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 You hit this error because the compiler lacks the information to
1356 determine a type for this expression. Erroneous code example:
1358 ```compile_fail,E0101
1359 let x = |_| {}; // error: cannot determine a type for this expression
1362 You have two possibilities to solve this situation:
1364 * Give an explicit definition of the expression
1365 * Infer the expression
1370 let x = |_ : u32| {}; // ok!
1378 You hit this error because the compiler lacks the information to
1379 determine the type of this variable. Erroneous code example:
1381 ```compile_fail,E0282
1382 // could be an array of anything
1383 let x = []; // error: cannot determine a type for this local variable
1386 To solve this situation, constrain the type of the variable.
1390 #![allow(unused_variables)]
1393 let x: [u8; 0] = [];
1399 This error means that an incorrect number of lifetime parameters were provided
1400 for a type (like a struct or enum) or trait:
1402 ```compile_fail,E0107
1403 struct Foo<'a, 'b>(&'a str, &'b str);
1404 enum Bar { A, B, C }
1407 foo: Foo<'a>, // error: expected 2, found 1
1408 bar: Bar<'a>, // error: expected 0, found 1
1414 You tried to give a type parameter to a type which doesn't need it. Erroneous
1417 ```compile_fail,E0109
1418 type X = u32<i32>; // error: type parameters are not allowed on this type
1421 Please check that you used the correct type and recheck its definition. Perhaps
1422 it doesn't need the type parameter.
1427 type X = u32; // this compiles
1430 Note that type parameters for enum-variant constructors go after the variant,
1431 not after the enum (`Option::None::<u32>`, not `Option::<u32>::None`).
1435 You tried to give a lifetime parameter to a type which doesn't need it.
1436 Erroneous code example:
1438 ```compile_fail,E0110
1439 type X = u32<'static>; // error: lifetime parameters are not allowed on
1443 Please check that the correct type was used and recheck its definition; perhaps
1444 it doesn't need the lifetime parameter. Example:
1447 type X = u32; // ok!
1452 You can only define an inherent implementation for a type in the same crate
1453 where the type was defined. For example, an `impl` block as below is not allowed
1454 since `Vec` is defined in the standard library:
1456 ```compile_fail,E0116
1457 impl Vec<u8> { } // error
1460 To fix this problem, you can do either of these things:
1462 - define a trait that has the desired associated functions/types/constants and
1463 implement the trait for the type in question
1464 - define a new type wrapping the type and define an implementation on the new
1467 Note that using the `type` keyword does not work here because `type` only
1468 introduces a type alias:
1470 ```compile_fail,E0116
1471 type Bytes = Vec<u8>;
1473 impl Bytes { } // error, same as above
1478 This error indicates a violation of one of Rust's orphan rules for trait
1479 implementations. The rule prohibits any implementation of a foreign trait (a
1480 trait defined in another crate) where
1482 - the type that is implementing the trait is foreign
1483 - all of the parameters being passed to the trait (if there are any) are also
1486 Here's one example of this error:
1488 ```compile_fail,E0117
1489 impl Drop for u32 {}
1492 To avoid this kind of error, ensure that at least one local type is referenced
1496 pub struct Foo; // you define your type in your crate
1498 impl Drop for Foo { // and you can implement the trait on it!
1499 // code of trait implementation here
1502 impl From<Foo> for i32 { // or you use a type from your crate as
1504 fn from(i: Foo) -> i32 {
1510 Alternatively, define a trait locally and implement that instead:
1514 fn get(&self) -> usize;
1518 fn get(&self) -> usize { 0 }
1522 For information on the design of the orphan rules, see [RFC 1023].
1524 [RFC 1023]: https://github.com/rust-lang/rfcs/blob/master/text/1023-rebalancing-coherence.md
1528 You're trying to write an inherent implementation for something which isn't a
1529 struct nor an enum. Erroneous code example:
1531 ```compile_fail,E0118
1532 impl (u8, u8) { // error: no base type found for inherent implementation
1533 fn get_state(&self) -> String {
1539 To fix this error, please implement a trait on the type or wrap it in a struct.
1543 // we create a trait here
1544 trait LiveLongAndProsper {
1545 fn get_state(&self) -> String;
1548 // and now you can implement it on (u8, u8)
1549 impl LiveLongAndProsper for (u8, u8) {
1550 fn get_state(&self) -> String {
1551 "He's dead, Jim!".to_owned()
1556 Alternatively, you can create a newtype. A newtype is a wrapping tuple-struct.
1557 For example, `NewType` is a newtype over `Foo` in `struct NewType(Foo)`.
1561 struct TypeWrapper((u8, u8));
1564 fn get_state(&self) -> String {
1565 "Fascinating!".to_owned()
1572 There are conflicting trait implementations for the same type.
1573 Example of erroneous code:
1575 ```compile_fail,E0119
1577 fn get(&self) -> usize;
1580 impl<T> MyTrait for T {
1581 fn get(&self) -> usize { 0 }
1588 impl MyTrait for Foo { // error: conflicting implementations of trait
1589 // `MyTrait` for type `Foo`
1590 fn get(&self) -> usize { self.value }
1594 When looking for the implementation for the trait, the compiler finds
1595 both the `impl<T> MyTrait for T` where T is all types and the `impl
1596 MyTrait for Foo`. Since a trait cannot be implemented multiple times,
1597 this is an error. So, when you write:
1601 fn get(&self) -> usize;
1604 impl<T> MyTrait for T {
1605 fn get(&self) -> usize { 0 }
1609 This makes the trait implemented on all types in the scope. So if you
1610 try to implement it on another one after that, the implementations will
1615 fn get(&self) -> usize;
1618 impl<T> MyTrait for T {
1619 fn get(&self) -> usize { 0 }
1627 f.get(); // the trait is implemented so we can use it
1633 An attempt was made to implement Drop on a trait, which is not allowed: only
1634 structs and enums can implement Drop. An example causing this error:
1636 ```compile_fail,E0120
1639 impl Drop for MyTrait {
1640 fn drop(&mut self) {}
1644 A workaround for this problem is to wrap the trait up in a struct, and implement
1645 Drop on that. An example is shown below:
1649 struct MyWrapper<T: MyTrait> { foo: T }
1651 impl <T: MyTrait> Drop for MyWrapper<T> {
1652 fn drop(&mut self) {}
1657 Alternatively, wrapping trait objects requires something like the following:
1662 //or Box<MyTrait>, if you wanted an owned trait object
1663 struct MyWrapper<'a> { foo: &'a MyTrait }
1665 impl <'a> Drop for MyWrapper<'a> {
1666 fn drop(&mut self) {}
1672 In order to be consistent with Rust's lack of global type inference, type
1673 placeholders are disallowed by design in item signatures.
1675 Examples of this error include:
1677 ```compile_fail,E0121
1678 fn foo() -> _ { 5 } // error, explicitly write out the return type instead
1680 static BAR: _ = "test"; // error, explicitly write out the type instead
1685 An attempt was made to add a generic constraint to a type alias. While Rust will
1686 allow this with a warning, it will not currently enforce the constraint.
