1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 #![allow(non_snake_case)]
13 // Error messages for EXXXX errors.
14 // Each message should start and end with a new line, and be wrapped to 80 characters.
15 // In vim you can `:set tw=80` and use `gq` to wrap paragraphs. Use `:set tw=0` to disable.
16 register_long_diagnostics! {
18 This error indicates that an attempt was made to divide by zero (or take the
19 remainder of a zero divisor) in a static or constant expression. Erroneous
25 const X: i32 = 42 / 0;
26 // error: attempt to divide by zero in a constant expression
31 Trait objects like `Box<Trait>` can only be constructed when certain
32 requirements are satisfied by the trait in question.
34 Trait objects are a form of dynamic dispatch and use a dynamically sized type
35 for the inner type. So, for a given trait `Trait`, when `Trait` is treated as a
36 type, as in `Box<Trait>`, the inner type is 'unsized'. In such cases the boxed
37 pointer is a 'fat pointer' that contains an extra pointer to a table of methods
38 (among other things) for dynamic dispatch. This design mandates some
39 restrictions on the types of traits that are allowed to be used in trait
40 objects, which are collectively termed as 'object safety' rules.
42 Attempting to create a trait object for a non object-safe trait will trigger
45 There are various rules:
47 ### The trait cannot require `Self: Sized`
49 When `Trait` is treated as a type, the type does not implement the special
50 `Sized` trait, because the type does not have a known size at compile time and
51 can only be accessed behind a pointer. Thus, if we have a trait like the
55 trait Foo where Self: Sized {
60 We cannot create an object of type `Box<Foo>` or `&Foo` since in this case
61 `Self` would not be `Sized`.
63 Generally, `Self : Sized` is used to indicate that the trait should not be used
64 as a trait object. If the trait comes from your own crate, consider removing
67 ### Method references the `Self` type in its arguments or return type
69 This happens when a trait has a method like the following:
73 fn foo(&self) -> Self;
76 impl Trait for String {
77 fn foo(&self) -> Self {
83 fn foo(&self) -> Self {
89 (Note that `&self` and `&mut self` are okay, it's additional `Self` types which
92 In such a case, the compiler cannot predict the return type of `foo()` in a
93 situation like the following:
97 fn foo(&self) -> Self;
100 fn call_foo(x: Box<Trait>) {
101 let y = x.foo(); // What type is y?
106 If only some methods aren't object-safe, you can add a `where Self: Sized` bound
107 on them to mark them as explicitly unavailable to trait objects. The
108 functionality will still be available to all other implementers, including
109 `Box<Trait>` which is itself sized (assuming you `impl Trait for Box<Trait>`).
113 fn foo(&self) -> Self where Self: Sized;
118 Now, `foo()` can no longer be called on a trait object, but you will now be
119 allowed to make a trait object, and that will be able to call any object-safe
120 methods. With such a bound, one can still call `foo()` on types implementing
121 that trait that aren't behind trait objects.
123 ### Method has generic type parameters
125 As mentioned before, trait objects contain pointers to method tables. So, if we
133 impl Trait for String {
147 At compile time each implementation of `Trait` will produce a table containing
148 the various methods (and other items) related to the implementation.
150 This works fine, but when the method gains generic parameters, we can have a
153 Usually, generic parameters get _monomorphized_. For example, if I have
161 The machine code for `foo::<u8>()`, `foo::<bool>()`, `foo::<String>()`, or any
162 other type substitution is different. Hence the compiler generates the
163 implementation on-demand. If you call `foo()` with a `bool` parameter, the
164 compiler will only generate code for `foo::<bool>()`. When we have additional
165 type parameters, the number of monomorphized implementations the compiler
166 generates does not grow drastically, since the compiler will only generate an
167 implementation if the function is called with unparametrized substitutions
168 (i.e., substitutions where none of the substituted types are themselves
171 However, with trait objects we have to make a table containing _every_ object
172 that implements the trait. Now, if it has type parameters, we need to add
173 implementations for every type that implements the trait, and there could
174 theoretically be an infinite number of types.
180 fn foo<T>(&self, on: T);
184 impl Trait for String {
185 fn foo<T>(&self, on: T) {
191 fn foo<T>(&self, on: T) {
196 // 8 more implementations
199 Now, if we have the following code:
202 fn call_foo(thing: Box<Trait>) {
203 thing.foo(true); // this could be any one of the 8 types above
209 We don't just need to create a table of all implementations of all methods of
210 `Trait`, we need to create such a table, for each different type fed to
211 `foo()`. In this case this turns out to be (10 types implementing `Trait`)*(3
212 types being fed to `foo()`) = 30 implementations!
214 With real world traits these numbers can grow drastically.
216 To fix this, it is suggested to use a `where Self: Sized` bound similar to the
217 fix for the sub-error above if you do not intend to call the method with type
222 fn foo<T>(&self, on: T) where Self: Sized;
227 If this is not an option, consider replacing the type parameter with another
228 trait object (e.g. if `T: OtherTrait`, use `on: Box<OtherTrait>`). If the number
229 of types you intend to feed to this method is limited, consider manually listing
230 out the methods of different types.
232 ### Method has no receiver
234 Methods that do not take a `self` parameter can't be called since there won't be
235 a way to get a pointer to the method table for them.
243 This could be called as `<Foo as Foo>::foo()`, which would not be able to pick
246 Adding a `Self: Sized` bound to these methods will generally make this compile.
250 fn foo() -> u8 where Self: Sized;
254 ### The trait cannot use `Self` as a type parameter in the supertrait listing
256 This is similar to the second sub-error, but subtler. It happens in situations
262 trait Trait: Super<Self> {
267 impl Super<Foo> for Foo{}
269 impl Trait for Foo {}
272 Here, the supertrait might have methods as follows:
276 fn get_a(&self) -> A; // note that this is object safe!
