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:
201 ```compile_fail,E0038
202 # trait Trait { fn foo<T>(&self, on: T); }
203 # impl Trait for String { fn foo<T>(&self, on: T) {} }
204 # impl Trait for u8 { fn foo<T>(&self, on: T) {} }
205 # impl Trait for bool { fn foo<T>(&self, on: T) {} }
207 fn call_foo(thing: Box<Trait>) {
208 thing.foo(true); // this could be any one of the 8 types above
214 We don't just need to create a table of all implementations of all methods of
215 `Trait`, we need to create such a table, for each different type fed to
216 `foo()`. In this case this turns out to be (10 types implementing `Trait`)*(3
217 types being fed to `foo()`) = 30 implementations!
219 With real world traits these numbers can grow drastically.
221 To fix this, it is suggested to use a `where Self: Sized` bound similar to the
222 fix for the sub-error above if you do not intend to call the method with type
227 fn foo<T>(&self, on: T) where Self: Sized;
232 If this is not an option, consider replacing the type parameter with another
233 trait object (e.g. if `T: OtherTrait`, use `on: Box<OtherTrait>`). If the number
234 of types you intend to feed to this method is limited, consider manually listing
235 out the methods of different types.
237 ### Method has no receiver
239 Methods that do not take a `self` parameter can't be called since there won't be
240 a way to get a pointer to the method table for them.
248 This could be called as `<Foo as Foo>::foo()`, which would not be able to pick
251 Adding a `Self: Sized` bound to these methods will generally make this compile.
255 fn foo() -> u8 where Self: Sized;
259 ### The trait cannot use `Self` as a type parameter in the supertrait listing
261 This is similar to the second sub-error, but subtler. It happens in situations
267 trait Trait: Super<Self> {
272 impl Super<Foo> for Foo{}
274 impl Trait for Foo {}
277 Here, the supertrait might have methods as follows:
281 fn get_a(&self) -> A; // note that this is object safe!
285 If the trait `Foo` was deriving from something like `Super<String>` or
286 `Super<T>` (where `Foo` itself is `Foo<T>`), this is okay, because given a type
287 `get_a()` will definitely return an object of that type.
289 However, if it derives from `Super<Self>`, even though `Super` is object safe,
290 the method `get_a()` would return an object of unknown type when called on the
291 function. `Self` type parameters let us make object safe traits no longer safe,
292 so they are forbidden when specifying supertraits.
294 There's no easy fix for this, generally code will need to be refactored so that
295 you no longer need to derive from `Super<Self>`.
299 When defining a recursive struct or enum, any use of the type being defined
300 from inside the definition must occur behind a pointer (like `Box` or `&`).
301 This is because structs and enums must have a well-defined size, and without
302 the pointer, the size of the type would need to be unbounded.
304 Consider the following erroneous definition of a type for a list of bytes:
306 ```compile_fail,E0072
307 // error, invalid recursive struct type
310 tail: Option<ListNode>,
314 This type cannot have a well-defined size, because it needs to be arbitrarily
315 large (since we would be able to nest `ListNode`s to any depth). Specifically,
318 size of `ListNode` = 1 byte for `head`
319 + 1 byte for the discriminant of the `Option`
323 One way to fix this is by wrapping `ListNode` in a `Box`, like so:
328 tail: Option<Box<ListNode>>,
332 This works because `Box` is a pointer, so its size is well-known.
336 This error indicates that the compiler was unable to sensibly evaluate an
337 constant expression that had to be evaluated. Attempting to divide by 0
338 or causing integer overflow are two ways to induce this error. For example:
340 ```compile_fail,E0080
347 Ensure that the expressions given can be evaluated as the desired integer type.
348 See the FFI section of the Reference for more information about using a custom
351 https://doc.rust-lang.org/reference.html#ffi-attributes
355 This error indicates that a lifetime is missing from a type. If it is an error
356 inside a function signature, the problem may be with failing to adhere to the
357 lifetime elision rules (see below).
359 Here are some simple examples of where you'll run into this error:
361 ```compile_fail,E0106
362 struct Foo { x: &bool } // error
363 struct Foo<'a> { x: &'a bool } // correct
365 struct Bar { x: Foo }
366 ^^^ expected lifetime parameter
367 struct Bar<'a> { x: Foo<'a> } // correct
369 enum Bar { A(u8), B(&bool), } // error
370 enum Bar<'a> { A(u8), B(&'a bool), } // correct
372 type MyStr = &str; // error
373 type MyStr<'a> = &'a str; // correct
376 Lifetime elision is a special, limited kind of inference for lifetimes in
377 function signatures which allows you to leave out lifetimes in certain cases.
378 For more background on lifetime elision see [the book][book-le].
380 The lifetime elision rules require that any function signature with an elided
381 output lifetime must either have
383 - exactly one input lifetime
384 - or, multiple input lifetimes, but the function must also be a method with a
385 `&self` or `&mut self` receiver
387 In the first case, the output lifetime is inferred to be the same as the unique
388 input lifetime. In the second case, the lifetime is instead inferred to be the
389 same as the lifetime on `&self` or `&mut self`.
