { foo: <T as Default>::default() }
2311 ### Additional information
2313 For more information, please see [RFC 447].
2315 [RFC 447]: https://github.com/rust-lang/rfcs/blob/master/text/0447-no-unused-impl-parameters.md
2319 This error indicates a violation of one of Rust's orphan rules for trait
2320 implementations. The rule concerns the use of type parameters in an
2321 implementation of a foreign trait (a trait defined in another crate), and
2322 states that type parameters must be "covered" by a local type. To understand
2323 what this means, it is perhaps easiest to consider a few examples.
2325 If `ForeignTrait` is a trait defined in some external crate `foo`, then the
2326 following trait `impl` is an error:
2328 ```compile_fail,E0210
2329 extern crate collections;
2330 use collections::range::RangeArgument;
2332 impl<T> RangeArgument<T> for T { } // error
2337 To work around this, it can be covered with a local type, `MyType`:
2340 struct MyType<T>(T);
2341 impl<T> ForeignTrait for MyType<T> { } // Ok
2344 Please note that a type alias is not sufficient.
2346 For another example of an error, suppose there's another trait defined in `foo`
2347 named `ForeignTrait2` that takes two type parameters. Then this `impl` results
2348 in the same rule violation:
2352 impl<T> ForeignTrait2<T, MyType<T>> for MyType2 { } // error
2355 The reason for this is that there are two appearances of type parameter `T` in
2356 the `impl` header, both as parameters for `ForeignTrait2`. The first appearance
2357 is uncovered, and so runs afoul of the orphan rule.
2359 Consider one more example:
2362 impl<T> ForeignTrait2<MyType<T>, T> for MyType2 { } // Ok
2365 This only differs from the previous `impl` in that the parameters `T` and
2366 `MyType<T>` for `ForeignTrait2` have been swapped. This example does *not*
2367 violate the orphan rule; it is permitted.
2369 To see why that last example was allowed, you need to understand the general
2370 rule. Unfortunately this rule is a bit tricky to state. Consider an `impl`:
2373 impl<P1, ..., Pm> ForeignTrait<T1, ..., Tn> for T0 { ... }
2376 where `P1, ..., Pm` are the type parameters of the `impl` and `T0, ..., Tn`
2377 are types. One of the types `T0, ..., Tn` must be a local type (this is another
2378 orphan rule, see the explanation for E0117). Let `i` be the smallest integer
2379 such that `Ti` is a local type. Then no type parameter can appear in any of the
2382 For information on the design of the orphan rules, see [RFC 1023].
2384 [RFC 1023]: https://github.com/rust-lang/rfcs/blob/master/text/1023-rebalancing-coherence.md
2389 You used a function or type which doesn't fit the requirements for where it was
2390 used. Erroneous code examples:
2393 #![feature(intrinsics)]
2395 extern "rust-intrinsic" {
2396 fn size_of<T>(); // error: intrinsic has wrong type
2401 fn main() -> i32 { 0 }
2402 // error: main function expects type: `fn() {main}`: expected (), found i32
2409 // error: mismatched types in range: expected u8, found i8
2419 fn x(self: Rc<Foo>) {}
2420 // error: mismatched self type: expected `Foo`: expected struct
2421 // `Foo`, found struct `alloc::rc::Rc`
2425 For the first code example, please check the function definition. Example:
2428 #![feature(intrinsics)]
2430 extern "rust-intrinsic" {
2431 fn size_of<T>() -> usize; // ok!
2435 The second case example is a bit particular : the main function must always
2436 have this definition:
2442 They never take parameters and never return types.
2444 For the third example, when you match, all patterns must have the same type
2445 as the type you're matching on. Example:
2451 0u8...3u8 => (), // ok!
2456 And finally, for the last example, only `Box<Self>`, `&Self`, `Self`,
2457 or `&mut Self` work as explicit self parameters. Example:
2463 fn x(self: Box<Foo>) {} // ok!
2470 A generic type was described using parentheses rather than angle brackets. For
2473 ```compile_fail,E0214
2475 let v: Vec(&str) = vec!["foo"];
2479 This is not currently supported: `v` should be defined as `Vec<&str>`.
2480 Parentheses are currently only used with generic types when defining parameters
2481 for `Fn`-family traits.
2485 You used an associated type which isn't defined in the trait.
2486 Erroneous code example:
2488 ```compile_fail,E0220
2493 type Foo = T1<F=i32>; // error: associated type `F` not found for `T1`
2500 // error: Baz is used but not declared
2501 fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
2505 Make sure that you have defined the associated type in the trait body.
2506 Also, verify that you used the right trait or you didn't misspell the
2507 associated type name. Example:
2514 type Foo = T1<Bar=i32>; // ok!
2520 type Baz; // we declare `Baz` in our trait.