1687 Consider the example below:
1692 type MyType<R: Foo> = (R, ());
1699 We're able to declare a variable of type `MyType<u32>`, despite the fact that
1700 `u32` does not implement `Foo`. As a result, one should avoid using generic
1701 constraints in concert with type aliases.
1705 You declared two fields of a struct with the same name. Erroneous code
1708 ```compile_fail,E0124
1711 field1: i32, // error: field is already declared
1715 Please verify that the field names have been correctly spelled. Example:
1726 It is not possible to define `main` with type parameters, or even with function
1727 parameters. When `main` is present, it must take no arguments and return `()`.
1728 Erroneous code example:
1730 ```compile_fail,E0131
1731 fn main<T>() { // error: main function is not allowed to have type parameters
1737 A function with the `start` attribute was declared with type parameters.
1739 Erroneous code example:
1741 ```compile_fail,E0132
1748 It is not possible to declare type parameters on a function that has the `start`
1749 attribute. Such a function must have the following type signature (for more
1750 information: http://doc.rust-lang.org/stable/book/no-stdlib.html):
1753 fn(isize, *const *const u8) -> isize;
1762 fn my_start(argc: isize, argv: *const *const u8) -> isize {
1769 This error means that an attempt was made to match a struct type enum
1770 variant as a non-struct type:
1772 ```compile_fail,E0164
1773 enum Foo { B { i: u32 } }
1775 fn bar(foo: Foo) -> u32 {
1777 Foo::B(i) => i, // error E0164
1782 Try using `{}` instead:
1785 enum Foo { B { i: u32 } }
1787 fn bar(foo: Foo) -> u32 {
1796 You bound an associated type in an expression path which is not
1799 Erroneous code example:
1801 ```compile_fail,E0182
1807 impl Foo for isize {
1809 fn bar() -> isize { 42 }
1812 // error: unexpected binding of associated item in expression path
1813 let x: isize = Foo::<A=usize>::bar();
1816 To give a concrete type when using the Universal Function Call Syntax,
1817 use "Type as Trait". Example:
1825 impl Foo for isize {
1827 fn bar() -> isize { 42 }
1830 let x: isize = <isize as Foo>::bar(); // ok!
1835 Explicitly implementing both Drop and Copy for a type is currently disallowed.
1836 This feature can make some sense in theory, but the current implementation is
1837 incorrect and can lead to memory unsafety (see [issue #20126][iss20126]), so
1838 it has been disabled for now.
1840 [iss20126]: https://github.com/rust-lang/rust/issues/20126
1844 An associated function for a trait was defined to be static, but an
1845 implementation of the trait declared the same function to be a method (i.e. to
1846 take a `self` parameter).
1848 Here's an example of this error:
1850 ```compile_fail,E0185
1858 // error, method `foo` has a `&self` declaration in the impl, but not in
1866 An associated function for a trait was defined to be a method (i.e. to take a
1867 `self` parameter), but an implementation of the trait declared the same function
1870 Here's an example of this error:
1872 ```compile_fail,E0186
1880 // error, method `foo` has a `&self` declaration in the trait, but not in
1888 Trait objects need to have all associated types specified. Erroneous code
1891 ```compile_fail,E0191
1896 type Foo = Trait; // error: the value of the associated type `Bar` (from
1897 // the trait `Trait`) must be specified
1900 Please verify you specified all associated types of the trait and that you
1901 used the right trait. Example:
1908 type Foo = Trait<Bar=i32>; // ok!
1913 Negative impls are only allowed for traits with default impls. For more
1914 information see the [opt-in builtin traits RFC][RFC 19].
1916 [RFC 19]: https://github.com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md
1920 `where` clauses must use generic type parameters: it does not make sense to use
1921 them otherwise. An example causing this error:
1928 #[derive(Copy,Clone)]
1933 impl Foo for Wrapper<u32> where Wrapper<u32>: Clone {
1938 This use of a `where` clause is strange - a more common usage would look
1939 something like the following:
1946 #[derive(Copy,Clone)]
1950 impl <T> Foo for Wrapper<T> where Wrapper<T>: Clone {
1955 Here, we're saying that the implementation exists on Wrapper only when the
1956 wrapped type `T` implements `Clone`. The `where` clause is important because
1957 some types will not implement `Clone`, and thus will not get this method.
1959 In our erroneous example, however, we're referencing a single concrete type.
1960 Since we know for certain that `Wrapper<u32>` implements `Clone`, there's no
1961 reason to also specify it in a `where` clause.
1965 A type parameter was declared which shadows an existing one. An example of this
1968 ```compile_fail,E0194
1970 fn do_something(&self) -> T;
1971 fn do_something_else<T: Clone>(&self, bar: T);
1975 In this example, the trait `Foo` and the trait method `do_something_else` both
1976 define a type parameter `T`. This is not allowed: if the method wishes to
1977 define a type parameter, it must use a different name for it.
1981 Your method's lifetime parameters do not match the trait declaration.
1982 Erroneous code example:
1984 ```compile_fail,E0195
1986 fn bar<'a,'b:'a>(x: &'a str, y: &'b str);
1991 impl Trait for Foo {
1992 fn bar<'a,'b>(x: &'a str, y: &'b str) {
1993 // error: lifetime parameters or bounds on method `bar`
1994 // do not match the trait declaration
1999 The lifetime constraint `'b` for bar() implementation does not match the
2000 trait declaration. Ensure lifetime declarations match exactly in both trait
2001 declaration and implementation. Example:
2005 fn t<'a,'b:'a>(x: &'a str, y: &'b str);
2010 impl Trait for Foo {
2011 fn t<'a,'b:'a>(x: &'a str, y: &'b str) { // ok!
2018 Inherent implementations (one that do not implement a trait but provide
2019 methods associated with a type) are always safe because they are not
2020 implementing an unsafe trait. Removing the `unsafe` keyword from the inherent
2021 implementation will resolve this error.
2023 ```compile_fail,E0197
2026 // this will cause this error
2028 // converting it to this will fix it
2034 A negative implementation is one that excludes a type from implementing a
2035 particular trait. Not being able to use a trait is always a safe operation,
2036 so negative implementations are always safe and never need to be marked as
2040 #![feature(optin_builtin_traits)]
2044 // unsafe is unnecessary
2045 unsafe impl !Clone for Foo { }
2051 #![feature(optin_builtin_traits)]
2057 impl Enterprise for .. { }
2059 impl !Enterprise for Foo { }
2062 Please note that negative impls are only allowed for traits with default impls.
2066 Safe traits should not have unsafe implementations, therefore marking an
2067 implementation for a safe trait unsafe will cause a compiler error. Removing
2068 the unsafe marker on the trait noted in the error will resolve this problem.
2070 ```compile_fail,E0199
2075 // this won't compile because Bar is safe
2076 unsafe impl Bar for Foo { }
2077 // this will compile
2078 impl Bar for Foo { }
2083 Unsafe traits must have unsafe implementations. This error occurs when an
2084 implementation for an unsafe trait isn't marked as unsafe. This may be resolved
2085 by marking the unsafe implementation as unsafe.
