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.
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.
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
205 // The ordering relation for strings can't be evaluated at compile time,
206 // so this doesn't work:
208 "hello" ... "world" => {}
212 // This is a more general version, using a guard:
214 s if s >= "hello" && s <= "world" => {}
221 This error indicates that a pointer to a trait type cannot be implicitly
222 dereferenced by a pattern. Every trait defines a type, but because the
223 size of trait implementors isn't fixed, this type has no compile-time size.
224 Therefore, all accesses to trait types must be through pointers. If you
225 encounter this error you should try to avoid dereferencing the pointer.
228 let trait_obj: &SomeTrait = ...;
230 // This tries to implicitly dereference to create an unsized local variable.
231 let &invalid = trait_obj;
233 // You can call methods without binding to the value being pointed at.
234 trait_obj.method_one();
235 trait_obj.method_two();
238 You can read more about trait objects in the Trait Object section of the
241 https://doc.rust-lang.org/reference.html#trait-objects
245 The compiler doesn't know what method to call because more than one method
246 has the same prototype. Erroneous code example:
259 impl Trait1 for Test { fn foo() {} }
260 impl Trait2 for Test { fn foo() {} }
263 Test::foo() // error, which foo() to call?
267 To avoid this error, you have to keep only one of them and remove the others.
268 So let's take our example and fix it:
277 impl Trait1 for Test { fn foo() {} }
280 Test::foo() // and now that's good!
284 However, a better solution would be using fully explicit naming of type and
298 impl Trait1 for Test { fn foo() {} }
299 impl Trait2 for Test { fn foo() {} }
302 <Test as Trait1>::foo()
319 impl F for X { fn m(&self) { println!("I am F"); } }
320 impl G for X { fn m(&self) { println!("I am G"); } }
325 F::m(&f); // it displays "I am F"
326 G::m(&f); // it displays "I am G"
332 You tried to give a type parameter where it wasn't needed. Erroneous code
345 x.method::<i32>(); // Error: Test::method doesn't need type parameter!
349 To fix this error, just remove the type parameter:
361 x.method(); // OK, we're good!
367 This error occurrs when you pass too many or not enough type parameters to
368 a method. Erroneous code example:
374 fn method<T>(&self, v: &[T]) -> usize {
383 x.method::<i32, i32>(v); // error: only one type parameter is expected!
387 To fix it, just specify a correct number of type parameters:
393 fn method<T>(&self, v: &[T]) -> usize {
402 x.method::<i32>(v); // OK, we're good!
406 Please note on the last example that we could have called `method` like this:
414 It is not allowed to manually call destructors in Rust. It is also not
415 necessary to do this since `drop` is called automatically whenever a value goes
418 Here's an example of this error:
432 let mut x = Foo { x: -7 };
433 x.drop(); // error: explicit use of destructor method
439 You can't use type parameters on foreign items. Example of erroneous code:
442 extern { fn some_func<T>(x: T); }
445 To fix this, replace the type parameter with the specializations that you
449 extern { fn some_func_i32(x: i32); }
450 extern { fn some_func_i64(x: i64); }
455 Rust only supports variadic parameters for interoperability with C code in its
456 FFI. As such, variadic parameters can only be used with functions which are
457 using the C ABI. Examples of erroneous code:
460 extern "rust-call" { fn foo(x: u8, ...); }
464 fn foo(x: u8, ...) {}
467 To fix such code, put them in an extern "C" block:
470 extern "C" fn foo(x: u8, ...);
483 Items are missing in a trait implementation. Erroneous code example:
493 // error: not all trait items implemented, missing: `foo`
496 When trying to make some type implement a trait `Foo`, you must, at minimum,
497 provide implementations for all of `Foo`'s required methods (meaning the
498 methods that do not have default implementations), as well as any required
499 trait items like associated types or constants. Example:
515 This error indicates that an attempted implementation of a trait method
516 has the wrong number of type parameters.
518 For example, the trait below has a method `foo` with a type parameter `T`,
519 but the implementation of `foo` for the type `Bar` is missing this parameter:
523 fn foo<T: Default>(x: T) -> Self;
528 // error: method `foo` has 0 type parameters but its trait declaration has 1
531 fn foo(x: bool) -> Self { Bar }
537 This error indicates that an attempted implementation of a trait method
538 has the wrong number of function parameters.
540 For example, the trait below has a method `foo` with two function parameters
541 (`&self` and `u8`), but the implementation of `foo` for the type `Bar` omits
546 fn foo(&self, x: u8) -> bool;
551 // error: method `foo` has 1 parameter but the declaration in trait `Foo::foo`
554 fn foo(&self) -> bool { true }
560 The parameters of any trait method must match between a trait implementation
561 and the trait definition.
563 Here are a couple examples of this error:
574 // error, expected u16, found i16
577 // error, values differ in mutability
578 fn bar(&mut self) { }
584 It is not allowed to cast to a bool. If you are trying to cast a numeric type
585 to a bool, you can compare it with zero instead:
590 // Not allowed, won't compile
591 let x_is_nonzero = x as bool;
598 let x_is_nonzero = x != 0;
603 During a method call, a value is automatically dereferenced as many times as
604 needed to make the value's type match the method's receiver. The catch is that
605 the compiler will only attempt to dereference a number of times up to the
606 recursion limit (which can be set via the `recursion_limit` attribute).
608 For a somewhat artificial example:
610 ```compile_fail,ignore
611 #![recursion_limit="2"]
623 // error, reached the recursion limit while auto-dereferencing &&Foo
628 One fix may be to increase the recursion limit. Note that it is possible to
629 create an infinite recursion of dereferencing, in which case the only fix is to
630 somehow break the recursion.
634 When invoking closures or other implementations of the function traits `Fn`,
635 `FnMut` or `FnOnce` using call notation, the number of parameters passed to the
636 function must match its definition.
638 An example using a closure:
642 let a = f(); // invalid, too few parameters
643 let b = f(4); // this works!
644 let c = f(2, 3); // invalid, too many parameters
647 A generic function must be treated similarly:
650 fn foo<F: Fn()>(f: F) {
651 f(); // this is valid, but f(3) would not work
657 The built-in function traits are generic over a tuple of the function arguments.
658 If one uses angle-bracket notation (`Fn<(T,), Output=U>`) instead of parentheses
659 (`Fn(T) -> U`) to denote the function trait, the type parameter should be a
660 tuple. Otherwise function call notation cannot be used and the trait will not be
661 implemented by closures.
663 The most likely source of this error is using angle-bracket notation without
664 wrapping the function argument type into a tuple, for example:
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 fn foo<F: Fn<(i32,)>>(f: F) -> F::Output { f(3) }
676 Note that `(T,)` always denotes the type of a 1-tuple containing an element of
677 type `T`. The comma is necessary for syntactic disambiguation.
681 External C functions are allowed to be variadic. However, a variadic function
682 takes a minimum number of arguments. For example, consider C's variadic `printf`
687 use libc::{ c_char, c_int };
690 fn printf(_: *const c_char, ...) -> c_int;
694 Using this declaration, it must be called with at least one argument, so
695 simply calling `printf()` is invalid. But the following uses are allowed:
699 use std::ffi::CString;
701 printf(CString::new("test\n").unwrap().as_ptr());
702 printf(CString::new("number = %d\n").unwrap().as_ptr(), 3);
703 printf(CString::new("%d, %d\n").unwrap().as_ptr(), 10, 5);
709 The number of arguments passed to a function must match the number of arguments
710 specified in the function signature.
712 For example, a function like:
715 fn f(a: u16, b: &str) {}
718 Must always be called with exactly two arguments, e.g. `f(2, "test")`.
720 Note that Rust does not have a notion of optional function arguments or
721 variadic functions (except for its C-FFI).
725 This error indicates that during an attempt to build a struct or struct-like
726 enum variant, one of the fields was specified more than once. Erroneous code
737 x: 0, // error: field `x` specified more than once
742 Each field should be specified exactly one time. Example:
750 let x = Foo { x: 0 }; // ok!
756 This error indicates that during an attempt to build a struct or struct-like
757 enum variant, one of the fields was not provided. Erroneous code example:
766 let x = Foo { x: 0 }; // error: missing field: `y`
770 Each field should be specified exactly once. Example:
779 let x = Foo { x: 0, y: 0 }; // ok!
785 Box placement expressions (like C++'s "placement new") do not yet support any
786 place expression except the exchange heap (i.e. `std::boxed::HEAP`).
787 Furthermore, the syntax is changing to use `in` instead of `box`. See [RFC 470]
788 and [RFC 809] for more details.
790 [RFC 470]: https://github.com/rust-lang/rfcs/pull/470
791 [RFC 809]: https://github.com/rust-lang/rfcs/pull/809
795 The left-hand side of a compound assignment expression must be an lvalue
796 expression. An lvalue expression represents a memory location and includes
797 item paths (ie, namespaced variables), dereferences, indexing expressions,
798 and field references.