2522 // and now we can use it here:
2523 fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
2529 An attempt was made to retrieve an associated type, but the type was ambiguous.
2532 ```compile_fail,E0221
2548 In this example, `Foo` defines an associated type `A`. `Bar` inherits that type
2549 from `Foo`, and defines another associated type of the same name. As a result,
2550 when we attempt to use `Self::A`, it's ambiguous whether we mean the `A` defined
2551 by `Foo` or the one defined by `Bar`.
2553 There are two options to work around this issue. The first is simply to rename
2554 one of the types. Alternatively, one can specify the intended type using the
2568 let _: <Self as Bar>::A;
2575 An attempt was made to retrieve an associated type, but the type was ambiguous.
2578 ```compile_fail,E0223
2579 trait MyTrait {type X; }
2582 let foo: MyTrait::X;
2586 The problem here is that we're attempting to take the type of X from MyTrait.
2587 Unfortunately, the type of X is not defined, because it's only made concrete in
2588 implementations of the trait. A working version of this code might look like:
2591 trait MyTrait {type X; }
2594 impl MyTrait for MyStruct {
2599 let foo: <MyStruct as MyTrait>::X;
2603 This syntax specifies that we want the X type from MyTrait, as made concrete in
2604 MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct
2605 might implement two different traits with identically-named associated types.
2606 This syntax allows disambiguation between the two.
2610 You attempted to use multiple types as bounds for a closure or trait object.
2611 Rust does not currently support this. A simple example that causes this error:
2613 ```compile_fail,E0225
2615 let _: Box<std::io::Read + std::io::Write>;
2619 Send and Sync are an exception to this rule: it's possible to have bounds of
2620 one non-builtin trait, plus either or both of Send and Sync. For example, the
2621 following compiles correctly:
2625 let _: Box<std::io::Read + Send + Sync>;
2631 An associated type binding was done outside of the type parameter declaration
2632 and `where` clause. Erroneous code example:
2634 ```compile_fail,E0229
2637 fn boo(&self) -> <Self as Foo>::A;
2642 impl Foo for isize {
2644 fn boo(&self) -> usize { 42 }
2647 fn baz<I>(x: &<I as Foo<A=Bar>>::A) {}
2648 // error: associated type bindings are not allowed here
2651 To solve this error, please move the type bindings in the type parameter
2655 fn baz<I: Foo<A=Bar>>(x: &<I as Foo>::A) {} // ok!
2658 Or in the `where` clause:
2661 fn baz<I>(x: &<I as Foo>::A) where I: Foo<A=Bar> {}
2666 The trait has more type parameters specified than appear in its definition.
2668 Erroneous example code:
2670 ```compile_fail,E0230
2671 #![feature(on_unimplemented)]
2672 #[rustc_on_unimplemented = "Trait error on `{Self}` with `<{A},{B},{C}>`"]
2673 // error: there is no type parameter C on trait TraitWithThreeParams
2674 trait TraitWithThreeParams<A,B>
2678 Include the correct number of type parameters and the compilation should
2682 #![feature(on_unimplemented)]
2683 #[rustc_on_unimplemented = "Trait error on `{Self}` with `<{A},{B},{C}>`"]
2684 trait TraitWithThreeParams<A,B,C> // ok!
2690 The attribute must have a value. Erroneous code example:
2692 ```compile_fail,E0232
2693 #![feature(on_unimplemented)]
2695 #[rustc_on_unimplemented] // error: this attribute must have a value
2699 Please supply the missing value of the attribute. Example:
2702 #![feature(on_unimplemented)]
2704 #[rustc_on_unimplemented = "foo"] // ok!
2710 This error indicates that not enough type parameters were found in a type or
2713 For example, the `Foo` struct below is defined to be generic in `T`, but the
2714 type parameter is missing in the definition of `Bar`:
2716 ```compile_fail,E0243
2717 struct Foo<T> { x: T }
2719 struct Bar { x: Foo }
2724 This error indicates that too many type parameters were found in a type or
2727 For example, the `Foo` struct below has no type parameters, but is supplied
2728 with two in the definition of `Bar`:
2730 ```compile_fail,E0244
2731 struct Foo { x: bool }
2733 struct Bar<S, T> { x: Foo<S, T> }
2738 If an impl has a generic parameter with the `#[may_dangle]` attribute, then
2739 that impl must be declared as an `unsafe impl. For example:
2741 ```compile_fail,E0569
2742 #![feature(generic_param_attrs)]
2743 #![feature(dropck_eyepatch)]
2746 impl<#[may_dangle] X> Drop for Foo<X> {
2747 fn drop(&mut self) { }
2751 In this example, we are asserting that the destructor for `Foo` will not
2752 access any data of type `X`, and require this assertion to be true for
2753 overall safety in our program. The compiler does not currently attempt to
2754 verify this assertion; therefore we must tag this `impl` as unsafe.