2087 ```compile_fail,E0200
2090 unsafe trait Bar { }
2092 // this won't compile because Bar is unsafe and impl isn't unsafe
2093 impl Bar for Foo { }
2094 // this will compile
2095 unsafe impl Bar for Foo { }
2100 It is an error to define two associated items (like methods, associated types,
2101 associated functions, etc.) with the same identifier.
2105 ```compile_fail,E0201
2109 fn bar(&self) -> bool { self.0 > 5 }
2110 fn bar() {} // error: duplicate associated function
2115 fn baz(&self) -> bool;
2121 fn baz(&self) -> bool { true }
2123 // error: duplicate method
2124 fn baz(&self) -> bool { self.0 > 5 }
2126 // error: duplicate associated type
2131 Note, however, that items with the same name are allowed for inherent `impl`
2132 blocks that don't overlap:
2138 fn bar(&self) -> bool { self.0 > 5 }
2142 fn bar(&self) -> bool { self.0 }
2148 Inherent associated types were part of [RFC 195] but are not yet implemented.
2149 See [the tracking issue][iss8995] for the status of this implementation.
2151 [RFC 195]: https://github.com/rust-lang/rfcs/blob/master/text/0195-associated-items.md
2152 [iss8995]: https://github.com/rust-lang/rust/issues/8995
2156 An attempt to implement the `Copy` trait for a struct failed because one of the
2157 fields does not implement `Copy`. To fix this, you must implement `Copy` for the
2158 mentioned field. Note that this may not be possible, as in the example of
2160 ```compile_fail,E0204
2165 impl Copy for Foo { }
2168 This fails because `Vec<T>` does not implement `Copy` for any `T`.
2170 Here's another example that will fail:
2172 ```compile_fail,E0204
2179 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
2180 differs from the behavior for `&T`, which is always `Copy`).
2185 An attempt to implement the `Copy` trait for an enum failed because one of the
2186 variants does not implement `Copy`. To fix this, you must implement `Copy` for
2187 the mentioned variant. Note that this may not be possible, as in the example of
2189 ```compile_fail,E0205
2195 impl Copy for Foo { }
2198 This fails because `Vec<T>` does not implement `Copy` for any `T`.
2200 Here's another example that will fail:
2202 ```compile_fail,E0205
2210 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
2211 differs from the behavior for `&T`, which is always `Copy`).
2216 You can only implement `Copy` for a struct or enum. Both of the following
2217 examples will fail, because neither `i32` (primitive type) nor `&'static Bar`
2218 (reference to `Bar`) is a struct or enum:
2220 ```compile_fail,E0206
2222 impl Copy for Foo { } // error
2224 #[derive(Copy, Clone)]
2226 impl Copy for &'static Bar { } // error
2231 Any type parameter or lifetime parameter of an `impl` must meet at least one of
2232 the following criteria:
2234 - it appears in the self type of the impl
2235 - for a trait impl, it appears in the trait reference
2236 - it is bound as an associated type
2240 Suppose we have a struct `Foo` and we would like to define some methods for it.
2241 The following definition leads to a compiler error:
2243 ```compile_fail,E0207
2246 impl<T: Default> Foo {
2247 // error: the type parameter `T` is not constrained by the impl trait, self
2248 // type, or predicates [E0207]
2249 fn get(&self) -> T {
2250 <T as Default>::default()
2255 The problem is that the parameter `T` does not appear in the self type (`Foo`)
2256 of the impl. In this case, we can fix the error by moving the type parameter
2257 from the `impl` to the method `get`:
2263 // Move the type parameter from the impl to the method
2265 fn get<T: Default>(&self) -> T {
2266 <T as Default>::default()
2273 As another example, suppose we have a `Maker` trait and want to establish a
2274 type `FooMaker` that makes `Foo`s:
2276 ```compile_fail,E0207
2279 fn make(&mut self) -> Self::Item;
2288 impl<T: Default> Maker for FooMaker {
2289 // error: the type parameter `T` is not constrained by the impl trait, self
2290 // type, or predicates [E0207]
2293 fn make(&mut self) -> Foo<T> {
2294 Foo { foo: <T as Default>::default() }
2299 This fails to compile because `T` does not appear in the trait or in the
2302 One way to work around this is to introduce a phantom type parameter into
2303 `FooMaker`, like so:
2306 use std::marker::PhantomData;
2310 fn make(&mut self) -> Self::Item;
2317 // Add a type parameter to `FooMaker`
2318 struct FooMaker<T> {
2319 phantom: PhantomData<T>,
2322 impl<T: Default> Maker for FooMaker<T> {
2325 fn make(&mut self) -> Foo<T> {
2327 foo: <T as Default>::default(),
2333 Another way is to do away with the associated type in `Maker` and use an input
2334 type parameter instead:
2337 // Use a type parameter instead of an associated type here
2339 fn make(&mut self) -> Item;
2348 impl<T: Default> Maker<Foo<T>> for FooMaker {
2349 fn make(&mut self) -> Foo<T> {
2350 Foo { foo: <T as Default>::default() }
2355 ### Additional information
2357 For more information, please see [RFC 447].
2359 [RFC 447]: https://github.com/rust-lang/rfcs/blob/master/text/0447-no-unused-impl-parameters.md
2363 This error indicates a violation of one of Rust's orphan rules for trait
2364 implementations. The rule concerns the use of type parameters in an
2365 implementation of a foreign trait (a trait defined in another crate), and
2366 states that type parameters must be "covered" by a local type. To understand
2367 what this means, it is perhaps easiest to consider a few examples.
2369 If `ForeignTrait` is a trait defined in some external crate `foo`, then the
2370 following trait `impl` is an error:
2372 ```compile_fail,E0210
2373 extern crate collections;
2374 use collections::range::RangeArgument;
2376 impl<T> RangeArgument<T> for T { } // error
2381 To work around this, it can be covered with a local type, `MyType`:
2384 struct MyType<T>(T);
2385 impl<T> ForeignTrait for MyType<T> { } // Ok
2388 Please note that a type alias is not sufficient.
2390 For another example of an error, suppose there's another trait defined in `foo`
2391 named `ForeignTrait2` that takes two type parameters. Then this `impl` results
2392 in the same rule violation:
2396 impl<T> ForeignTrait2<T, MyType<T>> for MyType2 { } // error
2399 The reason for this is that there are two appearances of type parameter `T` in
2400 the `impl` header, both as parameters for `ForeignTrait2`. The first appearance
2401 is uncovered, and so runs afoul of the orphan rule.
2403 Consider one more example:
2406 impl<T> ForeignTrait2<MyType<T>, T> for MyType2 { } // Ok
2409 This only differs from the previous `impl` in that the parameters `T` and
2410 `MyType<T>` for `ForeignTrait2` have been swapped. This example does *not*
2411 violate the orphan rule; it is permitted.
