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 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,E0102
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/pull/1023
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](https://github.com/rust-lang/
1915 rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
1919 `where` clauses must use generic type parameters: it does not make sense to use
1920 them otherwise. An example causing this error:
1927 #[derive(Copy,Clone)]
1932 impl Foo for Wrapper<u32> where Wrapper<u32>: Clone {
1937 This use of a `where` clause is strange - a more common usage would look
1938 something like the following:
1945 #[derive(Copy,Clone)]
1949 impl <T> Foo for Wrapper<T> where Wrapper<T>: Clone {
1954 Here, we're saying that the implementation exists on Wrapper only when the
1955 wrapped type `T` implements `Clone`. The `where` clause is important because
1956 some types will not implement `Clone`, and thus will not get this method.
1958 In our erroneous example, however, we're referencing a single concrete type.
1959 Since we know for certain that `Wrapper<u32>` implements `Clone`, there's no
1960 reason to also specify it in a `where` clause.
1964 A type parameter was declared which shadows an existing one. An example of this
1967 ```compile_fail,E0194
1969 fn do_something(&self) -> T;
1970 fn do_something_else<T: Clone>(&self, bar: T);
1974 In this example, the trait `Foo` and the trait method `do_something_else` both
1975 define a type parameter `T`. This is not allowed: if the method wishes to
1976 define a type parameter, it must use a different name for it.
1980 Your method's lifetime parameters do not match the trait declaration.
1981 Erroneous code example:
1983 ```compile_fail,E0195
1985 fn bar<'a,'b:'a>(x: &'a str, y: &'b str);
1990 impl Trait for Foo {
1991 fn bar<'a,'b>(x: &'a str, y: &'b str) {
1992 // error: lifetime parameters or bounds on method `bar`
1993 // do not match the trait declaration
1998 The lifetime constraint `'b` for bar() implementation does not match the
1999 trait declaration. Ensure lifetime declarations match exactly in both trait
2000 declaration and implementation. Example:
2004 fn t<'a,'b:'a>(x: &'a str, y: &'b str);
2009 impl Trait for Foo {
2010 fn t<'a,'b:'a>(x: &'a str, y: &'b str) { // ok!
2017 Inherent implementations (one that do not implement a trait but provide
2018 methods associated with a type) are always safe because they are not
2019 implementing an unsafe trait. Removing the `unsafe` keyword from the inherent
2020 implementation will resolve this error.
2022 ```compile_fail,E0197
2025 // this will cause this error
2027 // converting it to this will fix it
2033 A negative implementation is one that excludes a type from implementing a
2034 particular trait. Not being able to use a trait is always a safe operation,
2035 so negative implementations are always safe and never need to be marked as
2039 #![feature(optin_builtin_traits)]
2043 // unsafe is unnecessary
2044 unsafe impl !Clone for Foo { }
2050 #![feature(optin_builtin_traits)]
2056 impl Enterprise for .. { }
2058 impl !Enterprise for Foo { }
2061 Please note that negative impls are only allowed for traits with default impls.
2065 Safe traits should not have unsafe implementations, therefore marking an
2066 implementation for a safe trait unsafe will cause a compiler error. Removing
2067 the unsafe marker on the trait noted in the error will resolve this problem.
2069 ```compile_fail,E0199
2074 // this won't compile because Bar is safe
2075 unsafe impl Bar for Foo { }
2076 // this will compile
2077 impl Bar for Foo { }
2082 Unsafe traits must have unsafe implementations. This error occurs when an
2083 implementation for an unsafe trait isn't marked as unsafe. This may be resolved
2084 by marking the unsafe implementation as unsafe.
2086 ```compile_fail,E0200
2089 unsafe trait Bar { }
2091 // this won't compile because Bar is unsafe and impl isn't unsafe
2092 impl Bar for Foo { }
2093 // this will compile
2094 unsafe impl Bar for Foo { }
2099 It is an error to define two associated items (like methods, associated types,
2100 associated functions, etc.) with the same identifier.
2104 ```compile_fail,E0201
2108 fn bar(&self) -> bool { self.0 > 5 }
2109 fn bar() {} // error: duplicate associated function
2114 fn baz(&self) -> bool;
2120 fn baz(&self) -> bool { true }
2122 // error: duplicate method
2123 fn baz(&self) -> bool { self.0 > 5 }
2125 // error: duplicate associated type
2130 Note, however, that items with the same name are allowed for inherent `impl`
2131 blocks that don't overlap:
2137 fn bar(&self) -> bool { self.0 > 5 }
2141 fn bar(&self) -> bool { self.0 }
2147 Inherent associated types were part of [RFC 195] but are not yet implemented.
2148 See [the tracking issue][iss8995] for the status of this implementation.
2150 [RFC 195]: https://github.com/rust-lang/rfcs/pull/195
2151 [iss8995]: https://github.com/rust-lang/rust/issues/8995
2155 An attempt to implement the `Copy` trait for a struct failed because one of the
2156 fields does not implement `Copy`. To fix this, you must implement `Copy` for the
2157 mentioned field. Note that this may not be possible, as in the example of
2159 ```compile_fail,E0204
2164 impl Copy for Foo { }
2167 This fails because `Vec<T>` does not implement `Copy` for any `T`.
2169 Here's another example that will fail:
2171 ```compile_fail,E0204
2178 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
2179 differs from the behavior for `&T`, which is always `Copy`).
2184 An attempt to implement the `Copy` trait for an enum failed because one of the
2185 variants does not implement `Copy`. To fix this, you must implement `Copy` for
2186 the mentioned variant. Note that this may not be possible, as in the example of
2188 ```compile_fail,E0205
2194 impl Copy for Foo { }
2197 This fails because `Vec<T>` does not implement `Copy` for any `T`.
2199 Here's another example that will fail:
2201 ```compile_fail,E0205
2209 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
2210 differs from the behavior for `&T`, which is always `Copy`).