2758 Default impls for a trait must be located in the same crate where the trait was
2759 defined. For more information see the [opt-in builtin traits RFC][RFC 19].
2761 [RFC 19]: https://github.com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md
2765 A cross-crate opt-out trait was implemented on something which wasn't a struct
2766 or enum type. Erroneous code example:
2768 ```compile_fail,E0321
2769 #![feature(optin_builtin_traits)]
2773 impl !Sync for Foo {}
2775 unsafe impl Send for &'static Foo {}
2776 // error: cross-crate traits with a default impl, like `core::marker::Send`,
2777 // can only be implemented for a struct/enum type, not
2781 Only structs and enums are permitted to impl Send, Sync, and other opt-out
2782 trait, and the struct or enum must be local to the current crate. So, for
2783 example, `unsafe impl Send for Rc<Foo>` is not allowed.
2787 The `Sized` trait is a special trait built-in to the compiler for types with a
2788 constant size known at compile-time. This trait is automatically implemented
2789 for types as needed by the compiler, and it is currently disallowed to
2790 explicitly implement it for a type.
2794 An associated const was implemented when another trait item was expected.
2795 Erroneous code example:
2797 ```compile_fail,E0323
2798 #![feature(associated_consts)]
2808 // error: item `N` is an associated const, which doesn't match its
2809 // trait `<Bar as Foo>`
2813 Please verify that the associated const wasn't misspelled and the correct trait
2814 was implemented. Example:
2824 type N = u32; // ok!
2831 #![feature(associated_consts)]
2840 const N : u32 = 0; // ok!
2846 A method was implemented when another trait item was expected. Erroneous
2849 ```compile_fail,E0324
2850 #![feature(associated_consts)]
2862 // error: item `N` is an associated method, which doesn't match its
2863 // trait `<Bar as Foo>`
2867 To fix this error, please verify that the method name wasn't misspelled and
2868 verify that you are indeed implementing the correct trait items. Example:
2871 #![feature(associated_consts)]
2890 An associated type was implemented when another trait item was expected.
2891 Erroneous code example:
2893 ```compile_fail,E0325
2894 #![feature(associated_consts)]
2904 // error: item `N` is an associated type, which doesn't match its
2905 // trait `<Bar as Foo>`
2909 Please verify that the associated type name wasn't misspelled and your
2910 implementation corresponds to the trait definition. Example:
2920 type N = u32; // ok!
2927 #![feature(associated_consts)]
2936 const N : u32 = 0; // ok!
2942 The types of any associated constants in a trait implementation must match the
2943 types in the trait definition. This error indicates that there was a mismatch.
2945 Here's an example of this error:
2947 ```compile_fail,E0326
2948 #![feature(associated_consts)]
2957 const BAR: u32 = 5; // error, expected bool, found u32
2963 The Unsize trait should not be implemented directly. All implementations of
2964 Unsize are provided automatically by the compiler.
2966 Erroneous code example:
2968 ```compile_fail,E0328
2971 use std::marker::Unsize;
2975 impl<T> Unsize<T> for MyType {}
2978 If you are defining your own smart pointer type and would like to enable
2979 conversion from a sized to an unsized type with the
2980 [DST coercion system][RFC 982], use [`CoerceUnsized`] instead.
2983 #![feature(coerce_unsized)]
2985 use std::ops::CoerceUnsized;
2987 pub struct MyType<T: ?Sized> {
2988 field_with_unsized_type: T,
2991 impl<T, U> CoerceUnsized<MyType<U>> for MyType<T>
2992 where T: CoerceUnsized<U> {}
2995 [RFC 982]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md
2996 [`CoerceUnsized`]: https://doc.rust-lang.org/std/ops/trait.CoerceUnsized.html
3000 An attempt was made to access an associated constant through either a generic
3001 type parameter or `Self`. This is not supported yet. An example causing this
3002 error is shown below:
3005 #![feature(associated_consts)]
3013 impl Foo for MyStruct {
3014 const BAR: f64 = 0f64;
3017 fn get_bar_bad<F: Foo>(t: F) -> f64 {
3022 Currently, the value of `BAR` for a particular type can only be accessed
3023 through a concrete type, as shown below:
3026 #![feature(associated_consts)]
3034 fn get_bar_good() -> f64 {
3035 <MyStruct as Foo>::BAR
3041 An attempt was made to implement `Drop` on a concrete specialization of a
3042 generic type. An example is shown below:
3044 ```compile_fail,E0366
3049 impl Drop for Foo<u32> {
3050 fn drop(&mut self) {}
3054 This code is not legal: it is not possible to specialize `Drop` to a subset of
3055 implementations of a generic type. One workaround for this is to wrap the
3056 generic type, as shown below:
