1 #![allow(non_snake_case)]
3 // Error messages for EXXXX errors.
4 // Each message should start and end with a new line, and be wrapped to 80 characters.
5 // In vim you can `:set tw=80` and use `gq` to wrap paragraphs. Use `:set tw=0` to disable.
6 register_long_diagnostics! {
8 Trait objects like `Box<Trait>` can only be constructed when certain
9 requirements are satisfied by the trait in question.
11 Trait objects are a form of dynamic dispatch and use a dynamically sized type
12 for the inner type. So, for a given trait `Trait`, when `Trait` is treated as a
13 type, as in `Box<Trait>`, the inner type is 'unsized'. In such cases the boxed
14 pointer is a 'fat pointer' that contains an extra pointer to a table of methods
15 (among other things) for dynamic dispatch. This design mandates some
16 restrictions on the types of traits that are allowed to be used in trait
17 objects, which are collectively termed as 'object safety' rules.
19 Attempting to create a trait object for a non object-safe trait will trigger
22 There are various rules:
24 ### The trait cannot require `Self: Sized`
26 When `Trait` is treated as a type, the type does not implement the special
27 `Sized` trait, because the type does not have a known size at compile time and
28 can only be accessed behind a pointer. Thus, if we have a trait like the
32 trait Foo where Self: Sized {
37 We cannot create an object of type `Box<Foo>` or `&Foo` since in this case
38 `Self` would not be `Sized`.
40 Generally, `Self: Sized` is used to indicate that the trait should not be used
41 as a trait object. If the trait comes from your own crate, consider removing
44 ### Method references the `Self` type in its arguments or return type
46 This happens when a trait has a method like the following:
50 fn foo(&self) -> Self;
53 impl Trait for String {
54 fn foo(&self) -> Self {
60 fn foo(&self) -> Self {
66 (Note that `&self` and `&mut self` are okay, it's additional `Self` types which
69 In such a case, the compiler cannot predict the return type of `foo()` in a
70 situation like the following:
74 fn foo(&self) -> Self;
77 fn call_foo(x: Box<Trait>) {
78 let y = x.foo(); // What type is y?
83 If only some methods aren't object-safe, you can add a `where Self: Sized` bound
84 on them to mark them as explicitly unavailable to trait objects. The
85 functionality will still be available to all other implementers, including
86 `Box<Trait>` which is itself sized (assuming you `impl Trait for Box<Trait>`).
90 fn foo(&self) -> Self where Self: Sized;
95 Now, `foo()` can no longer be called on a trait object, but you will now be
96 allowed to make a trait object, and that will be able to call any object-safe
97 methods. With such a bound, one can still call `foo()` on types implementing
98 that trait that aren't behind trait objects.
100 ### Method has generic type parameters
102 As mentioned before, trait objects contain pointers to method tables. So, if we
110 impl Trait for String {
124 At compile time each implementation of `Trait` will produce a table containing
125 the various methods (and other items) related to the implementation.
127 This works fine, but when the method gains generic parameters, we can have a
130 Usually, generic parameters get _monomorphized_. For example, if I have
138 The machine code for `foo::<u8>()`, `foo::<bool>()`, `foo::<String>()`, or any
139 other type substitution is different. Hence the compiler generates the
140 implementation on-demand. If you call `foo()` with a `bool` parameter, the
141 compiler will only generate code for `foo::<bool>()`. When we have additional
142 type parameters, the number of monomorphized implementations the compiler
143 generates does not grow drastically, since the compiler will only generate an
144 implementation if the function is called with unparametrized substitutions
145 (i.e., substitutions where none of the substituted types are themselves
148 However, with trait objects we have to make a table containing _every_ object
149 that implements the trait. Now, if it has type parameters, we need to add
150 implementations for every type that implements the trait, and there could
151 theoretically be an infinite number of types.
157 fn foo<T>(&self, on: T);
161 impl Trait for String {
162 fn foo<T>(&self, on: T) {
168 fn foo<T>(&self, on: T) {
173 // 8 more implementations
176 Now, if we have the following code:
178 ```compile_fail,E0038
179 # trait Trait { fn foo<T>(&self, on: T); }
180 # impl Trait for String { fn foo<T>(&self, on: T) {} }
181 # impl Trait for u8 { fn foo<T>(&self, on: T) {} }
182 # impl Trait for bool { fn foo<T>(&self, on: T) {} }
184 fn call_foo(thing: Box<Trait>) {
185 thing.foo(true); // this could be any one of the 8 types above
191 We don't just need to create a table of all implementations of all methods of
192 `Trait`, we need to create such a table, for each different type fed to
193 `foo()`. In this case this turns out to be (10 types implementing `Trait`)*(3
194 types being fed to `foo()`) = 30 implementations!
196 With real world traits these numbers can grow drastically.
198 To fix this, it is suggested to use a `where Self: Sized` bound similar to the
199 fix for the sub-error above if you do not intend to call the method with type
204 fn foo<T>(&self, on: T) where Self: Sized;
209 If this is not an option, consider replacing the type parameter with another
210 trait object (e.g., if `T: OtherTrait`, use `on: Box<OtherTrait>`). If the
211 number of types you intend to feed to this method is limited, consider manually
212 listing out the methods of different types.
214 ### Method has no receiver
216 Methods that do not take a `self` parameter can't be called since there won't be
217 a way to get a pointer to the method table for them.
225 This could be called as `<Foo as Foo>::foo()`, which would not be able to pick
228 Adding a `Self: Sized` bound to these methods will generally make this compile.
232 fn foo() -> u8 where Self: Sized;
236 ### The trait cannot contain associated constants
238 Just like static functions, associated constants aren't stored on the method
239 table. If the trait or any subtrait contain an associated constant, they cannot
240 be made into an object.
242 ```compile_fail,E0038
250 A simple workaround is to use a helper method instead:
258 ### The trait cannot use `Self` as a type parameter in the supertrait listing
260 This is similar to the second sub-error, but subtler. It happens in situations
266 trait Trait: Super<Self> {
271 impl Super<Foo> for Foo{}
273 impl Trait for Foo {}
276 Here, the supertrait might have methods as follows:
280 fn get_a(&self) -> A; // note that this is object safe!
284 If the trait `Foo` was deriving from something like `Super<String>` or
285 `Super<T>` (where `Foo` itself is `Foo<T>`), this is okay, because given a type
286 `get_a()` will definitely return an object of that type.
288 However, if it derives from `Super<Self>`, even though `Super` is object safe,
289 the method `get_a()` would return an object of unknown type when called on the
290 function. `Self` type parameters let us make object safe traits no longer safe,
291 so they are forbidden when specifying supertraits.
293 There's no easy fix for this, generally code will need to be refactored so that
294 you no longer need to derive from `Super<Self>`.