391 Here are some examples of elision errors:
393 ```compile_fail,E0106
394 // error, no input lifetimes
397 // error, `x` and `y` have distinct lifetimes inferred
398 fn bar(x: &str, y: &str) -> &str { }
400 // error, `y`'s lifetime is inferred to be distinct from `x`'s
401 fn baz<'a>(x: &'a str, y: &str) -> &str { }
404 Lifetime elision in implementation headers was part of the lifetime elision
405 RFC. It is, however, [currently unimplemented][iss15872].
407 [book-le]: https://doc.rust-lang.org/nightly/book/first-edition/lifetimes.html#lifetime-elision
408 [iss15872]: https://github.com/rust-lang/rust/issues/15872
412 There are conflicting trait implementations for the same type.
413 Example of erroneous code:
415 ```compile_fail,E0119
417 fn get(&self) -> usize;
420 impl<T> MyTrait for T {
421 fn get(&self) -> usize { 0 }
428 impl MyTrait for Foo { // error: conflicting implementations of trait
429 // `MyTrait` for type `Foo`
430 fn get(&self) -> usize { self.value }
434 When looking for the implementation for the trait, the compiler finds
435 both the `impl<T> MyTrait for T` where T is all types and the `impl
436 MyTrait for Foo`. Since a trait cannot be implemented multiple times,
437 this is an error. So, when you write:
441 fn get(&self) -> usize;
444 impl<T> MyTrait for T {
445 fn get(&self) -> usize { 0 }
449 This makes the trait implemented on all types in the scope. So if you
450 try to implement it on another one after that, the implementations will
455 fn get(&self) -> usize;
458 impl<T> MyTrait for T {
459 fn get(&self) -> usize { 0 }
467 f.get(); // the trait is implemented so we can use it
472 // This shouldn't really ever trigger since the repeated value error comes first
474 A binary can only have one entry point, and by default that entry point is the
475 function `main()`. If there are multiple such functions, please rename one.
479 More than one function was declared with the `#[main]` attribute.
481 Erroneous code example:
483 ```compile_fail,E0137
490 fn f() {} // error: multiple functions with a #[main] attribute
493 This error indicates that the compiler found multiple functions with the
494 `#[main]` attribute. This is an error because there must be a unique entry
495 point into a Rust program. Example:
506 More than one function was declared with the `#[start]` attribute.
508 Erroneous code example:
510 ```compile_fail,E0138
514 fn foo(argc: isize, argv: *const *const u8) -> isize {}
517 fn f(argc: isize, argv: *const *const u8) -> isize {}
518 // error: multiple 'start' functions
521 This error indicates that the compiler found multiple functions with the
522 `#[start]` attribute. This is an error because there must be a unique entry
523 point into a Rust program. Example:
529 fn foo(argc: isize, argv: *const *const u8) -> isize { 0 } // ok!
534 #### Note: this error code is no longer emitted by the compiler.
536 There are various restrictions on transmuting between types in Rust; for example
537 types being transmuted must have the same size. To apply all these restrictions,
538 the compiler must know the exact types that may be transmuted. When type
539 parameters are involved, this cannot always be done.
541 So, for example, the following is not allowed:
544 use std::mem::transmute;
546 struct Foo<T>(Vec<T>);
548 fn foo<T>(x: Vec<T>) {
549 // we are transmuting between Vec<T> and Foo<F> here
550 let y: Foo<T> = unsafe { transmute(x) };
551 // do something with y
555 In this specific case there's a good chance that the transmute is harmless (but
556 this is not guaranteed by Rust). However, when alignment and enum optimizations
557 come into the picture, it's quite likely that the sizes may or may not match
558 with different type parameter substitutions. It's not possible to check this for
559 _all_ possible types, so `transmute()` simply only accepts types without any
560 unsubstituted type parameters.
562 If you need this, there's a good chance you're doing something wrong. Keep in
563 mind that Rust doesn't guarantee much about the layout of different structs
564 (even two structs with identical declarations may have different layouts). If
565 there is a solution that avoids the transmute entirely, try it instead.
567 If it's possible, hand-monomorphize the code by writing the function for each
568 possible type substitution. It's possible to use traits to do this cleanly,
572 use std::mem::transmute;
574 struct Foo<T>(Vec<T>);
576 trait MyTransmutableType: Sized {
577 fn transmute(_: Vec<Self>) -> Foo<Self>;
580 impl MyTransmutableType for u8 {
581 fn transmute(x: Vec<u8>) -> Foo<u8> {
582 unsafe { transmute(x) }
586 impl MyTransmutableType for String {
587 fn transmute(x: Vec<String>) -> Foo<String> {
588 unsafe { transmute(x) }
592 // ... more impls for the types you intend to transmute
594 fn foo<T: MyTransmutableType>(x: Vec<T>) {
595 let y: Foo<T> = <T as MyTransmutableType>::transmute(x);
596 // do something with y
600 Each impl will be checked for a size match in the transmute as usual, and since
601 there are no unbound type parameters involved, this should compile unless there
602 is a size mismatch in one of the impls.
604 It is also possible to manually transmute:
608 # let v = Some("value");
609 # type SomeType = &'static [u8];
611 ptr::read(&v as *const _ as *const SomeType) // `v` transmuted to `SomeType`
616 Note that this does not move `v` (unlike `transmute`), and may need a
617 call to `mem::forget(v)` in case you want to avoid destructors being called.
621 A lang item was redefined.
623 Erroneous code example:
625 ```compile_fail,E0152
626 #![feature(lang_items)]
628 #[lang = "panic_fmt"]
629 struct Foo; // error: duplicate lang item found: `panic_fmt`
632 Lang items are already implemented in the standard library. Unless you are
633 writing a free-standing application (e.g. a kernel), you do not need to provide
636 You can build a free-standing crate by adding `#![no_std]` to the crate
639 ```ignore (only-for-syntax-highlight)
643 See also https://doc.rust-lang.org/book/first-edition/no-stdlib.html
647 A generic type was described using parentheses rather than angle brackets.
650 ```compile_fail,E0214
652 let v: Vec(&str) = vec!["foo"];
656 This is not currently supported: `v` should be defined as `Vec<&str>`.
657 Parentheses are currently only used with generic types when defining parameters
658 for `Fn`-family traits.
