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 There are conflicting trait implementations for the same type.
414 Example of erroneous code:
416 ```compile_fail,E0119
418 fn get(&self) -> usize;
421 impl<T> MyTrait for T {
422 fn get(&self) -> usize { 0 }
429 impl MyTrait for Foo { // error: conflicting implementations of trait
430 // `MyTrait` for type `Foo`
431 fn get(&self) -> usize { self.value }
435 When looking for the implementation for the trait, the compiler finds
436 both the `impl<T> MyTrait for T` where T is all types and the `impl
437 MyTrait for Foo`. Since a trait cannot be implemented multiple times,
438 this is an error. So, when you write:
442 fn get(&self) -> usize;
445 impl<T> MyTrait for T {
446 fn get(&self) -> usize { 0 }
450 This makes the trait implemented on all types in the scope. So if you
451 try to implement it on another one after that, the implementations will
456 fn get(&self) -> usize;
459 impl<T> MyTrait for T {
460 fn get(&self) -> usize { 0 }
468 f.get(); // the trait is implemented so we can use it
474 Unsafe code was used outside of an unsafe function or block.
476 Erroneous code example:
478 ```compile_fail,E0133
479 unsafe fn f() { return; } // This is the unsafe code
482 f(); // error: call to unsafe function requires unsafe function or block
486 Using unsafe functionality is potentially dangerous and disallowed by safety
489 * Dereferencing raw pointers
490 * Calling functions via FFI
491 * Calling functions marked unsafe
493 These safety checks can be relaxed for a section of the code by wrapping the
494 unsafe instructions with an `unsafe` block. For instance:
497 unsafe fn f() { return; }
500 unsafe { f(); } // ok!
504 See also https://doc.rust-lang.org/book/unsafe.html
507 // This shouldn't really ever trigger since the repeated value error comes first
509 A binary can only have one entry point, and by default that entry point is the
510 function `main()`. If there are multiple such functions, please rename one.
514 More than one function was declared with the `#[main]` attribute.
516 Erroneous code example:
518 ```compile_fail,E0137
525 fn f() {} // error: multiple functions with a #[main] attribute
528 This error indicates that the compiler found multiple functions with the
529 `#[main]` attribute. This is an error because there must be a unique entry
530 point into a Rust program. Example:
541 More than one function was declared with the `#[start]` attribute.
543 Erroneous code example:
545 ```compile_fail,E0138
549 fn foo(argc: isize, argv: *const *const u8) -> isize {}
552 fn f(argc: isize, argv: *const *const u8) -> isize {}
553 // error: multiple 'start' functions
556 This error indicates that the compiler found multiple functions with the
557 `#[start]` attribute. This is an error because there must be a unique entry
558 point into a Rust program. Example:
564 fn foo(argc: isize, argv: *const *const u8) -> isize { 0 } // ok!
568 // isn't thrown anymore
570 There are various restrictions on transmuting between types in Rust; for example
571 types being transmuted must have the same size. To apply all these restrictions,
572 the compiler must know the exact types that may be transmuted. When type
573 parameters are involved, this cannot always be done.
575 So, for example, the following is not allowed:
578 use std::mem::transmute;
580 struct Foo<T>(Vec<T>);
582 fn foo<T>(x: Vec<T>) {
583 // we are transmuting between Vec<T> and Foo<F> here
584 let y: Foo<T> = unsafe { transmute(x) };
585 // do something with y
589 In this specific case there's a good chance that the transmute is harmless (but
590 this is not guaranteed by Rust). However, when alignment and enum optimizations
591 come into the picture, it's quite likely that the sizes may or may not match
592 with different type parameter substitutions. It's not possible to check this for
593 _all_ possible types, so `transmute()` simply only accepts types without any
594 unsubstituted type parameters.
596 If you need this, there's a good chance you're doing something wrong. Keep in
597 mind that Rust doesn't guarantee much about the layout of different structs
598 (even two structs with identical declarations may have different layouts). If
599 there is a solution that avoids the transmute entirely, try it instead.
