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/pull/809
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 an unnecessary type parameter in a type alias. Erroneous code
1230 ```compile_fail,E0091
1231 type Foo<T> = u32; // error: type parameter `T` is unused
1233 type Foo<A,B> = Box<A>; // error: type parameter `B` is unused
1236 Please check you didn't write too many type parameters. Example:
1239 type Foo = u32; // ok!
1240 type Foo2<A> = Box<A>; // ok!
1245 You tried to declare an undefined atomic operation function.
1246 Erroneous code example:
1248 ```compile_fail,E0092
1249 #![feature(intrinsics)]
1251 extern "rust-intrinsic" {
1252 fn atomic_foo(); // error: unrecognized atomic operation
1257 Please check you didn't make a mistake in the function's name. All intrinsic
1258 functions are defined in librustc_trans/trans/intrinsic.rs and in
1259 libcore/intrinsics.rs in the Rust source code. Example:
1262 #![feature(intrinsics)]
1264 extern "rust-intrinsic" {
1265 fn atomic_fence(); // ok!
1271 You declared an unknown intrinsic function. Erroneous code example:
1273 ```compile_fail,E0093
1274 #![feature(intrinsics)]
1276 extern "rust-intrinsic" {
1277 fn foo(); // error: unrecognized intrinsic function: `foo`
1287 Please check you didn't make a mistake in the function's name. All intrinsic
1288 functions are defined in librustc_trans/trans/intrinsic.rs and in
1289 libcore/intrinsics.rs in the Rust source code. Example:
1292 #![feature(intrinsics)]
1294 extern "rust-intrinsic" {
1295 fn atomic_fence(); // ok!
1307 You gave an invalid number of type parameters to an intrinsic function.
1308 Erroneous code example:
1310 ```compile_fail,E0094
1311 #![feature(intrinsics)]
1313 extern "rust-intrinsic" {
1314 fn size_of<T, U>() -> usize; // error: intrinsic has wrong number
1315 // of type parameters
1319 Please check that you provided the right number of type parameters
1320 and verify with the function declaration in the Rust source code.
1324 #![feature(intrinsics)]
1326 extern "rust-intrinsic" {
1327 fn size_of<T>() -> usize; // ok!
1333 You hit this error because the compiler lacks the information to
1334 determine a type for this expression. Erroneous code example:
1336 ```compile_fail,E0101
1337 let x = |_| {}; // error: cannot determine a type for this expression
1340 You have two possibilities to solve this situation:
1342 * Give an explicit definition of the expression
1343 * Infer the expression
1348 let x = |_ : u32| {}; // ok!
1356 You hit this error because the compiler lacks the information to
1357 determine the type of this variable. Erroneous code example:
1359 ```compile_fail,E0102
1360 // could be an array of anything
1361 let x = []; // error: cannot determine a type for this local variable
1364 To solve this situation, constrain the type of the variable.
1368 #![allow(unused_variables)]
1371 let x: [u8; 0] = [];
1377 This error means that an incorrect number of lifetime parameters were provided
1378 for a type (like a struct or enum) or trait:
1380 ```compile_fail,E0107
1381 struct Foo<'a, 'b>(&'a str, &'b str);
1382 enum Bar { A, B, C }
1385 foo: Foo<'a>, // error: expected 2, found 1
1386 bar: Bar<'a>, // error: expected 0, found 1
1392 You tried to give a type parameter to a type which doesn't need it. Erroneous
1395 ```compile_fail,E0109
1396 type X = u32<i32>; // error: type parameters are not allowed on this type
1399 Please check that you used the correct type and recheck its definition. Perhaps
1400 it doesn't need the type parameter.
1405 type X = u32; // this compiles
1408 Note that type parameters for enum-variant constructors go after the variant,
1409 not after the enum (Option::None::<u32>, not Option::<u32>::None).
1413 You tried to give a lifetime parameter to a type which doesn't need it.
1414 Erroneous code example:
1416 ```compile_fail,E0110
1417 type X = u32<'static>; // error: lifetime parameters are not allowed on
1421 Please check that the correct type was used and recheck its definition; perhaps
1422 it doesn't need the lifetime parameter. Example:
1425 type X = u32; // ok!
1430 You can only define an inherent implementation for a type in the same crate
1431 where the type was defined. For example, an `impl` block as below is not allowed
1432 since `Vec` is defined in the standard library:
1434 ```compile_fail,E0116
1435 impl Vec<u8> { } // error
1438 To fix this problem, you can do either of these things:
1440 - define a trait that has the desired associated functions/types/constants and
1441 implement the trait for the type in question
1442 - define a new type wrapping the type and define an implementation on the new
1445 Note that using the `type` keyword does not work here because `type` only
1446 introduces a type alias:
1448 ```compile_fail,E0116
1449 type Bytes = Vec<u8>;
1451 impl Bytes { } // error, same as above
1456 This error indicates a violation of one of Rust's orphan rules for trait
1457 implementations. The rule prohibits any implementation of a foreign trait (a
1458 trait defined in another crate) where
1460 - the type that is implementing the trait is foreign
1461 - all of the parameters being passed to the trait (if there are any) are also
1464 Here's one example of this error:
1466 ```compile_fail,E0117
1467 impl Drop for u32 {}
1470 To avoid this kind of error, ensure that at least one local type is referenced
1474 pub struct Foo; // you define your type in your crate
1476 impl Drop for Foo { // and you can implement the trait on it!
1477 // code of trait implementation here
1480 impl From<Foo> for i32 { // or you use a type from your crate as
1482 fn from(i: Foo) -> i32 {
1488 Alternatively, define a trait locally and implement that instead:
1492 fn get(&self) -> usize;
1496 fn get(&self) -> usize { 0 }
1500 For information on the design of the orphan rules, see [RFC 1023].
1502 [RFC 1023]: https://github.com/rust-lang/rfcs/pull/1023
1506 You're trying to write an inherent implementation for something which isn't a
1507 struct nor an enum. Erroneous code example:
1509 ```compile_fail,E0118
1510 impl (u8, u8) { // error: no base type found for inherent implementation
1511 fn get_state(&self) -> String {
1517 To fix this error, please implement a trait on the type or wrap it in a struct.
1521 // we create a trait here
1522 trait LiveLongAndProsper {
1523 fn get_state(&self) -> String;
1526 // and now you can implement it on (u8, u8)
1527 impl LiveLongAndProsper for (u8, u8) {
1528 fn get_state(&self) -> String {
1529 "He's dead, Jim!".to_owned()
1534 Alternatively, you can create a newtype. A newtype is a wrapping tuple-struct.
1535 For example, `NewType` is a newtype over `Foo` in `struct NewType(Foo)`.
1539 struct TypeWrapper((u8, u8));
1542 fn get_state(&self) -> String {
1543 "Fascinating!".to_owned()