280 If the trait `Foo` was deriving from something like `Super<String>` or
281 `Super<T>` (where `Foo` itself is `Foo<T>`), this is okay, because given a type
282 `get_a()` will definitely return an object of that type.
284 However, if it derives from `Super<Self>`, even though `Super` is object safe,
285 the method `get_a()` would return an object of unknown type when called on the
286 function. `Self` type parameters let us make object safe traits no longer safe,
287 so they are forbidden when specifying supertraits.
289 There's no easy fix for this, generally code will need to be refactored so that
290 you no longer need to derive from `Super<Self>`.
294 When defining a recursive struct or enum, any use of the type being defined
295 from inside the definition must occur behind a pointer (like `Box` or `&`).
296 This is because structs and enums must have a well-defined size, and without
297 the pointer, the size of the type would need to be unbounded.
299 Consider the following erroneous definition of a type for a list of bytes:
301 ```compile_fail,E0072
302 // error, invalid recursive struct type
305 tail: Option<ListNode>,
309 This type cannot have a well-defined size, because it needs to be arbitrarily
310 large (since we would be able to nest `ListNode`s to any depth). Specifically,
313 size of `ListNode` = 1 byte for `head`
314 + 1 byte for the discriminant of the `Option`
318 One way to fix this is by wrapping `ListNode` in a `Box`, like so:
323 tail: Option<Box<ListNode>>,
327 This works because `Box` is a pointer, so its size is well-known.
331 This error indicates that the compiler was unable to sensibly evaluate an
332 constant expression that had to be evaluated. Attempting to divide by 0
333 or causing integer overflow are two ways to induce this error. For example:
335 ```compile_fail,E0080
342 Ensure that the expressions given can be evaluated as the desired integer type.
343 See the FFI section of the Reference for more information about using a custom
346 https://doc.rust-lang.org/reference.html#ffi-attributes
350 This error indicates that a lifetime is missing from a type. If it is an error
351 inside a function signature, the problem may be with failing to adhere to the
352 lifetime elision rules (see below).
354 Here are some simple examples of where you'll run into this error:
356 ```compile_fail,E0106
357 struct Foo { x: &bool } // error
358 struct Foo<'a> { x: &'a bool } // correct
360 enum Bar { A(u8), B(&bool), } // error
361 enum Bar<'a> { A(u8), B(&'a bool), } // correct
363 type MyStr = &str; // error
364 type MyStr<'a> = &'a str; // correct
367 Lifetime elision is a special, limited kind of inference for lifetimes in
368 function signatures which allows you to leave out lifetimes in certain cases.
369 For more background on lifetime elision see [the book][book-le].
371 The lifetime elision rules require that any function signature with an elided
372 output lifetime must either have
374 - exactly one input lifetime
375 - or, multiple input lifetimes, but the function must also be a method with a
376 `&self` or `&mut self` receiver
378 In the first case, the output lifetime is inferred to be the same as the unique
379 input lifetime. In the second case, the lifetime is instead inferred to be the
380 same as the lifetime on `&self` or `&mut self`.
382 Here are some examples of elision errors:
384 ```compile_fail,E0106
385 // error, no input lifetimes
388 // error, `x` and `y` have distinct lifetimes inferred
389 fn bar(x: &str, y: &str) -> &str { }
391 // error, `y`'s lifetime is inferred to be distinct from `x`'s
392 fn baz<'a>(x: &'a str, y: &str) -> &str { }
395 Here's an example that is currently an error, but may work in a future version
398 ```compile_fail,E0106
399 struct Foo<'a>(&'a str);
402 impl Quux for Foo { }
405 Lifetime elision in implementation headers was part of the lifetime elision
406 RFC. It is, however, [currently unimplemented][iss15872].
408 [book-le]: https://doc.rust-lang.org/nightly/book/lifetimes.html#lifetime-elision
409 [iss15872]: https://github.com/rust-lang/rust/issues/15872
413 Unsafe code was used outside of an unsafe function or block.
415 Erroneous code example:
417 ```compile_fail,E0133
418 unsafe fn f() { return; } // This is the unsafe code
421 f(); // error: call to unsafe function requires unsafe function or block
425 Using unsafe functionality is potentially dangerous and disallowed by safety
428 * Dereferencing raw pointers
429 * Calling functions via FFI
430 * Calling functions marked unsafe
432 These safety checks can be relaxed for a section of the code by wrapping the
433 unsafe instructions with an `unsafe` block. For instance:
436 unsafe fn f() { return; }
439 unsafe { f(); } // ok!
443 See also https://doc.rust-lang.org/book/unsafe.html
446 // This shouldn't really ever trigger since the repeated value error comes first
448 A binary can only have one entry point, and by default that entry point is the
449 function `main()`. If there are multiple such functions, please rename one.
453 More than one function was declared with the `#[main]` attribute.
455 Erroneous code example:
457 ```compile_fail,E0137
464 fn f() {} // error: multiple functions with a #[main] attribute
467 This error indicates that the compiler found multiple functions with the
468 `#[main]` attribute. This is an error because there must be a unique entry
469 point into a Rust program. Example:
480 More than one function was declared with the `#[start]` attribute.
482 Erroneous code example:
484 ```compile_fail,E0138
488 fn foo(argc: isize, argv: *const *const u8) -> isize {}
491 fn f(argc: isize, argv: *const *const u8) -> isize {}
492 // error: multiple 'start' functions
495 This error indicates that the compiler found multiple functions with the
496 `#[start]` attribute. This is an error because there must be a unique entry
497 point into a Rust program. Example:
503 fn foo(argc: isize, argv: *const *const u8) -> isize { 0 } // ok!
