1 // Error messages for EXXXX errors.
2 // Each message should start and end with a new line, and be wrapped to 80
3 // characters. In vim you can `:set tw=80` and use `gq` to wrap paragraphs. Use
4 // `:set tw=0` to disable.
5 syntax::register_diagnostics! {
7 Trait objects like `Box<Trait>` can only be constructed when certain
8 requirements are satisfied by the trait in question.
10 Trait objects are a form of dynamic dispatch and use a dynamically sized type
11 for the inner type. So, for a given trait `Trait`, when `Trait` is treated as a
12 type, as in `Box<Trait>`, the inner type is 'unsized'. In such cases the boxed
13 pointer is a 'fat pointer' that contains an extra pointer to a table of methods
14 (among other things) for dynamic dispatch. This design mandates some
15 restrictions on the types of traits that are allowed to be used in trait
16 objects, which are collectively termed as 'object safety' rules.
18 Attempting to create a trait object for a non object-safe trait will trigger
21 There are various rules:
23 ### The trait cannot require `Self: Sized`
25 When `Trait` is treated as a type, the type does not implement the special
26 `Sized` trait, because the type does not have a known size at compile time and
27 can only be accessed behind a pointer. Thus, if we have a trait like the
31 trait Foo where Self: Sized {
36 We cannot create an object of type `Box<Foo>` or `&Foo` since in this case
37 `Self` would not be `Sized`.
39 Generally, `Self: Sized` is used to indicate that the trait should not be used
40 as a trait object. If the trait comes from your own crate, consider removing
43 ### Method references the `Self` type in its parameters or return type
45 This happens when a trait has a method like the following:
49 fn foo(&self) -> Self;
52 impl Trait for String {
53 fn foo(&self) -> Self {
59 fn foo(&self) -> Self {
65 (Note that `&self` and `&mut self` are okay, it's additional `Self` types which
68 In such a case, the compiler cannot predict the return type of `foo()` in a
69 situation like the following:
73 fn foo(&self) -> Self;
76 fn call_foo(x: Box<Trait>) {
77 let y = x.foo(); // What type is y?
82 If only some methods aren't object-safe, you can add a `where Self: Sized` bound
83 on them to mark them as explicitly unavailable to trait objects. The
84 functionality will still be available to all other implementers, including
85 `Box<Trait>` which is itself sized (assuming you `impl Trait for Box<Trait>`).
89 fn foo(&self) -> Self where Self: Sized;
94 Now, `foo()` can no longer be called on a trait object, but you will now be
95 allowed to make a trait object, and that will be able to call any object-safe
96 methods. With such a bound, one can still call `foo()` on types implementing
97 that trait that aren't behind trait objects.
99 ### Method has generic type parameters
101 As mentioned before, trait objects contain pointers to method tables. So, if we
109 impl Trait for String {
123 At compile time each implementation of `Trait` will produce a table containing
124 the various methods (and other items) related to the implementation.
126 This works fine, but when the method gains generic parameters, we can have a
129 Usually, generic parameters get _monomorphized_. For example, if I have
137 The machine code for `foo::<u8>()`, `foo::<bool>()`, `foo::<String>()`, or any
138 other type substitution is different. Hence the compiler generates the
139 implementation on-demand. If you call `foo()` with a `bool` parameter, the
140 compiler will only generate code for `foo::<bool>()`. When we have additional
141 type parameters, the number of monomorphized implementations the compiler
142 generates does not grow drastically, since the compiler will only generate an
143 implementation if the function is called with unparametrized substitutions
144 (i.e., substitutions where none of the substituted types are themselves
147 However, with trait objects we have to make a table containing _every_ object
148 that implements the trait. Now, if it has type parameters, we need to add
149 implementations for every type that implements the trait, and there could
150 theoretically be an infinite number of types.
156 fn foo<T>(&self, on: T);
160 impl Trait for String {
161 fn foo<T>(&self, on: T) {
167 fn foo<T>(&self, on: T) {
172 // 8 more implementations
175 Now, if we have the following code:
177 ```compile_fail,E0038
178 # trait Trait { fn foo<T>(&self, on: T); }
179 # impl Trait for String { fn foo<T>(&self, on: T) {} }
180 # impl Trait for u8 { fn foo<T>(&self, on: T) {} }
181 # impl Trait for bool { fn foo<T>(&self, on: T) {} }
183 fn call_foo(thing: Box<Trait>) {
184 thing.foo(true); // this could be any one of the 8 types above
190 We don't just need to create a table of all implementations of all methods of
191 `Trait`, we need to create such a table, for each different type fed to
192 `foo()`. In this case this turns out to be (10 types implementing `Trait`)*(3
193 types being fed to `foo()`) = 30 implementations!
195 With real world traits these numbers can grow drastically.
197 To fix this, it is suggested to use a `where Self: Sized` bound similar to the
198 fix for the sub-error above if you do not intend to call the method with type
203 fn foo<T>(&self, on: T) where Self: Sized;
208 If this is not an option, consider replacing the type parameter with another
209 trait object (e.g., if `T: OtherTrait`, use `on: Box<OtherTrait>`). If the
210 number of types you intend to feed to this method is limited, consider manually
211 listing out the methods of different types.
213 ### Method has no receiver
215 Methods that do not take a `self` parameter can't be called since there won't be
216 a way to get a pointer to the method table for them.
224 This could be called as `<Foo as Foo>::foo()`, which would not be able to pick
227 Adding a `Self: Sized` bound to these methods will generally make this compile.
231 fn foo() -> u8 where Self: Sized;
235 ### The trait cannot contain associated constants
237 Just like static functions, associated constants aren't stored on the method
238 table. If the trait or any subtrait contain an associated constant, they cannot
239 be made into an object.
241 ```compile_fail,E0038
249 A simple workaround is to use a helper method instead:
257 ### The trait cannot use `Self` as a type parameter in the supertrait listing
259 This is similar to the second sub-error, but subtler. It happens in situations
265 trait Trait: Super<Self> {
270 impl Super<Foo> for Foo{}
272 impl Trait for Foo {}
275 Here, the supertrait might have methods as follows:
279 fn get_a(&self) -> A; // note that this is object safe!
283 If the trait `Foo` was deriving from something like `Super<String>` or
284 `Super<T>` (where `Foo` itself is `Foo<T>`), this is okay, because given a type
285 `get_a()` will definitely return an object of that type.
287 However, if it derives from `Super<Self>`, even though `Super` is object safe,
288 the method `get_a()` would return an object of unknown type when called on the
289 function. `Self` type parameters let us make object safe traits no longer safe,
290 so they are forbidden when specifying supertraits.
292 There's no easy fix for this, generally code will need to be refactored so that
293 you no longer need to derive from `Super<Self>`.
297 When defining a recursive struct or enum, any use of the type being defined
298 from inside the definition must occur behind a pointer (like `Box` or `&`).