298 When defining a recursive struct or enum, any use of the type being defined
299 from inside the definition must occur behind a pointer (like `Box` or `&`).
300 This is because structs and enums must have a well-defined size, and without
301 the pointer, the size of the type would need to be unbounded.
303 Consider the following erroneous definition of a type for a list of bytes:
305 ```compile_fail,E0072
306 // error, invalid recursive struct type
309 tail: Option<ListNode>,
313 This type cannot have a well-defined size, because it needs to be arbitrarily
314 large (since we would be able to nest `ListNode`s to any depth). Specifically,
317 size of `ListNode` = 1 byte for `head`
318 + 1 byte for the discriminant of the `Option`
322 One way to fix this is by wrapping `ListNode` in a `Box`, like so:
327 tail: Option<Box<ListNode>>,
331 This works because `Box` is a pointer, so its size is well-known.
335 This error indicates that the compiler was unable to sensibly evaluate an
336 constant expression that had to be evaluated. Attempting to divide by 0
337 or causing integer overflow are two ways to induce this error. For example:
339 ```compile_fail,E0080
346 Ensure that the expressions given can be evaluated as the desired integer type.
347 See the FFI section of the Reference for more information about using a custom
350 https://doc.rust-lang.org/reference.html#ffi-attributes
354 This error indicates that a lifetime is missing from a type. If it is an error
355 inside a function signature, the problem may be with failing to adhere to the
356 lifetime elision rules (see below).
358 Here are some simple examples of where you'll run into this error:
360 ```compile_fail,E0106
361 struct Foo1 { x: &bool }
362 // ^ expected lifetime parameter
363 struct Foo2<'a> { x: &'a bool } // correct
365 struct Bar1 { x: Foo2 }
366 // ^^^^ expected lifetime parameter
367 struct Bar2<'a> { x: Foo2<'a> } // correct
369 enum Baz1 { A(u8), B(&bool), }
370 // ^ expected lifetime parameter
371 enum Baz2<'a> { A(u8), B(&'a bool), } // correct
374 // ^ expected lifetime parameter
375 type MyStr2<'a> = &'a str; // correct
378 Lifetime elision is a special, limited kind of inference for lifetimes in
379 function signatures which allows you to leave out lifetimes in certain cases.
380 For more background on lifetime elision see [the book][book-le].
382 The lifetime elision rules require that any function signature with an elided
383 output lifetime must either have
385 - exactly one input lifetime
386 - or, multiple input lifetimes, but the function must also be a method with a
387 `&self` or `&mut self` receiver
389 In the first case, the output lifetime is inferred to be the same as the unique
390 input lifetime. In the second case, the lifetime is instead inferred to be the
391 same as the lifetime on `&self` or `&mut self`.
393 Here are some examples of elision errors:
395 ```compile_fail,E0106
396 // error, no input lifetimes
399 // error, `x` and `y` have distinct lifetimes inferred
400 fn bar(x: &str, y: &str) -> &str { }
402 // error, `y`'s lifetime is inferred to be distinct from `x`'s
403 fn baz<'a>(x: &'a str, y: &str) -> &str { }
406 [book-le]: https://doc.rust-lang.org/book/ch10-03-lifetime-syntax.html#lifetime-elision
410 There are conflicting trait implementations for the same type.
411 Example of erroneous code:
413 ```compile_fail,E0119
415 fn get(&self) -> usize;
418 impl<T> MyTrait for T {
419 fn get(&self) -> usize { 0 }
426 impl MyTrait for Foo { // error: conflicting implementations of trait
427 // `MyTrait` for type `Foo`
428 fn get(&self) -> usize { self.value }
432 When looking for the implementation for the trait, the compiler finds
433 both the `impl<T> MyTrait for T` where T is all types and the `impl
434 MyTrait for Foo`. Since a trait cannot be implemented multiple times,
435 this is an error. So, when you write:
439 fn get(&self) -> usize;
442 impl<T> MyTrait for T {
443 fn get(&self) -> usize { 0 }
447 This makes the trait implemented on all types in the scope. So if you
448 try to implement it on another one after that, the implementations will
453 fn get(&self) -> usize;
456 impl<T> MyTrait for T {
457 fn get(&self) -> usize { 0 }
465 f.get(); // the trait is implemented so we can use it
470 // This shouldn't really ever trigger since the repeated value error comes first
472 A binary can only have one entry point, and by default that entry point is the
473 function `main()`. If there are multiple such functions, please rename one.
477 More than one function was declared with the `#[main]` attribute.
479 Erroneous code example:
481 ```compile_fail,E0137
488 fn f() {} // error: multiple functions with a #[main] attribute
491 This error indicates that the compiler found multiple functions with the
492 `#[main]` attribute. This is an error because there must be a unique entry
493 point into a Rust program. Example:
504 More than one function was declared with the `#[start]` attribute.
506 Erroneous code example:
508 ```compile_fail,E0138
512 fn foo(argc: isize, argv: *const *const u8) -> isize {}
515 fn f(argc: isize, argv: *const *const u8) -> isize {}
516 // error: multiple 'start' functions
519 This error indicates that the compiler found multiple functions with the
520 `#[start]` attribute. This is an error because there must be a unique entry
521 point into a Rust program. Example:
527 fn foo(argc: isize, argv: *const *const u8) -> isize { 0 } // ok!
532 #### Note: this error code is no longer emitted by the compiler.
534 There are various restrictions on transmuting between types in Rust; for example
535 types being transmuted must have the same size. To apply all these restrictions,
536 the compiler must know the exact types that may be transmuted. When type
537 parameters are involved, this cannot always be done.
539 So, for example, the following is not allowed:
542 use std::mem::transmute;
544 struct Foo<T>(Vec<T>);
546 fn foo<T>(x: Vec<T>) {
547 // we are transmuting between Vec<T> and Foo<F> here
548 let y: Foo<T> = unsafe { transmute(x) };
549 // do something with y
553 In this specific case there's a good chance that the transmute is harmless (but
554 this is not guaranteed by Rust). However, when alignment and enum optimizations
555 come into the picture, it's quite likely that the sizes may or may not match
556 with different type parameter substitutions. It's not possible to check this for
557 _all_ possible types, so `transmute()` simply only accepts types without any
558 unsubstituted type parameters.
560 If you need this, there's a good chance you're doing something wrong. Keep in
561 mind that Rust doesn't guarantee much about the layout of different structs
562 (even two structs with identical declarations may have different layouts). If
563 there is a solution that avoids the transmute entirely, try it instead.