800 Let's start with some erroneous code examples:
803 use std::collections::LinkedList;
805 // Bad: assignment to non-lvalue expression
806 LinkedList::new() += 1;
810 fn some_func(i: &mut i32) {
811 i += 12; // Error : '+=' operation cannot be applied on a reference !
815 And now some working examples:
824 fn some_func(i: &mut i32) {
831 The compiler found a function whose body contains a `return;` statement but
832 whose return type is not `()`. An example of this is:
841 Since `return;` is just like `return ();`, there is a mismatch between the
842 function's return type and the value being returned.
846 The left-hand side of an assignment operator must be an lvalue expression. An
847 lvalue expression represents a memory location and can be a variable (with
848 optional namespacing), a dereference, an indexing expression or a field
851 More details can be found here:
852 https://doc.rust-lang.org/reference.html#lvalues-rvalues-and-temporaries
854 Now, we can go further. Here are some erroneous code examples:
862 const SOME_CONST : i32 = 12;
864 fn some_other_func() {}
867 SOME_CONST = 14; // error : a constant value cannot be changed!
868 1 = 3; // error : 1 isn't a valid lvalue!
869 some_other_func() = 4; // error : we can't assign value to a function!
870 SomeStruct.x = 12; // error : SomeStruct a structure name but it is used
875 And now let's give working examples:
882 let mut s = SomeStruct {x: 0, y: 0};
884 s.x = 3; // that's good !
888 fn some_func(x: &mut i32) {
889 *x = 12; // that's good !
895 You tried to use structure-literal syntax to create an item that is
896 not a struct-style structure or enum variant.
898 Example of erroneous code:
901 enum Foo { FirstValue(i32) };
903 let u = Foo::FirstValue { value: 0 }; // error: Foo::FirstValue
904 // isn't a structure!
905 // or even simpler, if the name doesn't refer to a structure at all.
906 let t = u32 { value: 4 }; // error: `u32` does not name a structure.
909 To fix this, ensure that the name was correctly spelled, and that
910 the correct form of initializer was used.
912 For example, the code above can be fixed to:
920 let u = Foo::FirstValue(0i32);
928 You cannot define a struct (or enum) `Foo` that requires an instance of `Foo`
929 in order to make a new `Foo` value. This is because there would be no way a
930 first instance of `Foo` could be made to initialize another instance!
932 Here's an example of a struct that has this problem:
935 struct Foo { x: Box<Foo> } // error
938 One fix is to use `Option`, like so:
941 struct Foo { x: Option<Box<Foo>> }
944 Now it's possible to create at least one instance of `Foo`: `Foo { x: None }`.
948 When using the `#[simd]` attribute on a tuple struct, the components of the
949 tuple struct must all be of a concrete, nongeneric type so the compiler can
950 reason about how to use SIMD with them. This error will occur if the types
953 This will cause an error:
956 #![feature(repr_simd)]
959 struct Bad<T>(T, T, T);
965 #![feature(repr_simd)]
968 struct Good(u32, u32, u32);
973 The `#[simd]` attribute can only be applied to non empty tuple structs, because
974 it doesn't make sense to try to use SIMD operations when there are no values to
977 This will cause an error:
980 #![feature(repr_simd)]
989 #![feature(repr_simd)]
997 When using the `#[simd]` attribute to automatically use SIMD operations in tuple
998 struct, the types in the struct must all be of the same type, or the compiler
999 will trigger this error.
1001 This will cause an error:
1004 #![feature(repr_simd)]
1007 struct Bad(u16, u32, u32);
1013 #![feature(repr_simd)]
1016 struct Good(u32, u32, u32);
1021 When using the `#[simd]` attribute on a tuple struct, the elements in the tuple
1022 must be machine types so SIMD operations can be applied to them.
1024 This will cause an error:
1027 #![feature(repr_simd)]
1036 #![feature(repr_simd)]
1039 struct Good(u32, u32, u32);
1044 Enum variants which contain no data can be given a custom integer
1045 representation. This error indicates that the value provided is not an integer
1046 literal and is therefore invalid.
1048 For example, in the following code:
1056 We try to set the representation to a string.
1058 There's no general fix for this; if you can work with an integer then just set
1067 However if you actually wanted a mapping between variants and non-integer
1068 objects, it may be preferable to use a method with a match instead:
1073 fn get_str(&self) -> &'static str {
1083 This error indicates that the compiler was unable to sensibly evaluate an
1084 integer expression provided as an enum discriminant. Attempting to divide by 0
1085 or causing integer overflow are two ways to induce this error. For example:
1094 Ensure that the expressions given can be evaluated as the desired integer type.
1095 See the FFI section of the Reference for more information about using a custom
1098 https://doc.rust-lang.org/reference.html#ffi-attributes
1102 Enum discriminants are used to differentiate enum variants stored in memory.
1103 This error indicates that the same value was used for two or more variants,
1104 making them impossible to tell apart.
1124 Note that variants without a manually specified discriminant are numbered from
1125 top to bottom starting from 0, so clashes can occur with seemingly unrelated
1135 Here `X` will have already been specified the discriminant 0 by the time `Y` is
1136 encountered, so a conflict occurs.
1140 When you specify enum discriminants with `=`, the compiler expects `isize`
1141 values by default. Or you can add the `repr` attibute to the enum declaration
1142 for an explicit choice of the discriminant type. In either cases, the
1143 discriminant values must fall within a valid range for the expected type;
1144 otherwise this error is raised. For example:
1154 Here, 1024 lies outside the valid range for `u8`, so the discriminant for `A` is
1155 invalid. Here is another, more subtle example which depends on target word size:
1158 enum DependsOnPointerSize {
1163 Here, `1 << 32` is interpreted as an `isize` value. So it is invalid for 32 bit
1164 target (`target_pointer_width = "32"`) but valid for 64 bit target.
1166 You may want to change representation types to fix this, or else change invalid
1167 discriminant values so that they fit within the existing type.
1171 An unsupported representation was attempted on a zero-variant enum.
1173 Erroneous code example:
1177 enum NightsWatch {} // error: unsupported representation for zero-variant enum
1180 It is impossible to define an integer type to be used to represent zero-variant
1181 enum values because there are no zero-variant enum values. There is no way to
1182 construct an instance of the following type using only safe code. So you have
1183 two solutions. Either you add variants in your enum:
1193 or you remove the integer represention of your enum:
1201 Too many type parameters were supplied for a function. For example:
1207 foo::<f64, bool>(); // error, expected 1 parameter, found 2 parameters
1211 The number of supplied parameters must exactly match the number of defined type
1216 You gave too many lifetime parameters. Erroneous code example:
1222 f::<'static>() // error: too many lifetime parameters provided
1226 Please check you give the right number of lifetime parameters. Example:
1236 It's also important to note that the Rust compiler can generally
1237 determine the lifetime by itself. Example:
1245 // it can be written like this
1246 fn get_value<'a>(&'a self) -> &'a str { &self.value }
1247 // but the compiler works fine with this too:
1248 fn without_lifetime(&self) -> &str { &self.value }
1252 let f = Foo { value: "hello".to_owned() };
1254 println!("{}", f.get_value());
1255 println!("{}", f.without_lifetime());
1261 Not enough type parameters were supplied for a function. For example:
1267 foo::<f64>(); // error, expected 2 parameters, found 1 parameter
1271 Note that if a function takes multiple type parameters but you want the compiler
1272 to infer some of them, you can use type placeholders:
1275 fn foo<T, U>(x: T) {}
1279 foo::<f64>(x); // error, expected 2 parameters, found 1 parameter
1280 foo::<_, f64>(x); // same as `foo::<bool, f64>(x)`
1286 You gave an unnecessary type parameter in a type alias. Erroneous code
1290 type Foo<T> = u32; // error: type parameter `T` is unused
1292 type Foo<A,B> = Box<A>; // error: type parameter `B` is unused
1295 Please check you didn't write too many type parameters. Example:
1298 type Foo = u32; // ok!
1299 type Foo2<A> = Box<A>; // ok!
1304 You tried to declare an undefined atomic operation function.
1305 Erroneous code example:
1308 #![feature(intrinsics)]
1310 extern "rust-intrinsic" {
1311 fn atomic_foo(); // error: unrecognized atomic operation
1316 Please check you didn't make a mistake in the function's name. All intrinsic
1317 functions are defined in librustc_trans/trans/intrinsic.rs and in
1318 libcore/intrinsics.rs in the Rust source code. Example:
1321 #![feature(intrinsics)]
1323 extern "rust-intrinsic" {
1324 fn atomic_fence(); // ok!
1330 You declared an unknown intrinsic function. Erroneous code example:
1333 #![feature(intrinsics)]
1335 extern "rust-intrinsic" {
1336 fn foo(); // error: unrecognized intrinsic function: `foo`
1346 Please check you didn't make a mistake in the function's name. All intrinsic
1347 functions are defined in librustc_trans/trans/intrinsic.rs and in
1348 libcore/intrinsics.rs in the Rust source code. Example:
1351 #![feature(intrinsics)]
1353 extern "rust-intrinsic" {
1354 fn atomic_fence(); // ok!