299 This is because structs and enums must have a well-defined size, and without
300 the pointer, the size of the type would need to be unbounded.
302 Consider the following erroneous definition of a type for a list of bytes:
304 ```compile_fail,E0072
305 // error, invalid recursive struct type
308 tail: Option<ListNode>,
312 This type cannot have a well-defined size, because it needs to be arbitrarily
313 large (since we would be able to nest `ListNode`s to any depth). Specifically,
316 size of `ListNode` = 1 byte for `head`
317 + 1 byte for the discriminant of the `Option`
321 One way to fix this is by wrapping `ListNode` in a `Box`, like so:
326 tail: Option<Box<ListNode>>,
330 This works because `Box` is a pointer, so its size is well-known.
334 This error indicates that the compiler was unable to sensibly evaluate an
335 constant expression that had to be evaluated. Attempting to divide by 0
336 or causing integer overflow are two ways to induce this error. For example:
338 ```compile_fail,E0080
345 Ensure that the expressions given can be evaluated as the desired integer type.
346 See the FFI section of the Reference for more information about using a custom
349 https://doc.rust-lang.org/reference.html#ffi-attributes
353 This error indicates that a lifetime is missing from a type. If it is an error
354 inside a function signature, the problem may be with failing to adhere to the
355 lifetime elision rules (see below).
357 Here are some simple examples of where you'll run into this error:
359 ```compile_fail,E0106
360 struct Foo1 { x: &bool }
361 // ^ expected lifetime parameter
362 struct Foo2<'a> { x: &'a bool } // correct
364 struct Bar1 { x: Foo2 }
365 // ^^^^ expected lifetime parameter
366 struct Bar2<'a> { x: Foo2<'a> } // correct
368 enum Baz1 { A(u8), B(&bool), }
369 // ^ expected lifetime parameter
370 enum Baz2<'a> { A(u8), B(&'a bool), } // correct
373 // ^ expected lifetime parameter
374 type MyStr2<'a> = &'a str; // correct
377 Lifetime elision is a special, limited kind of inference for lifetimes in
378 function signatures which allows you to leave out lifetimes in certain cases.
379 For more background on lifetime elision see [the book][book-le].
381 The lifetime elision rules require that any function signature with an elided
382 output lifetime must either have
384 - exactly one input lifetime
385 - or, multiple input lifetimes, but the function must also be a method with a
386 `&self` or `&mut self` receiver
388 In the first case, the output lifetime is inferred to be the same as the unique
389 input lifetime. In the second case, the lifetime is instead inferred to be the
390 same as the lifetime on `&self` or `&mut self`.
392 Here are some examples of elision errors:
394 ```compile_fail,E0106
395 // error, no input lifetimes
398 // error, `x` and `y` have distinct lifetimes inferred
399 fn bar(x: &str, y: &str) -> &str { }
401 // error, `y`'s lifetime is inferred to be distinct from `x`'s
402 fn baz<'a>(x: &'a str, y: &str) -> &str { }
405 [book-le]: https://doc.rust-lang.org/book/ch10-03-lifetime-syntax.html#lifetime-elision
409 There are conflicting trait implementations for the same type.
410 Example of erroneous code:
412 ```compile_fail,E0119
414 fn get(&self) -> usize;
417 impl<T> MyTrait for T {
418 fn get(&self) -> usize { 0 }
425 impl MyTrait for Foo { // error: conflicting implementations of trait
426 // `MyTrait` for type `Foo`
427 fn get(&self) -> usize { self.value }
431 When looking for the implementation for the trait, the compiler finds
432 both the `impl<T> MyTrait for T` where T is all types and the `impl
433 MyTrait for Foo`. Since a trait cannot be implemented multiple times,
434 this is an error. So, when you write:
438 fn get(&self) -> usize;
441 impl<T> MyTrait for T {
442 fn get(&self) -> usize { 0 }
446 This makes the trait implemented on all types in the scope. So if you
447 try to implement it on another one after that, the implementations will
452 fn get(&self) -> usize;
455 impl<T> MyTrait for T {
456 fn get(&self) -> usize { 0 }
464 f.get(); // the trait is implemented so we can use it
469 // This shouldn't really ever trigger since the repeated value error comes first
471 A binary can only have one entry point, and by default that entry point is the
472 function `main()`. If there are multiple such functions, please rename one.
476 More than one function was declared with the `#[main]` attribute.
478 Erroneous code example:
480 ```compile_fail,E0137
487 fn f() {} // error: multiple functions with a `#[main]` attribute
490 This error indicates that the compiler found multiple functions with the
491 `#[main]` attribute. This is an error because there must be a unique entry
492 point into a Rust program. Example:
503 More than one function was declared with the `#[start]` attribute.
505 Erroneous code example:
507 ```compile_fail,E0138
511 fn foo(argc: isize, argv: *const *const u8) -> isize {}
514 fn f(argc: isize, argv: *const *const u8) -> isize {}
515 // error: multiple 'start' functions
518 This error indicates that the compiler found multiple functions with the
519 `#[start]` attribute. This is an error because there must be a unique entry
520 point into a Rust program. Example:
526 fn foo(argc: isize, argv: *const *const u8) -> isize { 0 } // ok!
531 #### Note: this error code is no longer emitted by the compiler.
533 There are various restrictions on transmuting between types in Rust; for example
534 types being transmuted must have the same size. To apply all these restrictions,
535 the compiler must know the exact types that may be transmuted. When type
536 parameters are involved, this cannot always be done.
538 So, for example, the following is not allowed:
541 use std::mem::transmute;
543 struct Foo<T>(Vec<T>);
545 fn foo<T>(x: Vec<T>) {
546 // we are transmuting between Vec<T> and Foo<F> here
547 let y: Foo<T> = unsafe { transmute(x) };
548 // do something with y
552 In this specific case there's a good chance that the transmute is harmless (but
553 this is not guaranteed by Rust). However, when alignment and enum optimizations
554 come into the picture, it's quite likely that the sizes may or may not match
555 with different type parameter substitutions. It's not possible to check this for
556 _all_ possible types, so `transmute()` simply only accepts types without any
557 unsubstituted type parameters.
559 If you need this, there's a good chance you're doing something wrong. Keep in
560 mind that Rust doesn't guarantee much about the layout of different structs
561 (even two structs with identical declarations may have different layouts). If
562 there is a solution that avoids the transmute entirely, try it instead.