565 If it's possible, hand-monomorphize the code by writing the function for each
566 possible type substitution. It's possible to use traits to do this cleanly,
570 use std::mem::transmute;
572 struct Foo<T>(Vec<T>);
574 trait MyTransmutableType: Sized {
575 fn transmute(_: Vec<Self>) -> Foo<Self>;
578 impl MyTransmutableType for u8 {
579 fn transmute(x: Vec<u8>) -> Foo<u8> {
580 unsafe { transmute(x) }
584 impl MyTransmutableType for String {
585 fn transmute(x: Vec<String>) -> Foo<String> {
586 unsafe { transmute(x) }
590 // ... more impls for the types you intend to transmute
592 fn foo<T: MyTransmutableType>(x: Vec<T>) {
593 let y: Foo<T> = <T as MyTransmutableType>::transmute(x);
594 // do something with y
598 Each impl will be checked for a size match in the transmute as usual, and since
599 there are no unbound type parameters involved, this should compile unless there
600 is a size mismatch in one of the impls.
602 It is also possible to manually transmute:
606 # let v = Some("value");
607 # type SomeType = &'static [u8];
609 ptr::read(&v as *const _ as *const SomeType) // `v` transmuted to `SomeType`
614 Note that this does not move `v` (unlike `transmute`), and may need a
615 call to `mem::forget(v)` in case you want to avoid destructors being called.
619 A lang item was redefined.
621 Erroneous code example:
623 ```compile_fail,E0152
624 #![feature(lang_items)]
627 struct Foo; // error: duplicate lang item found: `arc`
630 Lang items are already implemented in the standard library. Unless you are
631 writing a free-standing application (e.g., a kernel), you do not need to provide
634 You can build a free-standing crate by adding `#![no_std]` to the crate
637 ```ignore (only-for-syntax-highlight)
641 See also the [unstable book][1].
643 [1]: https://doc.rust-lang.org/unstable-book/language-features/lang-items.html#writing-an-executable-without-stdlib
647 A generic type was described using parentheses rather than angle brackets.
650 ```compile_fail,E0214
652 let v: Vec(&str) = vec!["foo"];
656 This is not currently supported: `v` should be defined as `Vec<&str>`.
657 Parentheses are currently only used with generic types when defining parameters
658 for `Fn`-family traits.
662 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
663 message for when a particular trait isn't implemented on a type placed in a
664 position that needs that trait. For example, when the following code is
668 #![feature(on_unimplemented)]
670 fn foo<T: Index<u8>>(x: T){}
672 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
673 trait Index<Idx> { /* ... */ }
675 foo(true); // `bool` does not implement `Index<u8>`
678 There will be an error about `bool` not implementing `Index<u8>`, followed by a
679 note saying "the type `bool` cannot be indexed by `u8`".
681 As you can see, you can specify type parameters in curly braces for
682 substitution with the actual types (using the regular format string syntax) in
683 a given situation. Furthermore, `{Self}` will substitute to the type (in this
684 case, `bool`) that we tried to use.
686 This error appears when the curly braces contain an identifier which doesn't
687 match with any of the type parameters or the string `Self`. This might happen
688 if you misspelled a type parameter, or if you intended to use literal curly
689 braces. If it is the latter, escape the curly braces with a second curly brace
690 of the same type; e.g., a literal `{` is `{{`.
694 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
695 message for when a particular trait isn't implemented on a type placed in a
696 position that needs that trait. For example, when the following code is
700 #![feature(on_unimplemented)]
702 fn foo<T: Index<u8>>(x: T){}
704 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
705 trait Index<Idx> { /* ... */ }
707 foo(true); // `bool` does not implement `Index<u8>`
710 there will be an error about `bool` not implementing `Index<u8>`, followed by a
711 note saying "the type `bool` cannot be indexed by `u8`".
713 As you can see, you can specify type parameters in curly braces for
714 substitution with the actual types (using the regular format string syntax) in
715 a given situation. Furthermore, `{Self}` will substitute to the type (in this
716 case, `bool`) that we tried to use.
718 This error appears when the curly braces do not contain an identifier. Please
719 add one of the same name as a type parameter. If you intended to use literal
720 braces, use `{{` and `}}` to escape them.
724 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
725 message for when a particular trait isn't implemented on a type placed in a
726 position that needs that trait. For example, when the following code is
730 #![feature(on_unimplemented)]
732 fn foo<T: Index<u8>>(x: T){}
734 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
735 trait Index<Idx> { /* ... */ }
737 foo(true); // `bool` does not implement `Index<u8>`
740 there will be an error about `bool` not implementing `Index<u8>`, followed by a
741 note saying "the type `bool` cannot be indexed by `u8`".
743 For this to work, some note must be specified. An empty attribute will not do
744 anything, please remove the attribute or add some helpful note for users of the
749 When using a lifetime like `'a` in a type, it must be declared before being
752 These two examples illustrate the problem:
754 ```compile_fail,E0261
755 // error, use of undeclared lifetime name `'a`
756 fn foo(x: &'a str) { }
759 // error, use of undeclared lifetime name `'a`
764 These can be fixed by declaring lifetime parameters:
771 fn foo<'a>(x: &'a str) {}
774 Impl blocks declare lifetime parameters separately. You need to add lifetime
775 parameters to an impl block if you're implementing a type that has a lifetime
776 parameter of its own.
779 ```compile_fail,E0261
784 // error, use of undeclared lifetime name `'a`
786 fn foo<'a>(x: &'a str) {}
790 This is fixed by declaring the impl block like this:
799 fn foo(x: &'a str) {}
805 Declaring certain lifetime names in parameters is disallowed. For example,
806 because the `'static` lifetime is a special built-in lifetime name denoting
807 the lifetime of the entire program, this is an error:
809 ```compile_fail,E0262
810 // error, invalid lifetime parameter name `'static`
811 fn foo<'static>(x: &'static str) { }
816 A lifetime name cannot be declared more than once in the same scope. For
819 ```compile_fail,E0263
820 // error, lifetime name `'a` declared twice in the same scope
821 fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str) { }
826 An unknown external lang item was used. Erroneous code example:
828 ```compile_fail,E0264
829 #![feature(lang_items)]
832 #[lang = "cake"] // error: unknown external lang item: `cake`
837 A list of available external lang items is available in
838 `src/librustc/middle/weak_lang_items.rs`. Example:
841 #![feature(lang_items)]
844 #[lang = "panic_impl"] // ok!
851 This is because of a type mismatch between the associated type of some
852 trait (e.g., `T::Bar`, where `T` implements `trait Quux { type Bar; }`)
853 and another type `U` that is required to be equal to `T::Bar`, but is not.
856 Here is a basic example:
858 ```compile_fail,E0271
859 trait Trait { type AssociatedType; }
861 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
865 impl Trait for i8 { type AssociatedType = &'static str; }
870 Here is that same example again, with some explanatory comments:
872 ```compile_fail,E0271
873 trait Trait { type AssociatedType; }
875 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
876 // ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
878 // This says `foo` can |
879 // only be used with |
881 // implements `Trait`. |
883 // This says not only must
884 // `T` be an impl of `Trait`
885 // but also that the impl
886 // must assign the type `u32`
887 // to the associated type.