1550 There are conflicting trait implementations for the same type.
1551 Example of erroneous code:
1553 ```compile_fail,E0119
1555 fn get(&self) -> usize;
1558 impl<T> MyTrait for T {
1559 fn get(&self) -> usize { 0 }
1566 impl MyTrait for Foo { // error: conflicting implementations of trait
1567 // `MyTrait` for type `Foo`
1568 fn get(&self) -> usize { self.value }
1572 When looking for the implementation for the trait, the compiler finds
1573 both the `impl<T> MyTrait for T` where T is all types and the `impl
1574 MyTrait for Foo`. Since a trait cannot be implemented multiple times,
1575 this is an error. So, when you write:
1579 fn get(&self) -> usize;
1582 impl<T> MyTrait for T {
1583 fn get(&self) -> usize { 0 }
1587 This makes the trait implemented on all types in the scope. So if you
1588 try to implement it on another one after that, the implementations will
1593 fn get(&self) -> usize;
1596 impl<T> MyTrait for T {
1597 fn get(&self) -> usize { 0 }
1605 f.get(); // the trait is implemented so we can use it
1611 An attempt was made to implement Drop on a trait, which is not allowed: only
1612 structs and enums can implement Drop. An example causing this error:
1614 ```compile_fail,E0120
1617 impl Drop for MyTrait {
1618 fn drop(&mut self) {}
1622 A workaround for this problem is to wrap the trait up in a struct, and implement
1623 Drop on that. An example is shown below:
1627 struct MyWrapper<T: MyTrait> { foo: T }
1629 impl <T: MyTrait> Drop for MyWrapper<T> {
1630 fn drop(&mut self) {}
1635 Alternatively, wrapping trait objects requires something like the following:
1640 //or Box<MyTrait>, if you wanted an owned trait object
1641 struct MyWrapper<'a> { foo: &'a MyTrait }
1643 impl <'a> Drop for MyWrapper<'a> {
1644 fn drop(&mut self) {}
1650 In order to be consistent with Rust's lack of global type inference, type
1651 placeholders are disallowed by design in item signatures.
1653 Examples of this error include:
1655 ```compile_fail,E0121
1656 fn foo() -> _ { 5 } // error, explicitly write out the return type instead
1658 static BAR: _ = "test"; // error, explicitly write out the type instead
1663 An attempt was made to add a generic constraint to a type alias. While Rust will
1664 allow this with a warning, it will not currently enforce the constraint.
1665 Consider the example below:
1670 type MyType<R: Foo> = (R, ());
1677 We're able to declare a variable of type `MyType<u32>`, despite the fact that
1678 `u32` does not implement `Foo`. As a result, one should avoid using generic
1679 constraints in concert with type aliases.
1683 You declared two fields of a struct with the same name. Erroneous code
1686 ```compile_fail,E0124
1689 field1: i32, // error: field is already declared
1693 Please verify that the field names have been correctly spelled. Example:
1704 It is not possible to define `main` with type parameters, or even with function
1705 parameters. When `main` is present, it must take no arguments and return `()`.
1706 Erroneous code example:
1708 ```compile_fail,E0131
1709 fn main<T>() { // error: main function is not allowed to have type parameters
1715 A function with the `start` attribute was declared with type parameters.
1717 Erroneous code example:
1719 ```compile_fail,E0132
1726 It is not possible to declare type parameters on a function that has the `start`
1727 attribute. Such a function must have the following type signature (for more
1728 information: http://doc.rust-lang.org/stable/book/no-stdlib.html):
1731 fn(isize, *const *const u8) -> isize;
1740 fn my_start(argc: isize, argv: *const *const u8) -> isize {
1747 This error means that an attempt was made to match a struct type enum
1748 variant as a non-struct type:
1750 ```compile_fail,E0164
1751 enum Foo { B { i: u32 } }
1753 fn bar(foo: Foo) -> u32 {
1755 Foo::B(i) => i, // error E0164
1760 Try using `{}` instead:
1763 enum Foo { B { i: u32 } }
1765 fn bar(foo: Foo) -> u32 {
1774 You bound an associated type in an expression path which is not
1777 Erroneous code example:
1779 ```compile_fail,E0182
1785 impl Foo for isize {
1787 fn bar() -> isize { 42 }
1790 // error: unexpected binding of associated item in expression path
1791 let x: isize = Foo::<A=usize>::bar();
1794 To give a concrete type when using the Universal Function Call Syntax,
1795 use "Type as Trait". Example:
1803 impl Foo for isize {
1805 fn bar() -> isize { 42 }
1808 let x: isize = <isize as Foo>::bar(); // ok!
1813 Explicitly implementing both Drop and Copy for a type is currently disallowed.
1814 This feature can make some sense in theory, but the current implementation is
1815 incorrect and can lead to memory unsafety (see [issue #20126][iss20126]), so
1816 it has been disabled for now.
1818 [iss20126]: https://github.com/rust-lang/rust/issues/20126
1822 An associated function for a trait was defined to be static, but an
1823 implementation of the trait declared the same function to be a method (i.e. to
1824 take a `self` parameter).
1826 Here's an example of this error:
1828 ```compile_fail,E0185
1836 // error, method `foo` has a `&self` declaration in the impl, but not in
1844 An associated function for a trait was defined to be a method (i.e. to take a
1845 `self` parameter), but an implementation of the trait declared the same function
1848 Here's an example of this error:
1850 ```compile_fail,E0186
1858 // error, method `foo` has a `&self` declaration in the trait, but not in
1866 Trait objects need to have all associated types specified. Erroneous code
1869 ```compile_fail,E0191
1874 type Foo = Trait; // error: the value of the associated type `Bar` (from
1875 // the trait `Trait`) must be specified
1878 Please verify you specified all associated types of the trait and that you
1879 used the right trait. Example:
1886 type Foo = Trait<Bar=i32>; // ok!
1891 Negative impls are only allowed for traits with default impls. For more
1892 information see the [opt-in builtin traits RFC](https://github.com/rust-lang/
1893 rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
1897 `where` clauses must use generic type parameters: it does not make sense to use
1898 them otherwise. An example causing this error:
1905 #[derive(Copy,Clone)]
1910 impl Foo for Wrapper<u32> where Wrapper<u32>: Clone {
1915 This use of a `where` clause is strange - a more common usage would look
1916 something like the following:
1923 #[derive(Copy,Clone)]
1927 impl <T> Foo for Wrapper<T> where Wrapper<T>: Clone {
1932 Here, we're saying that the implementation exists on Wrapper only when the
1933 wrapped type `T` implements `Clone`. The `where` clause is important because
1934 some types will not implement `Clone`, and thus will not get this method.
1936 In our erroneous example, however, we're referencing a single concrete type.
1937 Since we know for certain that `Wrapper<u32>` implements `Clone`, there's no
1938 reason to also specify it in a `where` clause.
1942 A type parameter was declared which shadows an existing one. An example of this
1945 ```compile_fail,E0194
1947 fn do_something(&self) -> T;
1948 fn do_something_else<T: Clone>(&self, bar: T);
1952 In this example, the trait `Foo` and the trait method `do_something_else` both
1953 define a type parameter `T`. This is not allowed: if the method wishes to
1954 define a type parameter, it must use a different name for it.
1958 Your method's lifetime parameters do not match the trait declaration.
1959 Erroneous code example:
1961 ```compile_fail,E0195
1963 fn bar<'a,'b:'a>(x: &'a str, y: &'b str);
1968 impl Trait for Foo {
1969 fn bar<'a,'b>(x: &'a str, y: &'b str) {
1970 // error: lifetime parameters or bounds on method `bar`
1971 // do not match the trait declaration
1976 The lifetime constraint `'b` for bar() implementation does not match the
1977 trait declaration. Ensure lifetime declarations match exactly in both trait
1978 declaration and implementation. Example:
1982 fn t<'a,'b:'a>(x: &'a str, y: &'b str);
1987 impl Trait for Foo {
1988 fn t<'a,'b:'a>(x: &'a str, y: &'b str) { // ok!