564 If it's possible, hand-monomorphize the code by writing the function for each
565 possible type substitution. It's possible to use traits to do this cleanly,
569 use std::mem::transmute;
571 struct Foo<T>(Vec<T>);
573 trait MyTransmutableType: Sized {
574 fn transmute(_: Vec<Self>) -> Foo<Self>;
577 impl MyTransmutableType for u8 {
578 fn transmute(x: Vec<u8>) -> Foo<u8> {
579 unsafe { transmute(x) }
583 impl MyTransmutableType for String {
584 fn transmute(x: Vec<String>) -> Foo<String> {
585 unsafe { transmute(x) }
589 // ... more impls for the types you intend to transmute
591 fn foo<T: MyTransmutableType>(x: Vec<T>) {
592 let y: Foo<T> = <T as MyTransmutableType>::transmute(x);
593 // do something with y
597 Each impl will be checked for a size match in the transmute as usual, and since
598 there are no unbound type parameters involved, this should compile unless there
599 is a size mismatch in one of the impls.
601 It is also possible to manually transmute:
605 # let v = Some("value");
606 # type SomeType = &'static [u8];
608 ptr::read(&v as *const _ as *const SomeType) // `v` transmuted to `SomeType`
613 Note that this does not move `v` (unlike `transmute`), and may need a
614 call to `mem::forget(v)` in case you want to avoid destructors being called.
618 A lang item was redefined.
620 Erroneous code example:
622 ```compile_fail,E0152
623 #![feature(lang_items)]
626 struct Foo; // error: duplicate lang item found: `arc`
629 Lang items are already implemented in the standard library. Unless you are
630 writing a free-standing application (e.g., a kernel), you do not need to provide
633 You can build a free-standing crate by adding `#![no_std]` to the crate
636 ```ignore (only-for-syntax-highlight)
640 See also the [unstable book][1].
642 [1]: https://doc.rust-lang.org/unstable-book/language-features/lang-items.html#writing-an-executable-without-stdlib
646 A generic type was described using parentheses rather than angle brackets.
649 ```compile_fail,E0214
651 let v: Vec(&str) = vec!["foo"];
655 This is not currently supported: `v` should be defined as `Vec<&str>`.
656 Parentheses are currently only used with generic types when defining parameters
657 for `Fn`-family traits.
661 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
662 message for when a particular trait isn't implemented on a type placed in a
663 position that needs that trait. For example, when the following code is
667 #![feature(on_unimplemented)]
669 fn foo<T: Index<u8>>(x: T){}
671 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
672 trait Index<Idx> { /* ... */ }
674 foo(true); // `bool` does not implement `Index<u8>`
677 There will be an error about `bool` not implementing `Index<u8>`, followed by a
678 note saying "the type `bool` cannot be indexed by `u8`".
680 As you can see, you can specify type parameters in curly braces for
681 substitution with the actual types (using the regular format string syntax) in
682 a given situation. Furthermore, `{Self}` will substitute to the type (in this
683 case, `bool`) that we tried to use.
685 This error appears when the curly braces contain an identifier which doesn't
686 match with any of the type parameters or the string `Self`. This might happen
687 if you misspelled a type parameter, or if you intended to use literal curly
688 braces. If it is the latter, escape the curly braces with a second curly brace
689 of the same type; e.g., a literal `{` is `{{`.
693 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
694 message for when a particular trait isn't implemented on a type placed in a
695 position that needs that trait. For example, when the following code is
699 #![feature(on_unimplemented)]
701 fn foo<T: Index<u8>>(x: T){}
703 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
704 trait Index<Idx> { /* ... */ }
706 foo(true); // `bool` does not implement `Index<u8>`
709 there will be an error about `bool` not implementing `Index<u8>`, followed by a
710 note saying "the type `bool` cannot be indexed by `u8`".
712 As you can see, you can specify type parameters in curly braces for
713 substitution with the actual types (using the regular format string syntax) in
714 a given situation. Furthermore, `{Self}` will substitute to the type (in this
715 case, `bool`) that we tried to use.
717 This error appears when the curly braces do not contain an identifier. Please
718 add one of the same name as a type parameter. If you intended to use literal
719 braces, use `{{` and `}}` to escape them.
723 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
724 message for when a particular trait isn't implemented on a type placed in a
725 position that needs that trait. For example, when the following code is
729 #![feature(on_unimplemented)]
731 fn foo<T: Index<u8>>(x: T){}
733 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
734 trait Index<Idx> { /* ... */ }
736 foo(true); // `bool` does not implement `Index<u8>`
739 there will be an error about `bool` not implementing `Index<u8>`, followed by a
740 note saying "the type `bool` cannot be indexed by `u8`".
742 For this to work, some note must be specified. An empty attribute will not do
743 anything, please remove the attribute or add some helpful note for users of the
748 When using a lifetime like `'a` in a type, it must be declared before being
751 These two examples illustrate the problem:
753 ```compile_fail,E0261
754 // error, use of undeclared lifetime name `'a`
755 fn foo(x: &'a str) { }
758 // error, use of undeclared lifetime name `'a`
763 These can be fixed by declaring lifetime parameters:
770 fn foo<'a>(x: &'a str) {}
773 Impl blocks declare lifetime parameters separately. You need to add lifetime
774 parameters to an impl block if you're implementing a type that has a lifetime
775 parameter of its own.
778 ```compile_fail,E0261
783 // error, use of undeclared lifetime name `'a`
785 fn foo<'a>(x: &'a str) {}
789 This is fixed by declaring the impl block like this:
798 fn foo(x: &'a str) {}
804 Declaring certain lifetime names in parameters is disallowed. For example,
805 because the `'static` lifetime is a special built-in lifetime name denoting
806 the lifetime of the entire program, this is an error:
808 ```compile_fail,E0262
809 // error, invalid lifetime parameter name `'static`
810 fn foo<'static>(x: &'static str) { }
815 A lifetime name cannot be declared more than once in the same scope. For
818 ```compile_fail,E0263
819 // error, lifetime name `'a` declared twice in the same scope
820 fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str) { }
825 An unknown external lang item was used. Erroneous code example:
827 ```compile_fail,E0264
828 #![feature(lang_items)]
831 #[lang = "cake"] // error: unknown external lang item: `cake`
836 A list of available external lang items is available in
837 `src/librustc/middle/weak_lang_items.rs`. Example:
840 #![feature(lang_items)]
843 #[lang = "panic_impl"] // ok!
850 This is because of a type mismatch between the associated type of some
851 trait (e.g., `T::Bar`, where `T` implements `trait Quux { type Bar; }`)
852 and another type `U` that is required to be equal to `T::Bar`, but is not.
855 Here is a basic example:
857 ```compile_fail,E0271
858 trait Trait { type AssociatedType; }
860 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
864 impl Trait for i8 { type AssociatedType = &'static str; }
869 Here is that same example again, with some explanatory comments:
871 ```compile_fail,E0271
872 trait Trait { type AssociatedType; }
874 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
875 // ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
877 // This says `foo` can |
878 // only be used with |
880 // implements `Trait`. |
882 // This says not only must
883 // `T` be an impl of `Trait`
884 // but also that the impl
885 // must assign the type `u32`
886 // to the associated type.
890 impl Trait for i8 { type AssociatedType = &'static str; }
891 //~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
896 // ... but it is an implementation
897 // that assigns `&'static str` to
898 // the associated type.