891 impl Trait for i8 { type AssociatedType = &'static str; }
892 //~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
897 // ... but it is an implementation
898 // that assigns `&'static str` to
899 // the associated type.
902 // Here, we invoke `foo` with an `i8`, which does not satisfy
903 // the constraint `<i8 as Trait>::AssociatedType=u32`, and
904 // therefore the type-checker complains with this error code.
907 To avoid those issues, you have to make the types match correctly.
908 So we can fix the previous examples like this:
912 trait Trait { type AssociatedType; }
914 fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
918 impl Trait for i8 { type AssociatedType = &'static str; }
923 let vs = vec![1, 2, 3, 4];
935 This error occurs when there was a recursive trait requirement that overflowed
936 before it could be evaluated. Often this means that there is unbounded
937 recursion in resolving some type bounds.
939 For example, in the following code:
941 ```compile_fail,E0275
946 impl<T> Foo for T where Bar<T>: Foo {}
949 To determine if a `T` is `Foo`, we need to check if `Bar<T>` is `Foo`. However,
950 to do this check, we need to determine that `Bar<Bar<T>>` is `Foo`. To
951 determine this, we check if `Bar<Bar<Bar<T>>>` is `Foo`, and so on. This is
952 clearly a recursive requirement that can't be resolved directly.
954 Consider changing your trait bounds so that they're less self-referential.
958 This error occurs when a bound in an implementation of a trait does not match
959 the bounds specified in the original trait. For example:
961 ```compile_fail,E0276
967 fn foo<T>(x: T) where T: Copy {}
971 Here, all types implementing `Foo` must have a method `foo<T>(x: T)` which can
972 take any type `T`. However, in the `impl` for `bool`, we have added an extra
973 bound that `T` is `Copy`, which isn't compatible with the original trait.
975 Consider removing the bound from the method or adding the bound to the original
976 method definition in the trait.
980 You tried to use a type which doesn't implement some trait in a place which
981 expected that trait. Erroneous code example:
983 ```compile_fail,E0277
984 // here we declare the Foo trait with a bar method
989 // we now declare a function which takes an object implementing the Foo trait
990 fn some_func<T: Foo>(foo: T) {
995 // we now call the method with the i32 type, which doesn't implement
997 some_func(5i32); // error: the trait bound `i32 : Foo` is not satisfied
1001 In order to fix this error, verify that the type you're using does implement
1009 fn some_func<T: Foo>(foo: T) {
1010 foo.bar(); // we can now use this method since i32 implements the
1014 // we implement the trait on the i32 type
1020 some_func(5i32); // ok!
1024 Or in a generic context, an erroneous code example would look like:
1026 ```compile_fail,E0277
1027 fn some_func<T>(foo: T) {
1028 println!("{:?}", foo); // error: the trait `core::fmt::Debug` is not
1029 // implemented for the type `T`
1033 // We now call the method with the i32 type,
1034 // which *does* implement the Debug trait.
1039 Note that the error here is in the definition of the generic function: Although
1040 we only call it with a parameter that does implement `Debug`, the compiler
1041 still rejects the function: It must work with all possible input types. In
1042 order to make this example compile, we need to restrict the generic type we're
1048 // Restrict the input type to types that implement Debug.
1049 fn some_func<T: fmt::Debug>(foo: T) {
1050 println!("{:?}", foo);
1054 // Calling the method is still fine, as i32 implements Debug.
1057 // This would fail to compile now:
1058 // struct WithoutDebug;
1059 // some_func(WithoutDebug);
1063 Rust only looks at the signature of the called function, as such it must
1064 already specify all requirements that will be used for every type parameter.
1068 #### Note: this error code is no longer emitted by the compiler.
1070 You tried to supply a type which doesn't implement some trait in a location
1071 which expected that trait. This error typically occurs when working with
1072 `Fn`-based types. Erroneous code example:
1075 fn foo<F: Fn(usize)>(x: F) { }
1078 // type mismatch: ... implements the trait `core::ops::Fn<(String,)>`,
1079 // but the trait `core::ops::Fn<(usize,)>` is required
1081 foo(|y: String| { });
1085 The issue in this case is that `foo` is defined as accepting a `Fn` with one
1086 argument of type `String`, but the closure we attempted to pass to it requires
1087 one arguments of type `usize`.
1091 This error indicates that type inference did not result in one unique possible
1092 type, and extra information is required. In most cases this can be provided
1093 by adding a type annotation. Sometimes you need to specify a generic type
1096 A common example is the `collect` method on `Iterator`. It has a generic type
1097 parameter with a `FromIterator` bound, which for a `char` iterator is
1098 implemented by `Vec` and `String` among others. Consider the following snippet
1099 that reverses the characters of a string:
1101 ```compile_fail,E0282
1102 let x = "hello".chars().rev().collect();
1105 In this case, the compiler cannot infer what the type of `x` should be:
1106 `Vec<char>` and `String` are both suitable candidates. To specify which type to
1107 use, you can use a type annotation on `x`:
1110 let x: Vec<char> = "hello".chars().rev().collect();
1113 It is not necessary to annotate the full type. Once the ambiguity is resolved,
1114 the compiler can infer the rest:
1117 let x: Vec<_> = "hello".chars().rev().collect();
1120 Another way to provide the compiler with enough information, is to specify the
1121 generic type parameter:
1124 let x = "hello".chars().rev().collect::<Vec<char>>();
1127 Again, you need not specify the full type if the compiler can infer it:
1130 let x = "hello".chars().rev().collect::<Vec<_>>();
1133 Apart from a method or function with a generic type parameter, this error can
1134 occur when a type parameter of a struct or trait cannot be inferred. In that
1135 case it is not always possible to use a type annotation, because all candidates
1136 have the same return type. For instance:
1138 ```compile_fail,E0282
1149 let number = Foo::bar();
1154 This will fail because the compiler does not know which instance of `Foo` to
1155 call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.
1159 This error occurs when the compiler doesn't have enough information
1160 to unambiguously choose an implementation.
1164 ```compile_fail,E0283
1171 impl Generator for Impl {
1172 fn create() -> u32 { 1 }
1177 impl Generator for AnotherImpl {
1178 fn create() -> u32 { 2 }
1182 let cont: u32 = Generator::create();
1183 // error, impossible to choose one of Generator trait implementation
1184 // Should it be Impl or AnotherImpl, maybe something else?