662 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
663 message for when a particular trait isn't implemented on a type placed in a
664 position that needs that trait. For example, when the following code is
668 #![feature(on_unimplemented)]
670 fn foo<T: Index<u8>>(x: T){}
672 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
673 trait Index<Idx> { /* ... */ }
675 foo(true); // `bool` does not implement `Index<u8>`
678 There will be an error about `bool` not implementing `Index<u8>`, followed by a
679 note saying "the type `bool` cannot be indexed by `u8`".
681 As you can see, you can specify type parameters in curly braces for
682 substitution with the actual types (using the regular format string syntax) in
683 a given situation. Furthermore, `{Self}` will substitute to the type (in this
684 case, `bool`) that we tried to use.
686 This error appears when the curly braces contain an identifier which doesn't
687 match with any of the type parameters or the string `Self`. This might happen
688 if you misspelled a type parameter, or if you intended to use literal curly
689 braces. If it is the latter, escape the curly braces with a second curly brace
690 of the same type; e.g. a literal `{` is `{{`.
694 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
695 message for when a particular trait isn't implemented on a type placed in a
696 position that needs that trait. For example, when the following code is
700 #![feature(on_unimplemented)]
702 fn foo<T: Index<u8>>(x: T){}
704 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
705 trait Index<Idx> { /* ... */ }
707 foo(true); // `bool` does not implement `Index<u8>`
710 there will be an error about `bool` not implementing `Index<u8>`, followed by a
711 note saying "the type `bool` cannot be indexed by `u8`".
713 As you can see, you can specify type parameters in curly braces for
714 substitution with the actual types (using the regular format string syntax) in
715 a given situation. Furthermore, `{Self}` will substitute to the type (in this
716 case, `bool`) that we tried to use.
718 This error appears when the curly braces do not contain an identifier. Please
719 add one of the same name as a type parameter. If you intended to use literal
720 braces, use `{{` and `}}` to escape them.
724 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
725 message for when a particular trait isn't implemented on a type placed in a
726 position that needs that trait. For example, when the following code is
730 #![feature(on_unimplemented)]
732 fn foo<T: Index<u8>>(x: T){}
734 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
735 trait Index<Idx> { /* ... */ }
737 foo(true); // `bool` does not implement `Index<u8>`
740 there will be an error about `bool` not implementing `Index<u8>`, followed by a
741 note saying "the type `bool` cannot be indexed by `u8`".
743 For this to work, some note must be specified. An empty attribute will not do
744 anything, please remove the attribute or add some helpful note for users of the
749 When using a lifetime like `'a` in a type, it must be declared before being
752 These two examples illustrate the problem:
754 ```compile_fail,E0261
755 // error, use of undeclared lifetime name `'a`
756 fn foo(x: &'a str) { }
759 // error, use of undeclared lifetime name `'a`
764 These can be fixed by declaring lifetime parameters:
767 fn foo<'a>(x: &'a str) {}
776 Declaring certain lifetime names in parameters is disallowed. For example,
777 because the `'static` lifetime is a special built-in lifetime name denoting
778 the lifetime of the entire program, this is an error:
780 ```compile_fail,E0262
781 // error, invalid lifetime parameter name `'static`
782 fn foo<'static>(x: &'static str) { }
787 A lifetime name cannot be declared more than once in the same scope. For
790 ```compile_fail,E0263
791 // error, lifetime name `'a` declared twice in the same scope
792 fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str) { }
797 An unknown external lang item was used. Erroneous code example:
799 ```compile_fail,E0264
800 #![feature(lang_items)]
803 #[lang = "cake"] // error: unknown external lang item: `cake`
808 A list of available external lang items is available in
809 `src/librustc/middle/weak_lang_items.rs`. Example:
812 #![feature(lang_items)]
815 #[lang = "panic_fmt"] // ok!
822 This is because of a type mismatch between the associated type of some
823 trait (e.g. `T::Bar`, where `T` implements `trait Quux { type Bar; }`)
824 and another type `U` that is required to be equal to `T::Bar`, but is not.
827 Here is a basic example:
829 ```compile_fail,E0271
830 trait Trait { type AssociatedType; }
832 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
836 impl Trait for i8 { type AssociatedType = &'static str; }
841 Here is that same example again, with some explanatory comments:
843 ```compile_fail,E0271
844 trait Trait { type AssociatedType; }
846 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
847 // ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
849 // This says `foo` can |
850 // only be used with |
852 // implements `Trait`. |
854 // This says not only must
855 // `T` be an impl of `Trait`
856 // but also that the impl
857 // must assign the type `u32`
858 // to the associated type.
862 impl Trait for i8 { type AssociatedType = &'static str; }
863 //~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
868 // ... but it is an implementation
869 // that assigns `&'static str` to
870 // the associated type.
873 // Here, we invoke `foo` with an `i8`, which does not satisfy
874 // the constraint `<i8 as Trait>::AssociatedType=u32`, and
875 // therefore the type-checker complains with this error code.
878 Here is a more subtle instance of the same problem, that can
879 arise with for-loops in Rust:
882 let vs: Vec<i32> = vec![1, 2, 3, 4];
891 The above fails because of an analogous type mismatch,
892 though may be harder to see. Again, here are some
893 explanatory comments for the same example:
897 let vs = vec![1, 2, 3, 4];
899 // `for`-loops use a protocol based on the `Iterator`
900 // trait. Each item yielded in a `for` loop has the
901 // type `Iterator::Item` -- that is, `Item` is the
902 // associated type of the concrete iterator impl.