601 If it's possible, hand-monomorphize the code by writing the function for each
602 possible type substitution. It's possible to use traits to do this cleanly,
606 struct Foo<T>(Vec<T>);
608 trait MyTransmutableType {
609 fn transmute(Vec<Self>) -> Foo<Self>;
612 impl MyTransmutableType for u8 {
613 fn transmute(x: Foo<u8>) -> Vec<u8> {
618 impl MyTransmutableType for String {
619 fn transmute(x: Foo<String>) -> Vec<String> {
624 // ... more impls for the types you intend to transmute
626 fn foo<T: MyTransmutableType>(x: Vec<T>) {
627 let y: Foo<T> = <T as MyTransmutableType>::transmute(x);
628 // do something with y
632 Each impl will be checked for a size match in the transmute as usual, and since
633 there are no unbound type parameters involved, this should compile unless there
634 is a size mismatch in one of the impls.
636 It is also possible to manually transmute:
639 ptr::read(&v as *const _ as *const SomeType) // `v` transmuted to `SomeType`
642 Note that this does not move `v` (unlike `transmute`), and may need a
643 call to `mem::forget(v)` in case you want to avoid destructors being called.
647 A lang item was redefined.
649 Erroneous code example:
651 ```compile_fail,E0152
652 #![feature(lang_items)]
654 #[lang = "panic_fmt"]
655 struct Foo; // error: duplicate lang item found: `panic_fmt`
658 Lang items are already implemented in the standard library. Unless you are
659 writing a free-standing application (e.g. a kernel), you do not need to provide
662 You can build a free-standing crate by adding `#![no_std]` to the crate
669 See also https://doc.rust-lang.org/book/no-stdlib.html
673 When using a lifetime like `'a` in a type, it must be declared before being
676 These two examples illustrate the problem:
678 ```compile_fail,E0261
679 // error, use of undeclared lifetime name `'a`
680 fn foo(x: &'a str) { }
683 // error, use of undeclared lifetime name `'a`
688 These can be fixed by declaring lifetime parameters:
691 fn foo<'a>(x: &'a str) {}
700 Declaring certain lifetime names in parameters is disallowed. For example,
701 because the `'static` lifetime is a special built-in lifetime name denoting
702 the lifetime of the entire program, this is an error:
704 ```compile_fail,E0262
705 // error, invalid lifetime parameter name `'static`
706 fn foo<'static>(x: &'static str) { }
711 A lifetime name cannot be declared more than once in the same scope. For
714 ```compile_fail,E0263
715 // error, lifetime name `'a` declared twice in the same scope
716 fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str) { }
721 An unknown external lang item was used. Erroneous code example:
723 ```compile_fail,E0264
724 #![feature(lang_items)]
727 #[lang = "cake"] // error: unknown external lang item: `cake`
732 A list of available external lang items is available in
733 `src/librustc/middle/weak_lang_items.rs`. Example:
736 #![feature(lang_items)]
739 #[lang = "panic_fmt"] // ok!
746 This is because of a type mismatch between the associated type of some
747 trait (e.g. `T::Bar`, where `T` implements `trait Quux { type Bar; }`)
748 and another type `U` that is required to be equal to `T::Bar`, but is not.
751 Here is a basic example:
753 ```compile_fail,E0271
754 trait Trait { type AssociatedType; }
756 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
760 impl Trait for i8 { type AssociatedType = &'static str; }
765 Here is that same example again, with some explanatory comments:
768 trait Trait { type AssociatedType; }
770 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
771 // ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
773 // This says `foo` can |
774 // only be used with |
776 // implements `Trait`. |
778 // This says not only must
779 // `T` be an impl of `Trait`
780 // but also that the impl
781 // must assign the type `u32`
782 // to the associated type.
786 impl Trait for i8 { type AssociatedType = &'static str; }
787 ~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
792 // ... but it is an implementation
793 // that assigns `&'static str` to
794 // the associated type.
797 // Here, we invoke `foo` with an `i8`, which does not satisfy
798 // the constraint `<i8 as Trait>::AssociatedType=u32`, and
799 // therefore the type-checker complains with this error code.
802 Here is a more subtle instance of the same problem, that can
803 arise with for-loops in Rust:
806 let vs: Vec<i32> = vec![1, 2, 3, 4];
815 The above fails because of an analogous type mismatch,
816 though may be harder to see. Again, here are some
817 explanatory comments for the same example:
821 let vs = vec![1, 2, 3, 4];
823 // `for`-loops use a protocol based on the `Iterator`
824 // trait. Each item yielded in a `for` loop has the
825 // type `Iterator::Item` -- that is, `Item` is the
826 // associated type of the concrete iterator impl.