1188 To resolve this error use the concrete type:
1197 impl Generator for AnotherImpl {
1198 fn create() -> u32 { 2 }
1202 let gen1 = AnotherImpl::create();
1204 // if there are multiple methods with same name (different traits)
1205 let gen2 = <AnotherImpl as Generator>::create();
1211 This error occurs when the compiler is unable to unambiguously infer the
1212 return type of a function or method which is generic on return type, such
1213 as the `collect` method for `Iterator`s.
1217 ```compile_fail,E0284
1218 fn foo() -> Result<bool, ()> {
1219 let results = [Ok(true), Ok(false), Err(())].iter().cloned();
1220 let v: Vec<bool> = results.collect()?;
1221 // Do things with v...
1226 Here we have an iterator `results` over `Result<bool, ()>`.
1227 Hence, `results.collect()` can return any type implementing
1228 `FromIterator<Result<bool, ()>>`. On the other hand, the
1229 `?` operator can accept any type implementing `Try`.
1231 The author of this code probably wants `collect()` to return a
1232 `Result<Vec<bool>, ()>`, but the compiler can't be sure
1233 that there isn't another type `T` implementing both `Try` and
1234 `FromIterator<Result<bool, ()>>` in scope such that
1235 `T::Ok == Vec<bool>`. Hence, this code is ambiguous and an error
1238 To resolve this error, use a concrete type for the intermediate expression:
1241 fn foo() -> Result<bool, ()> {
1242 let results = [Ok(true), Ok(false), Err(())].iter().cloned();
1244 let temp: Result<Vec<bool>, ()> = results.collect();
1247 // Do things with v...
1252 Note that the type of `v` can now be inferred from the type of `temp`.
1256 This error occurs when the compiler was unable to infer the concrete type of a
1257 variable. It can occur for several cases, the most common of which is a
1258 mismatch in the expected type that the compiler inferred for a variable's
1259 initializing expression, and the actual type explicitly assigned to the
1264 ```compile_fail,E0308
1265 let x: i32 = "I am not a number!";
1266 // ~~~ ~~~~~~~~~~~~~~~~~~~~
1268 // | initializing expression;
1269 // | compiler infers type `&str`
1271 // type `i32` assigned to variable `x`
1276 The type definition contains some field whose type
1277 requires an outlives annotation. Outlives annotations
1278 (e.g., `T: 'a`) are used to guarantee that all the data in T is valid
1279 for at least the lifetime `'a`. This scenario most commonly
1280 arises when the type contains an associated type reference
1281 like `<T as SomeTrait<'a>>::Output`, as shown in this example:
1283 ```compile_fail,E0309
1284 // This won't compile because the applicable impl of
1285 // `SomeTrait` (below) requires that `T: 'a`, but the struct does
1286 // not have a matching where-clause.
1288 foo: <T as SomeTrait<'a>>::Output,
1291 trait SomeTrait<'a> {
1295 impl<'a, T> SomeTrait<'a> for T
1303 Here, the where clause `T: 'a` that appears on the impl is not known to be
1304 satisfied on the struct. To make this example compile, you have to add
1305 a where-clause like `T: 'a` to the struct definition:
1312 foo: <T as SomeTrait<'a>>::Output
1315 trait SomeTrait<'a> {
1319 impl<'a, T> SomeTrait<'a> for T
1329 Types in type definitions have lifetimes associated with them that represent
1330 how long the data stored within them is guaranteed to be live. This lifetime
1331 must be as long as the data needs to be alive, and missing the constraint that
1332 denotes this will cause this error.
1334 ```compile_fail,E0310
1335 // This won't compile because T is not constrained to the static lifetime
1336 // the reference needs
1342 This will compile, because it has the constraint on the type parameter:
1345 struct Foo<T: 'static> {
1352 This error occurs when an `if` expression without an `else` block is used in a
1353 context where a type other than `()` is expected, for example a `let`
1356 ```compile_fail,E0317
1359 let a = if x == 5 { 1 };
1363 An `if` expression without an `else` block has the type `()`, so this is a type
1364 error. To resolve it, add an `else` block having the same type as the `if`
1369 This error indicates that some types or traits depend on each other
1370 and therefore cannot be constructed.
1372 The following example contains a circular dependency between two traits:
1374 ```compile_fail,E0391
1375 trait FirstTrait : SecondTrait {
1379 trait SecondTrait : FirstTrait {
1386 #### Note: this error code is no longer emitted by the compiler.
1388 In Rust 1.3, the default object lifetime bounds are expected to change, as
1389 described in [RFC 1156]. You are getting a warning because the compiler
1390 thinks it is possible that this change will cause a compilation error in your
1391 code. It is possible, though unlikely, that this is a false alarm.
1393 The heart of the change is that where `&'a Box<SomeTrait>` used to default to
1394 `&'a Box<SomeTrait+'a>`, it now defaults to `&'a Box<SomeTrait+'static>` (here,
1395 `SomeTrait` is the name of some trait type). Note that the only types which are
1396 affected are references to boxes, like `&Box<SomeTrait>` or
1397 `&[Box<SomeTrait>]`. More common types like `&SomeTrait` or `Box<SomeTrait>`
1400 To silence this warning, edit your code to use an explicit bound. Most of the
1401 time, this means that you will want to change the signature of a function that
1402 you are calling. For example, if the error is reported on a call like `foo(x)`,
1403 and `foo` is defined as follows:
1406 # trait SomeTrait {}
1407 fn foo(arg: &Box<SomeTrait>) { /* ... */ }
1410 You might change it to:
1413 # trait SomeTrait {}
1414 fn foo<'a>(arg: &'a Box<SomeTrait+'a>) { /* ... */ }
1417 This explicitly states that you expect the trait object `SomeTrait` to contain
1418 references (with a maximum lifetime of `'a`).
1420 [RFC 1156]: https://github.com/rust-lang/rfcs/blob/master/text/1156-adjust-default-object-bounds.md
1424 An invalid lint attribute has been given. Erroneous code example:
1426 ```compile_fail,E0452
1427 #![allow(foo = "")] // error: malformed lint attribute
1430 Lint attributes only accept a list of identifiers (where each identifier is a
1431 lint name). Ensure the attribute is of this form:
1434 #![allow(foo)] // ok!
1436 #![allow(foo, foo2)] // ok!
1441 A lint check attribute was overruled by a `forbid` directive set as an
1442 attribute on an enclosing scope, or on the command line with the `-F` option.
1444 Example of erroneous code:
1446 ```compile_fail,E0453
1447 #![forbid(non_snake_case)]
1449 #[allow(non_snake_case)]
1451 let MyNumber = 2; // error: allow(non_snake_case) overruled by outer
1452 // forbid(non_snake_case)
1456 The `forbid` lint setting, like `deny`, turns the corresponding compiler
1457 warning into a hard error. Unlike `deny`, `forbid` prevents itself from being
1458 overridden by inner attributes.