507 // isn't thrown anymore
509 There are various restrictions on transmuting between types in Rust; for example
510 types being transmuted must have the same size. To apply all these restrictions,
511 the compiler must know the exact types that may be transmuted. When type
512 parameters are involved, this cannot always be done.
514 So, for example, the following is not allowed:
517 use std::mem::transmute;
519 struct Foo<T>(Vec<T>);
521 fn foo<T>(x: Vec<T>) {
522 // we are transmuting between Vec<T> and Foo<F> here
523 let y: Foo<T> = unsafe { transmute(x) };
524 // do something with y
528 In this specific case there's a good chance that the transmute is harmless (but
529 this is not guaranteed by Rust). However, when alignment and enum optimizations
530 come into the picture, it's quite likely that the sizes may or may not match
531 with different type parameter substitutions. It's not possible to check this for
532 _all_ possible types, so `transmute()` simply only accepts types without any
533 unsubstituted type parameters.
535 If you need this, there's a good chance you're doing something wrong. Keep in
536 mind that Rust doesn't guarantee much about the layout of different structs
537 (even two structs with identical declarations may have different layouts). If
538 there is a solution that avoids the transmute entirely, try it instead.
540 If it's possible, hand-monomorphize the code by writing the function for each
541 possible type substitution. It's possible to use traits to do this cleanly,
545 struct Foo<T>(Vec<T>);
547 trait MyTransmutableType {
548 fn transmute(Vec<Self>) -> Foo<Self>;
551 impl MyTransmutableType for u8 {
552 fn transmute(x: Foo<u8>) -> Vec<u8> {
557 impl MyTransmutableType for String {
558 fn transmute(x: Foo<String>) -> Vec<String> {
563 // ... more impls for the types you intend to transmute
565 fn foo<T: MyTransmutableType>(x: Vec<T>) {
566 let y: Foo<T> = <T as MyTransmutableType>::transmute(x);
567 // do something with y
571 Each impl will be checked for a size match in the transmute as usual, and since
572 there are no unbound type parameters involved, this should compile unless there
573 is a size mismatch in one of the impls.
575 It is also possible to manually transmute:
578 ptr::read(&v as *const _ as *const SomeType) // `v` transmuted to `SomeType`
581 Note that this does not move `v` (unlike `transmute`), and may need a
582 call to `mem::forget(v)` in case you want to avoid destructors being called.
586 A lang item was redefined.
588 Erroneous code example:
590 ```compile_fail,E0152
591 #![feature(lang_items)]
593 #[lang = "panic_fmt"]
594 struct Foo; // error: duplicate lang item found: `panic_fmt`
597 Lang items are already implemented in the standard library. Unless you are
598 writing a free-standing application (e.g. a kernel), you do not need to provide
601 You can build a free-standing crate by adding `#![no_std]` to the crate
608 See also https://doc.rust-lang.org/book/no-stdlib.html
612 When using a lifetime like `'a` in a type, it must be declared before being
615 These two examples illustrate the problem:
617 ```compile_fail,E0261
618 // error, use of undeclared lifetime name `'a`
619 fn foo(x: &'a str) { }
622 // error, use of undeclared lifetime name `'a`
627 These can be fixed by declaring lifetime parameters:
630 fn foo<'a>(x: &'a str) {}
639 Declaring certain lifetime names in parameters is disallowed. For example,
640 because the `'static` lifetime is a special built-in lifetime name denoting
641 the lifetime of the entire program, this is an error:
643 ```compile_fail,E0262
644 // error, invalid lifetime parameter name `'static`
645 fn foo<'static>(x: &'static str) { }
650 A lifetime name cannot be declared more than once in the same scope. For
653 ```compile_fail,E0263
654 // error, lifetime name `'a` declared twice in the same scope
655 fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str) { }
660 An unknown external lang item was used. Erroneous code example:
662 ```compile_fail,E0264
663 #![feature(lang_items)]
666 #[lang = "cake"] // error: unknown external lang item: `cake`
671 A list of available external lang items is available in
672 `src/librustc/middle/weak_lang_items.rs`. Example:
675 #![feature(lang_items)]
678 #[lang = "panic_fmt"] // ok!
685 This is because of a type mismatch between the associated type of some
686 trait (e.g. `T::Bar`, where `T` implements `trait Quux { type Bar; }`)
687 and another type `U` that is required to be equal to `T::Bar`, but is not.
690 Here is a basic example:
692 ```compile_fail,E0271
693 trait Trait { type AssociatedType; }
695 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
699 impl Trait for i8 { type AssociatedType = &'static str; }
704 Here is that same example again, with some explanatory comments:
707 trait Trait { type AssociatedType; }
709 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
710 // ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
712 // This says `foo` can |
713 // only be used with |
715 // implements `Trait`. |
717 // This says not only must
718 // `T` be an impl of `Trait`
719 // but also that the impl
720 // must assign the type `u32`
721 // to the associated type.
725 impl Trait for i8 { type AssociatedType = &'static str; }
726 ~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
731 // ... but it is an implementation
732 // that assigns `&'static str` to
733 // the associated type.
736 // Here, we invoke `foo` with an `i8`, which does not satisfy
737 // the constraint `<i8 as Trait>::AssociatedType=u32`, and
738 // therefore the type-checker complains with this error code.