830 // | We borrow `vs`, iterating over a sequence of
831 // | *references* of type `&Elem` (where `Elem` is
832 // | vector's element type). Thus, the associated
833 // | type `Item` must be a reference `&`-type ...
835 // ... and `v` has the type `Iterator::Item`, as dictated by
836 // the `for`-loop protocol ...
842 // ... but *here*, `v` is forced to have some integral type;
843 // only types like `u8`,`i8`,`u16`,`i16`, et cetera can
844 // match the pattern `1` ...
849 // ... therefore, the compiler complains, because it sees
850 // an attempt to solve the equations
851 // `some integral-type` = type-of-`v`
852 // = `Iterator::Item`
853 // = `&Elem` (i.e. `some reference type`)
855 // which cannot possibly all be true.
861 To avoid those issues, you have to make the types match correctly.
862 So we can fix the previous examples like this:
866 trait Trait { type AssociatedType; }
868 fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
872 impl Trait for i8 { type AssociatedType = &'static str; }
877 let vs = vec![1, 2, 3, 4];
888 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
889 message for when a particular trait isn't implemented on a type placed in a
890 position that needs that trait. For example, when the following code is
894 #![feature(on_unimplemented)]
896 fn foo<T: Index<u8>>(x: T){}
898 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
899 trait Index<Idx> { /* ... */ }
901 foo(true); // `bool` does not implement `Index<u8>`
904 There will be an error about `bool` not implementing `Index<u8>`, followed by a
905 note saying "the type `bool` cannot be indexed by `u8`".
907 As you can see, you can specify type parameters in curly braces for
908 substitution with the actual types (using the regular format string syntax) in
909 a given situation. Furthermore, `{Self}` will substitute to the type (in this
910 case, `bool`) that we tried to use.
912 This error appears when the curly braces contain an identifier which doesn't
913 match with any of the type parameters or the string `Self`. This might happen
914 if you misspelled a type parameter, or if you intended to use literal curly
915 braces. If it is the latter, escape the curly braces with a second curly brace
916 of the same type; e.g. a literal `{` is `{{`.
920 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
921 message for when a particular trait isn't implemented on a type placed in a
922 position that needs that trait. For example, when the following code is
926 #![feature(on_unimplemented)]
928 fn foo<T: Index<u8>>(x: T){}
930 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
931 trait Index<Idx> { /* ... */ }
933 foo(true); // `bool` does not implement `Index<u8>`
936 there will be an error about `bool` not implementing `Index<u8>`, followed by a
937 note saying "the type `bool` cannot be indexed by `u8`".
939 As you can see, you can specify type parameters in curly braces for
940 substitution with the actual types (using the regular format string syntax) in
941 a given situation. Furthermore, `{Self}` will substitute to the type (in this
942 case, `bool`) that we tried to use.
944 This error appears when the curly braces do not contain an identifier. Please
945 add one of the same name as a type parameter. If you intended to use literal
946 braces, use `{{` and `}}` to escape them.
950 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
951 message for when a particular trait isn't implemented on a type placed in a
952 position that needs that trait. For example, when the following code is
956 #![feature(on_unimplemented)]
958 fn foo<T: Index<u8>>(x: T){}
960 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
961 trait Index<Idx> { /* ... */ }
963 foo(true); // `bool` does not implement `Index<u8>`
966 there will be an error about `bool` not implementing `Index<u8>`, followed by a
967 note saying "the type `bool` cannot be indexed by `u8`".
969 For this to work, some note must be specified. An empty attribute will not do
970 anything, please remove the attribute or add some helpful note for users of the
975 This error occurs when there was a recursive trait requirement that overflowed
976 before it could be evaluated. Often this means that there is unbounded
977 recursion in resolving some type bounds.
979 For example, in the following code:
981 ```compile_fail,E0275
986 impl<T> Foo for T where Bar<T>: Foo {}
989 To determine if a `T` is `Foo`, we need to check if `Bar<T>` is `Foo`. However,
990 to do this check, we need to determine that `Bar<Bar<T>>` is `Foo`. To
991 determine this, we check if `Bar<Bar<Bar<T>>>` is `Foo`, and so on. This is
992 clearly a recursive requirement that can't be resolved directly.