901 // Here, we invoke `foo` with an `i8`, which does not satisfy
902 // the constraint `<i8 as Trait>::AssociatedType=u32`, and
903 // therefore the type-checker complains with this error code.
906 To avoid those issues, you have to make the types match correctly.
907 So we can fix the previous examples like this:
911 trait Trait { type AssociatedType; }
913 fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
917 impl Trait for i8 { type AssociatedType = &'static str; }
922 let vs = vec![1, 2, 3, 4];
934 This error occurs when there was a recursive trait requirement that overflowed
935 before it could be evaluated. Often this means that there is unbounded
936 recursion in resolving some type bounds.
938 For example, in the following code:
940 ```compile_fail,E0275
945 impl<T> Foo for T where Bar<T>: Foo {}
948 To determine if a `T` is `Foo`, we need to check if `Bar<T>` is `Foo`. However,
949 to do this check, we need to determine that `Bar<Bar<T>>` is `Foo`. To
950 determine this, we check if `Bar<Bar<Bar<T>>>` is `Foo`, and so on. This is
951 clearly a recursive requirement that can't be resolved directly.
953 Consider changing your trait bounds so that they're less self-referential.
957 This error occurs when a bound in an implementation of a trait does not match
958 the bounds specified in the original trait. For example:
960 ```compile_fail,E0276
966 fn foo<T>(x: T) where T: Copy {}
970 Here, all types implementing `Foo` must have a method `foo<T>(x: T)` which can
971 take any type `T`. However, in the `impl` for `bool`, we have added an extra
972 bound that `T` is `Copy`, which isn't compatible with the original trait.
974 Consider removing the bound from the method or adding the bound to the original
975 method definition in the trait.
979 You tried to use a type which doesn't implement some trait in a place which
980 expected that trait. Erroneous code example:
982 ```compile_fail,E0277
983 // here we declare the Foo trait with a bar method
988 // we now declare a function which takes an object implementing the Foo trait
989 fn some_func<T: Foo>(foo: T) {
994 // we now call the method with the i32 type, which doesn't implement
996 some_func(5i32); // error: the trait bound `i32 : Foo` is not satisfied
1000 In order to fix this error, verify that the type you're using does implement
1008 fn some_func<T: Foo>(foo: T) {
1009 foo.bar(); // we can now use this method since i32 implements the
1013 // we implement the trait on the i32 type
1019 some_func(5i32); // ok!
1023 Or in a generic context, an erroneous code example would look like:
1025 ```compile_fail,E0277
1026 fn some_func<T>(foo: T) {
1027 println!("{:?}", foo); // error: the trait `core::fmt::Debug` is not
1028 // implemented for the type `T`
1032 // We now call the method with the i32 type,
1033 // which *does* implement the Debug trait.
1038 Note that the error here is in the definition of the generic function: Although
1039 we only call it with a parameter that does implement `Debug`, the compiler
1040 still rejects the function: It must work with all possible input types. In
1041 order to make this example compile, we need to restrict the generic type we're
1047 // Restrict the input type to types that implement Debug.
1048 fn some_func<T: fmt::Debug>(foo: T) {
1049 println!("{:?}", foo);
1053 // Calling the method is still fine, as i32 implements Debug.
1056 // This would fail to compile now:
1057 // struct WithoutDebug;
1058 // some_func(WithoutDebug);
1062 Rust only looks at the signature of the called function, as such it must
1063 already specify all requirements that will be used for every type parameter.
1067 #### Note: this error code is no longer emitted by the compiler.
1069 You tried to supply a type which doesn't implement some trait in a location
1070 which expected that trait. This error typically occurs when working with
1071 `Fn`-based types. Erroneous code example:
1074 fn foo<F: Fn(usize)>(x: F) { }
1077 // type mismatch: ... implements the trait `core::ops::Fn<(String,)>`,
1078 // but the trait `core::ops::Fn<(usize,)>` is required
1080 foo(|y: String| { });
1084 The issue in this case is that `foo` is defined as accepting a `Fn` with one
1085 argument of type `String`, but the closure we attempted to pass to it requires
1086 one arguments of type `usize`.
1090 This error indicates that type inference did not result in one unique possible
1091 type, and extra information is required. In most cases this can be provided
1092 by adding a type annotation. Sometimes you need to specify a generic type
1095 A common example is the `collect` method on `Iterator`. It has a generic type
1096 parameter with a `FromIterator` bound, which for a `char` iterator is
1097 implemented by `Vec` and `String` among others. Consider the following snippet
1098 that reverses the characters of a string:
1100 ```compile_fail,E0282
1101 let x = "hello".chars().rev().collect();
1104 In this case, the compiler cannot infer what the type of `x` should be:
1105 `Vec<char>` and `String` are both suitable candidates. To specify which type to
1106 use, you can use a type annotation on `x`:
1109 let x: Vec<char> = "hello".chars().rev().collect();
1112 It is not necessary to annotate the full type. Once the ambiguity is resolved,
1113 the compiler can infer the rest:
1116 let x: Vec<_> = "hello".chars().rev().collect();
1119 Another way to provide the compiler with enough information, is to specify the
1120 generic type parameter:
1123 let x = "hello".chars().rev().collect::<Vec<char>>();
1126 Again, you need not specify the full type if the compiler can infer it:
1129 let x = "hello".chars().rev().collect::<Vec<_>>();
1132 Apart from a method or function with a generic type parameter, this error can
1133 occur when a type parameter of a struct or trait cannot be inferred. In that
1134 case it is not always possible to use a type annotation, because all candidates
1135 have the same return type. For instance:
1137 ```compile_fail,E0282
1148 let number = Foo::bar();
1153 This will fail because the compiler does not know which instance of `Foo` to
1154 call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.
1158 This error occurs when the compiler doesn't have enough information
1159 to unambiguously choose an implementation.
1163 ```compile_fail,E0283
1170 impl Generator for Impl {
1171 fn create() -> u32 { 1 }
1176 impl Generator for AnotherImpl {
1177 fn create() -> u32 { 2 }
1181 let cont: u32 = Generator::create();
1182 // error, impossible to choose one of Generator trait implementation
1183 // Should it be Impl or AnotherImpl, maybe something else?
1187 To resolve this error use the concrete type:
1196 impl Generator for AnotherImpl {
1197 fn create() -> u32 { 2 }
1201 let gen1 = AnotherImpl::create();
1203 // if there are multiple methods with same name (different traits)
1204 let gen2 = <AnotherImpl as Generator>::create();
1210 This error occurs when the compiler is unable to unambiguously infer the
1211 return type of a function or method which is generic on return type, such
1212 as the `collect` method for `Iterator`s.
1216 ```compile_fail,E0284
1217 fn foo() -> Result<bool, ()> {
1218 let results = [Ok(true), Ok(false), Err(())].iter().cloned();
1219 let v: Vec<bool> = results.collect()?;
1220 // Do things with v...