3068 fn drop(&mut self) {}
3074 An attempt was made to implement `Drop` on a specialization of a generic type.
3075 An example is shown below:
3077 ```compile_fail,E0367
3080 struct MyStruct<T> {
3084 impl<T: Foo> Drop for MyStruct<T> {
3085 fn drop(&mut self) {}
3089 This code is not legal: it is not possible to specialize `Drop` to a subset of
3090 implementations of a generic type. In order for this code to work, `MyStruct`
3091 must also require that `T` implements `Foo`. Alternatively, another option is
3092 to wrap the generic type in another that specializes appropriately:
3097 struct MyStruct<T> {
3101 struct MyStructWrapper<T: Foo> {
3105 impl <T: Foo> Drop for MyStructWrapper<T> {
3106 fn drop(&mut self) {}
3112 This error indicates that a binary assignment operator like `+=` or `^=` was
3113 applied to a type that doesn't support it. For example:
3115 ```compile_fail,E0368
3116 let mut x = 12f32; // error: binary operation `<<` cannot be applied to
3122 To fix this error, please check that this type implements this binary
3126 let mut x = 12u32; // the `u32` type does implement the `ShlAssign` trait
3131 It is also possible to overload most operators for your own type by
3132 implementing the `[OP]Assign` traits from `std::ops`.
3134 Another problem you might be facing is this: suppose you've overloaded the `+`
3135 operator for some type `Foo` by implementing the `std::ops::Add` trait for
3136 `Foo`, but you find that using `+=` does not work, as in this example:
3138 ```compile_fail,E0368
3146 fn add(self, rhs: Foo) -> Foo {
3152 let mut x: Foo = Foo(5);
3153 x += Foo(7); // error, `+= cannot be applied to the type `Foo`
3157 This is because `AddAssign` is not automatically implemented, so you need to
3158 manually implement it for your type.
3162 A binary operation was attempted on a type which doesn't support it.
3163 Erroneous code example:
3165 ```compile_fail,E0369
3166 let x = 12f32; // error: binary operation `<<` cannot be applied to
3172 To fix this error, please check that this type implements this binary
3176 let x = 12u32; // the `u32` type does implement it:
3177 // https://doc.rust-lang.org/stable/std/ops/trait.Shl.html
3182 It is also possible to overload most operators for your own type by
3183 implementing traits from `std::ops`.
3187 The maximum value of an enum was reached, so it cannot be automatically
3188 set in the next enum value. Erroneous code example:
3191 #[deny(overflowing_literals)]
3193 X = 0x7fffffffffffffff,
3194 Y, // error: enum discriminant overflowed on value after
3195 // 9223372036854775807: i64; set explicitly via
3196 // Y = -9223372036854775808 if that is desired outcome
3200 To fix this, please set manually the next enum value or put the enum variant
3201 with the maximum value at the end of the enum. Examples:
3205 X = 0x7fffffffffffffff,
3215 X = 0x7fffffffffffffff,
3221 When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
3222 definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
3223 `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
3224 definition, so it is not useful to do this.
3228 ```compile_fail,E0371
3229 trait Foo { fn foo(&self) { } }
3233 impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
3234 impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
3235 impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
3236 impl Baz for Bar { } // Note: This is OK
3241 A struct without a field containing an unsized type cannot implement
3243 [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3244 is any type that the compiler doesn't know the length or alignment of at
3245 compile time. Any struct containing an unsized type is also unsized.
3247 Example of erroneous code:
3249 ```compile_fail,E0374
3250 #![feature(coerce_unsized)]
3251 use std::ops::CoerceUnsized;
3253 struct Foo<T: ?Sized> {
3257 // error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
3258 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
3259 where T: CoerceUnsized<U> {}
3262 `CoerceUnsized` is used to coerce one struct containing an unsized type
3263 into another struct containing a different unsized type. If the struct
3264 doesn't have any fields of unsized types then you don't need explicit
3265 coercion to get the types you want. To fix this you can either
3266 not try to implement `CoerceUnsized` or you can add a field that is
3267 unsized to the struct.
3272 #![feature(coerce_unsized)]
3273 use std::ops::CoerceUnsized;
3275 // We don't need to impl `CoerceUnsized` here.
3280 // We add the unsized type field to the struct.
3281 struct Bar<T: ?Sized> {
3286 // The struct has an unsized field so we can implement
3287 // `CoerceUnsized` for it.
3288 impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
3289 where T: CoerceUnsized<U> {}
3292 Note that `CoerceUnsized` is mainly used by smart pointers like `Box`, `Rc`
3293 and `Arc` to be able to mark that they can coerce unsized types that they
3298 A struct with more than one field containing an unsized type cannot implement
3299 `CoerceUnsized`. This only occurs when you are trying to coerce one of the
3300 types in your struct to another type in the struct. In this case we try to
3301 impl `CoerceUnsized` from `T` to `U` which are both types that the struct
3302 takes. An [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3303 is any type that the compiler doesn't know the length or alignment of at
3304 compile time. Any struct containing an unsized type is also unsized.