2413 To see why that last example was allowed, you need to understand the general
2414 rule. Unfortunately this rule is a bit tricky to state. Consider an `impl`:
2417 impl<P1, ..., Pm> ForeignTrait<T1, ..., Tn> for T0 { ... }
2420 where `P1, ..., Pm` are the type parameters of the `impl` and `T0, ..., Tn`
2421 are types. One of the types `T0, ..., Tn` must be a local type (this is another
2422 orphan rule, see the explanation for E0117). Let `i` be the smallest integer
2423 such that `Ti` is a local type. Then no type parameter can appear in any of the
2426 For information on the design of the orphan rules, see [RFC 1023].
2428 [RFC 1023]: https://github.com/rust-lang/rfcs/blob/master/text/1023-rebalancing-coherence.md
2433 You used a function or type which doesn't fit the requirements for where it was
2434 used. Erroneous code examples:
2437 #![feature(intrinsics)]
2439 extern "rust-intrinsic" {
2440 fn size_of<T>(); // error: intrinsic has wrong type
2445 fn main() -> i32 { 0 }
2446 // error: main function expects type: `fn() {main}`: expected (), found i32
2453 // error: mismatched types in range: expected u8, found i8
2463 fn x(self: Rc<Foo>) {}
2464 // error: mismatched self type: expected `Foo`: expected struct
2465 // `Foo`, found struct `alloc::rc::Rc`
2469 For the first code example, please check the function definition. Example:
2472 #![feature(intrinsics)]
2474 extern "rust-intrinsic" {
2475 fn size_of<T>() -> usize; // ok!
2479 The second case example is a bit particular : the main function must always
2480 have this definition:
2486 They never take parameters and never return types.
2488 For the third example, when you match, all patterns must have the same type
2489 as the type you're matching on. Example:
2495 0u8...3u8 => (), // ok!
2500 And finally, for the last example, only `Box<Self>`, `&Self`, `Self`,
2501 or `&mut Self` work as explicit self parameters. Example:
2507 fn x(self: Box<Foo>) {} // ok!
2514 A generic type was described using parentheses rather than angle brackets. For
2517 ```compile_fail,E0214
2519 let v: Vec(&str) = vec!["foo"];
2523 This is not currently supported: `v` should be defined as `Vec<&str>`.
2524 Parentheses are currently only used with generic types when defining parameters
2525 for `Fn`-family traits.
2529 You used an associated type which isn't defined in the trait.
2530 Erroneous code example:
2532 ```compile_fail,E0220
2537 type Foo = T1<F=i32>; // error: associated type `F` not found for `T1`
2544 // error: Baz is used but not declared
2545 fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
2549 Make sure that you have defined the associated type in the trait body.
2550 Also, verify that you used the right trait or you didn't misspell the
2551 associated type name. Example:
2558 type Foo = T1<Bar=i32>; // ok!
2564 type Baz; // we declare `Baz` in our trait.
2566 // and now we can use it here:
2567 fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
2573 An attempt was made to retrieve an associated type, but the type was ambiguous.
2576 ```compile_fail,E0221
2592 In this example, `Foo` defines an associated type `A`. `Bar` inherits that type
2593 from `Foo`, and defines another associated type of the same name. As a result,
2594 when we attempt to use `Self::A`, it's ambiguous whether we mean the `A` defined
2595 by `Foo` or the one defined by `Bar`.
2597 There are two options to work around this issue. The first is simply to rename
2598 one of the types. Alternatively, one can specify the intended type using the
2612 let _: <Self as Bar>::A;
2619 An attempt was made to retrieve an associated type, but the type was ambiguous.
2622 ```compile_fail,E0223
2623 trait MyTrait {type X; }
2626 let foo: MyTrait::X;
2630 The problem here is that we're attempting to take the type of X from MyTrait.
2631 Unfortunately, the type of X is not defined, because it's only made concrete in
2632 implementations of the trait. A working version of this code might look like:
2635 trait MyTrait {type X; }
2638 impl MyTrait for MyStruct {
2643 let foo: <MyStruct as MyTrait>::X;
2647 This syntax specifies that we want the X type from MyTrait, as made concrete in
2648 MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct
2649 might implement two different traits with identically-named associated types.
2650 This syntax allows disambiguation between the two.
2654 You attempted to use multiple types as bounds for a closure or trait object.
2655 Rust does not currently support this. A simple example that causes this error:
2657 ```compile_fail,E0225
2659 let _: Box<std::io::Read + std::io::Write>;
2663 Send and Sync are an exception to this rule: it's possible to have bounds of
2664 one non-builtin trait, plus either or both of Send and Sync. For example, the
2665 following compiles correctly:
2669 let _: Box<std::io::Read + Send + Sync>;
2675 An associated type binding was done outside of the type parameter declaration
2676 and `where` clause. Erroneous code example:
2678 ```compile_fail,E0229
2681 fn boo(&self) -> <Self as Foo>::A;
2686 impl Foo for isize {
2688 fn boo(&self) -> usize { 42 }
2691 fn baz<I>(x: &<I as Foo<A=Bar>>::A) {}
2692 // error: associated type bindings are not allowed here
2695 To solve this error, please move the type bindings in the type parameter
2699 fn baz<I: Foo<A=Bar>>(x: &<I as Foo>::A) {} // ok!
2702 Or in the `where` clause:
2705 fn baz<I>(x: &<I as Foo>::A) where I: Foo<A=Bar> {}
2710 The trait has more type parameters specified than appear in its definition.
2712 Erroneous example code:
2714 ```compile_fail,E0230
2715 #![feature(on_unimplemented)]
2716 #[rustc_on_unimplemented = "Trait error on `{Self}` with `<{A},{B},{C}>`"]
2717 // error: there is no type parameter C on trait TraitWithThreeParams
2718 trait TraitWithThreeParams<A,B>
2722 Include the correct number of type parameters and the compilation should
2726 #![feature(on_unimplemented)]
2727 #[rustc_on_unimplemented = "Trait error on `{Self}` with `<{A},{B},{C}>`"]
2728 trait TraitWithThreeParams<A,B,C> // ok!
2734 The attribute must have a value. Erroneous code example:
2736 ```compile_fail,E0232
2737 #![feature(on_unimplemented)]
2739 #[rustc_on_unimplemented] // error: this attribute must have a value
2743 Please supply the missing value of the attribute. Example:
2746 #![feature(on_unimplemented)]
2748 #[rustc_on_unimplemented = "foo"] // ok!
2754 This error indicates that not enough type parameters were found in a type or
2757 For example, the `Foo` struct below is defined to be generic in `T`, but the
2758 type parameter is missing in the definition of `Bar`:
2760 ```compile_fail,E0243
2761 struct Foo<T> { x: T }
2763 struct Bar { x: Foo }
2768 This error indicates that too many type parameters were found in a type or
2771 For example, the `Foo` struct below has no type parameters, but is supplied
2772 with two in the definition of `Bar`:
2774 ```compile_fail,E0244
2775 struct Foo { x: bool }
2777 struct Bar<S, T> { x: Foo<S, T> }
2782 If an impl has a generic parameter with the `#[may_dangle]` attribute, then
2783 that impl must be declared as an `unsafe impl. For example:
2785 ```compile_fail,E0569
2786 #![feature(generic_param_attrs)]
2787 #![feature(dropck_eyepatch)]
2790 impl<#[may_dangle] X> Drop for Foo<X> {
2791 fn drop(&mut self) { }
2795 In this example, we are asserting that the destructor for `Foo` will not
2796 access any data of type `X`, and require this assertion to be true for
2797 overall safety in our program. The compiler does not currently attempt to
2798 verify this assertion; therefore we must tag this `impl` as unsafe.