1460 If you're sure you want to override the lint check, you can change `forbid` to
1461 `deny` (or use `-D` instead of `-F` if the `forbid` setting was given as a
1462 command-line option) to allow the inner lint check attribute:
1465 #![deny(non_snake_case)]
1467 #[allow(non_snake_case)]
1469 let MyNumber = 2; // ok!
1473 Otherwise, edit the code to pass the lint check, and remove the overruled
1477 #![forbid(non_snake_case)]
1486 A lifetime bound was not satisfied.
1488 Erroneous code example:
1490 ```compile_fail,E0478
1491 // Check that the explicit lifetime bound (`'SnowWhite`, in this example) must
1492 // outlive all the superbounds from the trait (`'kiss`, in this example).
1494 trait Wedding<'t>: 't { }
1496 struct Prince<'kiss, 'SnowWhite> {
1497 child: Box<Wedding<'kiss> + 'SnowWhite>,
1498 // error: lifetime bound not satisfied
1502 In this example, the `'SnowWhite` lifetime is supposed to outlive the `'kiss`
1503 lifetime but the declaration of the `Prince` struct doesn't enforce it. To fix
1504 this issue, you need to specify it:
1507 trait Wedding<'t>: 't { }
1509 struct Prince<'kiss, 'SnowWhite: 'kiss> { // You say here that 'kiss must live
1510 // longer than 'SnowWhite.
1511 child: Box<Wedding<'kiss> + 'SnowWhite>, // And now it's all good!
1517 A reference has a longer lifetime than the data it references.
1519 Erroneous code example:
1521 ```compile_fail,E0491
1522 trait SomeTrait<'a> {
1526 impl<'a, T> SomeTrait<'a> for T {
1527 type Output = &'a T; // compile error E0491
1531 Here, the problem is that a reference type like `&'a T` is only valid
1532 if all the data in T outlives the lifetime `'a`. But this impl as written
1533 is applicable to any lifetime `'a` and any type `T` -- we have no guarantee
1534 that `T` outlives `'a`. To fix this, you can add a where clause like
1538 trait SomeTrait<'a> {
1542 impl<'a, T> SomeTrait<'a> for T
1546 type Output = &'a T; // compile error E0491
1552 A lifetime name is shadowing another lifetime name. Erroneous code example:
1554 ```compile_fail,E0496
1560 fn f<'a>(x: &'a i32) { // error: lifetime name `'a` shadows a lifetime
1561 // name that is already in scope
1566 Please change the name of one of the lifetimes to remove this error. Example:
1574 fn f<'b>(x: &'b i32) { // ok!
1584 A stability attribute was used outside of the standard library. Erroneous code
1588 #[stable] // error: stability attributes may not be used outside of the
1593 It is not possible to use stability attributes outside of the standard library.
1594 Also, for now, it is not possible to write deprecation messages either.
1598 Transmute with two differently sized types was attempted. Erroneous code
1601 ```compile_fail,E0512
1602 fn takes_u8(_: u8) {}
1605 unsafe { takes_u8(::std::mem::transmute(0u16)); }
1606 // error: cannot transmute between types of different sizes,
1607 // or dependently-sized types
1611 Please use types with same size or use the expected type directly. Example:
1614 fn takes_u8(_: u8) {}
1617 unsafe { takes_u8(::std::mem::transmute(0i8)); } // ok!
1619 unsafe { takes_u8(0u8); } // ok!
1625 This error indicates that a `#[repr(..)]` attribute was placed on an
1628 Examples of erroneous code:
1630 ```compile_fail,E0517
1638 struct Foo {bar: bool, baz: bool}
1646 * The `#[repr(C)]` attribute can only be placed on structs and enums.
1647 * The `#[repr(packed)]` and `#[repr(simd)]` attributes only work on structs.
1648 * The `#[repr(u8)]`, `#[repr(i16)]`, etc attributes only work on enums.
1650 These attributes do not work on typedefs, since typedefs are just aliases.
1652 Representations like `#[repr(u8)]`, `#[repr(i64)]` are for selecting the
1653 discriminant size for enums with no data fields on any of the variants, e.g.
1654 `enum Color {Red, Blue, Green}`, effectively setting the size of the enum to
1655 the size of the provided type. Such an enum can be cast to a value of the same
1656 type as well. In short, `#[repr(u8)]` makes the enum behave like an integer
1657 with a constrained set of allowed values.
1659 Only field-less enums can be cast to numerical primitives, so this attribute
1660 will not apply to structs.
1662 `#[repr(packed)]` reduces padding to make the struct size smaller. The
1663 representation of enums isn't strictly defined in Rust, and this attribute
1664 won't work on enums.
1666 `#[repr(simd)]` will give a struct consisting of a homogeneous series of machine
1667 types (i.e., `u8`, `i32`, etc) a representation that permits vectorization via
1668 SIMD. This doesn't make much sense for enums since they don't consist of a
1669 single list of data.
1673 This error indicates that an `#[inline(..)]` attribute was incorrectly placed
1674 on something other than a function or method.
1676 Examples of erroneous code:
1678 ```compile_fail,E0518
1688 `#[inline]` hints the compiler whether or not to attempt to inline a method or
1689 function. By default, the compiler does a pretty good job of figuring this out
1690 itself, but if you feel the need for annotations, `#[inline(always)]` and
1691 `#[inline(never)]` can override or force the compiler's decision.
1693 If you wish to apply this attribute to all methods in an impl, manually annotate
1694 each method; it is not possible to annotate the entire impl with an `#[inline]`
1699 The lang attribute is intended for marking special items that are built-in to
1700 Rust itself. This includes special traits (like `Copy` and `Sized`) that affect
1701 how the compiler behaves, as well as special functions that may be automatically
1702 invoked (such as the handler for out-of-bounds accesses when indexing a slice).
1703 Erroneous code example:
1705 ```compile_fail,E0522
1706 #![feature(lang_items)]
1709 fn cookie() -> ! { // error: definition of an unknown language item: `cookie`
1716 A closure was used but didn't implement the expected trait.
1718 Erroneous code example:
1720 ```compile_fail,E0525
1724 fn bar<T: Fn(u32)>(_: T) {}
1728 let closure = |_| foo(x); // error: expected a closure that implements
1729 // the `Fn` trait, but this closure only
1730 // implements `FnOnce`
1735 In the example above, `closure` is an `FnOnce` closure whereas the `bar`
1736 function expected an `Fn` closure. In this case, it's simple to fix the issue,
1737 you just have to implement `Copy` and `Clone` traits on `struct X` and it'll
1741 #[derive(Clone, Copy)] // We implement `Clone` and `Copy` traits.
1745 fn bar<T: Fn(u32)>(_: T) {}
1749 let closure = |_| foo(x);
1750 bar(closure); // ok!