1995 Inherent implementations (one that do not implement a trait but provide
1996 methods associated with a type) are always safe because they are not
1997 implementing an unsafe trait. Removing the `unsafe` keyword from the inherent
1998 implementation will resolve this error.
2000 ```compile_fail,E0197
2003 // this will cause this error
2005 // converting it to this will fix it
2011 A negative implementation is one that excludes a type from implementing a
2012 particular trait. Not being able to use a trait is always a safe operation,
2013 so negative implementations are always safe and never need to be marked as
2017 #![feature(optin_builtin_traits)]
2021 // unsafe is unnecessary
2022 unsafe impl !Clone for Foo { }
2028 #![feature(optin_builtin_traits)]
2034 impl Enterprise for .. { }
2036 impl !Enterprise for Foo { }
2039 Please note that negative impls are only allowed for traits with default impls.
2043 Safe traits should not have unsafe implementations, therefore marking an
2044 implementation for a safe trait unsafe will cause a compiler error. Removing
2045 the unsafe marker on the trait noted in the error will resolve this problem.
2047 ```compile_fail,E0199
2052 // this won't compile because Bar is safe
2053 unsafe impl Bar for Foo { }
2054 // this will compile
2055 impl Bar for Foo { }
2060 Unsafe traits must have unsafe implementations. This error occurs when an
2061 implementation for an unsafe trait isn't marked as unsafe. This may be resolved
2062 by marking the unsafe implementation as unsafe.
2064 ```compile_fail,E0200
2067 unsafe trait Bar { }
2069 // this won't compile because Bar is unsafe and impl isn't unsafe
2070 impl Bar for Foo { }
2071 // this will compile
2072 unsafe impl Bar for Foo { }
2077 It is an error to define two associated items (like methods, associated types,
2078 associated functions, etc.) with the same identifier.
2082 ```compile_fail,E0201
2086 fn bar(&self) -> bool { self.0 > 5 }
2087 fn bar() {} // error: duplicate associated function
2092 fn baz(&self) -> bool;
2098 fn baz(&self) -> bool { true }
2100 // error: duplicate method
2101 fn baz(&self) -> bool { self.0 > 5 }
2103 // error: duplicate associated type
2108 Note, however, that items with the same name are allowed for inherent `impl`
2109 blocks that don't overlap:
2115 fn bar(&self) -> bool { self.0 > 5 }
2119 fn bar(&self) -> bool { self.0 }
2125 Inherent associated types were part of [RFC 195] but are not yet implemented.
2126 See [the tracking issue][iss8995] for the status of this implementation.
2128 [RFC 195]: https://github.com/rust-lang/rfcs/pull/195
2129 [iss8995]: https://github.com/rust-lang/rust/issues/8995
2133 An attempt to implement the `Copy` trait for a struct failed because one of the
2134 fields does not implement `Copy`. To fix this, you must implement `Copy` for the
2135 mentioned field. Note that this may not be possible, as in the example of
2137 ```compile_fail,E0204
2142 impl Copy for Foo { }
2145 This fails because `Vec<T>` does not implement `Copy` for any `T`.
2147 Here's another example that will fail:
2149 ```compile_fail,E0204
2156 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
2157 differs from the behavior for `&T`, which is always `Copy`).
2162 An attempt to implement the `Copy` trait for an enum failed because one of the
2163 variants does not implement `Copy`. To fix this, you must implement `Copy` for
2164 the mentioned variant. Note that this may not be possible, as in the example of
2166 ```compile_fail,E0205
2172 impl Copy for Foo { }
2175 This fails because `Vec<T>` does not implement `Copy` for any `T`.
2177 Here's another example that will fail:
2179 ```compile_fail,E0205
2187 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
2188 differs from the behavior for `&T`, which is always `Copy`).
2193 You can only implement `Copy` for a struct or enum. Both of the following
2194 examples will fail, because neither `i32` (primitive type) nor `&'static Bar`
2195 (reference to `Bar`) is a struct or enum:
2197 ```compile_fail,E0206
2199 impl Copy for Foo { } // error
2201 #[derive(Copy, Clone)]
2203 impl Copy for &'static Bar { } // error
2208 Any type parameter or lifetime parameter of an `impl` must meet at least one of
2209 the following criteria:
2211 - it appears in the self type of the impl
2212 - for a trait impl, it appears in the trait reference
2213 - it is bound as an associated type
2217 Suppose we have a struct `Foo` and we would like to define some methods for it.
2218 The following definition leads to a compiler error:
2220 ```compile_fail,E0207
2223 impl<T: Default> Foo {
2224 // error: the type parameter `T` is not constrained by the impl trait, self
2225 // type, or predicates [E0207]
2226 fn get(&self) -> T {
2227 <T as Default>::default()
2232 The problem is that the parameter `T` does not appear in the self type (`Foo`)
2233 of the impl. In this case, we can fix the error by moving the type parameter
2234 from the `impl` to the method `get`:
2240 // Move the type parameter from the impl to the method
2242 fn get<T: Default>(&self) -> T {
2243 <T as Default>::default()
2250 As another example, suppose we have a `Maker` trait and want to establish a
2251 type `FooMaker` that makes `Foo`s:
2253 ```compile_fail,E0207
2256 fn make(&mut self) -> Self::Item;
2265 impl<T: Default> Maker for FooMaker {
2266 // error: the type parameter `T` is not constrained by the impl trait, self
2267 // type, or predicates [E0207]
2270 fn make(&mut self) -> Foo<T> {
2271 Foo { foo: <T as Default>::default() }
2276 This fails to compile because `T` does not appear in the trait or in the
2279 One way to work around this is to introduce a phantom type parameter into
2280 `FooMaker`, like so:
2283 use std::marker::PhantomData;
2287 fn make(&mut self) -> Self::Item;
2294 // Add a type parameter to `FooMaker`
2295 struct FooMaker<T> {
2296 phantom: PhantomData<T>,
2299 impl<T: Default> Maker for FooMaker<T> {
2302 fn make(&mut self) -> Foo<T> {
2304 foo: <T as Default>::default(),
2310 Another way is to do away with the associated type in `Maker` and use an input
2311 type parameter instead:
2314 // Use a type parameter instead of an associated type here
2316 fn make(&mut self) -> Item;
2325 impl<T: Default> Maker<Foo<T>> for FooMaker {
2326 fn make(&mut self) -> Foo<T> {
2327 Foo { foo: <T as Default>::default() }
2332 ### Additional information
2334 For more information, please see [RFC 447].
2336 [RFC 447]: https://github.com/rust-lang/rfcs/blob/master/text/0447-no-unused-impl-parameters.md
2340 This error indicates a violation of one of Rust's orphan rules for trait
2341 implementations. The rule concerns the use of type parameters in an
2342 implementation of a foreign trait (a trait defined in another crate), and
2343 states that type parameters must be "covered" by a local type. To understand
2344 what this means, it is perhaps easiest to consider a few examples.