1366 You gave an invalid number of type parameters to an intrinsic function.
1367 Erroneous code example:
1370 #![feature(intrinsics)]
1372 extern "rust-intrinsic" {
1373 fn size_of<T, U>() -> usize; // error: intrinsic has wrong number
1374 // of type parameters
1378 Please check that you provided the right number of lifetime parameters
1379 and verify with the function declaration in the Rust source code.
1383 #![feature(intrinsics)]
1385 extern "rust-intrinsic" {
1386 fn size_of<T>() -> usize; // ok!
1392 You hit this error because the compiler lacks the information to
1393 determine a type for this expression. Erroneous code example:
1397 let x = |_| {}; // error: cannot determine a type for this expression
1401 You have two possibilities to solve this situation:
1402 * Give an explicit definition of the expression
1403 * Infer the expression
1409 let x = |_ : u32| {}; // ok!
1418 You hit this error because the compiler lacks the information to
1419 determine the type of this variable. Erroneous code example:
1423 // could be an array of anything
1424 let x = []; // error: cannot determine a type for this local variable
1428 To solve this situation, constrain the type of the variable.
1432 #![allow(unused_variables)]
1435 let x: [u8; 0] = [];
1441 This error indicates that a lifetime is missing from a type. If it is an error
1442 inside a function signature, the problem may be with failing to adhere to the
1443 lifetime elision rules (see below).
1445 Here are some simple examples of where you'll run into this error:
1448 struct Foo { x: &bool } // error
1449 struct Foo<'a> { x: &'a bool } // correct
1451 enum Bar { A(u8), B(&bool), } // error
1452 enum Bar<'a> { A(u8), B(&'a bool), } // correct
1454 type MyStr = &str; // error
1455 type MyStr<'a> = &'a str; // correct
1458 Lifetime elision is a special, limited kind of inference for lifetimes in
1459 function signatures which allows you to leave out lifetimes in certain cases.
1460 For more background on lifetime elision see [the book][book-le].
1462 The lifetime elision rules require that any function signature with an elided
1463 output lifetime must either have
1465 - exactly one input lifetime
1466 - or, multiple input lifetimes, but the function must also be a method with a
1467 `&self` or `&mut self` receiver
1469 In the first case, the output lifetime is inferred to be the same as the unique
1470 input lifetime. In the second case, the lifetime is instead inferred to be the
1471 same as the lifetime on `&self` or `&mut self`.
1473 Here are some examples of elision errors:
1476 // error, no input lifetimes
1477 fn foo() -> &str { }
1479 // error, `x` and `y` have distinct lifetimes inferred
1480 fn bar(x: &str, y: &str) -> &str { }
1482 // error, `y`'s lifetime is inferred to be distinct from `x`'s
1483 fn baz<'a>(x: &'a str, y: &str) -> &str { }
1486 [book-le]: https://doc.rust-lang.org/nightly/book/lifetimes.html#lifetime-elision
1490 This error means that an incorrect number of lifetime parameters were provided
1491 for a type (like a struct or enum) or trait.
1493 Some basic examples include:
1496 struct Foo<'a>(&'a str);
1497 enum Bar { A, B, C }
1500 foo: Foo, // error: expected 1, found 0
1501 bar: Bar<'a>, // error: expected 0, found 1
1505 Here's an example that is currently an error, but may work in a future version
1509 struct Foo<'a>(&'a str);
1512 impl Quux for Foo { } // error: expected 1, found 0
1515 Lifetime elision in implementation headers was part of the lifetime elision
1516 RFC. It is, however, [currently unimplemented][iss15872].
1518 [iss15872]: https://github.com/rust-lang/rust/issues/15872
1522 You can only define an inherent implementation for a type in the same crate
1523 where the type was defined. For example, an `impl` block as below is not allowed
1524 since `Vec` is defined in the standard library:
1527 impl Vec<u8> { } // error
1530 To fix this problem, you can do either of these things:
1532 - define a trait that has the desired associated functions/types/constants and
1533 implement the trait for the type in question
1534 - define a new type wrapping the type and define an implementation on the new
1537 Note that using the `type` keyword does not work here because `type` only
1538 introduces a type alias:
1541 type Bytes = Vec<u8>;
1543 impl Bytes { } // error, same as above
1548 This error indicates a violation of one of Rust's orphan rules for trait
1549 implementations. The rule prohibits any implementation of a foreign trait (a
1550 trait defined in another crate) where
1552 - the type that is implementing the trait is foreign
1553 - all of the parameters being passed to the trait (if there are any) are also
1556 Here's one example of this error:
1559 impl Drop for u32 {}
1562 To avoid this kind of error, ensure that at least one local type is referenced
1566 pub struct Foo; // you define your type in your crate
1568 impl Drop for Foo { // and you can implement the trait on it!
1569 // code of trait implementation here
1572 impl From<Foo> for i32 { // or you use a type from your crate as
1574 fn from(i: Foo) -> i32 {
1580 Alternatively, define a trait locally and implement that instead:
1584 fn get(&self) -> usize;
1588 fn get(&self) -> usize { 0 }
1592 For information on the design of the orphan rules, see [RFC 1023].
1594 [RFC 1023]: https://github.com/rust-lang/rfcs/pull/1023
1598 You're trying to write an inherent implementation for something which isn't a
1599 struct nor an enum. Erroneous code example:
1602 impl (u8, u8) { // error: no base type found for inherent implementation
1603 fn get_state(&self) -> String {
1609 To fix this error, please implement a trait on the type or wrap it in a struct.
1613 // we create a trait here
1614 trait LiveLongAndProsper {
1615 fn get_state(&self) -> String;
1618 // and now you can implement it on (u8, u8)
1619 impl LiveLongAndProsper for (u8, u8) {
1620 fn get_state(&self) -> String {
1621 "He's dead, Jim!".to_owned()
1626 Alternatively, you can create a newtype. A newtype is a wrapping tuple-struct.
1627 For example, `NewType` is a newtype over `Foo` in `struct NewType(Foo)`.
1631 struct TypeWrapper((u8, u8));
1634 fn get_state(&self) -> String {
1635 "Fascinating!".to_owned()
1642 There are conflicting trait implementations for the same type.
1643 Example of erroneous code:
1647 fn get(&self) -> usize;
1650 impl<T> MyTrait for T {
1651 fn get(&self) -> usize { 0 }
1658 impl MyTrait for Foo { // error: conflicting implementations of trait
1659 // `MyTrait` for type `Foo`
1660 fn get(&self) -> usize { self.value }
1664 When looking for the implementation for the trait, the compiler finds
1665 both the `impl<T> MyTrait for T` where T is all types and the `impl
1666 MyTrait for Foo`. Since a trait cannot be implemented multiple times,
1667 this is an error. So, when you write:
1671 fn get(&self) -> usize;
1674 impl<T> MyTrait for T {
1675 fn get(&self) -> usize { 0 }
1679 This makes the trait implemented on all types in the scope. So if you
1680 try to implement it on another one after that, the implementations will
1685 fn get(&self) -> usize;
1688 impl<T> MyTrait for T {
1689 fn get(&self) -> usize { 0 }
1697 f.get(); // the trait is implemented so we can use it
1703 An attempt was made to implement Drop on a trait, which is not allowed: only
1704 structs and enums can implement Drop. An example causing this error:
1709 impl Drop for MyTrait {
1710 fn drop(&mut self) {}
1714 A workaround for this problem is to wrap the trait up in a struct, and implement
1715 Drop on that. An example is shown below:
1719 struct MyWrapper<T: MyTrait> { foo: T }
1721 impl <T: MyTrait> Drop for MyWrapper<T> {
1722 fn drop(&mut self) {}
1727 Alternatively, wrapping trait objects requires something like the following:
1732 //or Box<MyTrait>, if you wanted an owned trait object
1733 struct MyWrapper<'a> { foo: &'a MyTrait }
1735 impl <'a> Drop for MyWrapper<'a> {
1736 fn drop(&mut self) {}
1742 In order to be consistent with Rust's lack of global type inference, type
1743 placeholders are disallowed by design in item signatures.
1745 Examples of this error include:
1748 fn foo() -> _ { 5 } // error, explicitly write out the return type instead
1750 static BAR: _ = "test"; // error, explicitly write out the type instead
1755 An attempt was made to add a generic constraint to a type alias. While Rust will
1756 allow this with a warning, it will not currently enforce the constraint.
1757 Consider the example below:
1762 type MyType<R: Foo> = (R, ());
1769 We're able to declare a variable of type `MyType<u32>`, despite the fact that
1770 `u32` does not implement `Foo`. As a result, one should avoid using generic
1771 constraints in concert with type aliases.