2215 You can only implement `Copy` for a struct or enum. Both of the following
2216 examples will fail, because neither `i32` (primitive type) nor `&'static Bar`
2217 (reference to `Bar`) is a struct or enum:
2219 ```compile_fail,E0206
2221 impl Copy for Foo { } // error
2223 #[derive(Copy, Clone)]
2225 impl Copy for &'static Bar { } // error
2230 Any type parameter or lifetime parameter of an `impl` must meet at least one of
2231 the following criteria:
2233 - it appears in the self type of the impl
2234 - for a trait impl, it appears in the trait reference
2235 - it is bound as an associated type
2239 Suppose we have a struct `Foo` and we would like to define some methods for it.
2240 The following definition leads to a compiler error:
2242 ```compile_fail,E0207
2245 impl<T: Default> Foo {
2246 // error: the type parameter `T` is not constrained by the impl trait, self
2247 // type, or predicates [E0207]
2248 fn get(&self) -> T {
2249 <T as Default>::default()
2254 The problem is that the parameter `T` does not appear in the self type (`Foo`)
2255 of the impl. In this case, we can fix the error by moving the type parameter
2256 from the `impl` to the method `get`:
2262 // Move the type parameter from the impl to the method
2264 fn get<T: Default>(&self) -> T {
2265 <T as Default>::default()
2272 As another example, suppose we have a `Maker` trait and want to establish a
2273 type `FooMaker` that makes `Foo`s:
2275 ```compile_fail,E0207
2278 fn make(&mut self) -> Self::Item;
2287 impl<T: Default> Maker for FooMaker {
2288 // error: the type parameter `T` is not constrained by the impl trait, self
2289 // type, or predicates [E0207]
2292 fn make(&mut self) -> Foo<T> {
2293 Foo { foo: <T as Default>::default() }
2298 This fails to compile because `T` does not appear in the trait or in the
2301 One way to work around this is to introduce a phantom type parameter into
2302 `FooMaker`, like so:
2305 use std::marker::PhantomData;
2309 fn make(&mut self) -> Self::Item;
2316 // Add a type parameter to `FooMaker`
2317 struct FooMaker<T> {
2318 phantom: PhantomData<T>,
2321 impl<T: Default> Maker for FooMaker<T> {
2324 fn make(&mut self) -> Foo<T> {
2326 foo: <T as Default>::default(),
2332 Another way is to do away with the associated type in `Maker` and use an input
2333 type parameter instead:
2336 // Use a type parameter instead of an associated type here
2338 fn make(&mut self) -> Item;
2347 impl<T: Default> Maker<Foo<T>> for FooMaker {
2348 fn make(&mut self) -> Foo<T> {
2349 Foo { foo: <T as Default>::default() }
2354 ### Additional information
2356 For more information, please see [RFC 447].
2358 [RFC 447]: https://github.com/rust-lang/rfcs/blob/master/text/0447-no-unused-impl-parameters.md
2362 This error indicates a violation of one of Rust's orphan rules for trait
2363 implementations. The rule concerns the use of type parameters in an
2364 implementation of a foreign trait (a trait defined in another crate), and
2365 states that type parameters must be "covered" by a local type. To understand
2366 what this means, it is perhaps easiest to consider a few examples.
2368 If `ForeignTrait` is a trait defined in some external crate `foo`, then the
2369 following trait `impl` is an error:
2371 ```compile_fail,E0210
2372 extern crate collections;
2373 use collections::range::RangeArgument;
2375 impl<T> RangeArgument<T> for T { } // error
2380 To work around this, it can be covered with a local type, `MyType`:
2383 struct MyType<T>(T);
2384 impl<T> ForeignTrait for MyType<T> { } // Ok
2387 Please note that a type alias is not sufficient.
2389 For another example of an error, suppose there's another trait defined in `foo`
2390 named `ForeignTrait2` that takes two type parameters. Then this `impl` results
2391 in the same rule violation:
2395 impl<T> ForeignTrait2<T, MyType<T>> for MyType2 { } // error
2398 The reason for this is that there are two appearances of type parameter `T` in
2399 the `impl` header, both as parameters for `ForeignTrait2`. The first appearance
2400 is uncovered, and so runs afoul of the orphan rule.
2402 Consider one more example:
2405 impl<T> ForeignTrait2<MyType<T>, T> for MyType2 { } // Ok
2408 This only differs from the previous `impl` in that the parameters `T` and
2409 `MyType<T>` for `ForeignTrait2` have been swapped. This example does *not*
2410 violate the orphan rule; it is permitted.
2412 To see why that last example was allowed, you need to understand the general
2413 rule. Unfortunately this rule is a bit tricky to state. Consider an `impl`:
2416 impl<P1, ..., Pm> ForeignTrait<T1, ..., Tn> for T0 { ... }
2419 where `P1, ..., Pm` are the type parameters of the `impl` and `T0, ..., Tn`
2420 are types. One of the types `T0, ..., Tn` must be a local type (this is another
2421 orphan rule, see the explanation for E0117). Let `i` be the smallest integer
2422 such that `Ti` is a local type. Then no type parameter can appear in any of the
2425 For information on the design of the orphan rules, see [RFC 1023].
2427 [RFC 1023]: https://github.com/rust-lang/rfcs/pull/1023
2432 You used a function or type which doesn't fit the requirements for where it was
2433 used. Erroneous code examples:
2436 #![feature(intrinsics)]
2438 extern "rust-intrinsic" {
2439 fn size_of<T>(); // error: intrinsic has wrong type
2444 fn main() -> i32 { 0 }
2445 // error: main function expects type: `fn() {main}`: expected (), found i32
2452 // error: mismatched types in range: expected u8, found i8
2462 fn x(self: Rc<Foo>) {}
2463 // error: mismatched self type: expected `Foo`: expected struct
2464 // `Foo`, found struct `alloc::rc::Rc`
2468 For the first code example, please check the function definition. Example:
2471 #![feature(intrinsics)]
2473 extern "rust-intrinsic" {
2474 fn size_of<T>() -> usize; // ok!