2802 Default impls for a trait must be located in the same crate where the trait was
2803 defined. For more information see the [opt-in builtin traits RFC][RFC 19].
2805 [RFC 19]: https://github.com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md
2809 A cross-crate opt-out trait was implemented on something which wasn't a struct
2810 or enum type. Erroneous code example:
2812 ```compile_fail,E0321
2813 #![feature(optin_builtin_traits)]
2817 impl !Sync for Foo {}
2819 unsafe impl Send for &'static Foo {}
2820 // error: cross-crate traits with a default impl, like `core::marker::Send`,
2821 // can only be implemented for a struct/enum type, not
2825 Only structs and enums are permitted to impl Send, Sync, and other opt-out
2826 trait, and the struct or enum must be local to the current crate. So, for
2827 example, `unsafe impl Send for Rc<Foo>` is not allowed.
2831 The `Sized` trait is a special trait built-in to the compiler for types with a
2832 constant size known at compile-time. This trait is automatically implemented
2833 for types as needed by the compiler, and it is currently disallowed to
2834 explicitly implement it for a type.
2838 An associated const was implemented when another trait item was expected.
2839 Erroneous code example:
2841 ```compile_fail,E0323
2842 #![feature(associated_consts)]
2852 // error: item `N` is an associated const, which doesn't match its
2853 // trait `<Bar as Foo>`
2857 Please verify that the associated const wasn't misspelled and the correct trait
2858 was implemented. Example:
2868 type N = u32; // ok!
2875 #![feature(associated_consts)]
2884 const N : u32 = 0; // ok!
2890 A method was implemented when another trait item was expected. Erroneous
2893 ```compile_fail,E0324
2894 #![feature(associated_consts)]
2906 // error: item `N` is an associated method, which doesn't match its
2907 // trait `<Bar as Foo>`
2911 To fix this error, please verify that the method name wasn't misspelled and
2912 verify that you are indeed implementing the correct trait items. Example:
2915 #![feature(associated_consts)]
2934 An associated type was implemented when another trait item was expected.
2935 Erroneous code example:
2937 ```compile_fail,E0325
2938 #![feature(associated_consts)]
2948 // error: item `N` is an associated type, which doesn't match its
2949 // trait `<Bar as Foo>`
2953 Please verify that the associated type name wasn't misspelled and your
2954 implementation corresponds to the trait definition. Example:
2964 type N = u32; // ok!
2971 #![feature(associated_consts)]
2980 const N : u32 = 0; // ok!
2986 The types of any associated constants in a trait implementation must match the
2987 types in the trait definition. This error indicates that there was a mismatch.
2989 Here's an example of this error:
2991 ```compile_fail,E0326
2992 #![feature(associated_consts)]
3001 const BAR: u32 = 5; // error, expected bool, found u32
3007 The Unsize trait should not be implemented directly. All implementations of
3008 Unsize are provided automatically by the compiler.
3010 Erroneous code example:
3012 ```compile_fail,E0328
3015 use std::marker::Unsize;
3019 impl<T> Unsize<T> for MyType {}
3022 If you are defining your own smart pointer type and would like to enable
3023 conversion from a sized to an unsized type with the
3024 [DST coercion system][RFC 982], use [`CoerceUnsized`] instead.
3027 #![feature(coerce_unsized)]
3029 use std::ops::CoerceUnsized;
3031 pub struct MyType<T: ?Sized> {
3032 field_with_unsized_type: T,
3035 impl<T, U> CoerceUnsized<MyType<U>> for MyType<T>
3036 where T: CoerceUnsized<U> {}
3039 [RFC 982]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md
3040 [`CoerceUnsized`]: https://doc.rust-lang.org/std/ops/trait.CoerceUnsized.html
3044 An attempt was made to access an associated constant through either a generic
3045 type parameter or `Self`. This is not supported yet. An example causing this
3046 error is shown below:
3049 #![feature(associated_consts)]
3057 impl Foo for MyStruct {
3058 const BAR: f64 = 0f64;
3061 fn get_bar_bad<F: Foo>(t: F) -> f64 {
3066 Currently, the value of `BAR` for a particular type can only be accessed
3067 through a concrete type, as shown below:
3070 #![feature(associated_consts)]
3078 fn get_bar_good() -> f64 {
3079 <MyStruct as Foo>::BAR
3085 An attempt was made to implement `Drop` on a concrete specialization of a
3086 generic type. An example is shown below:
3088 ```compile_fail,E0366
3093 impl Drop for Foo<u32> {
3094 fn drop(&mut self) {}
3098 This code is not legal: it is not possible to specialize `Drop` to a subset of
3099 implementations of a generic type. One workaround for this is to wrap the
3100 generic type, as shown below:
3112 fn drop(&mut self) {}
3118 An attempt was made to implement `Drop` on a specialization of a generic type.
3119 An example is shown below:
3121 ```compile_fail,E0367
3124 struct MyStruct<T> {
3128 impl<T: Foo> Drop for MyStruct<T> {
3129 fn drop(&mut self) {}
3133 This code is not legal: it is not possible to specialize `Drop` to a subset of
3134 implementations of a generic type. In order for this code to work, `MyStruct`
3135 must also require that `T` implements `Foo`. Alternatively, another option is
3136 to wrap the generic type in another that specializes appropriately:
3141 struct MyStruct<T> {
3145 struct MyStructWrapper<T: Foo> {
3149 impl <T: Foo> Drop for MyStructWrapper<T> {
3150 fn drop(&mut self) {}
3156 This error indicates that a binary assignment operator like `+=` or `^=` was
3157 applied to a type that doesn't support it. For example:
3159 ```compile_fail,E0368
3160 let mut x = 12f32; // error: binary operation `<<` cannot be applied to
3166 To fix this error, please check that this type implements this binary
3170 let mut x = 12u32; // the `u32` type does implement the `ShlAssign` trait
3175 It is also possible to overload most operators for your own type by
3176 implementing the `[OP]Assign` traits from `std::ops`.
3178 Another problem you might be facing is this: suppose you've overloaded the `+`
3179 operator for some type `Foo` by implementing the `std::ops::Add` trait for
3180 `Foo`, but you find that using `+=` does not work, as in this example:
3182 ```compile_fail,E0368
3190 fn add(self, rhs: Foo) -> Foo {
3196 let mut x: Foo = Foo(5);
3197 x += Foo(7); // error, `+= cannot be applied to the type `Foo`
3201 This is because `AddAssign` is not automatically implemented, so you need to
3202 manually implement it for your type.
3206 A binary operation was attempted on a type which doesn't support it.
3207 Erroneous code example:
3209 ```compile_fail,E0369
3210 let x = 12f32; // error: binary operation `<<` cannot be applied to
3216 To fix this error, please check that this type implements this binary
3220 let x = 12u32; // the `u32` type does implement it:
3221 // https://doc.rust-lang.org/stable/std/ops/trait.Shl.html
3226 It is also possible to overload most operators for your own type by
3227 implementing traits from `std::ops`.