1775 You declared two fields of a struct with the same name. Erroneous code
1781 field1: i32, // error: field is already declared
1785 Please verify that the field names have been correctly spelled. Example:
1796 Type parameter defaults can only use parameters that occur before them.
1797 Erroneous code example:
1800 struct Foo<T=U, U=()> {
1804 // error: type parameters with a default cannot use forward declared
1808 Since type parameters are evaluated in-order, you may be able to fix this issue
1812 struct Foo<U=(), T=U> {
1818 Please also verify that this wasn't because of a name-clash and rename the type
1823 You declared a pattern as an argument in a foreign function declaration.
1824 Erroneous code example:
1828 fn foo((a, b): (u32, u32)); // error: patterns aren't allowed in foreign
1829 // function declarations
1833 Please replace the pattern argument with a regular one. Example:
1842 fn foo(s: SomeStruct); // ok!
1850 fn foo(a: (u32, u32)); // ok!
1856 It is not possible to define `main` with type parameters, or even with function
1857 parameters. When `main` is present, it must take no arguments and return `()`.
1858 Erroneous code example:
1861 fn main<T>() { // error: main function is not allowed to have type parameters
1867 A function with the `start` attribute was declared with type parameters.
1869 Erroneous code example:
1878 It is not possible to declare type parameters on a function that has the `start`
1879 attribute. Such a function must have the following type signature (for more
1880 information: http://doc.rust-lang.org/stable/book/no-stdlib.html):
1883 fn(isize, *const *const u8) -> isize;
1892 fn my_start(argc: isize, argv: *const *const u8) -> isize {
1899 This error means that an attempt was made to match an enum variant as a
1900 struct type when the variant isn't a struct type:
1905 fn bar(foo: Foo) -> u32 {
1907 B{i} => i, // error E0163
1912 Try using `()` instead:
1917 fn bar(foo: Foo) -> u32 {
1926 This error means that an attempt was made to match a struct type enum
1927 variant as a non-struct type:
1930 enum Foo { B { i: u32 } }
1932 fn bar(foo: Foo) -> u32 {
1934 Foo::B(i) => i, // error E0164
1939 Try using `{}` instead:
1942 enum Foo { B { i: u32 } }
1944 fn bar(foo: Foo) -> u32 {
1953 This error means that the compiler found a return expression in a function
1954 marked as diverging. A function diverges if it has `!` in the place of the
1955 return type in its signature. For example:
1958 fn foo() -> ! { return; } // error
1961 For a function that diverges, every control path in the function must never
1962 return, for example with a `loop` that never breaks or a call to another
1963 diverging function (such as `panic!()`).
1967 This error means that an attempt was made to specify the type of a variable with
1968 a combination of a concrete type and a trait. Consider the following example:
1971 fn foo(bar: i32+std::fmt::Display) {}
1974 The code is trying to specify that we want to receive a signed 32-bit integer
1975 which also implements `Display`. This doesn't make sense: when we pass `i32`, a
1976 concrete type, it implicitly includes all of the traits that it implements.
1977 This includes `Display`, `Debug`, `Clone`, and a host of others.
1979 If `i32` implements the trait we desire, there's no need to specify the trait
1980 separately. If it does not, then we need to `impl` the trait for `i32` before
1981 passing it into `foo`. Either way, a fixed definition for `foo` will look like
1988 To learn more about traits, take a look at the Book:
1990 https://doc.rust-lang.org/book/traits.html
1994 This error occurs because of the explicit use of unboxed closure methods
1995 that are an experimental feature in current Rust version.
1997 Example of erroneous code:
2000 fn foo<F: Fn(&str)>(mut f: F) {
2002 // error: explicit use of unboxed closure method `call`
2003 f.call_mut(("call_mut",));
2004 // error: explicit use of unboxed closure method `call_mut`
2005 f.call_once(("call_once",));
2006 // error: explicit use of unboxed closure method `call_once`
2009 fn bar(text: &str) {
2010 println!("Calling {} it works!", text);
2018 Rust's implementation of closures is a bit different than other languages.
2019 They are effectively syntax sugar for traits `Fn`, `FnMut` and `FnOnce`.
2020 To understand better how the closures are implemented see here:
2021 https://doc.rust-lang.org/book/closures.html#closure-implementation
2023 To fix this you can call them using parenthesis, like this: `foo()`.
2024 When you execute the closure with parenthesis, under the hood you are executing
2025 the method `call`, `call_mut` or `call_once`. However, using them explicitly is
2026 currently an experimental feature.
2028 Example of an implicit call:
2031 fn foo<F: Fn(&str)>(f: F) {
2032 f("using ()"); // Calling using () it works!
2035 fn bar(text: &str) {
2036 println!("Calling {} it works!", text);
2044 To enable the explicit calls you need to add `#![feature(unboxed_closures)]`.
2046 This feature is still unstable so you will also need to add
2047 `#![feature(fn_traits)]`.
2048 More details about this issue here:
2049 https://github.com/rust-lang/rust/issues/29625
2054 #![feature(fn_traits)]
2055 #![feature(unboxed_closures)]
2057 fn foo<F: Fn(&str)>(mut f: F) {
2058 f.call(("call",)); // Calling 'call' it works!
2059 f.call_mut(("call_mut",)); // Calling 'call_mut' it works!
2060 f.call_once(("call_once",)); // Calling 'call_once' it works!
2063 fn bar(text: &str) {
2064 println!("Calling '{}' it works!", text);
2072 To see more about closures take a look here:
2073 https://doc.rust-lang.org/book/closures.html`
2077 In types, the `+` type operator has low precedence, so it is often necessary
2086 w: &'a Foo + Copy, // error, use &'a (Foo + Copy)
2087 x: &'a Foo + 'a, // error, use &'a (Foo + 'a)
2088 y: &'a mut Foo + 'a, // error, use &'a mut (Foo + 'a)
2089 z: fn() -> Foo + 'a, // error, use fn() -> (Foo + 'a)
2093 More details can be found in [RFC 438].
2095 [RFC 438]: https://github.com/rust-lang/rfcs/pull/438
2099 Explicitly implementing both Drop and Copy for a type is currently disallowed.
2100 This feature can make some sense in theory, but the current implementation is
2101 incorrect and can lead to memory unsafety (see [issue #20126][iss20126]), so
2102 it has been disabled for now.
2104 [iss20126]: https://github.com/rust-lang/rust/issues/20126
2108 An associated function for a trait was defined to be static, but an
2109 implementation of the trait declared the same function to be a method (i.e. to
2110 take a `self` parameter).
2112 Here's an example of this error:
2122 // error, method `foo` has a `&self` declaration in the impl, but not in
2130 An associated function for a trait was defined to be a method (i.e. to take a
2131 `self` parameter), but an implementation of the trait declared the same function
2134 Here's an example of this error:
2144 // error, method `foo` has a `&self` declaration in the trait, but not in
2152 Trait objects need to have all associated types specified. Erroneous code
2160 type Foo = Trait; // error: the value of the associated type `Bar` (from
2161 // the trait `Trait`) must be specified
2164 Please verify you specified all associated types of the trait and that you
2165 used the right trait. Example:
2172 type Foo = Trait<Bar=i32>; // ok!
2177 Negative impls are only allowed for traits with default impls. For more
2178 information see the [opt-in builtin traits RFC](https://github.com/rust-lang/
2179 rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
2183 `where` clauses must use generic type parameters: it does not make sense to use
2184 them otherwise. An example causing this error:
2191 #[derive(Copy,Clone)]
2196 impl Foo for Wrapper<u32> where Wrapper<u32>: Clone {
2201 This use of a `where` clause is strange - a more common usage would look
2202 something like the following:
2209 #[derive(Copy,Clone)]
2213 impl <T> Foo for Wrapper<T> where Wrapper<T>: Clone {
2218 Here, we're saying that the implementation exists on Wrapper only when the
2219 wrapped type `T` implements `Clone`. The `where` clause is important because
2220 some types will not implement `Clone`, and thus will not get this method.
2222 In our erroneous example, however, we're referencing a single concrete type.
2223 Since we know for certain that `Wrapper<u32>` implements `Clone`, there's no
2224 reason to also specify it in a `where` clause.
2228 A type parameter was declared which shadows an existing one. An example of this
2233 fn do_something(&self) -> T;
2234 fn do_something_else<T: Clone>(&self, bar: T);
2238 In this example, the trait `Foo` and the trait method `do_something_else` both
2239 define a type parameter `T`. This is not allowed: if the method wishes to
2240 define a type parameter, it must use a different name for it.
2244 Your method's lifetime parameters do not match the trait declaration.
2245 Erroneous code example:
2249 fn bar<'a,'b:'a>(x: &'a str, y: &'b str);
2254 impl Trait for Foo {
2255 fn bar<'a,'b>(x: &'a str, y: &'b str) {
2256 // error: lifetime parameters or bounds on method `bar`
2257 // do not match the trait declaration
2262 The lifetime constraint `'b` for bar() implementation does not match the
2263 trait declaration. Ensure lifetime declarations match exactly in both trait
2264 declaration and implementation. Example:
2268 fn t<'a,'b:'a>(x: &'a str, y: &'b str);
2273 impl Trait for Foo {
2274 fn t<'a,'b:'a>(x: &'a str, y: &'b str) { // ok!