2478 The second case example is a bit particular : the main function must always
2479 have this definition:
2485 They never take parameters and never return types.
2487 For the third example, when you match, all patterns must have the same type
2488 as the type you're matching on. Example:
2494 0u8...3u8 => (), // ok!
2499 And finally, for the last example, only `Box<Self>`, `&Self`, `Self`,
2500 or `&mut Self` work as explicit self parameters. Example:
2506 fn x(self: Box<Foo>) {} // ok!
2513 A generic type was described using parentheses rather than angle brackets. For
2516 ```compile_fail,E0214
2518 let v: Vec(&str) = vec!["foo"];
2522 This is not currently supported: `v` should be defined as `Vec<&str>`.
2523 Parentheses are currently only used with generic types when defining parameters
2524 for `Fn`-family traits.
2528 You used an associated type which isn't defined in the trait.
2529 Erroneous code example:
2531 ```compile_fail,E0220
2536 type Foo = T1<F=i32>; // error: associated type `F` not found for `T1`
2543 // error: Baz is used but not declared
2544 fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
2548 Make sure that you have defined the associated type in the trait body.
2549 Also, verify that you used the right trait or you didn't misspell the
2550 associated type name. Example:
2557 type Foo = T1<Bar=i32>; // ok!
2563 type Baz; // we declare `Baz` in our trait.
2565 // and now we can use it here:
2566 fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
2572 An attempt was made to retrieve an associated type, but the type was ambiguous.
2575 ```compile_fail,E0221
2591 In this example, `Foo` defines an associated type `A`. `Bar` inherits that type
2592 from `Foo`, and defines another associated type of the same name. As a result,
2593 when we attempt to use `Self::A`, it's ambiguous whether we mean the `A` defined
2594 by `Foo` or the one defined by `Bar`.
2596 There are two options to work around this issue. The first is simply to rename
2597 one of the types. Alternatively, one can specify the intended type using the
2611 let _: <Self as Bar>::A;
2618 An attempt was made to retrieve an associated type, but the type was ambiguous.
2621 ```compile_fail,E0223
2622 trait MyTrait {type X; }
2625 let foo: MyTrait::X;
2629 The problem here is that we're attempting to take the type of X from MyTrait.
2630 Unfortunately, the type of X is not defined, because it's only made concrete in
2631 implementations of the trait. A working version of this code might look like:
2634 trait MyTrait {type X; }
2637 impl MyTrait for MyStruct {
2642 let foo: <MyStruct as MyTrait>::X;
2646 This syntax specifies that we want the X type from MyTrait, as made concrete in
2647 MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct
2648 might implement two different traits with identically-named associated types.
2649 This syntax allows disambiguation between the two.
2653 You attempted to use multiple types as bounds for a closure or trait object.
2654 Rust does not currently support this. A simple example that causes this error:
2656 ```compile_fail,E0225
2658 let _: Box<std::io::Read + std::io::Write>;
2662 Send and Sync are an exception to this rule: it's possible to have bounds of
2663 one non-builtin trait, plus either or both of Send and Sync. For example, the
2664 following compiles correctly:
2668 let _: Box<std::io::Read + Send + Sync>;
2674 An associated type binding was done outside of the type parameter declaration
2675 and `where` clause. Erroneous code example:
2677 ```compile_fail,E0229
2680 fn boo(&self) -> <Self as Foo>::A;
2685 impl Foo for isize {
2687 fn boo(&self) -> usize { 42 }
2690 fn baz<I>(x: &<I as Foo<A=Bar>>::A) {}
2691 // error: associated type bindings are not allowed here
2694 To solve this error, please move the type bindings in the type parameter
2698 fn baz<I: Foo<A=Bar>>(x: &<I as Foo>::A) {} // ok!
2701 Or in the `where` clause:
2704 fn baz<I>(x: &<I as Foo>::A) where I: Foo<A=Bar> {}
2709 The trait has more type parameters specified than appear in its definition.
2711 Erroneous example code:
2713 ```compile_fail,E0230
2714 #![feature(on_unimplemented)]
2715 #[rustc_on_unimplemented = "Trait error on `{Self}` with `<{A},{B},{C}>`"]
2716 // error: there is no type parameter C on trait TraitWithThreeParams
2717 trait TraitWithThreeParams<A,B>
2721 Include the correct number of type parameters and the compilation should
2725 #![feature(on_unimplemented)]
2726 #[rustc_on_unimplemented = "Trait error on `{Self}` with `<{A},{B},{C}>`"]
2727 trait TraitWithThreeParams<A,B,C> // ok!
2733 The attribute must have a value. Erroneous code example:
2735 ```compile_fail,E0232
2736 #![feature(on_unimplemented)]
2738 #[rustc_on_unimplemented] // error: this attribute must have a value
2742 Please supply the missing value of the attribute. Example:
2745 #![feature(on_unimplemented)]
2747 #[rustc_on_unimplemented = "foo"] // ok!