3306 Example of erroneous code:
3308 ```compile_fail,E0375
3309 #![feature(coerce_unsized)]
3310 use std::ops::CoerceUnsized;
3312 struct Foo<T: ?Sized, U: ?Sized> {
3318 // error: Struct `Foo` has more than one unsized field.
3319 impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
3322 `CoerceUnsized` only allows for coercion from a structure with a single
3323 unsized type field to another struct with a single unsized type field.
3324 In fact Rust only allows for a struct to have one unsized type in a struct
3325 and that unsized type must be the last field in the struct. So having two
3326 unsized types in a single struct is not allowed by the compiler. To fix this
3327 use only one field containing an unsized type in the struct and then use
3328 multiple structs to manage each unsized type field you need.
3333 #![feature(coerce_unsized)]
3334 use std::ops::CoerceUnsized;
3336 struct Foo<T: ?Sized> {
3341 impl <T, U> CoerceUnsized<Foo<U>> for Foo<T>
3342 where T: CoerceUnsized<U> {}
3344 fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
3345 Foo { a: 12i32, b: t } // we use coercion to get the `Foo<U>` type we need
3352 The type you are trying to impl `CoerceUnsized` for is not a struct.
3353 `CoerceUnsized` can only be implemented for a struct. Unsized types are
3354 already able to be coerced without an implementation of `CoerceUnsized`
3355 whereas a struct containing an unsized type needs to know the unsized type
3356 field it's containing is able to be coerced. An
3357 [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3358 is any type that the compiler doesn't know the length or alignment of at
3359 compile time. Any struct containing an unsized type is also unsized.
3361 Example of erroneous code:
3363 ```compile_fail,E0376
3364 #![feature(coerce_unsized)]
3365 use std::ops::CoerceUnsized;
3367 struct Foo<T: ?Sized> {
3371 // error: The type `U` is not a struct
3372 impl<T, U> CoerceUnsized<U> for Foo<T> {}
3375 The `CoerceUnsized` trait takes a struct type. Make sure the type you are
3376 providing to `CoerceUnsized` is a struct with only the last field containing an
3382 #![feature(coerce_unsized)]
3383 use std::ops::CoerceUnsized;
3389 // The `Foo<U>` is a struct so `CoerceUnsized` can be implemented
3390 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}
3393 Note that in Rust, structs can only contain an unsized type if the field
3394 containing the unsized type is the last and only unsized type field in the
3399 Default impls are only allowed for traits with no methods or associated items.
3400 For more information see the [opt-in builtin traits RFC][RFC 19].
3402 [RFC 19]: https://github.com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md
3406 You tried to implement methods for a primitive type. Erroneous code example:
3408 ```compile_fail,E0390
3414 // error: only a single inherent implementation marked with
3415 // `#[lang = "mut_ptr"]` is allowed for the `*mut T` primitive
3418 This isn't allowed, but using a trait to implement a method is a good solution.
3430 impl Bar for *mut Foo {
3437 This error indicates that a type or lifetime parameter has been declared
3438 but not actually used. Here is an example that demonstrates the error:
3440 ```compile_fail,E0392
3446 If the type parameter was included by mistake, this error can be fixed
3447 by simply removing the type parameter, as shown below:
3455 Alternatively, if the type parameter was intentionally inserted, it must be
3456 used. A simple fix is shown below:
3464 This error may also commonly be found when working with unsafe code. For
3465 example, when using raw pointers one may wish to specify the lifetime for
3466 which the pointed-at data is valid. An initial attempt (below) causes this
3469 ```compile_fail,E0392
3475 We want to express the constraint that Foo should not outlive `'a`, because
3476 the data pointed to by `T` is only valid for that lifetime. The problem is
3477 that there are no actual uses of `'a`. It's possible to work around this
3478 by adding a PhantomData type to the struct, using it to tell the compiler
3479 to act as if the struct contained a borrowed reference `&'a T`:
3482 use std::marker::PhantomData;
3484 struct Foo<'a, T: 'a> {
3486 phantom: PhantomData<&'a T>
3490 PhantomData can also be used to express information about unused type
3491 parameters. You can read more about it in the API documentation:
3493 https://doc.rust-lang.org/std/marker/struct.PhantomData.html
3497 A type parameter which references `Self` in its default value was not specified.
3498 Example of erroneous code:
3500 ```compile_fail,E0393
3503 fn together_we_will_rule_the_galaxy(son: &A) {}
3504 // error: the type parameter `T` must be explicitly specified in an
3505 // object type because its default value `Self` references the
3509 A trait object is defined over a single, fully-defined trait. With a regular
3510 default parameter, this parameter can just be substituted in. However, if the
3511 default parameter is `Self`, the trait changes for each concrete type; i.e.