2281 Inherent implementations (one that do not implement a trait but provide
2282 methods associated with a type) are always safe because they are not
2283 implementing an unsafe trait. Removing the `unsafe` keyword from the inherent
2284 implementation will resolve this error.
2289 // this will cause this error
2291 // converting it to this will fix it
2297 A negative implementation is one that excludes a type from implementing a
2298 particular trait. Not being able to use a trait is always a safe operation,
2299 so negative implementations are always safe and never need to be marked as
2303 #![feature(optin_builtin_traits)]
2307 // unsafe is unnecessary
2308 unsafe impl !Clone for Foo { }
2314 #![feature(optin_builtin_traits)]
2320 impl Enterprise for .. { }
2322 impl !Enterprise for Foo { }
2325 Please note that negative impls are only allowed for traits with default impls.
2329 Safe traits should not have unsafe implementations, therefore marking an
2330 implementation for a safe trait unsafe will cause a compiler error. Removing
2331 the unsafe marker on the trait noted in the error will resolve this problem.
2338 // this won't compile because Bar is safe
2339 unsafe impl Bar for Foo { }
2340 // this will compile
2341 impl Bar for Foo { }
2346 Unsafe traits must have unsafe implementations. This error occurs when an
2347 implementation for an unsafe trait isn't marked as unsafe. This may be resolved
2348 by marking the unsafe implementation as unsafe.
2353 unsafe trait Bar { }
2355 // this won't compile because Bar is unsafe and impl isn't unsafe
2356 impl Bar for Foo { }
2357 // this will compile
2358 unsafe impl Bar for Foo { }
2363 It is an error to define two associated items (like methods, associated types,
2364 associated functions, etc.) with the same identifier.
2372 fn bar(&self) -> bool { self.0 > 5 }
2373 fn bar() {} // error: duplicate associated function
2378 fn baz(&self) -> bool;
2384 fn baz(&self) -> bool { true }
2386 // error: duplicate method
2387 fn baz(&self) -> bool { self.0 > 5 }
2389 // error: duplicate associated type
2394 Note, however, that items with the same name are allowed for inherent `impl`
2395 blocks that don't overlap:
2401 fn bar(&self) -> bool { self.0 > 5 }
2405 fn bar(&self) -> bool { self.0 }
2411 Inherent associated types were part of [RFC 195] but are not yet implemented.
2412 See [the tracking issue][iss8995] for the status of this implementation.
2414 [RFC 195]: https://github.com/rust-lang/rfcs/pull/195
2415 [iss8995]: https://github.com/rust-lang/rust/issues/8995
2419 An attempt to implement the `Copy` trait for a struct failed because one of the
2420 fields does not implement `Copy`. To fix this, you must implement `Copy` for the
2421 mentioned field. Note that this may not be possible, as in the example of
2428 impl Copy for Foo { }
2431 This fails because `Vec<T>` does not implement `Copy` for any `T`.
2433 Here's another example that will fail:
2442 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
2443 differs from the behavior for `&T`, which is always `Copy`).
2447 An attempt to implement the `Copy` trait for an enum failed because one of the
2448 variants does not implement `Copy`. To fix this, you must implement `Copy` for
2449 the mentioned variant. Note that this may not be possible, as in the example of
2457 impl Copy for Foo { }
2460 This fails because `Vec<T>` does not implement `Copy` for any `T`.
2462 Here's another example that will fail:
2472 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
2473 differs from the behavior for `&T`, which is always `Copy`).
2477 You can only implement `Copy` for a struct or enum. Both of the following
2478 examples will fail, because neither `i32` (primitive type) nor `&'static Bar`
2479 (reference to `Bar`) is a struct or enum:
2483 impl Copy for Foo { } // error
2485 #[derive(Copy, Clone)]
2487 impl Copy for &'static Bar { } // error
2492 Any type parameter or lifetime parameter of an `impl` must meet at least one of
2493 the following criteria:
2495 - it appears in the self type of the impl
2496 - for a trait impl, it appears in the trait reference
2497 - it is bound as an associated type
2501 Suppose we have a struct `Foo` and we would like to define some methods for it.
2502 The following definition leads to a compiler error:
2507 impl<T: Default> Foo {
2508 // error: the type parameter `T` is not constrained by the impl trait, self
2509 // type, or predicates [E0207]
2510 fn get(&self) -> T {
2511 <T as Default>::default()
2516 The problem is that the parameter `T` does not appear in the self type (`Foo`)
2517 of the impl. In this case, we can fix the error by moving the type parameter
2518 from the `impl` to the method `get`:
2524 // Move the type parameter from the impl to the method
2526 fn get<T: Default>(&self) -> T {
2527 <T as Default>::default()
2534 As another example, suppose we have a `Maker` trait and want to establish a
2535 type `FooMaker` that makes `Foo`s:
2540 fn make(&mut self) -> Self::Item;
2549 impl<T: Default> Maker for FooMaker {
2550 // error: the type parameter `T` is not constrained by the impl trait, self
2551 // type, or predicates [E0207]
2554 fn make(&mut self) -> Foo<T> {
2555 Foo { foo: <T as Default>::default() }
2560 This fails to compile because `T` does not appear in the trait or in the
2563 One way to work around this is to introduce a phantom type parameter into
2564 `FooMaker`, like so:
2567 use std::marker::PhantomData;
2571 fn make(&mut self) -> Self::Item;
2578 // Add a type parameter to `FooMaker`
2579 struct FooMaker<T> {
2580 phantom: PhantomData<T>,
2583 impl<T: Default> Maker for FooMaker<T> {
2586 fn make(&mut self) -> Foo<T> {
2588 foo: <T as Default>::default(),
2594 Another way is to do away with the associated type in `Maker` and use an input
2595 type parameter instead:
2598 // Use a type parameter instead of an associated type here
2600 fn make(&mut self) -> Item;
2609 impl<T: Default> Maker<Foo<T>> for FooMaker {
2610 fn make(&mut self) -> Foo<T> {
2611 Foo { foo: <T as Default>::default() }
2616 ### Additional information
2618 For more information, please see [RFC 447].
2620 [RFC 447]: https://github.com/rust-lang/rfcs/blob/master/text/0447-no-unused-impl-parameters.md
2624 This error indicates a violation of one of Rust's orphan rules for trait
2625 implementations. The rule concerns the use of type parameters in an
2626 implementation of a foreign trait (a trait defined in another crate), and
2627 states that type parameters must be "covered" by a local type. To understand
2628 what this means, it is perhaps easiest to consider a few examples.
2630 If `ForeignTrait` is a trait defined in some external crate `foo`, then the
2631 following trait `impl` is an error:
2635 use foo::ForeignTrait;
2637 impl<T> ForeignTrait for T { } // error
2640 To work around this, it can be covered with a local type, `MyType`:
2643 struct MyType<T>(T);
2644 impl<T> ForeignTrait for MyType<T> { } // Ok
2647 Please note that a type alias is not sufficient.
2649 For another example of an error, suppose there's another trait defined in `foo`
2650 named `ForeignTrait2` that takes two type parameters. Then this `impl` results
2651 in the same rule violation:
2655 impl<T> ForeignTrait2<T, MyType<T>> for MyType2 { } // error
2658 The reason for this is that there are two appearances of type parameter `T` in
2659 the `impl` header, both as parameters for `ForeignTrait2`. The first appearance
2660 is uncovered, and so runs afoul of the orphan rule.
2662 Consider one more example:
2665 impl<T> ForeignTrait2<MyType<T>, T> for MyType2 { } // Ok
2668 This only differs from the previous `impl` in that the parameters `T` and
2669 `MyType<T>` for `ForeignTrait2` have been swapped. This example does *not*
2670 violate the orphan rule; it is permitted.
2672 To see why that last example was allowed, you need to understand the general
2673 rule. Unfortunately this rule is a bit tricky to state. Consider an `impl`:
2676 impl<P1, ..., Pm> ForeignTrait<T1, ..., Tn> for T0 { ... }
2679 where `P1, ..., Pm` are the type parameters of the `impl` and `T0, ..., Tn`
2680 are types. One of the types `T0, ..., Tn` must be a local type (this is another
2681 orphan rule, see the explanation for E0117). Let `i` be the smallest integer
2682 such that `Ti` is a local type. Then no type parameter can appear in any of the
2685 For information on the design of the orphan rules, see [RFC 1023].
2687 [RFC 1023]: https://github.com/rust-lang/rfcs/pull/1023
2691 You used a function or type which doesn't fit the requirements for where it was
2692 used. Erroneous code examples:
2695 #![feature(intrinsics)]
2697 extern "rust-intrinsic" {
2698 fn size_of<T>(); // error: intrinsic has wrong type
2703 fn main() -> i32 { 0 }
2704 // error: main function expects type: `fn() {main}`: expected (), found i32
2711 // error: mismatched types in range: expected u8, found i8
2721 fn x(self: Rc<Foo>) {}
2722 // error: mismatched self type: expected `Foo`: expected struct
2723 // `Foo`, found struct `alloc::rc::Rc`
2727 For the first code example, please check the function definition. Example:
2730 #![feature(intrinsics)]
2732 extern "rust-intrinsic" {
2733 fn size_of<T>() -> usize; // ok!