741 Here is a more subtle instance of the same problem, that can
742 arise with for-loops in Rust:
745 let vs: Vec<i32> = vec![1, 2, 3, 4];
754 The above fails because of an analogous type mismatch,
755 though may be harder to see. Again, here are some
756 explanatory comments for the same example:
760 let vs = vec![1, 2, 3, 4];
762 // `for`-loops use a protocol based on the `Iterator`
763 // trait. Each item yielded in a `for` loop has the
764 // type `Iterator::Item` -- that is, `Item` is the
765 // associated type of the concrete iterator impl.
769 // | We borrow `vs`, iterating over a sequence of
770 // | *references* of type `&Elem` (where `Elem` is
771 // | vector's element type). Thus, the associated
772 // | type `Item` must be a reference `&`-type ...
774 // ... and `v` has the type `Iterator::Item`, as dictated by
775 // the `for`-loop protocol ...
781 // ... but *here*, `v` is forced to have some integral type;
782 // only types like `u8`,`i8`,`u16`,`i16`, et cetera can
783 // match the pattern `1` ...
788 // ... therefore, the compiler complains, because it sees
789 // an attempt to solve the equations
790 // `some integral-type` = type-of-`v`
791 // = `Iterator::Item`
792 // = `&Elem` (i.e. `some reference type`)
794 // which cannot possibly all be true.
800 To avoid those issues, you have to make the types match correctly.
801 So we can fix the previous examples like this:
805 trait Trait { type AssociatedType; }
807 fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
811 impl Trait for i8 { type AssociatedType = &'static str; }
816 let vs = vec![1, 2, 3, 4];
827 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
828 message for when a particular trait isn't implemented on a type placed in a
829 position that needs that trait. For example, when the following code is
833 #![feature(on_unimplemented)]
835 fn foo<T: Index<u8>>(x: T){}
837 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
838 trait Index<Idx> { /* ... */ }
840 foo(true); // `bool` does not implement `Index<u8>`
843 There will be an error about `bool` not implementing `Index<u8>`, followed by a
844 note saying "the type `bool` cannot be indexed by `u8`".
846 As you can see, you can specify type parameters in curly braces for
847 substitution with the actual types (using the regular format string syntax) in
848 a given situation. Furthermore, `{Self}` will substitute to the type (in this
849 case, `bool`) that we tried to use.
851 This error appears when the curly braces contain an identifier which doesn't
852 match with any of the type parameters or the string `Self`. This might happen
853 if you misspelled a type parameter, or if you intended to use literal curly
854 braces. If it is the latter, escape the curly braces with a second curly brace
855 of the same type; e.g. a literal `{` is `{{`.
859 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
860 message for when a particular trait isn't implemented on a type placed in a
861 position that needs that trait. For example, when the following code is
865 #![feature(on_unimplemented)]
867 fn foo<T: Index<u8>>(x: T){}
869 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
870 trait Index<Idx> { /* ... */ }
872 foo(true); // `bool` does not implement `Index<u8>`
875 there will be an error about `bool` not implementing `Index<u8>`, followed by a
876 note saying "the type `bool` cannot be indexed by `u8`".
878 As you can see, you can specify type parameters in curly braces for
879 substitution with the actual types (using the regular format string syntax) in
880 a given situation. Furthermore, `{Self}` will substitute to the type (in this
881 case, `bool`) that we tried to use.
883 This error appears when the curly braces do not contain an identifier. Please
884 add one of the same name as a type parameter. If you intended to use literal
885 braces, use `{{` and `}}` to escape them.
889 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
890 message for when a particular trait isn't implemented on a type placed in a
891 position that needs that trait. For example, when the following code is
895 #![feature(on_unimplemented)]
897 fn foo<T: Index<u8>>(x: T){}
899 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
900 trait Index<Idx> { /* ... */ }
902 foo(true); // `bool` does not implement `Index<u8>`
905 there will be an error about `bool` not implementing `Index<u8>`, followed by a
906 note saying "the type `bool` cannot be indexed by `u8`".
908 For this to work, some note must be specified. An empty attribute will not do
909 anything, please remove the attribute or add some helpful note for users of the
914 This error occurs when there was a recursive trait requirement that overflowed
915 before it could be evaluated. Often this means that there is unbounded
916 recursion in resolving some type bounds.
918 For example, in the following code:
920 ```compile_fail,E0275
925 impl<T> Foo for T where Bar<T>: Foo {}
928 To determine if a `T` is `Foo`, we need to check if `Bar<T>` is `Foo`. However,
929 to do this check, we need to determine that `Bar<Bar<T>>` is `Foo`. To
930 determine this, we check if `Bar<Bar<Bar<T>>>` is `Foo`, and so on. This is
931 clearly a recursive requirement that can't be resolved directly.
933 Consider changing your trait bounds so that they're less self-referential.
937 This error occurs when a bound in an implementation of a trait does not match
938 the bounds specified in the original trait. For example:
940 ```compile_fail,E0276
946 fn foo<T>(x: T) where T: Copy {}
950 Here, all types implementing `Foo` must have a method `foo<T>(x: T)` which can
951 take any type `T`. However, in the `impl` for `bool`, we have added an extra
952 bound that `T` is `Copy`, which isn't compatible with the original trait.
954 Consider removing the bound from the method or adding the bound to the original
955 method definition in the trait.