2753 This error indicates that not enough type parameters were found in a type or
2756 For example, the `Foo` struct below is defined to be generic in `T`, but the
2757 type parameter is missing in the definition of `Bar`:
2759 ```compile_fail,E0243
2760 struct Foo<T> { x: T }
2762 struct Bar { x: Foo }
2767 This error indicates that too many type parameters were found in a type or
2770 For example, the `Foo` struct below has no type parameters, but is supplied
2771 with two in the definition of `Bar`:
2773 ```compile_fail,E0244
2774 struct Foo { x: bool }
2776 struct Bar<S, T> { x: Foo<S, T> }
2781 If an impl has a generic parameter with the `#[may_dangle]` attribute, then
2782 that impl must be declared as an `unsafe impl. For example:
2784 ```compile_fail,E0569
2785 #![feature(generic_param_attrs)]
2786 #![feature(dropck_eyepatch)]
2789 impl<#[may_dangle] X> Drop for Foo<X> {
2790 fn drop(&mut self) { }
2794 In this example, we are asserting that the destructor for `Foo` will not
2795 access any data of type `X`, and require this assertion to be true for
2796 overall safety in our program. The compiler does not currently attempt to
2797 verify this assertion; therefore we must tag this `impl` as unsafe.
2801 Default impls for a trait must be located in the same crate where the trait was
2802 defined. For more information see the [opt-in builtin traits RFC](https://github
2803 .com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
2807 A cross-crate opt-out trait was implemented on something which wasn't a struct
2808 or enum type. Erroneous code example:
2810 ```compile_fail,E0321
2811 #![feature(optin_builtin_traits)]
2815 impl !Sync for Foo {}
2817 unsafe impl Send for &'static Foo {}
2818 // error: cross-crate traits with a default impl, like `core::marker::Send`,
2819 // can only be implemented for a struct/enum type, not
2823 Only structs and enums are permitted to impl Send, Sync, and other opt-out
2824 trait, and the struct or enum must be local to the current crate. So, for
2825 example, `unsafe impl Send for Rc<Foo>` is not allowed.
2829 The `Sized` trait is a special trait built-in to the compiler for types with a
2830 constant size known at compile-time. This trait is automatically implemented
2831 for types as needed by the compiler, and it is currently disallowed to
2832 explicitly implement it for a type.
2836 An associated const was implemented when another trait item was expected.
2837 Erroneous code example:
2839 ```compile_fail,E0323
2840 #![feature(associated_consts)]
2850 // error: item `N` is an associated const, which doesn't match its
2851 // trait `<Bar as Foo>`
2855 Please verify that the associated const wasn't misspelled and the correct trait
2856 was implemented. Example:
2866 type N = u32; // ok!
2873 #![feature(associated_consts)]
2882 const N : u32 = 0; // ok!
2888 A method was implemented when another trait item was expected. Erroneous
2891 ```compile_fail,E0324
2892 #![feature(associated_consts)]
2904 // error: item `N` is an associated method, which doesn't match its
2905 // trait `<Bar as Foo>`
2909 To fix this error, please verify that the method name wasn't misspelled and
2910 verify that you are indeed implementing the correct trait items. Example:
2913 #![feature(associated_consts)]
2932 An associated type was implemented when another trait item was expected.
2933 Erroneous code example:
2935 ```compile_fail,E0325
2936 #![feature(associated_consts)]
2946 // error: item `N` is an associated type, which doesn't match its
2947 // trait `<Bar as Foo>`
2951 Please verify that the associated type name wasn't misspelled and your
2952 implementation corresponds to the trait definition. Example:
2962 type N = u32; // ok!
2969 #![feature(associated_consts)]
2978 const N : u32 = 0; // ok!
2984 The types of any associated constants in a trait implementation must match the
2985 types in the trait definition. This error indicates that there was a mismatch.
2987 Here's an example of this error:
2989 ```compile_fail,E0326
2990 #![feature(associated_consts)]
2999 const BAR: u32 = 5; // error, expected bool, found u32
3005 The Unsize trait should not be implemented directly. All implementations of
3006 Unsize are provided automatically by the compiler.
3008 Erroneous code example:
3010 ```compile_fail,E0328
3013 use std::marker::Unsize;
3017 impl<T> Unsize<T> for MyType {}
3020 If you are defining your own smart pointer type and would like to enable
3021 conversion from a sized to an unsized type with the [DST coercion system]
3022 (https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md), use
3023 [`CoerceUnsized`](https://doc.rust-lang.org/std/ops/trait.CoerceUnsized.html)
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> {}
3041 An attempt was made to access an associated constant through either a generic
3042 type parameter or `Self`. This is not supported yet. An example causing this
3043 error is shown below:
3046 #![feature(associated_consts)]
3054 impl Foo for MyStruct {
3055 const BAR: f64 = 0f64;
3058 fn get_bar_bad<F: Foo>(t: F) -> f64 {
3063 Currently, the value of `BAR` for a particular type can only be accessed
3064 through a concrete type, as shown below:
3067 #![feature(associated_consts)]
3075 fn get_bar_good() -> f64 {
3076 <MyStruct as Foo>::BAR
3082 An attempt was made to implement `Drop` on a concrete specialization of a
3083 generic type. An example is shown below:
3085 ```compile_fail,E0366
3090 impl Drop for Foo<u32> {
3091 fn drop(&mut self) {}
3095 This code is not legal: it is not possible to specialize `Drop` to a subset of
3096 implementations of a generic type. One workaround for this is to wrap the
3097 generic type, as shown below:
3109 fn drop(&mut self) {}
3115 An attempt was made to implement `Drop` on a specialization of a generic type.
3116 An example is shown below:
3118 ```compile_fail,E0367
3121 struct MyStruct<T> {
3125 impl<T: Foo> Drop for MyStruct<T> {
3126 fn drop(&mut self) {}
3130 This code is not legal: it is not possible to specialize `Drop` to a subset of
3131 implementations of a generic type. In order for this code to work, `MyStruct`
3132 must also require that `T` implements `Foo`. Alternatively, another option is
3133 to wrap the generic type in another that specializes appropriately:
3138 struct MyStruct<T> {
3142 struct MyStructWrapper<T: Foo> {
3146 impl <T: Foo> Drop for MyStructWrapper<T> {
3147 fn drop(&mut self) {}
3153 This error indicates that a binary assignment operator like `+=` or `^=` was
3154 applied to a type that doesn't support it. For example:
3156 ```compile_fail,E0368
3157 let mut x = 12f32; // error: binary operation `<<` cannot be applied to
3163 To fix this error, please check that this type implements this binary
3167 let mut x = 12u32; // the `u32` type does implement the `ShlAssign` trait
3172 It is also possible to overload most operators for your own type by
3173 implementing the `[OP]Assign` traits from `std::ops`.