2737 The second case example is a bit particular : the main function must always
2738 have this definition:
2744 They never take parameters and never return types.
2746 For the third example, when you match, all patterns must have the same type
2747 as the type you're matching on. Example:
2753 0u8...3u8 => (), // ok!
2758 And finally, for the last example, only `Box<Self>`, `&Self`, `Self`,
2759 or `&mut Self` work as explicit self parameters. Example:
2765 fn x(self: Box<Foo>) {} // ok!
2771 A generic type was described using parentheses rather than angle brackets. For
2776 let v: Vec(&str) = vec!["foo"];
2780 This is not currently supported: `v` should be defined as `Vec<&str>`.
2781 Parentheses are currently only used with generic types when defining parameters
2782 for `Fn`-family traits.
2786 You used an associated type which isn't defined in the trait.
2787 Erroneous code example:
2794 type Foo = Trait<F=i32>; // error: associated type `F` not found for
2798 Please verify you used the right trait or you didn't misspell the
2799 associated type name. Example:
2806 type Foo = Trait<Bar=i32>; // ok!
2811 An attempt was made to retrieve an associated type, but the type was ambiguous.
2830 In this example, `Foo` defines an associated type `A`. `Bar` inherits that type
2831 from `Foo`, and defines another associated type of the same name. As a result,
2832 when we attempt to use `Self::A`, it's ambiguous whether we mean the `A` defined
2833 by `Foo` or the one defined by `Bar`.
2835 There are two options to work around this issue. The first is simply to rename
2836 one of the types. Alternatively, one can specify the intended type using the
2850 let _: <Self as Bar>::A;
2857 An attempt was made to retrieve an associated type, but the type was ambiguous.
2861 trait MyTrait {type X; }
2864 let foo: MyTrait::X;
2868 The problem here is that we're attempting to take the type of X from MyTrait.
2869 Unfortunately, the type of X is not defined, because it's only made concrete in
2870 implementations of the trait. A working version of this code might look like:
2873 trait MyTrait {type X; }
2876 impl MyTrait for MyStruct {
2881 let foo: <MyStruct as MyTrait>::X;
2885 This syntax specifies that we want the X type from MyTrait, as made concrete in
2886 MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct
2887 might implement two different traits with identically-named associated types.
2888 This syntax allows disambiguation between the two.
2892 You attempted to use multiple types as bounds for a closure or trait object.
2893 Rust does not currently support this. A simple example that causes this error:
2897 let _: Box<std::io::Read + std::io::Write>;
2901 Builtin traits are an exception to this rule: it's possible to have bounds of
2902 one non-builtin type, plus any number of builtin types. For example, the
2903 following compiles correctly:
2907 let _: Box<std::io::Read + Send + Sync>;
2913 The attribute must have a value. Erroneous code example:
2916 #![feature(on_unimplemented)]
2918 #[rustc_on_unimplemented] // error: this attribute must have a value
2922 Please supply the missing value of the attribute. Example:
2925 #![feature(on_unimplemented)]
2927 #[rustc_on_unimplemented = "foo"] // ok!
2933 This error indicates that not enough type parameters were found in a type or
2936 For example, the `Foo` struct below is defined to be generic in `T`, but the
2937 type parameter is missing in the definition of `Bar`:
2940 struct Foo<T> { x: T }
2942 struct Bar { x: Foo }
2947 This error indicates that too many type parameters were found in a type or
2950 For example, the `Foo` struct below has no type parameters, but is supplied
2951 with two in the definition of `Bar`:
2954 struct Foo { x: bool }
2956 struct Bar<S, T> { x: Foo<S, T> }
2961 This error indicates an attempt to use a value where a type is expected. For
2969 fn do_something(x: Foo::Bar) { }
2972 In this example, we're attempting to take a type of `Foo::Bar` in the
2973 do_something function. This is not legal: `Foo::Bar` is a value of type `Foo`,
2974 not a distinct static type. Likewise, it's not legal to attempt to
2975 `impl Foo::Bar`: instead, you must `impl Foo` and then pattern match to specify
2976 behavior for specific enum variants.
2980 This error indicates a constant expression for the array length was found, but
2981 it was not an integer (signed or unsigned) expression.
2983 Some examples of code that produces this error are:
2986 const A: [u32; "hello"] = []; // error
2987 const B: [u32; true] = []; // error
2988 const C: [u32; 0.0] = []; // error
2992 There was an error while evaluating the expression for the length of a fixed-
2995 Some examples of this error are:
2998 // divide by zero in the length expression
2999 const A: [u32; 1/0] = [];
3001 // Rust currently will not evaluate the function `foo` at compile time
3002 fn foo() -> usize { 12 }
3003 const B: [u32; foo()] = [];
3005 // it is an error to try to add `u8` and `f64`
3007 const C: [u32; u8::MAX + f64::EPSILON] = [];
3012 Default impls for a trait must be located in the same crate where the trait was
3013 defined. For more information see the [opt-in builtin traits RFC](https://github
3014 .com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
3018 A cross-crate opt-out trait was implemented on something which wasn't a struct
3019 or enum type. Erroneous code example:
3022 #![feature(optin_builtin_traits)]
3026 impl !Sync for Foo {}
3028 unsafe impl Send for &'static Foo {
3029 // error: cross-crate traits with a default impl, like `core::marker::Send`,
3030 // can only be implemented for a struct/enum type, not
3034 Only structs and enums are permitted to impl Send, Sync, and other opt-out
3035 trait, and the struct or enum must be local to the current crate. So, for
3036 example, `unsafe impl Send for Rc<Foo>` is not allowed.
3040 The `Sized` trait is a special trait built-in to the compiler for types with a
3041 constant size known at compile-time. This trait is automatically implemented
3042 for types as needed by the compiler, and it is currently disallowed to
3043 explicitly implement it for a type.
3047 An associated const was implemented when another trait item was expected.
3048 Erroneous code example:
3051 #![feature(associated_consts)]
3061 // error: item `N` is an associated const, which doesn't match its
3062 // trait `<Bar as Foo>`
3066 Please verify that the associated const wasn't misspelled and the correct trait
3067 was implemented. Example:
3077 type N = u32; // ok!
3084 #![feature(associated_consts)]
3093 const N : u32 = 0; // ok!
3099 A method was implemented when another trait item was expected. Erroneous
3113 // error: item `N` is an associated method, which doesn't match its
3114 // trait `<Bar as Foo>`
3118 To fix this error, please verify that the method name wasn't misspelled and
3119 verify that you are indeed implementing the correct trait items. Example:
3122 #![feature(associated_consts)]
3141 An associated type was implemented when another trait item was expected.
3142 Erroneous code example:
3153 // error: item `N` is an associated type, which doesn't match its
3154 // trait `<Bar as Foo>`
3158 Please verify that the associated type name wasn't misspelled and your
3159 implementation corresponds to the trait definition. Example:
3169 type N = u32; // ok!
3176 #![feature(associated_consts)]
3185 const N : u32 = 0; // ok!
3191 The types of any associated constants in a trait implementation must match the
3192 types in the trait definition. This error indicates that there was a mismatch.
3194 Here's an example of this error:
3204 const BAR: u32 = 5; // error, expected bool, found u32
3210 You cannot use associated items other than constant items as patterns. This
3211 includes method items. Example of erroneous code:
3217 fn bb() -> i32 { 0 }
3222 B::bb => {} // error: associated items in match patterns must
3228 Please check that you're not using a method as a pattern. Example:
3246 An attempt was made to access an associated constant through either a generic
3247 type parameter or `Self`. This is not supported yet. An example causing this
3248 error is shown below:
3251 #![feature(associated_consts)]
3259 impl Foo for MyStruct {
3260 const BAR: f64 = 0f64;
3263 fn get_bar_bad<F: Foo>(t: F) -> f64 {
3268 Currently, the value of `BAR` for a particular type can only be accessed
3269 through a concrete type, as shown below:
3272 #![feature(associated_consts)]
3280 fn get_bar_good() -> f64 {
3281 <MyStruct as Foo>::BAR
3287 An attempt was made to implement `Drop` on a concrete specialization of a
3288 generic type. An example is shown below:
3295 impl Drop for Foo<u32> {
3296 fn drop(&mut self) {}
3300 This code is not legal: it is not possible to specialize `Drop` to a subset of
3301 implementations of a generic type. One workaround for this is to wrap the
3302 generic type, as shown below:
3314 fn drop(&mut self) {}
3320 An attempt was made to implement `Drop` on a specialization of a generic type.
3321 An example is shown below:
3326 struct MyStruct<T> {
3330 impl<T: Foo> Drop for MyStruct<T> {
3331 fn drop(&mut self) {}
3335 This code is not legal: it is not possible to specialize `Drop` to a subset of
3336 implementations of a generic type. In order for this code to work, `MyStruct`
3337 must also require that `T` implements `Foo`. Alternatively, another option is
3338 to wrap the generic type in another that specializes appropriately:
3343 struct MyStruct<T> {
3347 struct MyStructWrapper<T: Foo> {
3351 impl <T: Foo> Drop for MyStructWrapper<T> {
3352 fn drop(&mut self) {}
3358 This error indicates that a binary assignment operator like `+=` or `^=` was
3359 applied to a type that doesn't support it. For example:
3362 let mut x = 12f32; // error: binary operation `<<` cannot be applied to
3368 To fix this error, please check that this type implements this binary
3372 let mut x = 12u32; // the `u32` type does implement the `ShlAssign` trait
3377 It is also possible to overload most operators for your own type by
3378 implementing the `[OP]Assign` traits from `std::ops`.