959 You tried to use a type which doesn't implement some trait in a place which
960 expected that trait. Erroneous code example:
962 ```compile_fail,E0277
963 // here we declare the Foo trait with a bar method
968 // we now declare a function which takes an object implementing the Foo trait
969 fn some_func<T: Foo>(foo: T) {
974 // we now call the method with the i32 type, which doesn't implement
976 some_func(5i32); // error: the trait bound `i32 : Foo` is not satisfied
980 In order to fix this error, verify that the type you're using does implement
988 fn some_func<T: Foo>(foo: T) {
989 foo.bar(); // we can now use this method since i32 implements the
993 // we implement the trait on the i32 type
999 some_func(5i32); // ok!
1003 Or in a generic context, an erroneous code example would look like:
1005 ```compile_fail,E0277
1006 fn some_func<T>(foo: T) {
1007 println!("{:?}", foo); // error: the trait `core::fmt::Debug` is not
1008 // implemented for the type `T`
1012 // We now call the method with the i32 type,
1013 // which *does* implement the Debug trait.
1018 Note that the error here is in the definition of the generic function: Although
1019 we only call it with a parameter that does implement `Debug`, the compiler
1020 still rejects the function: It must work with all possible input types. In
1021 order to make this example compile, we need to restrict the generic type we're
1027 // Restrict the input type to types that implement Debug.
1028 fn some_func<T: fmt::Debug>(foo: T) {
1029 println!("{:?}", foo);
1033 // Calling the method is still fine, as i32 implements Debug.
1036 // This would fail to compile now:
1037 // struct WithoutDebug;
1038 // some_func(WithoutDebug);
1042 Rust only looks at the signature of the called function, as such it must
1043 already specify all requirements that will be used for every type parameter.
1047 You tried to supply a type which doesn't implement some trait in a location
1048 which expected that trait. This error typically occurs when working with
1049 `Fn`-based types. Erroneous code example:
1051 ```compile_fail,E0281
1052 fn foo<F: Fn(usize)>(x: F) { }
1055 // type mismatch: ... implements the trait `core::ops::Fn<(String,)>`,
1056 // but the trait `core::ops::Fn<(usize,)>` is required
1058 foo(|y: String| { });
1062 The issue in this case is that `foo` is defined as accepting a `Fn` with one
1063 argument of type `String`, but the closure we attempted to pass to it requires
1064 one arguments of type `usize`.
1068 This error indicates that type inference did not result in one unique possible
1069 type, and extra information is required. In most cases this can be provided
1070 by adding a type annotation. Sometimes you need to specify a generic type
1073 A common example is the `collect` method on `Iterator`. It has a generic type
1074 parameter with a `FromIterator` bound, which for a `char` iterator is
1075 implemented by `Vec` and `String` among others. Consider the following snippet
1076 that reverses the characters of a string:
1078 ```compile_fail,E0282
1079 let x = "hello".chars().rev().collect();
1082 In this case, the compiler cannot infer what the type of `x` should be:
1083 `Vec<char>` and `String` are both suitable candidates. To specify which type to
1084 use, you can use a type annotation on `x`:
1087 let x: Vec<char> = "hello".chars().rev().collect();
1090 It is not necessary to annotate the full type. Once the ambiguity is resolved,
1091 the compiler can infer the rest:
1094 let x: Vec<_> = "hello".chars().rev().collect();
1097 Another way to provide the compiler with enough information, is to specify the
1098 generic type parameter:
1101 let x = "hello".chars().rev().collect::<Vec<char>>();
1104 Again, you need not specify the full type if the compiler can infer it:
1107 let x = "hello".chars().rev().collect::<Vec<_>>();
1110 Apart from a method or function with a generic type parameter, this error can
1111 occur when a type parameter of a struct or trait cannot be inferred. In that
1112 case it is not always possible to use a type annotation, because all candidates
1113 have the same return type. For instance:
1115 ```compile_fail,E0282
1126 let number = Foo::bar();
1131 This will fail because the compiler does not know which instance of `Foo` to
1132 call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.
1136 This error occurs when the compiler doesn't have enough information
1137 to unambiguously choose an implementation.
1141 ```compile_fail,E0283
1148 impl Generator for Impl {
1149 fn create() -> u32 { 1 }
1154 impl Generator for AnotherImpl {
1155 fn create() -> u32 { 2 }
1159 let cont: u32 = Generator::create();
1160 // error, impossible to choose one of Generator trait implementation
1161 // Impl or AnotherImpl? Maybe anything else?
1165 To resolve this error use the concrete type:
1174 impl Generator for AnotherImpl {
1175 fn create() -> u32 { 2 }
1179 let gen1 = AnotherImpl::create();
1181 // if there are multiple methods with same name (different traits)
1182 let gen2 = <AnotherImpl as Generator>::create();
1188 This error indicates that the given recursion limit could not be parsed. Ensure
1189 that the value provided is a positive integer between quotes.
1191 Erroneous code example:
1193 ```compile_fail,E0296
1199 And a working example:
1202 #![recursion_limit="1000"]
1209 This error occurs when the compiler was unable to infer the concrete type of a
1210 variable. It can occur for several cases, the most common of which is a
1211 mismatch in the expected type that the compiler inferred for a variable's
1212 initializing expression, and the actual type explicitly assigned to the
1217 ```compile_fail,E0308
1218 let x: i32 = "I am not a number!";
1219 // ~~~ ~~~~~~~~~~~~~~~~~~~~
1221 // | initializing expression;
1222 // | compiler infers type `&str`
1224 // type `i32` assigned to variable `x`
1229 Types in type definitions have lifetimes associated with them that represent
1230 how long the data stored within them is guaranteed to be live. This lifetime
1231 must be as long as the data needs to be alive, and missing the constraint that
1232 denotes this will cause this error.