906 // | We borrow `vs`, iterating over a sequence of
907 // | *references* of type `&Elem` (where `Elem` is
908 // | vector's element type). Thus, the associated
909 // | type `Item` must be a reference `&`-type ...
911 // ... and `v` has the type `Iterator::Item`, as dictated by
912 // the `for`-loop protocol ...
918 // ... but *here*, `v` is forced to have some integral type;
919 // only types like `u8`,`i8`,`u16`,`i16`, et cetera can
920 // match the pattern `1` ...
925 // ... therefore, the compiler complains, because it sees
926 // an attempt to solve the equations
927 // `some integral-type` = type-of-`v`
928 // = `Iterator::Item`
929 // = `&Elem` (i.e. `some reference type`)
931 // which cannot possibly all be true.
937 To avoid those issues, you have to make the types match correctly.
938 So we can fix the previous examples like this:
942 trait Trait { type AssociatedType; }
944 fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
948 impl Trait for i8 { type AssociatedType = &'static str; }
953 let vs = vec![1, 2, 3, 4];
965 This error occurs when there was a recursive trait requirement that overflowed
966 before it could be evaluated. Often this means that there is unbounded
967 recursion in resolving some type bounds.
969 For example, in the following code:
971 ```compile_fail,E0275
976 impl<T> Foo for T where Bar<T>: Foo {}
979 To determine if a `T` is `Foo`, we need to check if `Bar<T>` is `Foo`. However,
980 to do this check, we need to determine that `Bar<Bar<T>>` is `Foo`. To
981 determine this, we check if `Bar<Bar<Bar<T>>>` is `Foo`, and so on. This is
982 clearly a recursive requirement that can't be resolved directly.
984 Consider changing your trait bounds so that they're less self-referential.
988 This error occurs when a bound in an implementation of a trait does not match
989 the bounds specified in the original trait. For example:
991 ```compile_fail,E0276
997 fn foo<T>(x: T) where T: Copy {}
1001 Here, all types implementing `Foo` must have a method `foo<T>(x: T)` which can
1002 take any type `T`. However, in the `impl` for `bool`, we have added an extra
1003 bound that `T` is `Copy`, which isn't compatible with the original trait.
1005 Consider removing the bound from the method or adding the bound to the original
1006 method definition in the trait.
1010 You tried to use a type which doesn't implement some trait in a place which
1011 expected that trait. Erroneous code example:
1013 ```compile_fail,E0277
1014 // here we declare the Foo trait with a bar method
1019 // we now declare a function which takes an object implementing the Foo trait
1020 fn some_func<T: Foo>(foo: T) {
1025 // we now call the method with the i32 type, which doesn't implement
1027 some_func(5i32); // error: the trait bound `i32 : Foo` is not satisfied
1031 In order to fix this error, verify that the type you're using does implement
1039 fn some_func<T: Foo>(foo: T) {
1040 foo.bar(); // we can now use this method since i32 implements the
1044 // we implement the trait on the i32 type
1050 some_func(5i32); // ok!
1054 Or in a generic context, an erroneous code example would look like:
1056 ```compile_fail,E0277
1057 fn some_func<T>(foo: T) {
1058 println!("{:?}", foo); // error: the trait `core::fmt::Debug` is not
1059 // implemented for the type `T`
1063 // We now call the method with the i32 type,
1064 // which *does* implement the Debug trait.
1069 Note that the error here is in the definition of the generic function: Although
1070 we only call it with a parameter that does implement `Debug`, the compiler
1071 still rejects the function: It must work with all possible input types. In
1072 order to make this example compile, we need to restrict the generic type we're
1078 // Restrict the input type to types that implement Debug.
1079 fn some_func<T: fmt::Debug>(foo: T) {
1080 println!("{:?}", foo);
1084 // Calling the method is still fine, as i32 implements Debug.
1087 // This would fail to compile now:
1088 // struct WithoutDebug;
1089 // some_func(WithoutDebug);
1093 Rust only looks at the signature of the called function, as such it must
1094 already specify all requirements that will be used for every type parameter.
1098 #### Note: this error code is no longer emitted by the compiler.
1100 You tried to supply a type which doesn't implement some trait in a location
1101 which expected that trait. This error typically occurs when working with
1102 `Fn`-based types. Erroneous code example:
1105 fn foo<F: Fn(usize)>(x: F) { }
1108 // type mismatch: ... implements the trait `core::ops::Fn<(String,)>`,
1109 // but the trait `core::ops::Fn<(usize,)>` is required
1111 foo(|y: String| { });
1115 The issue in this case is that `foo` is defined as accepting a `Fn` with one
1116 argument of type `String`, but the closure we attempted to pass to it requires
1117 one arguments of type `usize`.