1225 Here we have an iterator `results` over `Result<bool, ()>`.
1226 Hence, `results.collect()` can return any type implementing
1227 `FromIterator<Result<bool, ()>>`. On the other hand, the
1228 `?` operator can accept any type implementing `Try`.
1230 The author of this code probably wants `collect()` to return a
1231 `Result<Vec<bool>, ()>`, but the compiler can't be sure
1232 that there isn't another type `T` implementing both `Try` and
1233 `FromIterator<Result<bool, ()>>` in scope such that
1234 `T::Ok == Vec<bool>`. Hence, this code is ambiguous and an error
1237 To resolve this error, use a concrete type for the intermediate expression:
1240 fn foo() -> Result<bool, ()> {
1241 let results = [Ok(true), Ok(false), Err(())].iter().cloned();
1243 let temp: Result<Vec<bool>, ()> = results.collect();
1246 // Do things with v...
1251 Note that the type of `v` can now be inferred from the type of `temp`.
1255 This error occurs when the compiler was unable to infer the concrete type of a
1256 variable. It can occur for several cases, the most common of which is a
1257 mismatch in the expected type that the compiler inferred for a variable's
1258 initializing expression, and the actual type explicitly assigned to the
1263 ```compile_fail,E0308
1264 let x: i32 = "I am not a number!";
1265 // ~~~ ~~~~~~~~~~~~~~~~~~~~
1267 // | initializing expression;
1268 // | compiler infers type `&str`
1270 // type `i32` assigned to variable `x`
1275 The type definition contains some field whose type
1276 requires an outlives annotation. Outlives annotations
1277 (e.g., `T: 'a`) are used to guarantee that all the data in T is valid
1278 for at least the lifetime `'a`. This scenario most commonly
1279 arises when the type contains an associated type reference
1280 like `<T as SomeTrait<'a>>::Output`, as shown in this example:
1282 ```compile_fail,E0309
1283 // This won't compile because the applicable impl of
1284 // `SomeTrait` (below) requires that `T: 'a`, but the struct does
1285 // not have a matching where-clause.
1287 foo: <T as SomeTrait<'a>>::Output,
1290 trait SomeTrait<'a> {
1294 impl<'a, T> SomeTrait<'a> for T
1302 Here, the where clause `T: 'a` that appears on the impl is not known to be
1303 satisfied on the struct. To make this example compile, you have to add
1304 a where-clause like `T: 'a` to the struct definition:
1311 foo: <T as SomeTrait<'a>>::Output
1314 trait SomeTrait<'a> {
1318 impl<'a, T> SomeTrait<'a> for T
1328 Types in type definitions have lifetimes associated with them that represent
1329 how long the data stored within them is guaranteed to be live. This lifetime
1330 must be as long as the data needs to be alive, and missing the constraint that
1331 denotes this will cause this error.
1333 ```compile_fail,E0310
1334 // This won't compile because T is not constrained to the static lifetime
1335 // the reference needs
1341 This will compile, because it has the constraint on the type parameter:
1344 struct Foo<T: 'static> {
1351 Reference's lifetime of borrowed content doesn't match the expected lifetime.
1353 Erroneous code example:
1355 ```compile_fail,E0312
1356 pub fn opt_str<'a>(maybestr: &'a Option<String>) -> &'static str {
1357 if maybestr.is_none() {
1360 let s: &'a str = maybestr.as_ref().unwrap();
1361 s // Invalid lifetime!
1366 To fix this error, either lessen the expected lifetime or find a way to not have
1367 to use this reference outside of its current scope (by running the code directly
1368 in the same block for example?):
1371 // In this case, we can fix the issue by switching from "static" lifetime to 'a
1372 pub fn opt_str<'a>(maybestr: &'a Option<String>) -> &'a str {
1373 if maybestr.is_none() {
1376 let s: &'a str = maybestr.as_ref().unwrap();
1384 This error occurs when an `if` expression without an `else` block is used in a
1385 context where a type other than `()` is expected, for example a `let`
1388 ```compile_fail,E0317
1391 let a = if x == 5 { 1 };
1395 An `if` expression without an `else` block has the type `()`, so this is a type
1396 error. To resolve it, add an `else` block having the same type as the `if`
1401 This error indicates that some types or traits depend on each other
1402 and therefore cannot be constructed.
1404 The following example contains a circular dependency between two traits:
1406 ```compile_fail,E0391
1407 trait FirstTrait : SecondTrait {
1411 trait SecondTrait : FirstTrait {
1418 #### Note: this error code is no longer emitted by the compiler.
1420 In Rust 1.3, the default object lifetime bounds are expected to change, as
1421 described in [RFC 1156]. You are getting a warning because the compiler
1422 thinks it is possible that this change will cause a compilation error in your
1423 code. It is possible, though unlikely, that this is a false alarm.
1425 The heart of the change is that where `&'a Box<SomeTrait>` used to default to
1426 `&'a Box<SomeTrait+'a>`, it now defaults to `&'a Box<SomeTrait+'static>` (here,
1427 `SomeTrait` is the name of some trait type). Note that the only types which are
1428 affected are references to boxes, like `&Box<SomeTrait>` or
1429 `&[Box<SomeTrait>]`. More common types like `&SomeTrait` or `Box<SomeTrait>`
1432 To silence this warning, edit your code to use an explicit bound. Most of the
1433 time, this means that you will want to change the signature of a function that
1434 you are calling. For example, if the error is reported on a call like `foo(x)`,
1435 and `foo` is defined as follows:
1438 # trait SomeTrait {}
1439 fn foo(arg: &Box<SomeTrait>) { /* ... */ }
1442 You might change it to:
1445 # trait SomeTrait {}
1446 fn foo<'a>(arg: &'a Box<SomeTrait+'a>) { /* ... */ }
1449 This explicitly states that you expect the trait object `SomeTrait` to contain
1450 references (with a maximum lifetime of `'a`).
1452 [RFC 1156]: https://github.com/rust-lang/rfcs/blob/master/text/1156-adjust-default-object-bounds.md
1456 An invalid lint attribute has been given. Erroneous code example:
1458 ```compile_fail,E0452
1459 #![allow(foo = "")] // error: malformed lint attribute
1462 Lint attributes only accept a list of identifiers (where each identifier is a
1463 lint name). Ensure the attribute is of this form:
1466 #![allow(foo)] // ok!
1468 #![allow(foo, foo2)] // ok!
1473 A lint check attribute was overruled by a `forbid` directive set as an
1474 attribute on an enclosing scope, or on the command line with the `-F` option.
1476 Example of erroneous code:
1478 ```compile_fail,E0453
1479 #![forbid(non_snake_case)]
1481 #[allow(non_snake_case)]
1483 let MyNumber = 2; // error: allow(non_snake_case) overruled by outer
1484 // forbid(non_snake_case)
1488 The `forbid` lint setting, like `deny`, turns the corresponding compiler
1489 warning into a hard error. Unlike `deny`, `forbid` prevents itself from being
1490 overridden by inner attributes.