2346 If `ForeignTrait` is a trait defined in some external crate `foo`, then the
2347 following trait `impl` is an error:
2349 ```compile_fail,E0210
2350 extern crate collections;
2351 use collections::range::RangeArgument;
2353 impl<T> RangeArgument<T> for T { } // error
2358 To work around this, it can be covered with a local type, `MyType`:
2361 struct MyType<T>(T);
2362 impl<T> ForeignTrait for MyType<T> { } // Ok
2365 Please note that a type alias is not sufficient.
2367 For another example of an error, suppose there's another trait defined in `foo`
2368 named `ForeignTrait2` that takes two type parameters. Then this `impl` results
2369 in the same rule violation:
2373 impl<T> ForeignTrait2<T, MyType<T>> for MyType2 { } // error
2376 The reason for this is that there are two appearances of type parameter `T` in
2377 the `impl` header, both as parameters for `ForeignTrait2`. The first appearance
2378 is uncovered, and so runs afoul of the orphan rule.
2380 Consider one more example:
2383 impl<T> ForeignTrait2<MyType<T>, T> for MyType2 { } // Ok
2386 This only differs from the previous `impl` in that the parameters `T` and
2387 `MyType<T>` for `ForeignTrait2` have been swapped. This example does *not*
2388 violate the orphan rule; it is permitted.
2390 To see why that last example was allowed, you need to understand the general
2391 rule. Unfortunately this rule is a bit tricky to state. Consider an `impl`:
2394 impl<P1, ..., Pm> ForeignTrait<T1, ..., Tn> for T0 { ... }
2397 where `P1, ..., Pm` are the type parameters of the `impl` and `T0, ..., Tn`
2398 are types. One of the types `T0, ..., Tn` must be a local type (this is another
2399 orphan rule, see the explanation for E0117). Let `i` be the smallest integer
2400 such that `Ti` is a local type. Then no type parameter can appear in any of the
2403 For information on the design of the orphan rules, see [RFC 1023].
2405 [RFC 1023]: https://github.com/rust-lang/rfcs/pull/1023
2410 You used a function or type which doesn't fit the requirements for where it was
2411 used. Erroneous code examples:
2414 #![feature(intrinsics)]
2416 extern "rust-intrinsic" {
2417 fn size_of<T>(); // error: intrinsic has wrong type
2422 fn main() -> i32 { 0 }
2423 // error: main function expects type: `fn() {main}`: expected (), found i32
2430 // error: mismatched types in range: expected u8, found i8
2440 fn x(self: Rc<Foo>) {}
2441 // error: mismatched self type: expected `Foo`: expected struct
2442 // `Foo`, found struct `alloc::rc::Rc`
2446 For the first code example, please check the function definition. Example:
2449 #![feature(intrinsics)]
2451 extern "rust-intrinsic" {
2452 fn size_of<T>() -> usize; // ok!
2456 The second case example is a bit particular : the main function must always
2457 have this definition:
2463 They never take parameters and never return types.
2465 For the third example, when you match, all patterns must have the same type
2466 as the type you're matching on. Example:
2472 0u8...3u8 => (), // ok!
2477 And finally, for the last example, only `Box<Self>`, `&Self`, `Self`,
2478 or `&mut Self` work as explicit self parameters. Example:
2484 fn x(self: Box<Foo>) {} // ok!
2491 A generic type was described using parentheses rather than angle brackets. For
2494 ```compile_fail,E0214
2496 let v: Vec(&str) = vec!["foo"];
2500 This is not currently supported: `v` should be defined as `Vec<&str>`.
2501 Parentheses are currently only used with generic types when defining parameters
2502 for `Fn`-family traits.
2506 You used an associated type which isn't defined in the trait.
2507 Erroneous code example:
2509 ```compile_fail,E0220
2514 type Foo = T1<F=i32>; // error: associated type `F` not found for `T1`
2521 // error: Baz is used but not declared
2522 fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
2526 Make sure that you have defined the associated type in the trait body.
2527 Also, verify that you used the right trait or you didn't misspell the
2528 associated type name. Example:
2535 type Foo = T1<Bar=i32>; // ok!
2541 type Baz; // we declare `Baz` in our trait.
2543 // and now we can use it here:
2544 fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
2550 An attempt was made to retrieve an associated type, but the type was ambiguous.
2553 ```compile_fail,E0221
2569 In this example, `Foo` defines an associated type `A`. `Bar` inherits that type
2570 from `Foo`, and defines another associated type of the same name. As a result,
2571 when we attempt to use `Self::A`, it's ambiguous whether we mean the `A` defined
2572 by `Foo` or the one defined by `Bar`.
2574 There are two options to work around this issue. The first is simply to rename
2575 one of the types. Alternatively, one can specify the intended type using the
2589 let _: <Self as Bar>::A;
2596 An attempt was made to retrieve an associated type, but the type was ambiguous.
2599 ```compile_fail,E0223
2600 trait MyTrait {type X; }
2603 let foo: MyTrait::X;
2607 The problem here is that we're attempting to take the type of X from MyTrait.
2608 Unfortunately, the type of X is not defined, because it's only made concrete in
2609 implementations of the trait. A working version of this code might look like:
2612 trait MyTrait {type X; }
2615 impl MyTrait for MyStruct {
2620 let foo: <MyStruct as MyTrait>::X;
2624 This syntax specifies that we want the X type from MyTrait, as made concrete in
2625 MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct
2626 might implement two different traits with identically-named associated types.
2627 This syntax allows disambiguation between the two.
2631 You attempted to use multiple types as bounds for a closure or trait object.
2632 Rust does not currently support this. A simple example that causes this error:
2634 ```compile_fail,E0225
2636 let _: Box<std::io::Read + std::io::Write>;
2640 Send and Sync are an exception to this rule: it's possible to have bounds of
2641 one non-builtin trait, plus either or both of Send and Sync. For example, the
2642 following compiles correctly:
2646 let _: Box<std::io::Read + Send + Sync>;
2652 An associated type binding was done outside of the type parameter declaration
2653 and `where` clause. Erroneous code example:
2655 ```compile_fail,E0229
2658 fn boo(&self) -> <Self as Foo>::A;
2663 impl Foo for isize {
2665 fn boo(&self) -> usize { 42 }
2668 fn baz<I>(x: &<I as Foo<A=Bar>>::A) {}
2669 // error: associated type bindings are not allowed here
2672 To solve this error, please move the type bindings in the type parameter
2676 fn baz<I: Foo<A=Bar>>(x: &<I as Foo>::A) {} // ok!
2679 Or in the `where` clause:
2682 fn baz<I>(x: &<I as Foo>::A) where I: Foo<A=Bar> {}
2687 The trait has more type parameters specified than appear in its definition.
2689 Erroneous example code:
2691 ```compile_fail,E0230
2692 #![feature(on_unimplemented)]
2693 #[rustc_on_unimplemented = "Trait error on `{Self}` with `<{A},{B},{C}>`"]
2694 // error: there is no type parameter C on trait TraitWithThreeParams
2695 trait TraitWithThreeParams<A,B>
2699 Include the correct number of type parameters and the compilation should
2703 #![feature(on_unimplemented)]
2704 #[rustc_on_unimplemented = "Trait error on `{Self}` with `<{A},{B},{C}>`"]
2705 trait TraitWithThreeParams<A,B,C> // ok!
2711 The attribute must have a value. Erroneous code example:
2713 ```compile_fail,E0232
2714 #![feature(on_unimplemented)]
2716 #[rustc_on_unimplemented] // error: this attribute must have a value
2720 Please supply the missing value of the attribute. Example:
2723 #![feature(on_unimplemented)]
2725 #[rustc_on_unimplemented = "foo"] // ok!