1121 This error indicates that type inference did not result in one unique possible
1122 type, and extra information is required. In most cases this can be provided
1123 by adding a type annotation. Sometimes you need to specify a generic type
1126 A common example is the `collect` method on `Iterator`. It has a generic type
1127 parameter with a `FromIterator` bound, which for a `char` iterator is
1128 implemented by `Vec` and `String` among others. Consider the following snippet
1129 that reverses the characters of a string:
1131 ```compile_fail,E0282
1132 let x = "hello".chars().rev().collect();
1135 In this case, the compiler cannot infer what the type of `x` should be:
1136 `Vec<char>` and `String` are both suitable candidates. To specify which type to
1137 use, you can use a type annotation on `x`:
1140 let x: Vec<char> = "hello".chars().rev().collect();
1143 It is not necessary to annotate the full type. Once the ambiguity is resolved,
1144 the compiler can infer the rest:
1147 let x: Vec<_> = "hello".chars().rev().collect();
1150 Another way to provide the compiler with enough information, is to specify the
1151 generic type parameter:
1154 let x = "hello".chars().rev().collect::<Vec<char>>();
1157 Again, you need not specify the full type if the compiler can infer it:
1160 let x = "hello".chars().rev().collect::<Vec<_>>();
1163 Apart from a method or function with a generic type parameter, this error can
1164 occur when a type parameter of a struct or trait cannot be inferred. In that
1165 case it is not always possible to use a type annotation, because all candidates
1166 have the same return type. For instance:
1168 ```compile_fail,E0282
1179 let number = Foo::bar();
1184 This will fail because the compiler does not know which instance of `Foo` to
1185 call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.
1189 This error occurs when the compiler doesn't have enough information
1190 to unambiguously choose an implementation.
1194 ```compile_fail,E0283
1201 impl Generator for Impl {
1202 fn create() -> u32 { 1 }
1207 impl Generator for AnotherImpl {
1208 fn create() -> u32 { 2 }
1212 let cont: u32 = Generator::create();
1213 // error, impossible to choose one of Generator trait implementation
1214 // Impl or AnotherImpl? Maybe anything else?
1218 To resolve this error use the concrete type:
1227 impl Generator for AnotherImpl {
1228 fn create() -> u32 { 2 }
1232 let gen1 = AnotherImpl::create();
1234 // if there are multiple methods with same name (different traits)
1235 let gen2 = <AnotherImpl as Generator>::create();
1241 This error indicates that the given recursion limit could not be parsed. Ensure
1242 that the value provided is a positive integer between quotes.
1244 Erroneous code example:
1246 ```compile_fail,E0296
1252 And a working example:
1255 #![recursion_limit="1000"]
1262 This error occurs when the compiler was unable to infer the concrete type of a
1263 variable. It can occur for several cases, the most common of which is a
1264 mismatch in the expected type that the compiler inferred for a variable's
1265 initializing expression, and the actual type explicitly assigned to the
1270 ```compile_fail,E0308
1271 let x: i32 = "I am not a number!";
1272 // ~~~ ~~~~~~~~~~~~~~~~~~~~
1274 // | initializing expression;
1275 // | compiler infers type `&str`
1277 // type `i32` assigned to variable `x`
1282 Types in type definitions have lifetimes associated with them that represent
1283 how long the data stored within them is guaranteed to be live. This lifetime
1284 must be as long as the data needs to be alive, and missing the constraint that
1285 denotes this will cause this error.
1287 ```compile_fail,E0309
1288 // This won't compile because T is not constrained, meaning the data
1289 // stored in it is not guaranteed to last as long as the reference
1295 This will compile, because it has the constraint on the type parameter:
1298 struct Foo<'a, T: 'a> {
1303 To see why this is important, consider the case where `T` is itself a reference
1304 (e.g., `T = &str`). If we don't include the restriction that `T: 'a`, the
1305 following code would be perfectly legal:
1307 ```compile_fail,E0309
1313 let v = "42".to_string();
1314 let f = Foo{foo: &v};
1316 println!("{}", f.foo); // but we've already dropped v!
1322 Types in type definitions have lifetimes associated with them that represent
1323 how long the data stored within them is guaranteed to be live. This lifetime
1324 must be as long as the data needs to be alive, and missing the constraint that
1325 denotes this will cause this error.
1327 ```compile_fail,E0310
1328 // This won't compile because T is not constrained to the static lifetime
1329 // the reference needs
1335 This will compile, because it has the constraint on the type parameter:
1338 struct Foo<T: 'static> {
1345 This error occurs when an `if` expression without an `else` block is used in a
1346 context where a type other than `()` is expected, for example a `let`
1349 ```compile_fail,E0317
1352 let a = if x == 5 { 1 };
1356 An `if` expression without an `else` block has the type `()`, so this is a type
1357 error. To resolve it, add an `else` block having the same type as the `if`
1362 This error indicates that some types or traits depend on each other
1363 and therefore cannot be constructed.
1365 The following example contains a circular dependency between two traits:
1367 ```compile_fail,E0391
1368 trait FirstTrait : SecondTrait {
1372 trait SecondTrait : FirstTrait {
1379 #### Note: this error code is no longer emitted by the compiler.
1381 In Rust 1.3, the default object lifetime bounds are expected to change, as
1382 described in [RFC 1156]. You are getting a warning because the compiler
1383 thinks it is possible that this change will cause a compilation error in your
1384 code. It is possible, though unlikely, that this is a false alarm.
1386 The heart of the change is that where `&'a Box<SomeTrait>` used to default to
1387 `&'a Box<SomeTrait+'a>`, it now defaults to `&'a Box<SomeTrait+'static>` (here,
1388 `SomeTrait` is the name of some trait type). Note that the only types which are
1389 affected are references to boxes, like `&Box<SomeTrait>` or
1390 `&[Box<SomeTrait>]`. More common types like `&SomeTrait` or `Box<SomeTrait>`
1393 To silence this warning, edit your code to use an explicit bound. Most of the
1394 time, this means that you will want to change the signature of a function that
1395 you are calling. For example, if the error is reported on a call like `foo(x)`,
1396 and `foo` is defined as follows:
1399 # trait SomeTrait {}
1400 fn foo(arg: &Box<SomeTrait>) { /* ... */ }
1403 You might change it to:
1406 # trait SomeTrait {}
1407 fn foo<'a>(arg: &'a Box<SomeTrait+'a>) { /* ... */ }
1410 This explicitly states that you expect the trait object `SomeTrait` to contain
1411 references (with a maximum lifetime of `'a`).