3512 `i32` will be expected to implement `A<i32>`, `bool` will be expected to
3513 implement `A<bool>`, etc... These types will not share an implementation of a
3514 fully-defined trait; instead they share implementations of a trait with
3515 different parameters substituted in for each implementation. This is
3516 irreconcilable with what we need to make a trait object work, and is thus
3517 disallowed. Making the trait concrete by explicitly specifying the value of the
3518 defaulted parameter will fix this issue. Fixed example:
3523 fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!
3528 You implemented a trait, overriding one or more of its associated types but did
3529 not reimplement its default methods.
3531 Example of erroneous code:
3533 ```compile_fail,E0399
3534 #![feature(associated_type_defaults)]
3542 // error - the following trait items need to be reimplemented as
3543 // `Assoc` was overridden: `bar`
3548 To fix this, add an implementation for each default method from the trait:
3551 #![feature(associated_type_defaults)]
3560 fn bar(&self) {} // ok!
3566 The length of the platform-intrinsic function `simd_shuffle`
3567 wasn't specified. Erroneous code example:
3569 ```compile_fail,E0439
3570 #![feature(platform_intrinsics)]
3572 extern "platform-intrinsic" {
3573 fn simd_shuffle<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3574 // error: invalid `simd_shuffle`, needs length: `simd_shuffle`
3578 The `simd_shuffle` function needs the length of the array passed as
3579 last parameter in its name. Example:
3582 #![feature(platform_intrinsics)]
3584 extern "platform-intrinsic" {
3585 fn simd_shuffle8<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3591 A platform-specific intrinsic function has the wrong number of type
3592 parameters. Erroneous code example:
3594 ```compile_fail,E0440
3595 #![feature(repr_simd)]
3596 #![feature(platform_intrinsics)]
3599 struct f64x2(f64, f64);
3601 extern "platform-intrinsic" {
3602 fn x86_mm_movemask_pd<T>(x: f64x2) -> i32;
3603 // error: platform-specific intrinsic has wrong number of type
3608 Please refer to the function declaration to see if it corresponds
3609 with yours. Example:
3612 #![feature(repr_simd)]
3613 #![feature(platform_intrinsics)]
3616 struct f64x2(f64, f64);
3618 extern "platform-intrinsic" {
3619 fn x86_mm_movemask_pd(x: f64x2) -> i32;
3625 An unknown platform-specific intrinsic function was used. Erroneous
3628 ```compile_fail,E0441
3629 #![feature(repr_simd)]
3630 #![feature(platform_intrinsics)]
3633 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3635 extern "platform-intrinsic" {
3636 fn x86_mm_adds_ep16(x: i16x8, y: i16x8) -> i16x8;
3637 // error: unrecognized platform-specific intrinsic function
3641 Please verify that the function name wasn't misspelled, and ensure
3642 that it is declared in the rust source code (in the file
3643 src/librustc_platform_intrinsics/x86.rs). Example:
3646 #![feature(repr_simd)]
3647 #![feature(platform_intrinsics)]
3650 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3652 extern "platform-intrinsic" {
3653 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3659 Intrinsic argument(s) and/or return value have the wrong type.
3660 Erroneous code example:
3662 ```compile_fail,E0442
3663 #![feature(repr_simd)]
3664 #![feature(platform_intrinsics)]
3667 struct i8x16(i8, i8, i8, i8, i8, i8, i8, i8,
3668 i8, i8, i8, i8, i8, i8, i8, i8);
3670 struct i32x4(i32, i32, i32, i32);
3672 struct i64x2(i64, i64);
3674 extern "platform-intrinsic" {
3675 fn x86_mm_adds_epi16(x: i8x16, y: i32x4) -> i64x2;
3676 // error: intrinsic arguments/return value have wrong type
3680 To fix this error, please refer to the function declaration to give
3681 it the awaited types. Example:
3684 #![feature(repr_simd)]
3685 #![feature(platform_intrinsics)]
3688 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3690 extern "platform-intrinsic" {
3691 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3697 Intrinsic argument(s) and/or return value have the wrong type.
3698 Erroneous code example:
3700 ```compile_fail,E0443
3701 #![feature(repr_simd)]
3702 #![feature(platform_intrinsics)]
3705 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3707 struct i64x8(i64, i64, i64, i64, i64, i64, i64, i64);
3709 extern "platform-intrinsic" {
3710 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i64x8;
3711 // error: intrinsic argument/return value has wrong type
3715 To fix this error, please refer to the function declaration to give
3716 it the awaited types. Example:
3719 #![feature(repr_simd)]
3720 #![feature(platform_intrinsics)]
3723 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3725 extern "platform-intrinsic" {
3726 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3732 A platform-specific intrinsic function has wrong number of arguments.