2731 This error indicates that not enough type parameters were found in a type or
2734 For example, the `Foo` struct below is defined to be generic in `T`, but the
2735 type parameter is missing in the definition of `Bar`:
2737 ```compile_fail,E0243
2738 struct Foo<T> { x: T }
2740 struct Bar { x: Foo }
2745 This error indicates that too many type parameters were found in a type or
2748 For example, the `Foo` struct below has no type parameters, but is supplied
2749 with two in the definition of `Bar`:
2751 ```compile_fail,E0244
2752 struct Foo { x: bool }
2754 struct Bar<S, T> { x: Foo<S, T> }
2759 If an impl has a generic parameter with the `#[may_dangle]` attribute, then
2760 that impl must be declared as an `unsafe impl. For example:
2762 ```compile_fail,E0569
2763 #![feature(generic_param_attrs)]
2764 #![feature(dropck_eyepatch)]
2767 impl<#[may_dangle] X> Drop for Foo<X> {
2768 fn drop(&mut self) { }
2772 In this example, we are asserting that the destructor for `Foo` will not
2773 access any data of type `X`, and require this assertion to be true for
2774 overall safety in our program. The compiler does not currently attempt to
2775 verify this assertion; therefore we must tag this `impl` as unsafe.
2779 Default impls for a trait must be located in the same crate where the trait was
2780 defined. For more information see the [opt-in builtin traits RFC](https://github
2781 .com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
2785 A cross-crate opt-out trait was implemented on something which wasn't a struct
2786 or enum type. Erroneous code example:
2788 ```compile_fail,E0321
2789 #![feature(optin_builtin_traits)]
2793 impl !Sync for Foo {}
2795 unsafe impl Send for &'static Foo {}
2796 // error: cross-crate traits with a default impl, like `core::marker::Send`,
2797 // can only be implemented for a struct/enum type, not
2801 Only structs and enums are permitted to impl Send, Sync, and other opt-out
2802 trait, and the struct or enum must be local to the current crate. So, for
2803 example, `unsafe impl Send for Rc<Foo>` is not allowed.
2807 The `Sized` trait is a special trait built-in to the compiler for types with a
2808 constant size known at compile-time. This trait is automatically implemented
2809 for types as needed by the compiler, and it is currently disallowed to
2810 explicitly implement it for a type.
2814 An associated const was implemented when another trait item was expected.
2815 Erroneous code example:
2817 ```compile_fail,E0323
2818 #![feature(associated_consts)]
2828 // error: item `N` is an associated const, which doesn't match its
2829 // trait `<Bar as Foo>`
2833 Please verify that the associated const wasn't misspelled and the correct trait
2834 was implemented. Example:
2844 type N = u32; // ok!
2851 #![feature(associated_consts)]
2860 const N : u32 = 0; // ok!
2866 A method was implemented when another trait item was expected. Erroneous
2869 ```compile_fail,E0324
2870 #![feature(associated_consts)]
2882 // error: item `N` is an associated method, which doesn't match its
2883 // trait `<Bar as Foo>`
2887 To fix this error, please verify that the method name wasn't misspelled and
2888 verify that you are indeed implementing the correct trait items. Example:
2891 #![feature(associated_consts)]
2910 An associated type was implemented when another trait item was expected.
2911 Erroneous code example:
2913 ```compile_fail,E0325
2914 #![feature(associated_consts)]
2924 // error: item `N` is an associated type, which doesn't match its
2925 // trait `<Bar as Foo>`
2929 Please verify that the associated type name wasn't misspelled and your
2930 implementation corresponds to the trait definition. Example:
2940 type N = u32; // ok!
2947 #![feature(associated_consts)]
2956 const N : u32 = 0; // ok!
2962 The types of any associated constants in a trait implementation must match the
2963 types in the trait definition. This error indicates that there was a mismatch.
2965 Here's an example of this error:
2967 ```compile_fail,E0326
2968 #![feature(associated_consts)]
2977 const BAR: u32 = 5; // error, expected bool, found u32
2983 The Unsize trait should not be implemented directly. All implementations of
2984 Unsize are provided automatically by the compiler.
2986 Erroneous code example:
2988 ```compile_fail,E0328
2991 use std::marker::Unsize;
2995 impl<T> Unsize<T> for MyType {}
2998 If you are defining your own smart pointer type and would like to enable
2999 conversion from a sized to an unsized type with the [DST coercion system]
3000 (https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md), use
3001 [`CoerceUnsized`](https://doc.rust-lang.org/std/ops/trait.CoerceUnsized.html)
3005 #![feature(coerce_unsized)]
3007 use std::ops::CoerceUnsized;
3009 pub struct MyType<T: ?Sized> {
3010 field_with_unsized_type: T,
3013 impl<T, U> CoerceUnsized<MyType<U>> for MyType<T>
3014 where T: CoerceUnsized<U> {}
3019 An attempt was made to access an associated constant through either a generic
3020 type parameter or `Self`. This is not supported yet. An example causing this
3021 error is shown below:
3024 #![feature(associated_consts)]
3032 impl Foo for MyStruct {
3033 const BAR: f64 = 0f64;
3036 fn get_bar_bad<F: Foo>(t: F) -> f64 {
3041 Currently, the value of `BAR` for a particular type can only be accessed
3042 through a concrete type, as shown below:
3045 #![feature(associated_consts)]
3053 fn get_bar_good() -> f64 {
3054 <MyStruct as Foo>::BAR
3060 An attempt was made to implement `Drop` on a concrete specialization of a
3061 generic type. An example is shown below:
3063 ```compile_fail,E0366
3068 impl Drop for Foo<u32> {
3069 fn drop(&mut self) {}
3073 This code is not legal: it is not possible to specialize `Drop` to a subset of
3074 implementations of a generic type. One workaround for this is to wrap the
3075 generic type, as shown below:
3087 fn drop(&mut self) {}
3093 An attempt was made to implement `Drop` on a specialization of a generic type.
3094 An example is shown below:
3096 ```compile_fail,E0367
3099 struct MyStruct<T> {
3103 impl<T: Foo> Drop for MyStruct<T> {
3104 fn drop(&mut self) {}
3108 This code is not legal: it is not possible to specialize `Drop` to a subset of
3109 implementations of a generic type. In order for this code to work, `MyStruct`
3110 must also require that `T` implements `Foo`. Alternatively, another option is
3111 to wrap the generic type in another that specializes appropriately:
3116 struct MyStruct<T> {
3120 struct MyStructWrapper<T: Foo> {
3124 impl <T: Foo> Drop for MyStructWrapper<T> {
3125 fn drop(&mut self) {}
3131 This error indicates that a binary assignment operator like `+=` or `^=` was
3132 applied to a type that doesn't support it. For example:
3134 ```compile_fail,E0368
3135 let mut x = 12f32; // error: binary operation `<<` cannot be applied to
3141 To fix this error, please check that this type implements this binary
3145 let mut x = 12u32; // the `u32` type does implement the `ShlAssign` trait
3150 It is also possible to overload most operators for your own type by
3151 implementing the `[OP]Assign` traits from `std::ops`.
3153 Another problem you might be facing is this: suppose you've overloaded the `+`
3154 operator for some type `Foo` by implementing the `std::ops::Add` trait for
3155 `Foo`, but you find that using `+=` does not work, as in this example:
3157 ```compile_fail,E0368
3165 fn add(self, rhs: Foo) -> Foo {
3171 let mut x: Foo = Foo(5);
3172 x += Foo(7); // error, `+= cannot be applied to the type `Foo`
3176 This is because `AddAssign` is not automatically implemented, so you need to
3177 manually implement it for your type.