3231 The maximum value of an enum was reached, so it cannot be automatically
3232 set in the next enum value. Erroneous code example:
3235 #[deny(overflowing_literals)]
3237 X = 0x7fffffffffffffff,
3238 Y, // error: enum discriminant overflowed on value after
3239 // 9223372036854775807: i64; set explicitly via
3240 // Y = -9223372036854775808 if that is desired outcome
3244 To fix this, please set manually the next enum value or put the enum variant
3245 with the maximum value at the end of the enum. Examples:
3249 X = 0x7fffffffffffffff,
3259 X = 0x7fffffffffffffff,
3265 When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
3266 definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
3267 `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
3268 definition, so it is not useful to do this.
3272 ```compile_fail,E0371
3273 trait Foo { fn foo(&self) { } }
3277 impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
3278 impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
3279 impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
3280 impl Baz for Bar { } // Note: This is OK
3285 A struct without a field containing an unsized type cannot implement
3287 [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3288 is any type that the compiler doesn't know the length or alignment of at
3289 compile time. Any struct containing an unsized type is also unsized.
3291 Example of erroneous code:
3293 ```compile_fail,E0374
3294 #![feature(coerce_unsized)]
3295 use std::ops::CoerceUnsized;
3297 struct Foo<T: ?Sized> {
3301 // error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
3302 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
3303 where T: CoerceUnsized<U> {}
3306 `CoerceUnsized` is used to coerce one struct containing an unsized type
3307 into another struct containing a different unsized type. If the struct
3308 doesn't have any fields of unsized types then you don't need explicit
3309 coercion to get the types you want. To fix this you can either
3310 not try to implement `CoerceUnsized` or you can add a field that is
3311 unsized to the struct.
3316 #![feature(coerce_unsized)]
3317 use std::ops::CoerceUnsized;
3319 // We don't need to impl `CoerceUnsized` here.
3324 // We add the unsized type field to the struct.
3325 struct Bar<T: ?Sized> {
3330 // The struct has an unsized field so we can implement
3331 // `CoerceUnsized` for it.
3332 impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
3333 where T: CoerceUnsized<U> {}
3336 Note that `CoerceUnsized` is mainly used by smart pointers like `Box`, `Rc`
3337 and `Arc` to be able to mark that they can coerce unsized types that they
3342 A struct with more than one field containing an unsized type cannot implement
3343 `CoerceUnsized`. This only occurs when you are trying to coerce one of the
3344 types in your struct to another type in the struct. In this case we try to
3345 impl `CoerceUnsized` from `T` to `U` which are both types that the struct
3346 takes. An [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3347 is any type that the compiler doesn't know the length or alignment of at
3348 compile time. Any struct containing an unsized type is also unsized.
3350 Example of erroneous code:
3352 ```compile_fail,E0375
3353 #![feature(coerce_unsized)]
3354 use std::ops::CoerceUnsized;
3356 struct Foo<T: ?Sized, U: ?Sized> {
3362 // error: Struct `Foo` has more than one unsized field.
3363 impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
3366 `CoerceUnsized` only allows for coercion from a structure with a single
3367 unsized type field to another struct with a single unsized type field.
3368 In fact Rust only allows for a struct to have one unsized type in a struct
3369 and that unsized type must be the last field in the struct. So having two
3370 unsized types in a single struct is not allowed by the compiler. To fix this
3371 use only one field containing an unsized type in the struct and then use
3372 multiple structs to manage each unsized type field you need.
3377 #![feature(coerce_unsized)]
3378 use std::ops::CoerceUnsized;
3380 struct Foo<T: ?Sized> {
3385 impl <T, U> CoerceUnsized<Foo<U>> for Foo<T>
3386 where T: CoerceUnsized<U> {}
3388 fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
3389 Foo { a: 12i32, b: t } // we use coercion to get the `Foo<U>` type we need
3396 The type you are trying to impl `CoerceUnsized` for is not a struct.
3397 `CoerceUnsized` can only be implemented for a struct. Unsized types are
3398 already able to be coerced without an implementation of `CoerceUnsized`
3399 whereas a struct containing an unsized type needs to know the unsized type
3400 field it's containing is able to be coerced. An
3401 [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3402 is any type that the compiler doesn't know the length or alignment of at
3403 compile time. Any struct containing an unsized type is also unsized.
3405 Example of erroneous code:
3407 ```compile_fail,E0376
3408 #![feature(coerce_unsized)]
3409 use std::ops::CoerceUnsized;
3411 struct Foo<T: ?Sized> {
3415 // error: The type `U` is not a struct
3416 impl<T, U> CoerceUnsized<U> for Foo<T> {}
3419 The `CoerceUnsized` trait takes a struct type. Make sure the type you are
3420 providing to `CoerceUnsized` is a struct with only the last field containing an
3426 #![feature(coerce_unsized)]
3427 use std::ops::CoerceUnsized;
3433 // The `Foo<U>` is a struct so `CoerceUnsized` can be implemented
3434 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}
3437 Note that in Rust, structs can only contain an unsized type if the field
3438 containing the unsized type is the last and only unsized type field in the
3443 Default impls are only allowed for traits with no methods or associated items.
3444 For more information see the [opt-in builtin traits RFC][RFC 19].
3446 [RFC 19]: https://github.com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md
3450 You tried to implement methods for a primitive type. Erroneous code example:
3452 ```compile_fail,E0390
3458 // error: only a single inherent implementation marked with
3459 // `#[lang = "mut_ptr"]` is allowed for the `*mut T` primitive
3462 This isn't allowed, but using a trait to implement a method is a good solution.
3474 impl Bar for *mut Foo {
3481 This error indicates that a type or lifetime parameter has been declared
3482 but not actually used. Here is an example that demonstrates the error:
3484 ```compile_fail,E0392
3490 If the type parameter was included by mistake, this error can be fixed
3491 by simply removing the type parameter, as shown below:
3499 Alternatively, if the type parameter was intentionally inserted, it must be
3500 used. A simple fix is shown below:
3508 This error may also commonly be found when working with unsafe code. For
3509 example, when using raw pointers one may wish to specify the lifetime for
3510 which the pointed-at data is valid. An initial attempt (below) causes this
3513 ```compile_fail,E0392
3519 We want to express the constraint that Foo should not outlive `'a`, because
3520 the data pointed to by `T` is only valid for that lifetime. The problem is
3521 that there are no actual uses of `'a`. It's possible to work around this
3522 by adding a PhantomData type to the struct, using it to tell the compiler
3523 to act as if the struct contained a borrowed reference `&'a T`:
3526 use std::marker::PhantomData;
3528 struct Foo<'a, T: 'a> {
3530 phantom: PhantomData<&'a T>
3534 PhantomData can also be used to express information about unused type
3535 parameters. You can read more about it in the API documentation:
3537 https://doc.rust-lang.org/std/marker/struct.PhantomData.html
3541 A type parameter which references `Self` in its default value was not specified.
3542 Example of erroneous code:
3544 ```compile_fail,E0393
3547 fn together_we_will_rule_the_galaxy(son: &A) {}
3548 // error: the type parameter `T` must be explicitly specified in an
3549 // object type because its default value `Self` references the
3553 A trait object is defined over a single, fully-defined trait. With a regular
3554 default parameter, this parameter can just be substituted in. However, if the
3555 default parameter is `Self`, the trait changes for each concrete type; i.e.