1234 ```compile_fail,E0309
1235 // This won't compile because T is not constrained, meaning the data
1236 // stored in it is not guaranteed to last as long as the reference
1242 This will compile, because it has the constraint on the type parameter:
1245 struct Foo<'a, T: 'a> {
1250 To see why this is important, consider the case where `T` is itself a reference
1251 (e.g., `T = &str`). If we don't include the restriction that `T: 'a`, the
1252 following code would be perfectly legal:
1254 ```compile_fail,E0309
1260 let v = "42".to_string();
1261 let f = Foo{foo: &v};
1263 println!("{}", f.foo); // but we've already dropped v!
1269 Types in type definitions have lifetimes associated with them that represent
1270 how long the data stored within them is guaranteed to be live. This lifetime
1271 must be as long as the data needs to be alive, and missing the constraint that
1272 denotes this will cause this error.
1274 ```compile_fail,E0310
1275 // This won't compile because T is not constrained to the static lifetime
1276 // the reference needs
1282 This will compile, because it has the constraint on the type parameter:
1285 struct Foo<T: 'static> {
1292 A lifetime of reference outlives lifetime of borrowed content.
1294 Erroneous code example:
1296 ```compile_fail,E0312
1297 fn make_child<'human, 'elve>(x: &mut &'human isize, y: &mut &'elve isize) {
1299 // error: lifetime of reference outlives lifetime of borrowed content
1303 The compiler cannot determine if the `human` lifetime will live long enough
1304 to keep up on the elve one. To solve this error, you have to give an
1305 explicit lifetime hierarchy:
1308 fn make_child<'human, 'elve: 'human>(x: &mut &'human isize,
1309 y: &mut &'elve isize) {
1314 Or use the same lifetime for every variable:
1317 fn make_child<'elve>(x: &mut &'elve isize, y: &mut &'elve isize) {
1324 This error occurs when an `if` expression without an `else` block is used in a
1325 context where a type other than `()` is expected, for example a `let`
1328 ```compile_fail,E0317
1331 let a = if x == 5 { 1 };
1335 An `if` expression without an `else` block has the type `()`, so this is a type
1336 error. To resolve it, add an `else` block having the same type as the `if`
1341 This error indicates that some types or traits depend on each other
1342 and therefore cannot be constructed.
1344 The following example contains a circular dependency between two traits:
1346 ```compile_fail,E0391
1347 trait FirstTrait : SecondTrait {
1351 trait SecondTrait : FirstTrait {
1358 In Rust 1.3, the default object lifetime bounds are expected to change, as
1359 described in [RFC 1156]. You are getting a warning because the compiler
1360 thinks it is possible that this change will cause a compilation error in your
1361 code. It is possible, though unlikely, that this is a false alarm.
1363 The heart of the change is that where `&'a Box<SomeTrait>` used to default to
1364 `&'a Box<SomeTrait+'a>`, it now defaults to `&'a Box<SomeTrait+'static>` (here,
1365 `SomeTrait` is the name of some trait type). Note that the only types which are
1366 affected are references to boxes, like `&Box<SomeTrait>` or
1367 `&[Box<SomeTrait>]`. More common types like `&SomeTrait` or `Box<SomeTrait>`
1370 To silence this warning, edit your code to use an explicit bound. Most of the
1371 time, this means that you will want to change the signature of a function that
1372 you are calling. For example, if the error is reported on a call like `foo(x)`,
1373 and `foo` is defined as follows:
1376 fn foo(arg: &Box<SomeTrait>) { ... }
1379 You might change it to:
1382 fn foo<'a>(arg: &Box<SomeTrait+'a>) { ... }
1385 This explicitly states that you expect the trait object `SomeTrait` to contain
1386 references (with a maximum lifetime of `'a`).
1388 [RFC 1156]: https://github.com/rust-lang/rfcs/blob/master/text/1156-adjust-default-object-bounds.md
1392 An invalid lint attribute has been given. Erroneous code example:
1394 ```compile_fail,E0452
1395 #![allow(foo = "")] // error: malformed lint attribute
1398 Lint attributes only accept a list of identifiers (where each identifier is a
1399 lint name). Ensure the attribute is of this form:
1402 #![allow(foo)] // ok!
1404 #![allow(foo, foo2)] // ok!
1409 A lint check attribute was overruled by a `forbid` directive set as an
1410 attribute on an enclosing scope, or on the command line with the `-F` option.
1412 Example of erroneous code:
1414 ```compile_fail,E0453
1415 #![forbid(non_snake_case)]
1417 #[allow(non_snake_case)]
1419 let MyNumber = 2; // error: allow(non_snake_case) overruled by outer
1420 // forbid(non_snake_case)
1424 The `forbid` lint setting, like `deny`, turns the corresponding compiler
1425 warning into a hard error. Unlike `deny`, `forbid` prevents itself from being
1426 overridden by inner attributes.
1428 If you're sure you want to override the lint check, you can change `forbid` to
1429 `deny` (or use `-D` instead of `-F` if the `forbid` setting was given as a
1430 command-line option) to allow the inner lint check attribute:
1433 #![deny(non_snake_case)]
1435 #[allow(non_snake_case)]
1437 let MyNumber = 2; // ok!
1441 Otherwise, edit the code to pass the lint check, and remove the overruled
1445 #![forbid(non_snake_case)]
1454 A lifetime bound was not satisfied.
1456 Erroneous code example:
1458 ```compile_fail,E0478
1459 // Check that the explicit lifetime bound (`'SnowWhite`, in this example) must
1460 // outlive all the superbounds from the trait (`'kiss`, in this example).