1492 If you're sure you want to override the lint check, you can change `forbid` to
1493 `deny` (or use `-D` instead of `-F` if the `forbid` setting was given as a
1494 command-line option) to allow the inner lint check attribute:
1497 #![deny(non_snake_case)]
1499 #[allow(non_snake_case)]
1501 let MyNumber = 2; // ok!
1505 Otherwise, edit the code to pass the lint check, and remove the overruled
1509 #![forbid(non_snake_case)]
1518 A lifetime bound was not satisfied.
1520 Erroneous code example:
1522 ```compile_fail,E0478
1523 // Check that the explicit lifetime bound (`'SnowWhite`, in this example) must
1524 // outlive all the superbounds from the trait (`'kiss`, in this example).
1526 trait Wedding<'t>: 't { }
1528 struct Prince<'kiss, 'SnowWhite> {
1529 child: Box<Wedding<'kiss> + 'SnowWhite>,
1530 // error: lifetime bound not satisfied
1534 In this example, the `'SnowWhite` lifetime is supposed to outlive the `'kiss`
1535 lifetime but the declaration of the `Prince` struct doesn't enforce it. To fix
1536 this issue, you need to specify it:
1539 trait Wedding<'t>: 't { }
1541 struct Prince<'kiss, 'SnowWhite: 'kiss> { // You say here that 'kiss must live
1542 // longer than 'SnowWhite.
1543 child: Box<Wedding<'kiss> + 'SnowWhite>, // And now it's all good!
1549 A reference has a longer lifetime than the data it references.
1551 Erroneous code example:
1553 ```compile_fail,E0491
1554 trait SomeTrait<'a> {
1558 impl<'a, T> SomeTrait<'a> for T {
1559 type Output = &'a T; // compile error E0491
1563 Here, the problem is that a reference type like `&'a T` is only valid
1564 if all the data in T outlives the lifetime `'a`. But this impl as written
1565 is applicable to any lifetime `'a` and any type `T` -- we have no guarantee
1566 that `T` outlives `'a`. To fix this, you can add a where clause like
1570 trait SomeTrait<'a> {
1574 impl<'a, T> SomeTrait<'a> for T
1578 type Output = &'a T; // compile error E0491
1584 A lifetime name is shadowing another lifetime name. Erroneous code example:
1586 ```compile_fail,E0496
1592 fn f<'a>(x: &'a i32) { // error: lifetime name `'a` shadows a lifetime
1593 // name that is already in scope
1598 Please change the name of one of the lifetimes to remove this error. Example:
1606 fn f<'b>(x: &'b i32) { // ok!
1616 A stability attribute was used outside of the standard library. Erroneous code
1620 #[stable] // error: stability attributes may not be used outside of the
1625 It is not possible to use stability attributes outside of the standard library.
1626 Also, for now, it is not possible to write deprecation messages either.
1630 Transmute with two differently sized types was attempted. Erroneous code
1633 ```compile_fail,E0512
1634 fn takes_u8(_: u8) {}
1637 unsafe { takes_u8(::std::mem::transmute(0u16)); }
1638 // error: cannot transmute between types of different sizes,
1639 // or dependently-sized types
1643 Please use types with same size or use the expected type directly. Example:
1646 fn takes_u8(_: u8) {}
1649 unsafe { takes_u8(::std::mem::transmute(0i8)); } // ok!
1651 unsafe { takes_u8(0u8); } // ok!
1657 This error indicates that a `#[repr(..)]` attribute was placed on an
1660 Examples of erroneous code:
1662 ```compile_fail,E0517
1670 struct Foo {bar: bool, baz: bool}
1678 * The `#[repr(C)]` attribute can only be placed on structs and enums.
1679 * The `#[repr(packed)]` and `#[repr(simd)]` attributes only work on structs.
1680 * The `#[repr(u8)]`, `#[repr(i16)]`, etc attributes only work on enums.
1682 These attributes do not work on typedefs, since typedefs are just aliases.
1684 Representations like `#[repr(u8)]`, `#[repr(i64)]` are for selecting the
1685 discriminant size for enums with no data fields on any of the variants, e.g.
1686 `enum Color {Red, Blue, Green}`, effectively setting the size of the enum to
1687 the size of the provided type. Such an enum can be cast to a value of the same
1688 type as well. In short, `#[repr(u8)]` makes the enum behave like an integer
1689 with a constrained set of allowed values.
1691 Only field-less enums can be cast to numerical primitives, so this attribute
1692 will not apply to structs.
1694 `#[repr(packed)]` reduces padding to make the struct size smaller. The
1695 representation of enums isn't strictly defined in Rust, and this attribute
1696 won't work on enums.
1698 `#[repr(simd)]` will give a struct consisting of a homogeneous series of machine
1699 types (i.e., `u8`, `i32`, etc) a representation that permits vectorization via
1700 SIMD. This doesn't make much sense for enums since they don't consist of a
1701 single list of data.
1705 This error indicates that an `#[inline(..)]` attribute was incorrectly placed
1706 on something other than a function or method.
1708 Examples of erroneous code:
1710 ```compile_fail,E0518
1720 `#[inline]` hints the compiler whether or not to attempt to inline a method or
1721 function. By default, the compiler does a pretty good job of figuring this out
1722 itself, but if you feel the need for annotations, `#[inline(always)]` and
1723 `#[inline(never)]` can override or force the compiler's decision.
1725 If you wish to apply this attribute to all methods in an impl, manually annotate
1726 each method; it is not possible to annotate the entire impl with an `#[inline]`
1731 The lang attribute is intended for marking special items that are built-in to
1732 Rust itself. This includes special traits (like `Copy` and `Sized`) that affect
1733 how the compiler behaves, as well as special functions that may be automatically
1734 invoked (such as the handler for out-of-bounds accesses when indexing a slice).
1735 Erroneous code example:
1737 ```compile_fail,E0522
1738 #![feature(lang_items)]
1741 fn cookie() -> ! { // error: definition of an unknown language item: `cookie`
1748 A closure was used but didn't implement the expected trait.
1750 Erroneous code example:
1752 ```compile_fail,E0525
1756 fn bar<T: Fn(u32)>(_: T) {}
1760 let closure = |_| foo(x); // error: expected a closure that implements
1761 // the `Fn` trait, but this closure only
1762 // implements `FnOnce`
1767 In the example above, `closure` is an `FnOnce` closure whereas the `bar`
1768 function expected an `Fn` closure. In this case, it's simple to fix the issue,
1769 you just have to implement `Copy` and `Clone` traits on `struct X` and it'll
1773 #[derive(Clone, Copy)] // We implement `Clone` and `Copy` traits.