1413 [RFC 1156]: https://github.com/rust-lang/rfcs/blob/master/text/1156-adjust-default-object-bounds.md
1417 An invalid lint attribute has been given. Erroneous code example:
1419 ```compile_fail,E0452
1420 #![allow(foo = "")] // error: malformed lint attribute
1423 Lint attributes only accept a list of identifiers (where each identifier is a
1424 lint name). Ensure the attribute is of this form:
1427 #![allow(foo)] // ok!
1429 #![allow(foo, foo2)] // ok!
1434 A lint check attribute was overruled by a `forbid` directive set as an
1435 attribute on an enclosing scope, or on the command line with the `-F` option.
1437 Example of erroneous code:
1439 ```compile_fail,E0453
1440 #![forbid(non_snake_case)]
1442 #[allow(non_snake_case)]
1444 let MyNumber = 2; // error: allow(non_snake_case) overruled by outer
1445 // forbid(non_snake_case)
1449 The `forbid` lint setting, like `deny`, turns the corresponding compiler
1450 warning into a hard error. Unlike `deny`, `forbid` prevents itself from being
1451 overridden by inner attributes.
1453 If you're sure you want to override the lint check, you can change `forbid` to
1454 `deny` (or use `-D` instead of `-F` if the `forbid` setting was given as a
1455 command-line option) to allow the inner lint check attribute:
1458 #![deny(non_snake_case)]
1460 #[allow(non_snake_case)]
1462 let MyNumber = 2; // ok!
1466 Otherwise, edit the code to pass the lint check, and remove the overruled
1470 #![forbid(non_snake_case)]
1479 A lifetime bound was not satisfied.
1481 Erroneous code example:
1483 ```compile_fail,E0478
1484 // Check that the explicit lifetime bound (`'SnowWhite`, in this example) must
1485 // outlive all the superbounds from the trait (`'kiss`, in this example).
1487 trait Wedding<'t>: 't { }
1489 struct Prince<'kiss, 'SnowWhite> {
1490 child: Box<Wedding<'kiss> + 'SnowWhite>,
1491 // error: lifetime bound not satisfied
1495 In this example, the `'SnowWhite` lifetime is supposed to outlive the `'kiss`
1496 lifetime but the declaration of the `Prince` struct doesn't enforce it. To fix
1497 this issue, you need to specify it:
1500 trait Wedding<'t>: 't { }
1502 struct Prince<'kiss, 'SnowWhite: 'kiss> { // You say here that 'kiss must live
1503 // longer than 'SnowWhite.
1504 child: Box<Wedding<'kiss> + 'SnowWhite>, // And now it's all good!
1510 A reference has a longer lifetime than the data it references.
1512 Erroneous code example:
1514 ```compile_fail,E0491
1515 // struct containing a reference requires a lifetime parameter,
1516 // because the data the reference points to must outlive the struct (see E0106)
1521 // However, a nested struct like this, the signature itself does not tell
1522 // whether 'a outlives 'b or the other way around.
1523 // So it could be possible that 'b of reference outlives 'a of the data.
1524 struct Nested<'a, 'b> {
1525 ref_struct: &'b Struct<'a>, // compile error E0491
1529 To fix this issue, you can specify a bound to the lifetime like below:
1536 // 'a: 'b means 'a outlives 'b
1537 struct Nested<'a: 'b, 'b> {
1538 ref_struct: &'b Struct<'a>,
1544 A lifetime name is shadowing another lifetime name. Erroneous code example:
1546 ```compile_fail,E0496
1552 fn f<'a>(x: &'a i32) { // error: lifetime name `'a` shadows a lifetime
1553 // name that is already in scope
1558 Please change the name of one of the lifetimes to remove this error. Example:
1566 fn f<'b>(x: &'b i32) { // ok!
1576 A stability attribute was used outside of the standard library. Erroneous code
1580 #[stable] // error: stability attributes may not be used outside of the
1585 It is not possible to use stability attributes outside of the standard library.
1586 Also, for now, it is not possible to write deprecation messages either.
1590 Transmute with two differently sized types was attempted. Erroneous code
1593 ```compile_fail,E0512
1594 fn takes_u8(_: u8) {}
1597 unsafe { takes_u8(::std::mem::transmute(0u16)); }
1598 // error: transmute called with types of different sizes
1602 Please use types with same size or use the expected type directly. Example:
1605 fn takes_u8(_: u8) {}
1608 unsafe { takes_u8(::std::mem::transmute(0i8)); } // ok!
1610 unsafe { takes_u8(0u8); } // ok!
1616 This error indicates that a `#[repr(..)]` attribute was placed on an
1619 Examples of erroneous code:
1621 ```compile_fail,E0517
1629 struct Foo {bar: bool, baz: bool}
1637 * The `#[repr(C)]` attribute can only be placed on structs and enums.
1638 * The `#[repr(packed)]` and `#[repr(simd)]` attributes only work on structs.
1639 * The `#[repr(u8)]`, `#[repr(i16)]`, etc attributes only work on enums.
1641 These attributes do not work on typedefs, since typedefs are just aliases.
1643 Representations like `#[repr(u8)]`, `#[repr(i64)]` are for selecting the
1644 discriminant size for C-like enums (when there is no associated data, e.g.