3175 Another problem you might be facing is this: suppose you've overloaded the `+`
3176 operator for some type `Foo` by implementing the `std::ops::Add` trait for
3177 `Foo`, but you find that using `+=` does not work, as in this example:
3179 ```compile_fail,E0368
3187 fn add(self, rhs: Foo) -> Foo {
3193 let mut x: Foo = Foo(5);
3194 x += Foo(7); // error, `+= cannot be applied to the type `Foo`
3198 This is because `AddAssign` is not automatically implemented, so you need to
3199 manually implement it for your type.
3203 A binary operation was attempted on a type which doesn't support it.
3204 Erroneous code example:
3206 ```compile_fail,E0369
3207 let x = 12f32; // error: binary operation `<<` cannot be applied to
3213 To fix this error, please check that this type implements this binary
3217 let x = 12u32; // the `u32` type does implement it:
3218 // https://doc.rust-lang.org/stable/std/ops/trait.Shl.html
3223 It is also possible to overload most operators for your own type by
3224 implementing traits from `std::ops`.
3228 The maximum value of an enum was reached, so it cannot be automatically
3229 set in the next enum value. Erroneous code example:
3232 #[deny(overflowing_literals)]
3234 X = 0x7fffffffffffffff,
3235 Y, // error: enum discriminant overflowed on value after
3236 // 9223372036854775807: i64; set explicitly via
3237 // Y = -9223372036854775808 if that is desired outcome
3241 To fix this, please set manually the next enum value or put the enum variant
3242 with the maximum value at the end of the enum. Examples:
3246 X = 0x7fffffffffffffff,
3256 X = 0x7fffffffffffffff,
3262 When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
3263 definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
3264 `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
3265 definition, so it is not useful to do this.
3269 ```compile_fail,E0371
3270 trait Foo { fn foo(&self) { } }
3274 impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
3275 impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
3276 impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
3277 impl Baz for Bar { } // Note: This is OK
3282 A struct without a field containing an unsized type cannot implement
3284 [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3285 is any type that the compiler doesn't know the length or alignment of at
3286 compile time. Any struct containing an unsized type is also unsized.
3288 Example of erroneous code:
3290 ```compile_fail,E0374
3291 #![feature(coerce_unsized)]
3292 use std::ops::CoerceUnsized;
3294 struct Foo<T: ?Sized> {
3298 // error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
3299 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
3300 where T: CoerceUnsized<U> {}
3303 `CoerceUnsized` is used to coerce one struct containing an unsized type
3304 into another struct containing a different unsized type. If the struct
3305 doesn't have any fields of unsized types then you don't need explicit
3306 coercion to get the types you want. To fix this you can either
3307 not try to implement `CoerceUnsized` or you can add a field that is
3308 unsized to the struct.
3313 #![feature(coerce_unsized)]
3314 use std::ops::CoerceUnsized;
3316 // We don't need to impl `CoerceUnsized` here.
3321 // We add the unsized type field to the struct.
3322 struct Bar<T: ?Sized> {
3327 // The struct has an unsized field so we can implement
3328 // `CoerceUnsized` for it.
3329 impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
3330 where T: CoerceUnsized<U> {}
3333 Note that `CoerceUnsized` is mainly used by smart pointers like `Box`, `Rc`
3334 and `Arc` to be able to mark that they can coerce unsized types that they
3339 A struct with more than one field containing an unsized type cannot implement
3340 `CoerceUnsized`. This only occurs when you are trying to coerce one of the
3341 types in your struct to another type in the struct. In this case we try to
3342 impl `CoerceUnsized` from `T` to `U` which are both types that the struct
3343 takes. An [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3344 is any type that the compiler doesn't know the length or alignment of at
3345 compile time. Any struct containing an unsized type is also unsized.
3347 Example of erroneous code:
3349 ```compile_fail,E0375
3350 #![feature(coerce_unsized)]
3351 use std::ops::CoerceUnsized;
3353 struct Foo<T: ?Sized, U: ?Sized> {
3359 // error: Struct `Foo` has more than one unsized field.
3360 impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
3363 `CoerceUnsized` only allows for coercion from a structure with a single
3364 unsized type field to another struct with a single unsized type field.
3365 In fact Rust only allows for a struct to have one unsized type in a struct
3366 and that unsized type must be the last field in the struct. So having two
3367 unsized types in a single struct is not allowed by the compiler. To fix this
3368 use only one field containing an unsized type in the struct and then use
3369 multiple structs to manage each unsized type field you need.
3374 #![feature(coerce_unsized)]
3375 use std::ops::CoerceUnsized;
3377 struct Foo<T: ?Sized> {
3382 impl <T, U> CoerceUnsized<Foo<U>> for Foo<T>
3383 where T: CoerceUnsized<U> {}
3385 fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
3386 Foo { a: 12i32, b: t } // we use coercion to get the `Foo<U>` type we need
3393 The type you are trying to impl `CoerceUnsized` for is not a struct.
3394 `CoerceUnsized` can only be implemented for a struct. Unsized types are
3395 already able to be coerced without an implementation of `CoerceUnsized`
3396 whereas a struct containing an unsized type needs to know the unsized type
3397 field it's containing is able to be coerced. An
3398 [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3399 is any type that the compiler doesn't know the length or alignment of at
3400 compile time. Any struct containing an unsized type is also unsized.