3181 A binary operation was attempted on a type which doesn't support it.
3182 Erroneous code example:
3184 ```compile_fail,E0369
3185 let x = 12f32; // error: binary operation `<<` cannot be applied to
3191 To fix this error, please check that this type implements this binary
3195 let x = 12u32; // the `u32` type does implement it:
3196 // https://doc.rust-lang.org/stable/std/ops/trait.Shl.html
3201 It is also possible to overload most operators for your own type by
3202 implementing traits from `std::ops`.
3206 The maximum value of an enum was reached, so it cannot be automatically
3207 set in the next enum value. Erroneous code example:
3210 #[deny(overflowing_literals)]
3212 X = 0x7fffffffffffffff,
3213 Y, // error: enum discriminant overflowed on value after
3214 // 9223372036854775807: i64; set explicitly via
3215 // Y = -9223372036854775808 if that is desired outcome
3219 To fix this, please set manually the next enum value or put the enum variant
3220 with the maximum value at the end of the enum. Examples:
3224 X = 0x7fffffffffffffff,
3234 X = 0x7fffffffffffffff,
3240 When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
3241 definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
3242 `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
3243 definition, so it is not useful to do this.
3247 ```compile_fail,E0371
3248 trait Foo { fn foo(&self) { } }
3252 impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
3253 impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
3254 impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
3255 impl Baz for Bar { } // Note: This is OK
3260 A struct without a field containing an unsized type cannot implement
3262 [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3263 is any type that the compiler doesn't know the length or alignment of at
3264 compile time. Any struct containing an unsized type is also unsized.
3266 Example of erroneous code:
3268 ```compile_fail,E0374
3269 #![feature(coerce_unsized)]
3270 use std::ops::CoerceUnsized;
3272 struct Foo<T: ?Sized> {
3276 // error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
3277 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
3278 where T: CoerceUnsized<U> {}
3281 `CoerceUnsized` is used to coerce one struct containing an unsized type
3282 into another struct containing a different unsized type. If the struct
3283 doesn't have any fields of unsized types then you don't need explicit
3284 coercion to get the types you want. To fix this you can either
3285 not try to implement `CoerceUnsized` or you can add a field that is
3286 unsized to the struct.
3291 #![feature(coerce_unsized)]
3292 use std::ops::CoerceUnsized;
3294 // We don't need to impl `CoerceUnsized` here.
3299 // We add the unsized type field to the struct.
3300 struct Bar<T: ?Sized> {
3305 // The struct has an unsized field so we can implement
3306 // `CoerceUnsized` for it.
3307 impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
3308 where T: CoerceUnsized<U> {}
3311 Note that `CoerceUnsized` is mainly used by smart pointers like `Box`, `Rc`
3312 and `Arc` to be able to mark that they can coerce unsized types that they
3317 A struct with more than one field containing an unsized type cannot implement
3318 `CoerceUnsized`. This only occurs when you are trying to coerce one of the
3319 types in your struct to another type in the struct. In this case we try to
3320 impl `CoerceUnsized` from `T` to `U` which are both types that the struct
3321 takes. An [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3322 is any type that the compiler doesn't know the length or alignment of at
3323 compile time. Any struct containing an unsized type is also unsized.
3325 Example of erroneous code:
3327 ```compile_fail,E0375
3328 #![feature(coerce_unsized)]
3329 use std::ops::CoerceUnsized;
3331 struct Foo<T: ?Sized, U: ?Sized> {
3337 // error: Struct `Foo` has more than one unsized field.
3338 impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
3341 `CoerceUnsized` only allows for coercion from a structure with a single
3342 unsized type field to another struct with a single unsized type field.
3343 In fact Rust only allows for a struct to have one unsized type in a struct
3344 and that unsized type must be the last field in the struct. So having two
3345 unsized types in a single struct is not allowed by the compiler. To fix this
3346 use only one field containing an unsized type in the struct and then use
3347 multiple structs to manage each unsized type field you need.
3352 #![feature(coerce_unsized)]
3353 use std::ops::CoerceUnsized;
3355 struct Foo<T: ?Sized> {
3360 impl <T, U> CoerceUnsized<Foo<U>> for Foo<T>
3361 where T: CoerceUnsized<U> {}
3363 fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
3364 Foo { a: 12i32, b: t } // we use coercion to get the `Foo<U>` type we need
3371 The type you are trying to impl `CoerceUnsized` for is not a struct.
3372 `CoerceUnsized` can only be implemented for a struct. Unsized types are
3373 already able to be coerced without an implementation of `CoerceUnsized`
3374 whereas a struct containing an unsized type needs to know the unsized type
3375 field it's containing is able to be coerced. An
3376 [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3377 is any type that the compiler doesn't know the length or alignment of at
3378 compile time. Any struct containing an unsized type is also unsized.
3380 Example of erroneous code:
3382 ```compile_fail,E0376
3383 #![feature(coerce_unsized)]
3384 use std::ops::CoerceUnsized;
3386 struct Foo<T: ?Sized> {
3390 // error: The type `U` is not a struct
3391 impl<T, U> CoerceUnsized<U> for Foo<T> {}
3394 The `CoerceUnsized` trait takes a struct type. Make sure the type you are
3395 providing to `CoerceUnsized` is a struct with only the last field containing an
3401 #![feature(coerce_unsized)]
3402 use std::ops::CoerceUnsized;
3408 // The `Foo<U>` is a struct so `CoerceUnsized` can be implemented
3409 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}
3412 Note that in Rust, structs can only contain an unsized type if the field
3413 containing the unsized type is the last and only unsized type field in the
3418 Default impls are only allowed for traits with no methods or associated items.
3419 For more information see the [opt-in builtin traits RFC](https://github.com/rust
3420 -lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
3424 You tried to implement methods for a primitive type. Erroneous code example:
3426 ```compile_fail,E0390
3432 // error: only a single inherent implementation marked with
3433 // `#[lang = "mut_ptr"]` is allowed for the `*mut T` primitive
3436 This isn't allowed, but using a trait to implement a method is a good solution.
3448 impl Bar for *mut Foo {
3455 This error indicates that a type or lifetime parameter has been declared
3456 but not actually used. Here is an example that demonstrates the error:
3458 ```compile_fail,E0392
3464 If the type parameter was included by mistake, this error can be fixed
3465 by simply removing the type parameter, as shown below:
3473 Alternatively, if the type parameter was intentionally inserted, it must be
3474 used. A simple fix is shown below:
3482 This error may also commonly be found when working with unsafe code. For
3483 example, when using raw pointers one may wish to specify the lifetime for
3484 which the pointed-at data is valid. An initial attempt (below) causes this
3487 ```compile_fail,E0392
3493 We want to express the constraint that Foo should not outlive `'a`, because
3494 the data pointed to by `T` is only valid for that lifetime. The problem is
3495 that there are no actual uses of `'a`. It's possible to work around this
3496 by adding a PhantomData type to the struct, using it to tell the compiler
3497 to act as if the struct contained a borrowed reference `&'a T`:
3500 use std::marker::PhantomData;
3502 struct Foo<'a, T: 'a> {
3504 phantom: PhantomData<&'a T>
3508 PhantomData can also be used to express information about unused type
3509 parameters. You can read more about it in the API documentation:
3511 https://doc.rust-lang.org/std/marker/struct.PhantomData.html
3515 A type parameter which references `Self` in its default value was not specified.
3516 Example of erroneous code:
3518 ```compile_fail,E0393
3521 fn together_we_will_rule_the_galaxy(son: &A) {}
3522 // error: the type parameter `T` must be explicitly specified in an
3523 // object type because its default value `Self` references the
3527 A trait object is defined over a single, fully-defined trait. With a regular
3528 default parameter, this parameter can just be substituted in. However, if the
3529 default parameter is `Self`, the trait changes for each concrete type; i.e.