1645 `enum Color {Red, Blue, Green}`), effectively setting the size of the enum to
1646 the size of the provided type. Such an enum can be cast to a value of the same
1647 type as well. In short, `#[repr(u8)]` makes the enum behave like an integer
1648 with a constrained set of allowed values.
1650 Only C-like enums can be cast to numerical primitives, so this attribute will
1651 not apply to structs.
1653 `#[repr(packed)]` reduces padding to make the struct size smaller. The
1654 representation of enums isn't strictly defined in Rust, and this attribute
1655 won't work on enums.
1657 `#[repr(simd)]` will give a struct consisting of a homogeneous series of machine
1658 types (i.e. `u8`, `i32`, etc) a representation that permits vectorization via
1659 SIMD. This doesn't make much sense for enums since they don't consist of a
1660 single list of data.
1664 This error indicates that an `#[inline(..)]` attribute was incorrectly placed
1665 on something other than a function or method.
1667 Examples of erroneous code:
1669 ```compile_fail,E0518
1679 `#[inline]` hints the compiler whether or not to attempt to inline a method or
1680 function. By default, the compiler does a pretty good job of figuring this out
1681 itself, but if you feel the need for annotations, `#[inline(always)]` and
1682 `#[inline(never)]` can override or force the compiler's decision.
1684 If you wish to apply this attribute to all methods in an impl, manually annotate
1685 each method; it is not possible to annotate the entire impl with an `#[inline]`
1690 The lang attribute is intended for marking special items that are built-in to
1691 Rust itself. This includes special traits (like `Copy` and `Sized`) that affect
1692 how the compiler behaves, as well as special functions that may be automatically
1693 invoked (such as the handler for out-of-bounds accesses when indexing a slice).
1694 Erroneous code example:
1696 ```compile_fail,E0522
1697 #![feature(lang_items)]
1700 fn cookie() -> ! { // error: definition of an unknown language item: `cookie`
1707 A closure was used but didn't implement the expected trait.
1709 Erroneous code example:
1711 ```compile_fail,E0525
1715 fn bar<T: Fn(u32)>(_: T) {}
1719 let closure = |_| foo(x); // error: expected a closure that implements
1720 // the `Fn` trait, but this closure only
1721 // implements `FnOnce`
1726 In the example above, `closure` is an `FnOnce` closure whereas the `bar`
1727 function expected an `Fn` closure. In this case, it's simple to fix the issue,
1728 you just have to implement `Copy` and `Clone` traits on `struct X` and it'll
1732 #[derive(Clone, Copy)] // We implement `Clone` and `Copy` traits.
1736 fn bar<T: Fn(u32)>(_: T) {}
1740 let closure = |_| foo(x);
1741 bar(closure); // ok!
1745 To understand better how closures work in Rust, read:
1746 https://doc.rust-lang.org/book/first-edition/closures.html
1750 The `main` function was incorrectly declared.
1752 Erroneous code example:
1754 ```compile_fail,E0580
1755 fn main() -> i32 { // error: main function has wrong type
1760 The `main` function prototype should never take arguments or return type.
1769 If you want to get command-line arguments, use `std::env::args`. To exit with a
1770 specified exit code, use `std::process::exit`.
1774 Abstract return types (written `impl Trait` for some trait `Trait`) are only
1775 allowed as function return types.
1777 Erroneous code example:
1779 ```compile_fail,E0562
1780 #![feature(conservative_impl_trait)]
1783 let count_to_ten: impl Iterator<Item=usize> = 0..10;
1784 // error: `impl Trait` not allowed outside of function and inherent method
1786 for i in count_to_ten {
1792 Make sure `impl Trait` only appears in return-type position.
1795 #![feature(conservative_impl_trait)]
1797 fn count_to_n(n: usize) -> impl Iterator<Item=usize> {
1802 for i in count_to_n(10) { // ok!
1808 See [RFC 1522] for more details.
1810 [RFC 1522]: https://github.com/rust-lang/rfcs/blob/master/text/1522-conservative-impl-trait.md
1814 Per [RFC 401][rfc401], if you have a function declaration `foo`:
1817 // For the purposes of this explanation, all of these
1818 // different kinds of `fn` declarations are equivalent:
1820 fn foo(x: S) { /* ... */ }
1821 # #[cfg(for_demonstration_only)]
1822 extern "C" { fn foo(x: S); }
1823 # #[cfg(for_demonstration_only)]
1824 impl S { fn foo(self) { /* ... */ } }
1827 the type of `foo` is **not** `fn(S)`, as one might expect.
1828 Rather, it is a unique, zero-sized marker type written here as `typeof(foo)`.
1829 However, `typeof(foo)` can be _coerced_ to a function pointer `fn(S)`,
1830 so you rarely notice this:
1835 let x: fn(S) = foo; // OK, coerces
1838 The reason that this matter is that the type `fn(S)` is not specific to
1839 any particular function: it's a function _pointer_. So calling `x()` results
1840 in a virtual call, whereas `foo()` is statically dispatched, because the type
1841 of `foo` tells us precisely what function is being called.
1843 As noted above, coercions mean that most code doesn't have to be
1844 concerned with this distinction. However, you can tell the difference
1845 when using **transmute** to convert a fn item into a fn pointer.