1754 To understand better how closures work in Rust, read:
1755 https://doc.rust-lang.org/book/ch13-01-closures.html
1759 The `main` function was incorrectly declared.
1761 Erroneous code example:
1763 ```compile_fail,E0580
1764 fn main(x: i32) { // error: main function has wrong type
1769 The `main` function prototype should never take arguments.
1778 If you want to get command-line arguments, use `std::env::args`. To exit with a
1779 specified exit code, use `std::process::exit`.
1783 Abstract return types (written `impl Trait` for some trait `Trait`) are only
1784 allowed as function and inherent impl return types.
1786 Erroneous code example:
1788 ```compile_fail,E0562
1790 let count_to_ten: impl Iterator<Item=usize> = 0..10;
1791 // error: `impl Trait` not allowed outside of function and inherent method
1793 for i in count_to_ten {
1799 Make sure `impl Trait` only appears in return-type position.
1802 fn count_to_n(n: usize) -> impl Iterator<Item=usize> {
1807 for i in count_to_n(10) { // ok!
1813 See [RFC 1522] for more details.
1815 [RFC 1522]: https://github.com/rust-lang/rfcs/blob/master/text/1522-conservative-impl-trait.md
1819 Per [RFC 401][rfc401], if you have a function declaration `foo`:
1822 // For the purposes of this explanation, all of these
1823 // different kinds of `fn` declarations are equivalent:
1825 fn foo(x: S) { /* ... */ }
1826 # #[cfg(for_demonstration_only)]
1827 extern "C" { fn foo(x: S); }
1828 # #[cfg(for_demonstration_only)]
1829 impl S { fn foo(self) { /* ... */ } }
1832 the type of `foo` is **not** `fn(S)`, as one might expect.
1833 Rather, it is a unique, zero-sized marker type written here as `typeof(foo)`.
1834 However, `typeof(foo)` can be _coerced_ to a function pointer `fn(S)`,
1835 so you rarely notice this:
1840 let x: fn(S) = foo; // OK, coerces
1843 The reason that this matter is that the type `fn(S)` is not specific to
1844 any particular function: it's a function _pointer_. So calling `x()` results
1845 in a virtual call, whereas `foo()` is statically dispatched, because the type
1846 of `foo` tells us precisely what function is being called.
1848 As noted above, coercions mean that most code doesn't have to be
1849 concerned with this distinction. However, you can tell the difference
1850 when using **transmute** to convert a fn item into a fn pointer.
1852 This is sometimes done as part of an FFI:
1854 ```compile_fail,E0591
1855 extern "C" fn foo(userdata: Box<i32>) {
1859 # fn callback(_: extern "C" fn(*mut i32)) {}
1860 # use std::mem::transmute;
1862 let f: extern "C" fn(*mut i32) = transmute(foo);
1867 Here, transmute is being used to convert the types of the fn arguments.
1868 This pattern is incorrect because, because the type of `foo` is a function
1869 **item** (`typeof(foo)`), which is zero-sized, and the target type (`fn()`)
1870 is a function pointer, which is not zero-sized.
1871 This pattern should be rewritten. There are a few possible ways to do this:
1873 - change the original fn declaration to match the expected signature,
1874 and do the cast in the fn body (the preferred option)
1875 - cast the fn item fo a fn pointer before calling transmute, as shown here:
1878 # extern "C" fn foo(_: Box<i32>) {}
1879 # use std::mem::transmute;
1881 let f: extern "C" fn(*mut i32) = transmute(foo as extern "C" fn(_));
1882 let f: extern "C" fn(*mut i32) = transmute(foo as usize); // works too
1886 The same applies to transmutes to `*mut fn()`, which were observedin practice.
1887 Note though that use of this type is generally incorrect.
1888 The intention is typically to describe a function pointer, but just `fn()`
1889 alone suffices for that. `*mut fn()` is a pointer to a fn pointer.
1890 (Since these values are typically just passed to C code, however, this rarely
1891 makes a difference in practice.)
1893 [rfc401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md
1897 You tried to supply an `Fn`-based type with an incorrect number of arguments
1898 than what was expected.
1900 Erroneous code example:
1902 ```compile_fail,E0593
1903 fn foo<F: Fn()>(x: F) { }
1906 // [E0593] closure takes 1 argument but 0 arguments are required
1913 No `main` function was found in a binary crate. To fix this error, add a
1914 `main` function. For example:
1918 // Your program will start here.
1919 println!("Hello world!");
1923 If you don't know the basics of Rust, you can go look to the Rust Book to get
1924 started: https://doc.rust-lang.org/book/
1928 An unknown lint was used on the command line.
1933 rustc -D bogus omse_file.rs
1936 Maybe you just misspelled the lint name or the lint doesn't exist anymore.
1937 Either way, try to update/remove it in order to fix the error.
1941 This error code indicates a mismatch between the lifetimes appearing in the
1942 function signature (i.e., the parameter types and the return type) and the
1943 data-flow found in the function body.
1945 Erroneous code example:
1947 ```compile_fail,E0621
1948 fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 { // error: explicit lifetime
1949 // required in the type of
1951 if x > y { x } else { y }
1955 In the code above, the function is returning data borrowed from either `x` or
1956 `y`, but the `'a` annotation indicates that it is returning data only from `x`.
1957 To fix the error, the signature and the body must be made to match. Typically,
1958 this is done by updating the function signature. So, in this case, we change
1959 the type of `y` to `&'a i32`, like so:
1962 fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {
1963 if x > y { x } else { y }
1967 Now the signature indicates that the function data borrowed from either `x` or
1968 `y`. Alternatively, you could change the body to not return data from `y`:
1971 fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 {
1978 The `#![feature]` attribute specified an unknown feature.
1980 Erroneous code example:
1982 ```compile_fail,E0635
1983 #![feature(nonexistent_rust_feature)] // error: unknown feature
1989 A `#![feature]` attribute was declared multiple times.
1991 Erroneous code example:
1993 ```compile_fail,E0636
1994 #![allow(stable_features)]
1996 #![feature(rust1)] // error: the feature `rust1` has already been declared
2002 A closure or generator was constructed that references its own type.
2006 ```compile-fail,E0644
2015 // Here, when `x` is called, the parameter `y` is equal to `x`.
2020 Rust does not permit a closure to directly reference its own type,
2021 either through an argument (as in the example above) or by capturing
2022 itself through its environment. This restriction helps keep closure
2023 inference tractable.
2025 The easiest fix is to rewrite your closure into a top-level function,
2026 or into a method. In some cases, you may also be able to have your
2027 closure call itself by capturing a `&Fn()` object or `fn()` pointer
2028 that refers to itself. That is permitting, since the closure would be
2029 invoking itself via a virtual call, and hence does not directly
2030 reference its own *type*.