3530 `i32` will be expected to implement `A<i32>`, `bool` will be expected to
3531 implement `A<bool>`, etc... These types will not share an implementation of a
3532 fully-defined trait; instead they share implementations of a trait with
3533 different parameters substituted in for each implementation. This is
3534 irreconcilable with what we need to make a trait object work, and is thus
3535 disallowed. Making the trait concrete by explicitly specifying the value of the
3536 defaulted parameter will fix this issue. Fixed example:
3541 fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!
3546 You implemented a trait, overriding one or more of its associated types but did
3547 not reimplement its default methods.
3549 Example of erroneous code:
3551 ```compile_fail,E0399
3552 #![feature(associated_type_defaults)]
3560 // error - the following trait items need to be reimplemented as
3561 // `Assoc` was overridden: `bar`
3566 To fix this, add an implementation for each default method from the trait:
3569 #![feature(associated_type_defaults)]
3578 fn bar(&self) {} // ok!
3584 The length of the platform-intrinsic function `simd_shuffle`
3585 wasn't specified. Erroneous code example:
3587 ```compile_fail,E0439
3588 #![feature(platform_intrinsics)]
3590 extern "platform-intrinsic" {
3591 fn simd_shuffle<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3592 // error: invalid `simd_shuffle`, needs length: `simd_shuffle`
3596 The `simd_shuffle` function needs the length of the array passed as
3597 last parameter in its name. Example:
3600 #![feature(platform_intrinsics)]
3602 extern "platform-intrinsic" {
3603 fn simd_shuffle8<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3609 A platform-specific intrinsic function has the wrong number of type
3610 parameters. Erroneous code example:
3612 ```compile_fail,E0440
3613 #![feature(repr_simd)]
3614 #![feature(platform_intrinsics)]
3617 struct f64x2(f64, f64);
3619 extern "platform-intrinsic" {
3620 fn x86_mm_movemask_pd<T>(x: f64x2) -> i32;
3621 // error: platform-specific intrinsic has wrong number of type
3626 Please refer to the function declaration to see if it corresponds
3627 with yours. Example:
3630 #![feature(repr_simd)]
3631 #![feature(platform_intrinsics)]
3634 struct f64x2(f64, f64);
3636 extern "platform-intrinsic" {
3637 fn x86_mm_movemask_pd(x: f64x2) -> i32;
3643 An unknown platform-specific intrinsic function was used. Erroneous
3646 ```compile_fail,E0441
3647 #![feature(repr_simd)]
3648 #![feature(platform_intrinsics)]
3651 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3653 extern "platform-intrinsic" {
3654 fn x86_mm_adds_ep16(x: i16x8, y: i16x8) -> i16x8;
3655 // error: unrecognized platform-specific intrinsic function
3659 Please verify that the function name wasn't misspelled, and ensure
3660 that it is declared in the rust source code (in the file
3661 src/librustc_platform_intrinsics/x86.rs). Example:
3664 #![feature(repr_simd)]
3665 #![feature(platform_intrinsics)]
3668 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3670 extern "platform-intrinsic" {
3671 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3677 Intrinsic argument(s) and/or return value have the wrong type.
3678 Erroneous code example:
3680 ```compile_fail,E0442
3681 #![feature(repr_simd)]
3682 #![feature(platform_intrinsics)]
3685 struct i8x16(i8, i8, i8, i8, i8, i8, i8, i8,
3686 i8, i8, i8, i8, i8, i8, i8, i8);
3688 struct i32x4(i32, i32, i32, i32);
3690 struct i64x2(i64, i64);
3692 extern "platform-intrinsic" {
3693 fn x86_mm_adds_epi16(x: i8x16, y: i32x4) -> i64x2;
3694 // error: intrinsic arguments/return value have wrong type
3698 To fix this error, please refer to the function declaration to give
3699 it the awaited types. Example:
3702 #![feature(repr_simd)]
3703 #![feature(platform_intrinsics)]
3706 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3708 extern "platform-intrinsic" {
3709 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3715 Intrinsic argument(s) and/or return value have the wrong type.
3716 Erroneous code example:
3718 ```compile_fail,E0443
3719 #![feature(repr_simd)]
3720 #![feature(platform_intrinsics)]
3723 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3725 struct i64x8(i64, i64, i64, i64, i64, i64, i64, i64);
3727 extern "platform-intrinsic" {
3728 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i64x8;
3729 // error: intrinsic argument/return value has wrong type
3733 To fix this error, please refer to the function declaration to give
3734 it the awaited types. Example:
3737 #![feature(repr_simd)]
3738 #![feature(platform_intrinsics)]
3741 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3743 extern "platform-intrinsic" {
3744 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3750 A platform-specific intrinsic function has wrong number of arguments.
3751 Erroneous code example:
3753 ```compile_fail,E0444
3754 #![feature(repr_simd)]
3755 #![feature(platform_intrinsics)]
3758 struct f64x2(f64, f64);
3760 extern "platform-intrinsic" {
3761 fn x86_mm_movemask_pd(x: f64x2, y: f64x2, z: f64x2) -> i32;
3762 // error: platform-specific intrinsic has invalid number of arguments
3766 Please refer to the function declaration to see if it corresponds
3767 with yours. Example:
3770 #![feature(repr_simd)]
3771 #![feature(platform_intrinsics)]
3774 struct f64x2(f64, f64);
3776 extern "platform-intrinsic" {
3777 fn x86_mm_movemask_pd(x: f64x2) -> i32; // ok!
3783 The `typeof` keyword is currently reserved but unimplemented.
3784 Erroneous code example:
3786 ```compile_fail,E0516
3788 let x: typeof(92) = 92;
3792 Try using type inference instead. Example:
3802 A non-default implementation was already made on this type so it cannot be
3803 specialized further. Erroneous code example:
3805 ```compile_fail,E0520
3806 #![feature(specialization)]
3813 impl<T> SpaceLlama for T {
3814 default fn fly(&self) {}
3818 // applies to all `Clone` T and overrides the previous impl
3819 impl<T: Clone> SpaceLlama for T {
3823 // since `i32` is clone, this conflicts with the previous implementation
3824 impl SpaceLlama for i32 {
3825 default fn fly(&self) {}
3826 // error: item `fly` is provided by an `impl` that specializes
3827 // another, but the item in the parent `impl` is not marked
3828 // `default` and so it cannot be specialized.
3832 Specialization only allows you to override `default` functions in
3835 To fix this error, you need to mark all the parent implementations as default.
3839 #![feature(specialization)]
3846 impl<T> SpaceLlama for T {
3847 default fn fly(&self) {} // This is a parent implementation.
3850 // applies to all `Clone` T; overrides the previous impl
3851 impl<T: Clone> SpaceLlama for T {
3852 default fn fly(&self) {} // This is a parent implementation but was
3853 // previously not a default one, causing the error
3856 // applies to i32, overrides the previous two impls
3857 impl SpaceLlama for i32 {
3858 fn fly(&self) {} // And now that's ok!