1462 trait Wedding<'t>: 't { }
1464 struct Prince<'kiss, 'SnowWhite> {
1465 child: Box<Wedding<'kiss> + 'SnowWhite>,
1466 // error: lifetime bound not satisfied
1470 In this example, the `'SnowWhite` lifetime is supposed to outlive the `'kiss`
1471 lifetime but the declaration of the `Prince` struct doesn't enforce it. To fix
1472 this issue, you need to specify it:
1475 trait Wedding<'t>: 't { }
1477 struct Prince<'kiss, 'SnowWhite: 'kiss> { // You say here that 'kiss must live
1478 // longer than 'SnowWhite.
1479 child: Box<Wedding<'kiss> + 'SnowWhite>, // And now it's all good!
1485 A reference has a longer lifetime than the data it references.
1487 Erroneous code example:
1489 ```compile_fail,E0491
1490 // struct containing a reference requires a lifetime parameter,
1491 // because the data the reference points to must outlive the struct (see E0106)
1496 // However, a nested struct like this, the signature itself does not tell
1497 // whether 'a outlives 'b or the other way around.
1498 // So it could be possible that 'b of reference outlives 'a of the data.
1499 struct Nested<'a, 'b> {
1500 ref_struct: &'b Struct<'a>, // compile error E0491
1504 To fix this issue, you can specify a bound to the lifetime like below:
1511 // 'a: 'b means 'a outlives 'b
1512 struct Nested<'a: 'b, 'b> {
1513 ref_struct: &'b Struct<'a>,
1519 A lifetime name is shadowing another lifetime name. Erroneous code example:
1521 ```compile_fail,E0496
1527 fn f<'a>(x: &'a i32) { // error: lifetime name `'a` shadows a lifetime
1528 // name that is already in scope
1533 Please change the name of one of the lifetimes to remove this error. Example:
1541 fn f<'b>(x: &'b i32) { // ok!
1551 A stability attribute was used outside of the standard library. Erroneous code
1555 #[stable] // error: stability attributes may not be used outside of the
1560 It is not possible to use stability attributes outside of the standard library.
1561 Also, for now, it is not possible to write deprecation messages either.
1565 Transmute with two differently sized types was attempted. Erroneous code
1568 ```compile_fail,E0512
1569 fn takes_u8(_: u8) {}
1572 unsafe { takes_u8(::std::mem::transmute(0u16)); }
1573 // error: transmute called with differently sized types
1577 Please use types with same size or use the expected type directly. Example:
1580 fn takes_u8(_: u8) {}
1583 unsafe { takes_u8(::std::mem::transmute(0i8)); } // ok!
1585 unsafe { takes_u8(0u8); } // ok!
1591 This error indicates that a `#[repr(..)]` attribute was placed on an
1594 Examples of erroneous code:
1596 ```compile_fail,E0517
1604 struct Foo {bar: bool, baz: bool}
1612 * The `#[repr(C)]` attribute can only be placed on structs and enums.
1613 * The `#[repr(packed)]` and `#[repr(simd)]` attributes only work on structs.
1614 * The `#[repr(u8)]`, `#[repr(i16)]`, etc attributes only work on enums.
1616 These attributes do not work on typedefs, since typedefs are just aliases.
1618 Representations like `#[repr(u8)]`, `#[repr(i64)]` are for selecting the
1619 discriminant size for C-like enums (when there is no associated data, e.g.
1620 `enum Color {Red, Blue, Green}`), effectively setting the size of the enum to
1621 the size of the provided type. Such an enum can be cast to a value of the same
1622 type as well. In short, `#[repr(u8)]` makes the enum behave like an integer
1623 with a constrained set of allowed values.
1625 Only C-like enums can be cast to numerical primitives, so this attribute will
1626 not apply to structs.
1628 `#[repr(packed)]` reduces padding to make the struct size smaller. The
1629 representation of enums isn't strictly defined in Rust, and this attribute
1630 won't work on enums.
1632 `#[repr(simd)]` will give a struct consisting of a homogenous series of machine
1633 types (i.e. `u8`, `i32`, etc) a representation that permits vectorization via
1634 SIMD. This doesn't make much sense for enums since they don't consist of a
1635 single list of data.
1639 This error indicates that an `#[inline(..)]` attribute was incorrectly placed
1640 on something other than a function or method.
1642 Examples of erroneous code:
1644 ```compile_fail,E0518
1654 `#[inline]` hints the compiler whether or not to attempt to inline a method or
1655 function. By default, the compiler does a pretty good job of figuring this out
1656 itself, but if you feel the need for annotations, `#[inline(always)]` and
1657 `#[inline(never)]` can override or force the compiler's decision.
1659 If you wish to apply this attribute to all methods in an impl, manually annotate
1660 each method; it is not possible to annotate the entire impl with an `#[inline]`
1665 The lang attribute is intended for marking special items that are built-in to
1666 Rust itself. This includes special traits (like `Copy` and `Sized`) that affect
1667 how the compiler behaves, as well as special functions that may be automatically
1668 invoked (such as the handler for out-of-bounds accesses when indexing a slice).
1669 Erroneous code example:
1671 ```compile_fail,E0522
1672 #![feature(lang_items)]
1675 fn cookie() -> ! { // error: definition of an unknown language item: `cookie`
1682 A closure was used but didn't implement the expected trait.
1684 Erroneous code example:
1686 ```compile_fail,E0525
1690 fn bar<T: Fn(u32)>(_: T) {}
1694 let closure = |_| foo(x); // error: expected a closure that implements
1695 // the `Fn` trait, but this closure only
1696 // implements `FnOnce`
1701 In the example above, `closure` is an `FnOnce` closure whereas the `bar`
1702 function expected an `Fn` closure. In this case, it's simple to fix the issue,
1703 you just have to implement `Copy` and `Clone` traits on `struct X` and it'll
1707 #[derive(Clone, Copy)] // We implement `Clone` and `Copy` traits.