3733 Erroneous code example:
3735 ```compile_fail,E0444
3736 #![feature(repr_simd)]
3737 #![feature(platform_intrinsics)]
3740 struct f64x2(f64, f64);
3742 extern "platform-intrinsic" {
3743 fn x86_mm_movemask_pd(x: f64x2, y: f64x2, z: f64x2) -> i32;
3744 // error: platform-specific intrinsic has invalid number of arguments
3748 Please refer to the function declaration to see if it corresponds
3749 with yours. Example:
3752 #![feature(repr_simd)]
3753 #![feature(platform_intrinsics)]
3756 struct f64x2(f64, f64);
3758 extern "platform-intrinsic" {
3759 fn x86_mm_movemask_pd(x: f64x2) -> i32; // ok!
3765 The `typeof` keyword is currently reserved but unimplemented.
3766 Erroneous code example:
3768 ```compile_fail,E0516
3770 let x: typeof(92) = 92;
3774 Try using type inference instead. Example:
3784 A non-default implementation was already made on this type so it cannot be
3785 specialized further. Erroneous code example:
3787 ```compile_fail,E0520
3788 #![feature(specialization)]
3795 impl<T> SpaceLlama for T {
3796 default fn fly(&self) {}
3800 // applies to all `Clone` T and overrides the previous impl
3801 impl<T: Clone> SpaceLlama for T {
3805 // since `i32` is clone, this conflicts with the previous implementation
3806 impl SpaceLlama for i32 {
3807 default fn fly(&self) {}
3808 // error: item `fly` is provided by an `impl` that specializes
3809 // another, but the item in the parent `impl` is not marked
3810 // `default` and so it cannot be specialized.
3814 Specialization only allows you to override `default` functions in
3817 To fix this error, you need to mark all the parent implementations as default.
3821 #![feature(specialization)]
3828 impl<T> SpaceLlama for T {
3829 default fn fly(&self) {} // This is a parent implementation.
3832 // applies to all `Clone` T; overrides the previous impl
3833 impl<T: Clone> SpaceLlama for T {
3834 default fn fly(&self) {} // This is a parent implementation but was
3835 // previously not a default one, causing the error
3838 // applies to i32, overrides the previous two impls
3839 impl SpaceLlama for i32 {
3840 fn fly(&self) {} // And now that's ok!
3846 The number of elements in an array or slice pattern differed from the number of
3847 elements in the array being matched.
3849 Example of erroneous code:
3851 ```compile_fail,E0527
3852 #![feature(slice_patterns)]
3854 let r = &[1, 2, 3, 4];
3856 &[a, b] => { // error: pattern requires 2 elements but array
3858 println!("a={}, b={}", a, b);
3863 Ensure that the pattern is consistent with the size of the matched
3864 array. Additional elements can be matched with `..`:
3867 #![feature(slice_patterns)]
3869 let r = &[1, 2, 3, 4];
3871 &[a, b, ..] => { // ok!
3872 println!("a={}, b={}", a, b);
3879 An array or slice pattern required more elements than were present in the
3882 Example of erroneous code:
3884 ```compile_fail,E0528
3885 #![feature(slice_patterns)]
3889 &[a, b, c, rest..] => { // error: pattern requires at least 3
3890 // elements but array has 2
3891 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3896 Ensure that the matched array has at least as many elements as the pattern
3897 requires. You can match an arbitrary number of remaining elements with `..`:
3900 #![feature(slice_patterns)]
3902 let r = &[1, 2, 3, 4, 5];
3904 &[a, b, c, rest..] => { // ok!
3905 // prints `a=1, b=2, c=3 rest=[4, 5]`
3906 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3913 An array or slice pattern was matched against some other type.
3915 Example of erroneous code:
3917 ```compile_fail,E0529
3918 #![feature(slice_patterns)]
3922 [a, b] => { // error: expected an array or slice, found `f32`
3923 println!("a={}, b={}", a, b);
3928 Ensure that the pattern and the expression being matched on are of consistent
3932 #![feature(slice_patterns)]
3937 println!("a={}, b={}", a, b);
3944 An unknown field was specified into an enum's structure variant.
3946 Erroneous code example:
3948 ```compile_fail,E0559
3953 let s = Field::Fool { joke: 0 };
3954 // error: struct variant `Field::Fool` has no field named `joke`
3957 Verify you didn't misspell the field's name or that the field exists. Example:
3964 let s = Field::Fool { joke: 0 }; // ok!
3969 An unknown field was specified into a structure.
3971 Erroneous code example:
3973 ```compile_fail,E0560
3978 let s = Simba { mother: 1, father: 0 };
3979 // error: structure `Simba` has no field named `father`
3982 Verify you didn't misspell the field's name or that the field exists. Example:
3990 let s = Simba { mother: 1, father: 0 }; // ok!