1847 This is sometimes done as part of an FFI:
1849 ```compile_fail,E0591
1850 extern "C" fn foo(userdata: Box<i32>) {
1854 # fn callback(_: extern "C" fn(*mut i32)) {}
1855 # use std::mem::transmute;
1857 let f: extern "C" fn(*mut i32) = transmute(foo);
1862 Here, transmute is being used to convert the types of the fn arguments.
1863 This pattern is incorrect because, because the type of `foo` is a function
1864 **item** (`typeof(foo)`), which is zero-sized, and the target type (`fn()`)
1865 is a function pointer, which is not zero-sized.
1866 This pattern should be rewritten. There are a few possible ways to do this:
1868 - change the original fn declaration to match the expected signature,
1869 and do the cast in the fn body (the prefered option)
1870 - cast the fn item fo a fn pointer before calling transmute, as shown here:
1873 # extern "C" fn foo(_: Box<i32>) {}
1874 # use std::mem::transmute;
1876 let f: extern "C" fn(*mut i32) = transmute(foo as extern "C" fn(_));
1877 let f: extern "C" fn(*mut i32) = transmute(foo as usize); // works too
1881 The same applies to transmutes to `*mut fn()`, which were observedin practice.
1882 Note though that use of this type is generally incorrect.
1883 The intention is typically to describe a function pointer, but just `fn()`
1884 alone suffices for that. `*mut fn()` is a pointer to a fn pointer.
1885 (Since these values are typically just passed to C code, however, this rarely
1886 makes a difference in practice.)
1888 [rfc401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md
1892 You tried to supply an `Fn`-based type with an incorrect number of arguments
1893 than what was expected.
1895 Erroneous code example:
1897 ```compile_fail,E0593
1898 fn foo<F: Fn()>(x: F) { }
1901 // [E0593] closure takes 1 argument but 0 arguments are required
1908 No `main` function was found in a binary crate. To fix this error, add a
1909 `main` function. For example:
1913 // Your program will start here.
1914 println!("Hello world!");
1918 If you don't know the basics of Rust, you can go look to the Rust Book to get
1919 started: https://doc.rust-lang.org/book/
1923 An unknown lint was used on the command line.
1928 rustc -D bogus omse_file.rs
1931 Maybe you just misspelled the lint name or the lint doesn't exist anymore.
1932 Either way, try to update/remove it in order to fix the error.
1936 This error code indicates a mismatch between the lifetimes appearing in the
1937 function signature (i.e., the parameter types and the return type) and the
1938 data-flow found in the function body.
1940 Erroneous code example:
1942 ```compile_fail,E0621
1943 fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 { // error: explicit lifetime
1944 // required in the type of
1946 if x > y { x } else { y }
1950 In the code above, the function is returning data borrowed from either `x` or
1951 `y`, but the `'a` annotation indicates that it is returning data only from `x`.
1952 To fix the error, the signature and the body must be made to match. Typically,
1953 this is done by updating the function signature. So, in this case, we change
1954 the type of `y` to `&'a i32`, like so:
1957 fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {
1958 if x > y { x } else { y }
1962 Now the signature indicates that the function data borrowed from either `x` or
1963 `y`. Alternatively, you could change the body to not return data from `y`:
1966 fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 {
1975 register_diagnostics! {
1976 // E0006 // merged with E0005
1977 // E0101, // replaced with E0282
1978 // E0102, // replaced with E0282
1981 // E0272, // on_unimplemented #0
1982 // E0273, // on_unimplemented #1
1983 // E0274, // on_unimplemented #2
1984 E0278, // requirement is not satisfied
1985 E0279, // requirement is not satisfied
1986 E0280, // requirement is not satisfied
1987 E0284, // cannot resolve type
1988 // E0285, // overflow evaluation builtin bounds
1989 // E0300, // unexpanded macro
1990 // E0304, // expected signed integer constant
1991 // E0305, // expected constant
1992 E0311, // thing may not live long enough
1993 E0312, // lifetime of reference outlives lifetime of borrowed content
1994 E0313, // lifetime of borrowed pointer outlives lifetime of captured variable
1995 E0314, // closure outlives stack frame
1996 E0315, // cannot invoke closure outside of its lifetime
1997 E0316, // nested quantification of lifetimes
1998 E0320, // recursive overflow during dropck
1999 E0473, // dereference of reference outside its lifetime
2000 E0474, // captured variable `..` does not outlive the enclosing closure
2001 E0475, // index of slice outside its lifetime
2002 E0476, // lifetime of the source pointer does not outlive lifetime bound...
2003 E0477, // the type `..` does not fulfill the required lifetime...
2004 E0479, // the type `..` (provided as the value of a type parameter) is...
2005 E0480, // lifetime of method receiver does not outlive the method call
2006 E0481, // lifetime of function argument does not outlive the function call
2007 E0482, // lifetime of return value does not outlive the function call
2008 E0483, // lifetime of operand does not outlive the operation
2009 E0484, // reference is not valid at the time of borrow
2010 E0485, // automatically reference is not valid at the time of borrow
2011 E0486, // type of expression contains references that are not valid during...
2012 E0487, // unsafe use of destructor: destructor might be called while...
2013 E0488, // lifetime of variable does not enclose its declaration
2014 E0489, // type/lifetime parameter not in scope here
2015 E0490, // a value of type `..` is borrowed for too long
2016 E0495, // cannot infer an appropriate lifetime due to conflicting requirements
2017 E0566, // conflicting representation hints
2018 E0623, // lifetime mismatch where both parameters are anonymous regions
2019 E0628, // generators cannot have explicit arguments
2020 E0631, // type mismatch in closure arguments
2021 E0637, // "'_" is not a valid lifetime bound
2022 E0657, // `impl Trait` can only capture lifetimes bound at the fn level