2035 A `repr(transparent)` type was also annotated with other, incompatible
2036 representation hints.
2038 Erroneous code example:
2040 ```compile_fail,E0692
2041 #[repr(transparent, C)] // error: incompatible representation hints
2045 A type annotated as `repr(transparent)` delegates all representation concerns to
2046 another type, so adding more representation hints is contradictory. Remove
2047 either the `transparent` hint or the other hints, like this:
2050 #[repr(transparent)]
2054 Alternatively, move the other attributes to the contained type:
2063 #[repr(transparent)]
2064 struct FooWrapper(Foo);
2067 Note that introducing another `struct` just to have a place for the other
2068 attributes may have unintended side effects on the representation:
2071 #[repr(transparent)]
2077 #[repr(transparent)]
2078 struct Grams2(Float); // this is not equivalent to `Grams` above
2081 Here, `Grams2` is a not equivalent to `Grams` -- the former transparently wraps
2082 a (non-transparent) struct containing a single float, while `Grams` is a
2083 transparent wrapper around a float. This can make a difference for the ABI.
2087 When using generators (or async) all type variables must be bound so a
2088 generator can be constructed.
2090 Erroneous code example:
2092 ```edition2018,compile-fail,E0698
2093 #![feature(futures_api, async_await, await_macro)]
2094 async fn bar<T>() -> () {}
2097 await!(bar()); // error: cannot infer type for `T`
2101 In the above example `T` is unknowable by the compiler.
2102 To fix this you must bind `T` to a concrete type such as `String`
2103 so that a generator can then be constructed:
2106 #![feature(futures_api, async_await, await_macro)]
2107 async fn bar<T>() -> () {}
2110 await!(bar::<String>());
2111 // ^^^^^^^^ specify type explicitly
2117 The `impl Trait` return type captures lifetime parameters that do not
2118 appear within the `impl Trait` itself.
2120 Erroneous code example:
2122 ```compile-fail,E0700
2123 use std::cell::Cell;
2127 impl<'a, 'b> Trait<'b> for Cell<&'a u32> { }
2129 fn foo<'x, 'y>(x: Cell<&'x u32>) -> impl Trait<'y>
2136 Here, the function `foo` returns a value of type `Cell<&'x u32>`,
2137 which references the lifetime `'x`. However, the return type is
2138 declared as `impl Trait<'y>` -- this indicates that `foo` returns
2139 "some type that implements `Trait<'y>`", but it also indicates that
2140 the return type **only captures data referencing the lifetime `'y`**.
2141 In this case, though, we are referencing data with lifetime `'x`, so
2142 this function is in error.
2144 To fix this, you must reference the lifetime `'x` from the return
2145 type. For example, changing the return type to `impl Trait<'y> + 'x`
2149 use std::cell::Cell;
2153 impl<'a,'b> Trait<'b> for Cell<&'a u32> { }
2155 fn foo<'x, 'y>(x: Cell<&'x u32>) -> impl Trait<'y> + 'x
2164 This error indicates that a `#[non_exhaustive]` attribute was incorrectly placed
2165 on something other than a struct or enum.
2167 Examples of erroneous code:
2169 ```compile_fail,E0701
2170 # #![feature(non_exhaustive)]
2178 This error indicates that a `#[lang = ".."]` attribute was placed
2179 on the wrong type of item.
2181 Examples of erroneous code:
2183 ```compile_fail,E0718
2184 #![feature(lang_items)]
2194 register_diagnostics! {
2195 // E0006, // merged with E0005
2196 // E0101, // replaced with E0282
2197 // E0102, // replaced with E0282
2200 // E0272, // on_unimplemented #0
2201 // E0273, // on_unimplemented #1
2202 // E0274, // on_unimplemented #2
2203 E0278, // requirement is not satisfied
2204 E0279, // requirement is not satisfied
2205 E0280, // requirement is not satisfied
2206 // E0285, // overflow evaluation builtin bounds
2207 // E0296, // replaced with a generic attribute input check
2208 // E0300, // unexpanded macro
2209 // E0304, // expected signed integer constant
2210 // E0305, // expected constant
2211 E0311, // thing may not live long enough
2212 E0312, // lifetime of reference outlives lifetime of borrowed content
2213 E0313, // lifetime of borrowed pointer outlives lifetime of captured variable
2214 E0314, // closure outlives stack frame
2215 E0315, // cannot invoke closure outside of its lifetime
2216 E0316, // nested quantification of lifetimes
2217 E0320, // recursive overflow during dropck
2218 E0473, // dereference of reference outside its lifetime
2219 E0474, // captured variable `..` does not outlive the enclosing closure
2220 E0475, // index of slice outside its lifetime
2221 E0476, // lifetime of the source pointer does not outlive lifetime bound...
2222 E0477, // the type `..` does not fulfill the required lifetime...
2223 E0479, // the type `..` (provided as the value of a type parameter) is...
2224 E0480, // lifetime of method receiver does not outlive the method call
2225 E0481, // lifetime of function argument does not outlive the function call
2226 E0482, // lifetime of return value does not outlive the function call
2227 E0483, // lifetime of operand does not outlive the operation
2228 E0484, // reference is not valid at the time of borrow
2229 E0485, // automatically reference is not valid at the time of borrow
2230 E0486, // type of expression contains references that are not valid during...
2231 E0487, // unsafe use of destructor: destructor might be called while...
2232 E0488, // lifetime of variable does not enclose its declaration
2233 E0489, // type/lifetime parameter not in scope here
2234 E0490, // a value of type `..` is borrowed for too long
2235 E0495, // cannot infer an appropriate lifetime due to conflicting requirements
2236 E0566, // conflicting representation hints
2237 E0623, // lifetime mismatch where both parameters are anonymous regions
2238 E0628, // generators cannot have explicit arguments
2239 E0631, // type mismatch in closure arguments
2240 E0637, // "'_" is not a valid lifetime bound
2241 E0657, // `impl Trait` can only capture lifetimes bound at the fn level
2242 E0687, // in-band lifetimes cannot be used in `fn`/`Fn` syntax
2243 E0688, // in-band lifetimes cannot be mixed with explicit lifetime binders
2244 E0697, // closures cannot be static
2245 E0707, // multiple elided lifetimes used in arguments of `async fn`
2246 E0708, // `async` non-`move` closures with arguments are not currently supported
2247 E0709, // multiple different lifetimes used in arguments of `async fn`
2248 E0710, // an unknown tool name found in scoped lint
2249 E0711, // a feature has been declared with conflicting stability attributes
2250 // E0702, // replaced with a generic attribute input check
2251 E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
2252 E0727, // `async` generators are not yet supported
2253 E0728, // `await` must be in an `async` function or block