3402 Example of erroneous code:
3404 ```compile_fail,E0376
3405 #![feature(coerce_unsized)]
3406 use std::ops::CoerceUnsized;
3408 struct Foo<T: ?Sized> {
3412 // error: The type `U` is not a struct
3413 impl<T, U> CoerceUnsized<U> for Foo<T> {}
3416 The `CoerceUnsized` trait takes a struct type. Make sure the type you are
3417 providing to `CoerceUnsized` is a struct with only the last field containing an
3423 #![feature(coerce_unsized)]
3424 use std::ops::CoerceUnsized;
3430 // The `Foo<U>` is a struct so `CoerceUnsized` can be implemented
3431 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}
3434 Note that in Rust, structs can only contain an unsized type if the field
3435 containing the unsized type is the last and only unsized type field in the
3440 Default impls are only allowed for traits with no methods or associated items.
3441 For more information see the [opt-in builtin traits RFC](https://github.com/rust
3442 -lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
3446 You tried to implement methods for a primitive type. Erroneous code example:
3448 ```compile_fail,E0390
3454 // error: only a single inherent implementation marked with
3455 // `#[lang = "mut_ptr"]` is allowed for the `*mut T` primitive
3458 This isn't allowed, but using a trait to implement a method is a good solution.
3470 impl Bar for *mut Foo {
3477 This error indicates that a type or lifetime parameter has been declared
3478 but not actually used. Here is an example that demonstrates the error:
3480 ```compile_fail,E0392
3486 If the type parameter was included by mistake, this error can be fixed
3487 by simply removing the type parameter, as shown below:
3495 Alternatively, if the type parameter was intentionally inserted, it must be
3496 used. A simple fix is shown below:
3504 This error may also commonly be found when working with unsafe code. For
3505 example, when using raw pointers one may wish to specify the lifetime for
3506 which the pointed-at data is valid. An initial attempt (below) causes this
3509 ```compile_fail,E0392
3515 We want to express the constraint that Foo should not outlive `'a`, because
3516 the data pointed to by `T` is only valid for that lifetime. The problem is
3517 that there are no actual uses of `'a`. It's possible to work around this
3518 by adding a PhantomData type to the struct, using it to tell the compiler
3519 to act as if the struct contained a borrowed reference `&'a T`:
3522 use std::marker::PhantomData;
3524 struct Foo<'a, T: 'a> {
3526 phantom: PhantomData<&'a T>
3530 PhantomData can also be used to express information about unused type
3531 parameters. You can read more about it in the API documentation:
3533 https://doc.rust-lang.org/std/marker/struct.PhantomData.html
3537 A type parameter which references `Self` in its default value was not specified.
3538 Example of erroneous code:
3540 ```compile_fail,E0393
3543 fn together_we_will_rule_the_galaxy(son: &A) {}
3544 // error: the type parameter `T` must be explicitly specified in an
3545 // object type because its default value `Self` references the
3549 A trait object is defined over a single, fully-defined trait. With a regular
3550 default parameter, this parameter can just be substituted in. However, if the
3551 default parameter is `Self`, the trait changes for each concrete type; i.e.
3552 `i32` will be expected to implement `A<i32>`, `bool` will be expected to
3553 implement `A<bool>`, etc... These types will not share an implementation of a
3554 fully-defined trait; instead they share implementations of a trait with
3555 different parameters substituted in for each implementation. This is
3556 irreconcilable with what we need to make a trait object work, and is thus
3557 disallowed. Making the trait concrete by explicitly specifying the value of the
3558 defaulted parameter will fix this issue. Fixed example:
3563 fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!
3568 You implemented a trait, overriding one or more of its associated types but did
3569 not reimplement its default methods.
3571 Example of erroneous code:
3573 ```compile_fail,E0399
3574 #![feature(associated_type_defaults)]
3582 // error - the following trait items need to be reimplemented as
3583 // `Assoc` was overridden: `bar`
3588 To fix this, add an implementation for each default method from the trait:
3591 #![feature(associated_type_defaults)]
3600 fn bar(&self) {} // ok!
3606 The length of the platform-intrinsic function `simd_shuffle`
3607 wasn't specified. Erroneous code example:
3609 ```compile_fail,E0439
3610 #![feature(platform_intrinsics)]
3612 extern "platform-intrinsic" {
3613 fn simd_shuffle<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3614 // error: invalid `simd_shuffle`, needs length: `simd_shuffle`
3618 The `simd_shuffle` function needs the length of the array passed as
3619 last parameter in its name. Example:
3622 #![feature(platform_intrinsics)]
3624 extern "platform-intrinsic" {
3625 fn simd_shuffle8<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3631 A platform-specific intrinsic function has the wrong number of type
3632 parameters. Erroneous code example:
3634 ```compile_fail,E0440
3635 #![feature(repr_simd)]
3636 #![feature(platform_intrinsics)]
3639 struct f64x2(f64, f64);
3641 extern "platform-intrinsic" {
3642 fn x86_mm_movemask_pd<T>(x: f64x2) -> i32;
3643 // error: platform-specific intrinsic has wrong number of type
3648 Please refer to the function declaration to see if it corresponds
3649 with yours. Example:
3652 #![feature(repr_simd)]
3653 #![feature(platform_intrinsics)]
3656 struct f64x2(f64, f64);
3658 extern "platform-intrinsic" {
3659 fn x86_mm_movemask_pd(x: f64x2) -> i32;
3665 An unknown platform-specific intrinsic function was used. Erroneous
3668 ```compile_fail,E0441
3669 #![feature(repr_simd)]
3670 #![feature(platform_intrinsics)]
3673 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3675 extern "platform-intrinsic" {
3676 fn x86_mm_adds_ep16(x: i16x8, y: i16x8) -> i16x8;
3677 // error: unrecognized platform-specific intrinsic function
3681 Please verify that the function name wasn't misspelled, and ensure
3682 that it is declared in the rust source code (in the file
3683 src/librustc_platform_intrinsics/x86.rs). Example:
3686 #![feature(repr_simd)]
3687 #![feature(platform_intrinsics)]
3690 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3692 extern "platform-intrinsic" {
3693 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3699 Intrinsic argument(s) and/or return value have the wrong type.