3864 The number of elements in an array or slice pattern differed from the number of
3865 elements in the array being matched.
3867 Example of erroneous code:
3869 ```compile_fail,E0527
3870 #![feature(slice_patterns)]
3872 let r = &[1, 2, 3, 4];
3874 &[a, b] => { // error: pattern requires 2 elements but array
3876 println!("a={}, b={}", a, b);
3881 Ensure that the pattern is consistent with the size of the matched
3882 array. Additional elements can be matched with `..`:
3885 #![feature(slice_patterns)]
3887 let r = &[1, 2, 3, 4];
3889 &[a, b, ..] => { // ok!
3890 println!("a={}, b={}", a, b);
3897 An array or slice pattern required more elements than were present in the
3900 Example of erroneous code:
3902 ```compile_fail,E0528
3903 #![feature(slice_patterns)]
3907 &[a, b, c, rest..] => { // error: pattern requires at least 3
3908 // elements but array has 2
3909 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3914 Ensure that the matched array has at least as many elements as the pattern
3915 requires. You can match an arbitrary number of remaining elements with `..`:
3918 #![feature(slice_patterns)]
3920 let r = &[1, 2, 3, 4, 5];
3922 &[a, b, c, rest..] => { // ok!
3923 // prints `a=1, b=2, c=3 rest=[4, 5]`
3924 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3931 An array or slice pattern was matched against some other type.
3933 Example of erroneous code:
3935 ```compile_fail,E0529
3936 #![feature(slice_patterns)]
3940 [a, b] => { // error: expected an array or slice, found `f32`
3941 println!("a={}, b={}", a, b);
3946 Ensure that the pattern and the expression being matched on are of consistent
3950 #![feature(slice_patterns)]
3955 println!("a={}, b={}", a, b);
3962 An unknown field was specified into an enum's structure variant.
3964 Erroneous code example:
3966 ```compile_fail,E0559
3971 let s = Field::Fool { joke: 0 };
3972 // error: struct variant `Field::Fool` has no field named `joke`
3975 Verify you didn't misspell the field's name or that the field exists. Example:
3982 let s = Field::Fool { joke: 0 }; // ok!
3987 An unknown field was specified into a structure.
3989 Erroneous code example:
3991 ```compile_fail,E0560
3996 let s = Simba { mother: 1, father: 0 };
3997 // error: structure `Simba` has no field named `father`
4000 Verify you didn't misspell the field's name or that the field exists. Example:
4008 let s = Simba { mother: 1, father: 0 }; // ok!
4013 The requested ABI is unsupported by the current target.
4015 The rust compiler maintains for each target a blacklist of ABIs unsupported on
4016 that target. If an ABI is present in such a list this usually means that the
4017 target / ABI combination is currently unsupported by llvm.
4019 If necessary, you can circumvent this check using custom target specifications.
4023 A return statement was found outside of a function body.
4025 Erroneous code example:
4027 ```compile_fail,E0572
4028 const FOO: u32 = return 0; // error: return statement outside of function body
4033 To fix this issue, just remove the return keyword or move the expression into a
4039 fn some_fn() -> u32 {
4050 In a `fn` type, a lifetime appears only in the return type,
4051 and not in the arguments types.
4053 Erroneous code example:
4055 ```compile_fail,E0581
4057 // Here, `'a` appears only in the return type:
4058 let x: for<'a> fn() -> &'a i32;
4062 To fix this issue, either use the lifetime in the arguments, or use
4067 // Here, `'a` appears only in the return type:
4068 let x: for<'a> fn(&'a i32) -> &'a i32;
4069 let y: fn() -> &'static i32;
4073 Note: The examples above used to be (erroneously) accepted by the
4074 compiler, but this was since corrected. See [issue #33685] for more
4077 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
4081 A lifetime appears only in an associated-type binding,
4082 and not in the input types to the trait.
4084 Erroneous code example:
4086 ```compile_fail,E0582
4088 // No type can satisfy this requirement, since `'a` does not
4089 // appear in any of the input types (here, `i32`):
4090 where F: for<'a> Fn(i32) -> Option<&'a i32>
4097 To fix this issue, either use the lifetime in the inputs, or use
4101 fn bar<F, G>(t: F, u: G)
4102 where F: for<'a> Fn(&'a i32) -> Option<&'a i32>,
4103 G: Fn(i32) -> Option<&'static i32>,
4110 Note: The examples above used to be (erroneously) accepted by the
4111 compiler, but this was since corrected. See [issue #33685] for more
4114 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
4119 register_diagnostics! {
4124 E0103, // @GuillaumeGomez: I was unable to get this error, try your best!
4130 // E0159, // use of trait `{}` as struct constructor
4131 // E0163, // merged into E0071
4134 // E0172, // non-trait found in a type sum, moved to resolve
4135 // E0173, // manual implementations of unboxed closure traits are experimental
4138 // E0187, // can't infer the kind of the closure
4139 // E0188, // can not cast an immutable reference to a mutable pointer
4140 // E0189, // deprecated: can only cast a boxed pointer to a boxed object
4141 // E0190, // deprecated: can only cast a &-pointer to an &-object
4142 E0196, // cannot determine a type for this closure
4143 E0203, // type parameter has more than one relaxed default bound,
4144 // and only one is supported
4146 // E0209, // builtin traits can only be implemented on structs or enums
4147 E0212, // cannot extract an associated type from a higher-ranked trait bound
4148 // E0213, // associated types are not accepted in this context
4149 // E0215, // angle-bracket notation is not stable with `Fn`
4150 // E0216, // parenthetical notation is only stable with `Fn`
4151 // E0217, // ambiguous associated type, defined in multiple supertraits
4152 // E0218, // no associated type defined
4153 // E0219, // associated type defined in higher-ranked supertrait
4154 // E0222, // Error code E0045 (variadic function must have C calling
4155 // convention) duplicate
4156 E0224, // at least one non-builtin train is required for an object type
4157 E0227, // ambiguous lifetime bound, explicit lifetime bound required
4158 E0228, // explicit lifetime bound required
4159 E0231, // only named substitution parameters are allowed
4162 // E0235, // structure constructor specifies a structure of type but
4163 // E0236, // no lang item for range syntax
4164 // E0237, // no lang item for range syntax
4165 // E0238, // parenthesized parameters may only be used with a trait
4166 // E0239, // `next` method of `Iterator` trait has unexpected type
4170 E0245, // not a trait
4171 // E0246, // invalid recursive type
4173 // E0248, // value used as a type, now reported earlier during resolution as E0412
4175 // E0319, // trait impls for defaulted traits allowed just for structs/enums
4176 E0320, // recursive overflow during dropck
4177 // E0372, // coherence not object safe
4178 E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
4179 // between structures with the same definition
4180 E0436, // functional record update requires a struct
4181 E0521, // redundant default implementations of trait
4182 E0533, // `{}` does not name a unit variant, unit struct or a constant
4183 E0562, // `impl Trait` not allowed outside of function
4184 // and inherent method return types
4185 E0563, // cannot determine a type for this `impl Trait`: {}
4186 E0564, // only named lifetimes are allowed in `impl Trait`,
4187 // but `{}` was found in the type `{}`
4188 E0567, // auto traits can not have type parameters
4189 E0568, // auto-traits can not have predicates,