3380 Another problem you might be facing is this: suppose you've overloaded the `+`
3381 operator for some type `Foo` by implementing the `std::ops::Add` trait for
3382 `Foo`, but you find that using `+=` does not work, as in this example:
3392 fn add(self, rhs: Foo) -> Foo {
3398 let mut x: Foo = Foo(5);
3399 x += Foo(7); // error, `+= cannot be applied to the type `Foo`
3403 This is because `AddAssign` is not automatically implemented, so you need to
3404 manually implement it for your type.
3408 A binary operation was attempted on a type which doesn't support it.
3409 Erroneous code example:
3412 let x = 12f32; // error: binary operation `<<` cannot be applied to
3418 To fix this error, please check that this type implements this binary
3422 let x = 12u32; // the `u32` type does implement it:
3423 // https://doc.rust-lang.org/stable/std/ops/trait.Shl.html
3428 It is also possible to overload most operators for your own type by
3429 implementing traits from `std::ops`.
3433 The maximum value of an enum was reached, so it cannot be automatically
3434 set in the next enum value. Erroneous code example:
3437 #[deny(overflowing_literals)]
3439 X = 0x7fffffffffffffff,
3440 Y, // error: enum discriminant overflowed on value after
3441 // 9223372036854775807: i64; set explicitly via
3442 // Y = -9223372036854775808 if that is desired outcome
3446 To fix this, please set manually the next enum value or put the enum variant
3447 with the maximum value at the end of the enum. Examples:
3451 X = 0x7fffffffffffffff,
3461 X = 0x7fffffffffffffff,
3467 When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
3468 definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
3469 `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
3470 definition, so it is not useful to do this.
3475 trait Foo { fn foo(&self) { } }
3479 impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
3480 impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
3481 impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
3482 impl Baz for Bar { } // Note: This is OK
3487 A struct without a field containing an unsized type cannot implement
3489 [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3490 is any type that the compiler doesn't know the length or alignment of at
3491 compile time. Any struct containing an unsized type is also unsized.
3493 Example of erroneous code:
3496 #![feature(coerce_unsized)]
3497 use std::ops::CoerceUnsized;
3499 struct Foo<T: ?Sized> {
3503 // error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
3504 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
3505 where T: CoerceUnsized<U> {}
3508 `CoerceUnsized` is used to coerce one struct containing an unsized type
3509 into another struct containing a different unsized type. If the struct
3510 doesn't have any fields of unsized types then you don't need explicit
3511 coercion to get the types you want. To fix this you can either
3512 not try to implement `CoerceUnsized` or you can add a field that is
3513 unsized to the struct.
3518 #![feature(coerce_unsized)]
3519 use std::ops::CoerceUnsized;
3521 // We don't need to impl `CoerceUnsized` here.
3526 // We add the unsized type field to the struct.
3527 struct Bar<T: ?Sized> {
3532 // The struct has an unsized field so we can implement
3533 // `CoerceUnsized` for it.
3534 impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
3535 where T: CoerceUnsized<U> {}
3538 Note that `CoerceUnsized` is mainly used by smart pointers like `Box`, `Rc`
3539 and `Arc` to be able to mark that they can coerce unsized types that they
3544 A struct with more than one field containing an unsized type cannot implement
3545 `CoerceUnsized`. This only occurs when you are trying to coerce one of the
3546 types in your struct to another type in the struct. In this case we try to
3547 impl `CoerceUnsized` from `T` to `U` which are both types that the struct
3548 takes. An [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3549 is any type that the compiler doesn't know the length or alignment of at
3550 compile time. Any struct containing an unsized type is also unsized.
3552 Example of erroneous code:
3555 #![feature(coerce_unsized)]
3556 use std::ops::CoerceUnsized;
3558 struct Foo<T: ?Sized, U: ?Sized> {
3564 // error: Struct `Foo` has more than one unsized field.
3565 impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
3568 `CoerceUnsized` only allows for coercion from a structure with a single
3569 unsized type field to another struct with a single unsized type field.
3570 In fact Rust only allows for a struct to have one unsized type in a struct
3571 and that unsized type must be the last field in the struct. So having two
3572 unsized types in a single struct is not allowed by the compiler. To fix this
3573 use only one field containing an unsized type in the struct and then use
3574 multiple structs to manage each unsized type field you need.
3579 #![feature(coerce_unsized)]
3580 use std::ops::CoerceUnsized;
3582 struct Foo<T: ?Sized> {
3587 impl <T, U> CoerceUnsized<Foo<U>> for Foo<T>
3588 where T: CoerceUnsized<U> {}
3590 fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
3591 Foo { a: 12i32, b: t } // we use coercion to get the `Foo<U>` type we need
3598 The type you are trying to impl `CoerceUnsized` for is not a struct.
3599 `CoerceUnsized` can only be implemented for a struct. Unsized types are
3600 already able to be coerced without an implementation of `CoerceUnsized`
3601 whereas a struct containing an unsized type needs to know the unsized type
3602 field it's containing is able to be coerced. An
3603 [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
3604 is any type that the compiler doesn't know the length or alignment of at
3605 compile time. Any struct containing an unsized type is also unsized.
3607 Example of erroneous code:
3610 #![feature(coerce_unsized)]
3611 use std::ops::CoerceUnsized;
3613 struct Foo<T: ?Sized> {
3617 // error: The type `U` is not a struct
3618 impl<T, U> CoerceUnsized<U> for Foo<T> {}
3621 The `CoerceUnsized` trait takes a struct type. Make sure the type you are
3622 providing to `CoerceUnsized` is a struct with only the last field containing an
3628 #![feature(coerce_unsized)]
3629 use std::ops::CoerceUnsized;
3635 // The `Foo<U>` is a struct so `CoerceUnsized` can be implemented
3636 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}
3639 Note that in Rust, structs can only contain an unsized type if the field
3640 containing the unsized type is the last and only unsized type field in the
3645 Trait methods cannot be declared `const` by design. For more information, see
3648 [RFC 911]: https://github.com/rust-lang/rfcs/pull/911
3652 Default impls are only allowed for traits with no methods or associated items.
3653 For more information see the [opt-in builtin traits RFC](https://github.com/rust
3654 -lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
3658 You tried to implement methods for a primitive type. Erroneous code example:
3666 // error: only a single inherent implementation marked with
3667 // `#[lang = "mut_ptr"]` is allowed for the `*mut T` primitive
3670 This isn't allowed, but using a trait to implement a method is a good solution.
3682 impl Bar for *mut Foo {
3689 This error indicates that some types or traits depend on each other
3690 and therefore cannot be constructed.
3692 The following example contains a circular dependency between two traits:
3695 trait FirstTrait : SecondTrait {
3699 trait SecondTrait : FirstTrait {
3706 This error indicates that a type or lifetime parameter has been declared
3707 but not actually used. Here is an example that demonstrates the error:
3715 If the type parameter was included by mistake, this error can be fixed
3716 by simply removing the type parameter, as shown below:
3724 Alternatively, if the type parameter was intentionally inserted, it must be
3725 used. A simple fix is shown below:
3733 This error may also commonly be found when working with unsafe code. For
3734 example, when using raw pointers one may wish to specify the lifetime for
3735 which the pointed-at data is valid. An initial attempt (below) causes this
3744 We want to express the constraint that Foo should not outlive `'a`, because
3745 the data pointed to by `T` is only valid for that lifetime. The problem is
3746 that there are no actual uses of `'a`. It's possible to work around this
3747 by adding a PhantomData type to the struct, using it to tell the compiler
3748 to act as if the struct contained a borrowed reference `&'a T`:
3751 use std::marker::PhantomData;
3753 struct Foo<'a, T: 'a> {
3755 phantom: PhantomData<&'a T>
3759 PhantomData can also be used to express information about unused type
3760 parameters. You can read more about it in the API documentation:
3762 https://doc.rust-lang.org/std/marker/struct.PhantomData.html
3766 A type parameter which references `Self` in its default value was not specified.
3767 Example of erroneous code:
3772 fn together_we_will_rule_the_galaxy(son: &A) {}
3773 // error: the type parameter `T` must be explicitly specified in an
3774 // object type because its default value `Self` references the
3778 A trait object is defined over a single, fully-defined trait. With a regular
3779 default parameter, this parameter can just be substituted in. However, if the
3780 default parameter is `Self`, the trait changes for each concrete type; i.e.