3700 Erroneous code example:
3702 ```compile_fail,E0442
3703 #![feature(repr_simd)]
3704 #![feature(platform_intrinsics)]
3707 struct i8x16(i8, i8, i8, i8, i8, i8, i8, i8,
3708 i8, i8, i8, i8, i8, i8, i8, i8);
3710 struct i32x4(i32, i32, i32, i32);
3712 struct i64x2(i64, i64);
3714 extern "platform-intrinsic" {
3715 fn x86_mm_adds_epi16(x: i8x16, y: i32x4) -> i64x2;
3716 // error: intrinsic arguments/return value have wrong type
3720 To fix this error, please refer to the function declaration to give
3721 it the awaited types. Example:
3724 #![feature(repr_simd)]
3725 #![feature(platform_intrinsics)]
3728 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3730 extern "platform-intrinsic" {
3731 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3737 Intrinsic argument(s) and/or return value have the wrong type.
3738 Erroneous code example:
3740 ```compile_fail,E0443
3741 #![feature(repr_simd)]
3742 #![feature(platform_intrinsics)]
3745 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3747 struct i64x8(i64, i64, i64, i64, i64, i64, i64, i64);
3749 extern "platform-intrinsic" {
3750 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i64x8;
3751 // error: intrinsic argument/return value has wrong type
3755 To fix this error, please refer to the function declaration to give
3756 it the awaited types. Example:
3759 #![feature(repr_simd)]
3760 #![feature(platform_intrinsics)]
3763 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3765 extern "platform-intrinsic" {
3766 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3772 A platform-specific intrinsic function has wrong number of arguments.
3773 Erroneous code example:
3775 ```compile_fail,E0444
3776 #![feature(repr_simd)]
3777 #![feature(platform_intrinsics)]
3780 struct f64x2(f64, f64);
3782 extern "platform-intrinsic" {
3783 fn x86_mm_movemask_pd(x: f64x2, y: f64x2, z: f64x2) -> i32;
3784 // error: platform-specific intrinsic has invalid number of arguments
3788 Please refer to the function declaration to see if it corresponds
3789 with yours. Example:
3792 #![feature(repr_simd)]
3793 #![feature(platform_intrinsics)]
3796 struct f64x2(f64, f64);
3798 extern "platform-intrinsic" {
3799 fn x86_mm_movemask_pd(x: f64x2) -> i32; // ok!
3805 The `typeof` keyword is currently reserved but unimplemented.
3806 Erroneous code example:
3808 ```compile_fail,E0516
3810 let x: typeof(92) = 92;
3814 Try using type inference instead. Example:
3824 A non-default implementation was already made on this type so it cannot be
3825 specialized further. Erroneous code example:
3827 ```compile_fail,E0520
3828 #![feature(specialization)]
3835 impl<T> SpaceLlama for T {
3836 default fn fly(&self) {}
3840 // applies to all `Clone` T and overrides the previous impl
3841 impl<T: Clone> SpaceLlama for T {
3845 // since `i32` is clone, this conflicts with the previous implementation
3846 impl SpaceLlama for i32 {
3847 default fn fly(&self) {}
3848 // error: item `fly` is provided by an `impl` that specializes
3849 // another, but the item in the parent `impl` is not marked
3850 // `default` and so it cannot be specialized.
3854 Specialization only allows you to override `default` functions in
3857 To fix this error, you need to mark all the parent implementations as default.
3861 #![feature(specialization)]
3868 impl<T> SpaceLlama for T {
3869 default fn fly(&self) {} // This is a parent implementation.
3872 // applies to all `Clone` T; overrides the previous impl
3873 impl<T: Clone> SpaceLlama for T {
3874 default fn fly(&self) {} // This is a parent implementation but was
3875 // previously not a default one, causing the error
3878 // applies to i32, overrides the previous two impls
3879 impl SpaceLlama for i32 {
3880 fn fly(&self) {} // And now that's ok!
3886 The number of elements in an array or slice pattern differed from the number of
3887 elements in the array being matched.
3889 Example of erroneous code:
3891 ```compile_fail,E0527
3892 #![feature(slice_patterns)]
3894 let r = &[1, 2, 3, 4];
3896 &[a, b] => { // error: pattern requires 2 elements but array
3898 println!("a={}, b={}", a, b);
3903 Ensure that the pattern is consistent with the size of the matched
3904 array. Additional elements can be matched with `..`:
3907 #![feature(slice_patterns)]
3909 let r = &[1, 2, 3, 4];
3911 &[a, b, ..] => { // ok!
3912 println!("a={}, b={}", a, b);
3919 An array or slice pattern required more elements than were present in the
3922 Example of erroneous code:
3924 ```compile_fail,E0528
3925 #![feature(slice_patterns)]
3929 &[a, b, c, rest..] => { // error: pattern requires at least 3
3930 // elements but array has 2
3931 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3936 Ensure that the matched array has at least as many elements as the pattern
3937 requires. You can match an arbitrary number of remaining elements with `..`:
3940 #![feature(slice_patterns)]
3942 let r = &[1, 2, 3, 4, 5];
3944 &[a, b, c, rest..] => { // ok!