3556 `i32` will be expected to implement `A<i32>`, `bool` will be expected to
3557 implement `A<bool>`, etc... These types will not share an implementation of a
3558 fully-defined trait; instead they share implementations of a trait with
3559 different parameters substituted in for each implementation. This is
3560 irreconcilable with what we need to make a trait object work, and is thus
3561 disallowed. Making the trait concrete by explicitly specifying the value of the
3562 defaulted parameter will fix this issue. Fixed example:
3567 fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!
3572 You implemented a trait, overriding one or more of its associated types but did
3573 not reimplement its default methods.
3575 Example of erroneous code:
3577 ```compile_fail,E0399
3578 #![feature(associated_type_defaults)]
3586 // error - the following trait items need to be reimplemented as
3587 // `Assoc` was overridden: `bar`
3592 To fix this, add an implementation for each default method from the trait:
3595 #![feature(associated_type_defaults)]
3604 fn bar(&self) {} // ok!
3610 The length of the platform-intrinsic function `simd_shuffle`
3611 wasn't specified. Erroneous code example:
3613 ```compile_fail,E0439
3614 #![feature(platform_intrinsics)]
3616 extern "platform-intrinsic" {
3617 fn simd_shuffle<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3618 // error: invalid `simd_shuffle`, needs length: `simd_shuffle`
3622 The `simd_shuffle` function needs the length of the array passed as
3623 last parameter in its name. Example:
3626 #![feature(platform_intrinsics)]
3628 extern "platform-intrinsic" {
3629 fn simd_shuffle8<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3635 A platform-specific intrinsic function has the wrong number of type
3636 parameters. Erroneous code example:
3638 ```compile_fail,E0440
3639 #![feature(repr_simd)]
3640 #![feature(platform_intrinsics)]
3643 struct f64x2(f64, f64);
3645 extern "platform-intrinsic" {
3646 fn x86_mm_movemask_pd<T>(x: f64x2) -> i32;
3647 // error: platform-specific intrinsic has wrong number of type
3652 Please refer to the function declaration to see if it corresponds
3653 with yours. Example:
3656 #![feature(repr_simd)]
3657 #![feature(platform_intrinsics)]
3660 struct f64x2(f64, f64);
3662 extern "platform-intrinsic" {
3663 fn x86_mm_movemask_pd(x: f64x2) -> i32;
3669 An unknown platform-specific intrinsic function was used. Erroneous
3672 ```compile_fail,E0441
3673 #![feature(repr_simd)]
3674 #![feature(platform_intrinsics)]
3677 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3679 extern "platform-intrinsic" {
3680 fn x86_mm_adds_ep16(x: i16x8, y: i16x8) -> i16x8;
3681 // error: unrecognized platform-specific intrinsic function
3685 Please verify that the function name wasn't misspelled, and ensure
3686 that it is declared in the rust source code (in the file
3687 src/librustc_platform_intrinsics/x86.rs). Example:
3690 #![feature(repr_simd)]
3691 #![feature(platform_intrinsics)]
3694 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3696 extern "platform-intrinsic" {
3697 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3703 Intrinsic argument(s) and/or return value have the wrong type.
3704 Erroneous code example:
3706 ```compile_fail,E0442
3707 #![feature(repr_simd)]
3708 #![feature(platform_intrinsics)]
3711 struct i8x16(i8, i8, i8, i8, i8, i8, i8, i8,
3712 i8, i8, i8, i8, i8, i8, i8, i8);
3714 struct i32x4(i32, i32, i32, i32);
3716 struct i64x2(i64, i64);
3718 extern "platform-intrinsic" {
3719 fn x86_mm_adds_epi16(x: i8x16, y: i32x4) -> i64x2;
3720 // error: intrinsic arguments/return value have wrong type
3724 To fix this error, please refer to the function declaration to give
3725 it the awaited types. Example:
3728 #![feature(repr_simd)]
3729 #![feature(platform_intrinsics)]
3732 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3734 extern "platform-intrinsic" {
3735 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3741 Intrinsic argument(s) and/or return value have the wrong type.
3742 Erroneous code example:
3744 ```compile_fail,E0443
3745 #![feature(repr_simd)]
3746 #![feature(platform_intrinsics)]
3749 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3751 struct i64x8(i64, i64, i64, i64, i64, i64, i64, i64);
3753 extern "platform-intrinsic" {
3754 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i64x8;
3755 // error: intrinsic argument/return value has wrong type
3759 To fix this error, please refer to the function declaration to give
3760 it the awaited types. Example:
3763 #![feature(repr_simd)]
3764 #![feature(platform_intrinsics)]
3767 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3769 extern "platform-intrinsic" {
3770 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3776 A platform-specific intrinsic function has wrong number of arguments.
3777 Erroneous code example:
3779 ```compile_fail,E0444
3780 #![feature(repr_simd)]
3781 #![feature(platform_intrinsics)]
3784 struct f64x2(f64, f64);
3786 extern "platform-intrinsic" {
3787 fn x86_mm_movemask_pd(x: f64x2, y: f64x2, z: f64x2) -> i32;
3788 // error: platform-specific intrinsic has invalid number of arguments
3792 Please refer to the function declaration to see if it corresponds
3793 with yours. Example:
3796 #![feature(repr_simd)]
3797 #![feature(platform_intrinsics)]
3800 struct f64x2(f64, f64);
3802 extern "platform-intrinsic" {
3803 fn x86_mm_movemask_pd(x: f64x2) -> i32; // ok!
3809 The `typeof` keyword is currently reserved but unimplemented.
3810 Erroneous code example:
3812 ```compile_fail,E0516
3814 let x: typeof(92) = 92;
3818 Try using type inference instead. Example:
3828 A non-default implementation was already made on this type so it cannot be
3829 specialized further. Erroneous code example:
3831 ```compile_fail,E0520
3832 #![feature(specialization)]
3839 impl<T> SpaceLlama for T {
3840 default fn fly(&self) {}
3844 // applies to all `Clone` T and overrides the previous impl
3845 impl<T: Clone> SpaceLlama for T {
3849 // since `i32` is clone, this conflicts with the previous implementation
3850 impl SpaceLlama for i32 {
3851 default fn fly(&self) {}
3852 // error: item `fly` is provided by an `impl` that specializes
3853 // another, but the item in the parent `impl` is not marked
3854 // `default` and so it cannot be specialized.
3858 Specialization only allows you to override `default` functions in
3861 To fix this error, you need to mark all the parent implementations as default.
3865 #![feature(specialization)]
3872 impl<T> SpaceLlama for T {
3873 default fn fly(&self) {} // This is a parent implementation.
3876 // applies to all `Clone` T; overrides the previous impl
3877 impl<T: Clone> SpaceLlama for T {
3878 default fn fly(&self) {} // This is a parent implementation but was
3879 // previously not a default one, causing the error
3882 // applies to i32, overrides the previous two impls
3883 impl SpaceLlama for i32 {
3884 fn fly(&self) {} // And now that's ok!
3890 The number of elements in an array or slice pattern differed from the number of
3891 elements in the array being matched.