3781 `i32` will be expected to implement `A<i32>`, `bool` will be expected to
3782 implement `A<bool>`, etc... These types will not share an implementation of a
3783 fully-defined trait; instead they share implementations of a trait with
3784 different parameters substituted in for each implementation. This is
3785 irreconcilable with what we need to make a trait object work, and is thus
3786 disallowed. Making the trait concrete by explicitly specifying the value of the
3787 defaulted parameter will fix this issue. Fixed example:
3792 fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!
3797 The length of the platform-intrinsic function `simd_shuffle`
3798 wasn't specified. Erroneous code example:
3801 #![feature(platform_intrinsics)]
3803 extern "platform-intrinsic" {
3804 fn simd_shuffle<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3805 // error: invalid `simd_shuffle`, needs length: `simd_shuffle`
3809 The `simd_shuffle` function needs the length of the array passed as
3810 last parameter in its name. Example:
3813 #![feature(platform_intrinsics)]
3815 extern "platform-intrinsic" {
3816 fn simd_shuffle8<A,B>(a: A, b: A, c: [u32; 8]) -> B;
3822 A platform-specific intrinsic function has the wrong number of type
3823 parameters. Erroneous code example:
3826 #![feature(repr_simd)]
3827 #![feature(platform_intrinsics)]
3830 struct f64x2(f64, f64);
3832 extern "platform-intrinsic" {
3833 fn x86_mm_movemask_pd<T>(x: f64x2) -> i32;
3834 // error: platform-specific intrinsic has wrong number of type
3839 Please refer to the function declaration to see if it corresponds
3840 with yours. Example:
3843 #![feature(repr_simd)]
3844 #![feature(platform_intrinsics)]
3847 struct f64x2(f64, f64);
3849 extern "platform-intrinsic" {
3850 fn x86_mm_movemask_pd(x: f64x2) -> i32;
3856 An unknown platform-specific intrinsic function was used. Erroneous
3860 #![feature(repr_simd)]
3861 #![feature(platform_intrinsics)]
3864 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3866 extern "platform-intrinsic" {
3867 fn x86_mm_adds_ep16(x: i16x8, y: i16x8) -> i16x8;
3868 // error: unrecognized platform-specific intrinsic function
3872 Please verify that the function name wasn't misspelled, and ensure
3873 that it is declared in the rust source code (in the file
3874 src/librustc_platform_intrinsics/x86.rs). Example:
3877 #![feature(repr_simd)]
3878 #![feature(platform_intrinsics)]
3881 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3883 extern "platform-intrinsic" {
3884 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3890 Intrinsic argument(s) and/or return value have the wrong type.
3891 Erroneous code example:
3894 #![feature(repr_simd)]
3895 #![feature(platform_intrinsics)]
3898 struct i8x16(i8, i8, i8, i8, i8, i8, i8, i8,
3899 i8, i8, i8, i8, i8, i8, i8, i8);
3901 struct i32x4(i32, i32, i32, i32);
3903 struct i64x2(i64, i64);
3905 extern "platform-intrinsic" {
3906 fn x86_mm_adds_epi16(x: i8x16, y: i32x4) -> i64x2;
3907 // error: intrinsic arguments/return value have wrong type
3911 To fix this error, please refer to the function declaration to give
3912 it the awaited types. Example:
3915 #![feature(repr_simd)]
3916 #![feature(platform_intrinsics)]
3919 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3921 extern "platform-intrinsic" {
3922 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3928 Intrinsic argument(s) and/or return value have the wrong type.
3929 Erroneous code example:
3932 #![feature(repr_simd)]
3933 #![feature(platform_intrinsics)]
3936 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3938 struct i64x8(i64, i64, i64, i64, i64, i64, i64, i64);
3940 extern "platform-intrinsic" {
3941 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i64x8;
3942 // error: intrinsic argument/return value has wrong type
3946 To fix this error, please refer to the function declaration to give
3947 it the awaited types. Example:
3950 #![feature(repr_simd)]
3951 #![feature(platform_intrinsics)]
3954 struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
3956 extern "platform-intrinsic" {
3957 fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
3963 A platform-specific intrinsic function has wrong number of arguments.
3964 Erroneous code example:
3967 #![feature(repr_simd)]
3968 #![feature(platform_intrinsics)]
3971 struct f64x2(f64, f64);
3973 extern "platform-intrinsic" {
3974 fn x86_mm_movemask_pd(x: f64x2, y: f64x2, z: f64x2) -> i32;
3975 // error: platform-specific intrinsic has invalid number of arguments
3979 Please refer to the function declaration to see if it corresponds
3980 with yours. Example:
3983 #![feature(repr_simd)]
3984 #![feature(platform_intrinsics)]
3987 struct f64x2(f64, f64);
3989 extern "platform-intrinsic" {
3990 fn x86_mm_movemask_pd(x: f64x2) -> i32; // ok!
3996 The `typeof` keyword is currently reserved but unimplemented.
3997 Erroneous code example:
4001 let x: typeof(92) = 92;
4005 Try using type inference instead. Example:
4015 A non-default implementation was already made on this type so it cannot be
4016 specialized further. Erroneous code example:
4019 #![feature(specialization)]
4026 impl<T> SpaceLlama for T {
4027 default fn fly(&self) {}
4031 // applies to all `Clone` T and overrides the previous impl
4032 impl<T: Clone> SpaceLlama for T {
4036 // since `i32` is clone, this conflicts with the previous implementation
4037 impl SpaceLlama for i32 {
4038 default fn fly(&self) {}
4039 // error: item `fly` is provided by an `impl` that specializes
4040 // another, but the item in the parent `impl` is not marked
4041 // `default` and so it cannot be specialized.
4045 Specialization only allows you to override `default` functions in
4048 To fix this error, you need to mark all the parent implementations as default.
4052 #![feature(specialization)]
4059 impl<T> SpaceLlama for T {
4060 default fn fly(&self) {} // This is a parent implementation.
4063 // applies to all `Clone` T; overrides the previous impl
4064 impl<T: Clone> SpaceLlama for T {
4065 default fn fly(&self) {} // This is a parent implementation but was
4066 // previously not a default one, causing the error
4069 // applies to i32, overrides the previous two impls
4070 impl SpaceLlama for i32 {
4071 fn fly(&self) {} // And now that's ok!
4078 register_diagnostics! {
4083 E0103, // @GuillaumeGomez: I was unable to get this error, try your best!
4089 // E0159, // use of trait `{}` as struct constructor
4092 // E0173, // manual implementations of unboxed closure traits are experimental
4095 // E0187, // can't infer the kind of the closure
4096 // E0188, // can not cast an immutable reference to a mutable pointer
4097 // E0189, // deprecated: can only cast a boxed pointer to a boxed object
4098 // E0190, // deprecated: can only cast a &-pointer to an &-object
4099 E0196, // cannot determine a type for this closure
4100 E0203, // type parameter has more than one relaxed default bound,
4101 // and only one is supported
4103 // E0209, // builtin traits can only be implemented on structs or enums
4104 E0212, // cannot extract an associated type from a higher-ranked trait bound
4105 // E0213, // associated types are not accepted in this context
4106 // E0215, // angle-bracket notation is not stable with `Fn`
4107 // E0216, // parenthetical notation is only stable with `Fn`
4108 // E0217, // ambiguous associated type, defined in multiple supertraits
4109 // E0218, // no associated type defined
4110 // E0219, // associated type defined in higher-ranked supertrait
4111 // E0222, // Error code E0045 (variadic function must have C calling
4112 // convention) duplicate
4113 E0224, // at least one non-builtin train is required for an object type
4114 E0226, // only a single explicit lifetime bound is permitted
4115 E0227, // ambiguous lifetime bound, explicit lifetime bound required
4116 E0228, // explicit lifetime bound required
4117 E0230, // there is no type parameter on trait
4118 E0231, // only named substitution parameters are allowed
4121 // E0235, // structure constructor specifies a structure of type but
4122 // E0236, // no lang item for range syntax
4123 // E0237, // no lang item for range syntax
4124 E0238, // parenthesized parameters may only be used with a trait
4125 // E0239, // `next` method of `Iterator` trait has unexpected type
4129 E0245, // not a trait
4130 // E0246, // invalid recursive type
4132 // E0319, // trait impls for defaulted traits allowed just for structs/enums
4133 E0320, // recursive overflow during dropck
4134 E0328, // cannot implement Unsize explicitly
4135 // E0372, // coherence not object safe
4136 E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
4137 // between structures with the same definition
4138 E0399, // trait items need to be implemented because the associated
4139 // type `{}` was overridden
4140 E0436, // functional record update requires a struct
4141 E0513, // no type for local variable ..
4142 E0521, // redundant default implementations of trait
4143 E0527, // expected {} elements, found {}
4144 E0528, // expected at least {} elements, found {}
4145 E0529, // slice pattern expects array or slice, not `{}`