1777 fn bar<T: Fn(u32)>(_: T) {}
1781 let closure = |_| foo(x);
1782 bar(closure); // ok!
1786 To understand better how closures work in Rust, read:
1787 https://doc.rust-lang.org/book/ch13-01-closures.html
1791 The `main` function was incorrectly declared.
1793 Erroneous code example:
1795 ```compile_fail,E0580
1796 fn main(x: i32) { // error: main function has wrong type
1801 The `main` function prototype should never take arguments.
1810 If you want to get command-line arguments, use `std::env::args`. To exit with a
1811 specified exit code, use `std::process::exit`.
1815 Abstract return types (written `impl Trait` for some trait `Trait`) are only
1816 allowed as function and inherent impl return types.
1818 Erroneous code example:
1820 ```compile_fail,E0562
1822 let count_to_ten: impl Iterator<Item=usize> = 0..10;
1823 // error: `impl Trait` not allowed outside of function and inherent method
1825 for i in count_to_ten {
1831 Make sure `impl Trait` only appears in return-type position.
1834 fn count_to_n(n: usize) -> impl Iterator<Item=usize> {
1839 for i in count_to_n(10) { // ok!
1845 See [RFC 1522] for more details.
1847 [RFC 1522]: https://github.com/rust-lang/rfcs/blob/master/text/1522-conservative-impl-trait.md
1851 Per [RFC 401][rfc401], if you have a function declaration `foo`:
1854 // For the purposes of this explanation, all of these
1855 // different kinds of `fn` declarations are equivalent:
1857 fn foo(x: S) { /* ... */ }
1858 # #[cfg(for_demonstration_only)]
1859 extern "C" { fn foo(x: S); }
1860 # #[cfg(for_demonstration_only)]
1861 impl S { fn foo(self) { /* ... */ } }
1864 the type of `foo` is **not** `fn(S)`, as one might expect.
1865 Rather, it is a unique, zero-sized marker type written here as `typeof(foo)`.
1866 However, `typeof(foo)` can be _coerced_ to a function pointer `fn(S)`,
1867 so you rarely notice this:
1872 let x: fn(S) = foo; // OK, coerces
1875 The reason that this matter is that the type `fn(S)` is not specific to
1876 any particular function: it's a function _pointer_. So calling `x()` results
1877 in a virtual call, whereas `foo()` is statically dispatched, because the type
1878 of `foo` tells us precisely what function is being called.
1880 As noted above, coercions mean that most code doesn't have to be
1881 concerned with this distinction. However, you can tell the difference
1882 when using **transmute** to convert a fn item into a fn pointer.
1884 This is sometimes done as part of an FFI:
1886 ```compile_fail,E0591
1887 extern "C" fn foo(userdata: Box<i32>) {
1891 # fn callback(_: extern "C" fn(*mut i32)) {}
1892 # use std::mem::transmute;
1894 let f: extern "C" fn(*mut i32) = transmute(foo);
1899 Here, transmute is being used to convert the types of the fn arguments.
1900 This pattern is incorrect because, because the type of `foo` is a function
1901 **item** (`typeof(foo)`), which is zero-sized, and the target type (`fn()`)
1902 is a function pointer, which is not zero-sized.
1903 This pattern should be rewritten. There are a few possible ways to do this:
1905 - change the original fn declaration to match the expected signature,
1906 and do the cast in the fn body (the preferred option)
1907 - cast the fn item fo a fn pointer before calling transmute, as shown here:
1910 # extern "C" fn foo(_: Box<i32>) {}
1911 # use std::mem::transmute;
1913 let f: extern "C" fn(*mut i32) = transmute(foo as extern "C" fn(_));
1914 let f: extern "C" fn(*mut i32) = transmute(foo as usize); // works too
1918 The same applies to transmutes to `*mut fn()`, which were observed in practice.
1919 Note though that use of this type is generally incorrect.
1920 The intention is typically to describe a function pointer, but just `fn()`
1921 alone suffices for that. `*mut fn()` is a pointer to a fn pointer.
1922 (Since these values are typically just passed to C code, however, this rarely
1923 makes a difference in practice.)
1925 [rfc401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md
1929 You tried to supply an `Fn`-based type with an incorrect number of arguments
1930 than what was expected.
1932 Erroneous code example:
1934 ```compile_fail,E0593
1935 fn foo<F: Fn()>(x: F) { }
1938 // [E0593] closure takes 1 argument but 0 arguments are required
1945 No `main` function was found in a binary crate. To fix this error, add a
1946 `main` function. For example:
1950 // Your program will start here.
1951 println!("Hello world!");
1955 If you don't know the basics of Rust, you can go look to the Rust Book to get
1956 started: https://doc.rust-lang.org/book/
1960 An unknown lint was used on the command line.
1965 rustc -D bogus omse_file.rs
1968 Maybe you just misspelled the lint name or the lint doesn't exist anymore.
1969 Either way, try to update/remove it in order to fix the error.
1973 This error code indicates a mismatch between the lifetimes appearing in the
1974 function signature (i.e., the parameter types and the return type) and the
1975 data-flow found in the function body.
1977 Erroneous code example:
1979 ```compile_fail,E0621
1980 fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 { // error: explicit lifetime
1981 // required in the type of
1983 if x > y { x } else { y }
1987 In the code above, the function is returning data borrowed from either `x` or
1988 `y`, but the `'a` annotation indicates that it is returning data only from `x`.
1989 To fix the error, the signature and the body must be made to match. Typically,
1990 this is done by updating the function signature. So, in this case, we change
1991 the type of `y` to `&'a i32`, like so:
1994 fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {
1995 if x > y { x } else { y }
1999 Now the signature indicates that the function data borrowed from either `x` or
2000 `y`. Alternatively, you could change the body to not return data from `y`:
2003 fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 {
2010 The `#![feature]` attribute specified an unknown feature.
2012 Erroneous code example:
2014 ```compile_fail,E0635
2015 #![feature(nonexistent_rust_feature)] // error: unknown feature
2021 A `#![feature]` attribute was declared multiple times.
2023 Erroneous code example:
2025 ```compile_fail,E0636
2026 #![allow(stable_features)]
2028 #![feature(rust1)] // error: the feature `rust1` has already been declared
2034 A closure or generator was constructed that references its own type.
2038 ```compile-fail,E0644
2047 // Here, when `x` is called, the parameter `y` is equal to `x`.
2052 Rust does not permit a closure to directly reference its own type,
2053 either through an argument (as in the example above) or by capturing
2054 itself through its environment. This restriction helps keep closure
2055 inference tractable.
2057 The easiest fix is to rewrite your closure into a top-level function,
2058 or into a method. In some cases, you may also be able to have your
2059 closure call itself by capturing a `&Fn()` object or `fn()` pointer
2060 that refers to itself. That is permitting, since the closure would be
2061 invoking itself via a virtual call, and hence does not directly
2062 reference its own *type*.