3945 // prints `a=1, b=2, c=3 rest=[4, 5]`
3946 println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
3953 An array or slice pattern was matched against some other type.
3955 Example of erroneous code:
3957 ```compile_fail,E0529
3958 #![feature(slice_patterns)]
3962 [a, b] => { // error: expected an array or slice, found `f32`
3963 println!("a={}, b={}", a, b);
3968 Ensure that the pattern and the expression being matched on are of consistent
3972 #![feature(slice_patterns)]
3977 println!("a={}, b={}", a, b);
3984 An unknown field was specified into an enum's structure variant.
3986 Erroneous code example:
3988 ```compile_fail,E0559
3993 let s = Field::Fool { joke: 0 };
3994 // error: struct variant `Field::Fool` has no field named `joke`
3997 Verify you didn't misspell the field's name or that the field exists. Example:
4004 let s = Field::Fool { joke: 0 }; // ok!
4009 An unknown field was specified into a structure.
4011 Erroneous code example:
4013 ```compile_fail,E0560
4018 let s = Simba { mother: 1, father: 0 };
4019 // error: structure `Simba` has no field named `father`
4022 Verify you didn't misspell the field's name or that the field exists. Example:
4030 let s = Simba { mother: 1, father: 0 }; // ok!
4035 The requested ABI is unsupported by the current target.
4037 The rust compiler maintains for each target a blacklist of ABIs unsupported on
4038 that target. If an ABI is present in such a list this usually means that the
4039 target / ABI combination is currently unsupported by llvm.
4041 If necessary, you can circumvent this check using custom target specifications.
4045 A return statement was found outside of a function body.
4047 Erroneous code example:
4049 ```compile_fail,E0572
4050 const FOO: u32 = return 0; // error: return statement outside of function body
4055 To fix this issue, just remove the return keyword or move the expression into a
4061 fn some_fn() -> u32 {
4072 In a `fn` type, a lifetime appears only in the return type,
4073 and not in the arguments types.
4075 Erroneous code example:
4077 ```compile_fail,E0581
4079 // Here, `'a` appears only in the return type:
4080 let x: for<'a> fn() -> &'a i32;
4084 To fix this issue, either use the lifetime in the arguments, or use
4089 // Here, `'a` appears only in the return type:
4090 let x: for<'a> fn(&'a i32) -> &'a i32;
4091 let y: fn() -> &'static i32;
4095 Note: The examples above used to be (erroneously) accepted by the
4096 compiler, but this was since corrected. See [issue #33685] for more
4099 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
4103 A lifetime appears only in an associated-type binding,
4104 and not in the input types to the trait.
4106 Erroneous code example:
4108 ```compile_fail,E0582
4110 // No type can satisfy this requirement, since `'a` does not
4111 // appear in any of the input types (here, `i32`):
4112 where F: for<'a> Fn(i32) -> Option<&'a i32>
4119 To fix this issue, either use the lifetime in the inputs, or use
4123 fn bar<F, G>(t: F, u: G)
4124 where F: for<'a> Fn(&'a i32) -> Option<&'a i32>,
4125 G: Fn(i32) -> Option<&'static i32>,
4132 Note: The examples above used to be (erroneously) accepted by the
4133 compiler, but this was since corrected. See [issue #33685] for more
4136 [issue #33685]: https://github.com/rust-lang/rust/issues/33685
4141 register_diagnostics! {
4145 E0103, // @GuillaumeGomez: I was unable to get this error, try your best!
4151 // E0159, // use of trait `{}` as struct constructor
4152 // E0163, // merged into E0071
4155 // E0172, // non-trait found in a type sum, moved to resolve
4156 // E0173, // manual implementations of unboxed closure traits are experimental
4159 // E0187, // can't infer the kind of the closure
4160 // E0188, // can not cast an immutable reference to a mutable pointer
4161 // E0189, // deprecated: can only cast a boxed pointer to a boxed object
4162 // E0190, // deprecated: can only cast a &-pointer to an &-object
4163 E0196, // cannot determine a type for this closure
4164 E0203, // type parameter has more than one relaxed default bound,
4165 // and only one is supported
4167 // E0209, // builtin traits can only be implemented on structs or enums
4168 E0212, // cannot extract an associated type from a higher-ranked trait bound
4169 // E0213, // associated types are not accepted in this context
4170 // E0215, // angle-bracket notation is not stable with `Fn`
4171 // E0216, // parenthetical notation is only stable with `Fn`
4172 // E0217, // ambiguous associated type, defined in multiple supertraits
4173 // E0218, // no associated type defined
4174 // E0219, // associated type defined in higher-ranked supertrait
4175 // E0222, // Error code E0045 (variadic function must have C calling
4176 // convention) duplicate
4177 E0224, // at least one non-builtin train is required for an object type
4178 E0227, // ambiguous lifetime bound, explicit lifetime bound required
4179 E0228, // explicit lifetime bound required
4180 E0231, // only named substitution parameters are allowed
4183 // E0235, // structure constructor specifies a structure of type but
4184 // E0236, // no lang item for range syntax
4185 // E0237, // no lang item for range syntax
4186 // E0238, // parenthesized parameters may only be used with a trait
4187 // E0239, // `next` method of `Iterator` trait has unexpected type
4191 E0245, // not a trait
4192 // E0246, // invalid recursive type
4194 // E0248, // value used as a type, now reported earlier during resolution as E0412
4196 // E0319, // trait impls for defaulted traits allowed just for structs/enums
4197 E0320, // recursive overflow during dropck
4198 // E0372, // coherence not object safe
4199 E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
4200 // between structures with the same definition
4201 E0436, // functional record update requires a struct
4202 E0521, // redundant default implementations of trait
4203 E0533, // `{}` does not name a unit variant, unit struct or a constant
4204 E0562, // `impl Trait` not allowed outside of function
4205 // and inherent method return types
4206 E0563, // cannot determine a type for this `impl Trait`: {}
4207 E0564, // only named lifetimes are allowed in `impl Trait`,
4208 // but `{}` was found in the type `{}`
4209 E0567, // auto traits can not have type parameters
4210 E0568, // auto-traits can not have predicates,