3995 The requested ABI is unsupported by the current target.
3997 The rust compiler maintains for each target a blacklist of ABIs unsupported on
3998 that target. If an ABI is present in such a list this usually means that the
3999 target / ABI combination is currently unsupported by llvm.
4001 If necessary, you can circumvent this check using custom target specifications.
4005 A return statement was found outside of a function body.
4007 Erroneous code example:
4009 ```compile_fail,E0572
4010 const FOO: u32 = return 0; // error: return statement outside of function body
4015 To fix this issue, just remove the return keyword or move the expression into a
4021 fn some_fn() -> u32 {
4032 In a `fn` type, a lifetime appears only in the return type,
4033 and not in the arguments types.
4035 Erroneous code example:
4037 ```compile_fail,E0581
4039 // Here, `'a` appears only in the return type:
4040 let x: for<'a> fn() -> &'a i32;
4044 To fix this issue, either use the lifetime in the arguments, or use
4049 // Here, `'a` appears only in the return type:
4050 let x: for<'a> fn(&'a i32) -> &'a i32;
4051 let y: fn() -> &'static i32;
4055 Note: The examples above used to be (erroneously) accepted by the
4056 compiler, but this was since corrected. See [issue #33685] for more
4059 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
4063 A lifetime appears only in an associated-type binding,
4064 and not in the input types to the trait.
4066 Erroneous code example:
4068 ```compile_fail,E0582
4070 // No type can satisfy this requirement, since `'a` does not
4071 // appear in any of the input types (here, `i32`):
4072 where F: for<'a> Fn(i32) -> Option<&'a i32>
4079 To fix this issue, either use the lifetime in the inputs, or use
4083 fn bar<F, G>(t: F, u: G)
4084 where F: for<'a> Fn(&'a i32) -> Option<&'a i32>,
4085 G: Fn(i32) -> Option<&'static i32>,
4092 Note: The examples above used to be (erroneously) accepted by the
4093 compiler, but this was since corrected. See [issue #33685] for more
4096 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
4101 register_diagnostics! {
4111 // E0159, // use of trait `{}` as struct constructor
4112 // E0163, // merged into E0071
4115 // E0172, // non-trait found in a type sum, moved to resolve
4116 // E0173, // manual implementations of unboxed closure traits are experimental
4119 // E0187, // can't infer the kind of the closure
4120 // E0188, // can not cast an immutable reference to a mutable pointer
4121 // E0189, // deprecated: can only cast a boxed pointer to a boxed object
4122 // E0190, // deprecated: can only cast a &-pointer to an &-object
4123 // E0196, // cannot determine a type for this closure
4124 E0203, // type parameter has more than one relaxed default bound,
4125 // and only one is supported
4127 // E0209, // builtin traits can only be implemented on structs or enums
4128 E0212, // cannot extract an associated type from a higher-ranked trait bound
4129 // E0213, // associated types are not accepted in this context
4130 // E0215, // angle-bracket notation is not stable with `Fn`
4131 // E0216, // parenthetical notation is only stable with `Fn`
4132 // E0217, // ambiguous associated type, defined in multiple supertraits
4133 // E0218, // no associated type defined
4134 // E0219, // associated type defined in higher-ranked supertrait
4135 // E0222, // Error code E0045 (variadic function must have C calling
4136 // convention) duplicate
4137 E0224, // at least one non-builtin train is required for an object type
4138 E0227, // ambiguous lifetime bound, explicit lifetime bound required
4139 E0228, // explicit lifetime bound required
4140 E0231, // only named substitution parameters are allowed
4143 // E0235, // structure constructor specifies a structure of type but
4144 // E0236, // no lang item for range syntax
4145 // E0237, // no lang item for range syntax
4146 // E0238, // parenthesized parameters may only be used with a trait
4147 // E0239, // `next` method of `Iterator` trait has unexpected type
4151 E0245, // not a trait
4152 // E0246, // invalid recursive type
4154 // E0248, // value used as a type, now reported earlier during resolution as E0412
4156 // E0319, // trait impls for defaulted traits allowed just for structs/enums
4157 E0320, // recursive overflow during dropck
4158 // E0372, // coherence not object safe
4159 E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
4160 // between structures with the same definition
4161 E0436, // functional record update requires a struct
4162 E0521, // redundant default implementations of trait
4163 E0533, // `{}` does not name a unit variant, unit struct or a constant
4164 E0562, // `impl Trait` not allowed outside of function
4165 // and inherent method return types
4166 E0563, // cannot determine a type for this `impl Trait`: {}
4167 E0564, // only named lifetimes are allowed in `impl Trait`,
4168 // but `{}` was found in the type `{}`
4169 E0567, // auto traits can not have type parameters
4170 E0568, // auto-traits can not have predicates,
4171 E0588, // packed struct cannot transitively contain a `[repr(align)]` struct
4172 E0592, // duplicate definitions with name `{}`