2067 A `repr(transparent)` type was also annotated with other, incompatible
2068 representation hints.
2070 Erroneous code example:
2072 ```compile_fail,E0692
2073 #[repr(transparent, C)] // error: incompatible representation hints
2077 A type annotated as `repr(transparent)` delegates all representation concerns to
2078 another type, so adding more representation hints is contradictory. Remove
2079 either the `transparent` hint or the other hints, like this:
2082 #[repr(transparent)]
2086 Alternatively, move the other attributes to the contained type:
2095 #[repr(transparent)]
2096 struct FooWrapper(Foo);
2099 Note that introducing another `struct` just to have a place for the other
2100 attributes may have unintended side effects on the representation:
2103 #[repr(transparent)]
2109 #[repr(transparent)]
2110 struct Grams2(Float); // this is not equivalent to `Grams` above
2113 Here, `Grams2` is a not equivalent to `Grams` -- the former transparently wraps
2114 a (non-transparent) struct containing a single float, while `Grams` is a
2115 transparent wrapper around a float. This can make a difference for the ABI.
2119 When using generators (or async) all type variables must be bound so a
2120 generator can be constructed.
2122 Erroneous code example:
2124 ```edition2018,compile-fail,E0698
2125 async fn bar<T>() -> () {}
2128 bar().await; // error: cannot infer type for `T`
2132 In the above example `T` is unknowable by the compiler.
2133 To fix this you must bind `T` to a concrete type such as `String`
2134 so that a generator can then be constructed:
2137 async fn bar<T>() -> () {}
2140 bar::<String>().await;
2141 // ^^^^^^^^ specify type explicitly
2147 The `impl Trait` return type captures lifetime parameters that do not
2148 appear within the `impl Trait` itself.
2150 Erroneous code example:
2152 ```compile-fail,E0700
2153 use std::cell::Cell;
2157 impl<'a, 'b> Trait<'b> for Cell<&'a u32> { }
2159 fn foo<'x, 'y>(x: Cell<&'x u32>) -> impl Trait<'y>
2166 Here, the function `foo` returns a value of type `Cell<&'x u32>`,
2167 which references the lifetime `'x`. However, the return type is
2168 declared as `impl Trait<'y>` -- this indicates that `foo` returns
2169 "some type that implements `Trait<'y>`", but it also indicates that
2170 the return type **only captures data referencing the lifetime `'y`**.
2171 In this case, though, we are referencing data with lifetime `'x`, so
2172 this function is in error.
2174 To fix this, you must reference the lifetime `'x` from the return
2175 type. For example, changing the return type to `impl Trait<'y> + 'x`
2179 use std::cell::Cell;
2183 impl<'a,'b> Trait<'b> for Cell<&'a u32> { }
2185 fn foo<'x, 'y>(x: Cell<&'x u32>) -> impl Trait<'y> + 'x
2194 This error indicates that a `#[non_exhaustive]` attribute was incorrectly placed
2195 on something other than a struct or enum.
2197 Examples of erroneous code:
2199 ```compile_fail,E0701
2200 # #![feature(non_exhaustive)]
2208 This error indicates that a `#[lang = ".."]` attribute was placed
2209 on the wrong type of item.
2211 Examples of erroneous code:
2213 ```compile_fail,E0718
2214 #![feature(lang_items)]
2222 A stability attribute has been used outside of the standard library.
2224 Erroneous code examples:
2226 ```compile_fail,E0734
2227 #[rustc_deprecated(since = "b", reason = "text")] // invalid
2228 #[stable(feature = "a", since = "b")] // invalid
2229 #[unstable(feature = "b", issue = "0")] // invalid
2233 These attributes are meant to only be used by the standard library and are
2234 rejected in your own crates.
2238 // E0006, // merged with E0005
2239 // E0101, // replaced with E0282
2240 // E0102, // replaced with E0282
2243 // E0272, // on_unimplemented #0
2244 // E0273, // on_unimplemented #1
2245 // E0274, // on_unimplemented #2
2246 E0278, // requirement is not satisfied
2247 E0279, // requirement is not satisfied
2248 E0280, // requirement is not satisfied
2249 // E0285, // overflow evaluation builtin bounds
2250 // E0296, // replaced with a generic attribute input check
2251 // E0300, // unexpanded macro
2252 // E0304, // expected signed integer constant
2253 // E0305, // expected constant
2254 E0311, // thing may not live long enough
2255 E0313, // lifetime of borrowed pointer outlives lifetime of captured
2257 E0314, // closure outlives stack frame
2258 E0315, // cannot invoke closure outside of its lifetime
2259 E0316, // nested quantification of lifetimes
2260 E0320, // recursive overflow during dropck
2261 E0473, // dereference of reference outside its lifetime
2262 E0474, // captured variable `..` does not outlive the enclosing closure
2263 E0475, // index of slice outside its lifetime
2264 E0476, // lifetime of the source pointer does not outlive lifetime bound...
2265 E0477, // the type `..` does not fulfill the required lifetime...
2266 E0479, // the type `..` (provided as the value of a type parameter) is...
2267 E0480, // lifetime of method receiver does not outlive the method call
2268 E0481, // lifetime of function argument does not outlive the function call
2269 E0482, // lifetime of return value does not outlive the function call
2270 E0483, // lifetime of operand does not outlive the operation
2271 E0484, // reference is not valid at the time of borrow
2272 E0485, // automatically reference is not valid at the time of borrow
2273 E0486, // type of expression contains references that are not valid during..
2274 E0487, // unsafe use of destructor: destructor might be called while...
2275 E0488, // lifetime of variable does not enclose its declaration
2276 E0489, // type/lifetime parameter not in scope here
2277 E0490, // a value of type `..` is borrowed for too long
2278 E0495, // cannot infer an appropriate lifetime due to conflicting
2280 E0566, // conflicting representation hints
2281 E0623, // lifetime mismatch where both parameters are anonymous regions
2282 E0628, // generators cannot have explicit parameters
2283 E0631, // type mismatch in closure arguments
2284 E0637, // "'_" is not a valid lifetime bound
2285 E0657, // `impl Trait` can only capture lifetimes bound at the fn level
2286 E0687, // in-band lifetimes cannot be used in `fn`/`Fn` syntax
2287 E0688, // in-band lifetimes cannot be mixed with explicit lifetime binders
2288 E0697, // closures cannot be static
2289 E0707, // multiple elided lifetimes used in arguments of `async fn`
2290 E0708, // `async` non-`move` closures with parameters are not currently
2292 E0709, // multiple different lifetimes used in arguments of `async fn`
2293 E0710, // an unknown tool name found in scoped lint
2294 E0711, // a feature has been declared with conflicting stability attributes
2295 // E0702, // replaced with a generic attribute input check
2296 E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
2297 E0727, // `async` generators are not yet supported
2298 E0728, // `await` must be in an `async` function or block