1711 fn bar<T: Fn(u32)>(_: T) {}
1715 let closure = |_| foo(x);
1716 bar(closure); // ok!
1720 To understand better how closures work in Rust, read:
1721 https://doc.rust-lang.org/book/closures.html
1725 The `main` function was incorrectly declared.
1727 Erroneous code example:
1729 ```compile_fail,E0580
1730 fn main() -> i32 { // error: main function has wrong type
1735 The `main` function prototype should never take arguments or return type.
1744 If you want to get command-line arguments, use `std::env::args`. To exit with a
1745 specified exit code, use `std::process::exit`.
1749 Per [RFC 401][rfc401], if you have a function declaration `foo`:
1752 // For the purposes of this explanation, all of these
1753 // different kinds of `fn` declarations are equivalent:
1754 fn foo(x: i32) { ... }
1755 extern "C" fn foo(x: i32);
1756 impl i32 { fn foo(x: self) { ... } }
1759 the type of `foo` is **not** `fn(i32)`, as one might expect.
1760 Rather, it is a unique, zero-sized marker type written here as `typeof(foo)`.
1761 However, `typeof(foo)` can be _coerced_ to a function pointer `fn(i32)`,
1762 so you rarely notice this:
1765 let x: fn(i32) = foo; // OK, coerces
1768 The reason that this matter is that the type `fn(i32)` is not specific to
1769 any particular function: it's a function _pointer_. So calling `x()` results
1770 in a virtual call, whereas `foo()` is statically dispatched, because the type
1771 of `foo` tells us precisely what function is being called.
1773 As noted above, coercions mean that most code doesn't have to be
1774 concerned with this distinction. However, you can tell the difference
1775 when using **transmute** to convert a fn item into a fn pointer.
1777 This is sometimes done as part of an FFI:
1780 extern "C" fn foo(userdata: Box<i32>) {
1784 let f: extern "C" fn(*mut i32) = transmute(foo);
1789 Here, transmute is being used to convert the types of the fn arguments.
1790 This pattern is incorrect because, because the type of `foo` is a function
1791 **item** (`typeof(foo)`), which is zero-sized, and the target type (`fn()`)
1792 is a function pointer, which is not zero-sized.
1793 This pattern should be rewritten. There are a few possible ways to do this:
1795 - change the original fn declaration to match the expected signature,
1796 and do the cast in the fn body (the prefered option)
1797 - cast the fn item fo a fn pointer before calling transmute, as shown here:
1798 - `let f: extern "C" fn(*mut i32) = transmute(foo as extern "C" fn(_))`
1799 - `let f: extern "C" fn(*mut i32) = transmute(foo as usize) /* works too */`
1801 The same applies to transmutes to `*mut fn()`, which were observedin practice.
1802 Note though that use of this type is generally incorrect.
1803 The intention is typically to describe a function pointer, but just `fn()`
1804 alone suffices for that. `*mut fn()` is a pointer to a fn pointer.
1805 (Since these values are typically just passed to C code, however, this rarely
1806 makes a difference in practice.)
1808 [rfc401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md
1812 You tried to supply an `Fn`-based type with an incorrect number of arguments
1813 than what was expected. Erroneous code example:
1815 ```compile_fail,E0593
1816 fn foo<F: Fn()>(x: F) { }
1819 // [E0593] closure takes 1 argument but 0 arguments are required
1828 register_diagnostics! {
1829 // E0006 // merged with E0005
1830 // E0101, // replaced with E0282
1831 // E0102, // replaced with E0282
1834 E0278, // requirement is not satisfied
1835 E0279, // requirement is not satisfied
1836 E0280, // requirement is not satisfied
1837 E0284, // cannot resolve type
1838 // E0285, // overflow evaluation builtin bounds
1839 // E0300, // unexpanded macro
1840 // E0304, // expected signed integer constant
1841 // E0305, // expected constant
1842 E0311, // thing may not live long enough
1843 E0313, // lifetime of borrowed pointer outlives lifetime of captured variable
1844 E0314, // closure outlives stack frame
1845 E0315, // cannot invoke closure outside of its lifetime
1846 E0316, // nested quantification of lifetimes
1847 E0320, // recursive overflow during dropck
1848 E0473, // dereference of reference outside its lifetime
1849 E0474, // captured variable `..` does not outlive the enclosing closure
1850 E0475, // index of slice outside its lifetime
1851 E0476, // lifetime of the source pointer does not outlive lifetime bound...
1852 E0477, // the type `..` does not fulfill the required lifetime...
1853 E0479, // the type `..` (provided as the value of a type parameter) is...
1854 E0480, // lifetime of method receiver does not outlive the method call
1855 E0481, // lifetime of function argument does not outlive the function call
1856 E0482, // lifetime of return value does not outlive the function call
1857 E0483, // lifetime of operand does not outlive the operation
1858 E0484, // reference is not valid at the time of borrow
1859 E0485, // automatically reference is not valid at the time of borrow
1860 E0486, // type of expression contains references that are not valid during...
1861 E0487, // unsafe use of destructor: destructor might be called while...
1862 E0488, // lifetime of variable does not enclose its declaration
1863 E0489, // type/lifetime parameter not in scope here
1864 E0490, // a value of type `..` is borrowed for too long
1865 E0495, // cannot infer an appropriate lifetime due to conflicting requirements
1866 E0566, // conflicting representation hints
1867 E0587, // conflicting packed and align representation hints