3893 Example of erroneous code:
3895 ```compile_fail,E0527
3896 #![feature(slice_patterns)]
3898 let r = &[1, 2, 3, 4];
3900 &[a, b] => { // error: pattern requires 2 elements but array
3902 println!("a={}, b={}", a, b);
3907 Ensure that the pattern is consistent with the size of the matched
3908 array. Additional elements can be matched with `..`:
3911 #![feature(slice_patterns)]
3913 let r = &[1, 2, 3, 4];
3915 &[a, b, ..] => { // ok!
3916 println!("a={}, b={}", a, b);
3923 An array or slice pattern required more elements than were present in the
3926 Example of erroneous code:
3928 ```compile_fail,E0528
3929 #![feature(slice_patterns)]
3933 &[a, b, c, rest..] => { // error: pattern requires at least 3
3934 // elements but array has 2
3935 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3940 Ensure that the matched array has at least as many elements as the pattern
3941 requires. You can match an arbitrary number of remaining elements with `..`:
3944 #![feature(slice_patterns)]
3946 let r = &[1, 2, 3, 4, 5];
3948 &[a, b, c, rest..] => { // ok!
3949 // prints `a=1, b=2, c=3 rest=[4, 5]`
3950 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3957 An array or slice pattern was matched against some other type.
3959 Example of erroneous code:
3961 ```compile_fail,E0529
3962 #![feature(slice_patterns)]
3966 [a, b] => { // error: expected an array or slice, found `f32`
3967 println!("a={}, b={}", a, b);
3972 Ensure that the pattern and the expression being matched on are of consistent
3976 #![feature(slice_patterns)]
3981 println!("a={}, b={}", a, b);
3988 An unknown field was specified into an enum's structure variant.
3990 Erroneous code example:
3992 ```compile_fail,E0559
3997 let s = Field::Fool { joke: 0 };
3998 // error: struct variant `Field::Fool` has no field named `joke`
4001 Verify you didn't misspell the field's name or that the field exists. Example:
4008 let s = Field::Fool { joke: 0 }; // ok!
4013 An unknown field was specified into a structure.
4015 Erroneous code example:
4017 ```compile_fail,E0560
4022 let s = Simba { mother: 1, father: 0 };
4023 // error: structure `Simba` has no field named `father`
4026 Verify you didn't misspell the field's name or that the field exists. Example:
4034 let s = Simba { mother: 1, father: 0 }; // ok!
4039 The requested ABI is unsupported by the current target.
4041 The rust compiler maintains for each target a blacklist of ABIs unsupported on
4042 that target. If an ABI is present in such a list this usually means that the
4043 target / ABI combination is currently unsupported by llvm.
4045 If necessary, you can circumvent this check using custom target specifications.
4049 A return statement was found outside of a function body.
4051 Erroneous code example:
4053 ```compile_fail,E0572
4054 const FOO: u32 = return 0; // error: return statement outside of function body
4059 To fix this issue, just remove the return keyword or move the expression into a
4065 fn some_fn() -> u32 {
4076 In a `fn` type, a lifetime appears only in the return type,
4077 and not in the arguments types.
4079 Erroneous code example:
4081 ```compile_fail,E0581
4083 // Here, `'a` appears only in the return type:
4084 let x: for<'a> fn() -> &'a i32;
4088 To fix this issue, either use the lifetime in the arguments, or use
4093 // Here, `'a` appears only in the return type:
4094 let x: for<'a> fn(&'a i32) -> &'a i32;
4095 let y: fn() -> &'static i32;
4099 Note: The examples above used to be (erroneously) accepted by the
4100 compiler, but this was since corrected. See [issue #33685] for more
4103 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
4107 A lifetime appears only in an associated-type binding,
4108 and not in the input types to the trait.
4110 Erroneous code example:
4112 ```compile_fail,E0582
4114 // No type can satisfy this requirement, since `'a` does not
4115 // appear in any of the input types (here, `i32`):
4116 where F: for<'a> Fn(i32) -> Option<&'a i32>
4123 To fix this issue, either use the lifetime in the inputs, or use
4127 fn bar<F, G>(t: F, u: G)
4128 where F: for<'a> Fn(&'a i32) -> Option<&'a i32>,
4129 G: Fn(i32) -> Option<&'static i32>,
4136 Note: The examples above used to be (erroneously) accepted by the
4137 compiler, but this was since corrected. See [issue #33685] for more
4140 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
4145 register_diagnostics! {
4149 E0103, // @GuillaumeGomez: I was unable to get this error, try your best!
4155 // E0159, // use of trait `{}` as struct constructor
4156 // E0163, // merged into E0071
4159 // E0172, // non-trait found in a type sum, moved to resolve
4160 // E0173, // manual implementations of unboxed closure traits are experimental
4163 // E0187, // can't infer the kind of the closure
4164 // E0188, // can not cast an immutable reference to a mutable pointer
4165 // E0189, // deprecated: can only cast a boxed pointer to a boxed object
4166 // E0190, // deprecated: can only cast a &-pointer to an &-object
4167 E0196, // cannot determine a type for this closure
4168 E0203, // type parameter has more than one relaxed default bound,
4169 // and only one is supported
4171 // E0209, // builtin traits can only be implemented on structs or enums
4172 E0212, // cannot extract an associated type from a higher-ranked trait bound
4173 // E0213, // associated types are not accepted in this context
4174 // E0215, // angle-bracket notation is not stable with `Fn`
4175 // E0216, // parenthetical notation is only stable with `Fn`
4176 // E0217, // ambiguous associated type, defined in multiple supertraits
4177 // E0218, // no associated type defined
4178 // E0219, // associated type defined in higher-ranked supertrait
4179 // E0222, // Error code E0045 (variadic function must have C calling
4180 // convention) duplicate
4181 E0224, // at least one non-builtin train is required for an object type
4182 E0227, // ambiguous lifetime bound, explicit lifetime bound required
4183 E0228, // explicit lifetime bound required
4184 E0231, // only named substitution parameters are allowed
4187 // E0235, // structure constructor specifies a structure of type but
4188 // E0236, // no lang item for range syntax
4189 // E0237, // no lang item for range syntax
4190 // E0238, // parenthesized parameters may only be used with a trait
4191 // E0239, // `next` method of `Iterator` trait has unexpected type
4195 E0245, // not a trait
4196 // E0246, // invalid recursive type
4198 // E0248, // value used as a type, now reported earlier during resolution as E0412
4200 // E0319, // trait impls for defaulted traits allowed just for structs/enums
4201 E0320, // recursive overflow during dropck
4202 // E0372, // coherence not object safe
4203 E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
4204 // between structures with the same definition
4205 E0436, // functional record update requires a struct
4206 E0521, // redundant default implementations of trait
4207 E0533, // `{}` does not name a unit variant, unit struct or a constant
4208 E0562, // `impl Trait` not allowed outside of function
4209 // and inherent method return types
4210 E0563, // cannot determine a type for this `impl Trait`: {}
4211 E0564, // only named lifetimes are allowed in `impl Trait`,
4212 // but `{}` was found in the type `{}`
4213 E0567, // auto traits can not have type parameters
4214 E0568, // auto-traits can not have predicates,
4215 E0592, // duplicate definitions with name `{}`