994 Consider changing your trait bounds so that they're less self-referential.
998 This error occurs when a bound in an implementation of a trait does not match
999 the bounds specified in the original trait. For example:
1001 ```compile_fail,E0276
1007 fn foo<T>(x: T) where T: Copy {}
1011 Here, all types implementing `Foo` must have a method `foo<T>(x: T)` which can
1012 take any type `T`. However, in the `impl` for `bool`, we have added an extra
1013 bound that `T` is `Copy`, which isn't compatible with the original trait.
1015 Consider removing the bound from the method or adding the bound to the original
1016 method definition in the trait.
1020 You tried to use a type which doesn't implement some trait in a place which
1021 expected that trait. Erroneous code example:
1023 ```compile_fail,E0277
1024 // here we declare the Foo trait with a bar method
1029 // we now declare a function which takes an object implementing the Foo trait
1030 fn some_func<T: Foo>(foo: T) {
1035 // we now call the method with the i32 type, which doesn't implement
1037 some_func(5i32); // error: the trait bound `i32 : Foo` is not satisfied
1041 In order to fix this error, verify that the type you're using does implement
1049 fn some_func<T: Foo>(foo: T) {
1050 foo.bar(); // we can now use this method since i32 implements the
1054 // we implement the trait on the i32 type
1060 some_func(5i32); // ok!
1064 Or in a generic context, an erroneous code example would look like:
1066 ```compile_fail,E0277
1067 fn some_func<T>(foo: T) {
1068 println!("{:?}", foo); // error: the trait `core::fmt::Debug` is not
1069 // implemented for the type `T`
1073 // We now call the method with the i32 type,
1074 // which *does* implement the Debug trait.
1079 Note that the error here is in the definition of the generic function: Although
1080 we only call it with a parameter that does implement `Debug`, the compiler
1081 still rejects the function: It must work with all possible input types. In
1082 order to make this example compile, we need to restrict the generic type we're
1088 // Restrict the input type to types that implement Debug.
1089 fn some_func<T: fmt::Debug>(foo: T) {
1090 println!("{:?}", foo);
1094 // Calling the method is still fine, as i32 implements Debug.
1097 // This would fail to compile now:
1098 // struct WithoutDebug;
1099 // some_func(WithoutDebug);
1103 Rust only looks at the signature of the called function, as such it must
1104 already specify all requirements that will be used for every type parameter.
1108 You tried to supply a type which doesn't implement some trait in a location
1109 which expected that trait. This error typically occurs when working with
1110 `Fn`-based types. Erroneous code example:
1112 ```compile_fail,E0281
1113 fn foo<F: Fn(usize)>(x: F) { }
1116 // type mismatch: ... implements the trait `core::ops::Fn<(String,)>`,
1117 // but the trait `core::ops::Fn<(usize,)>` is required
1119 foo(|y: String| { });
1123 The issue in this case is that `foo` is defined as accepting a `Fn` with one
1124 argument of type `String`, but the closure we attempted to pass to it requires
1125 one arguments of type `usize`.
1129 This error indicates that type inference did not result in one unique possible
1130 type, and extra information is required. In most cases this can be provided
1131 by adding a type annotation. Sometimes you need to specify a generic type
1134 A common example is the `collect` method on `Iterator`. It has a generic type
1135 parameter with a `FromIterator` bound, which for a `char` iterator is
1136 implemented by `Vec` and `String` among others. Consider the following snippet
1137 that reverses the characters of a string:
1139 ```compile_fail,E0282
1140 let x = "hello".chars().rev().collect();
1143 In this case, the compiler cannot infer what the type of `x` should be:
1144 `Vec<char>` and `String` are both suitable candidates. To specify which type to
1145 use, you can use a type annotation on `x`:
1148 let x: Vec<char> = "hello".chars().rev().collect();
1151 It is not necessary to annotate the full type. Once the ambiguity is resolved,
1152 the compiler can infer the rest:
1155 let x: Vec<_> = "hello".chars().rev().collect();
1158 Another way to provide the compiler with enough information, is to specify the
1159 generic type parameter:
1162 let x = "hello".chars().rev().collect::<Vec<char>>();
1165 Again, you need not specify the full type if the compiler can infer it:
1168 let x = "hello".chars().rev().collect::<Vec<_>>();
1171 Apart from a method or function with a generic type parameter, this error can
1172 occur when a type parameter of a struct or trait cannot be inferred. In that
1173 case it is not always possible to use a type annotation, because all candidates
1174 have the same return type. For instance:
1176 ```compile_fail,E0282
1187 let number = Foo::bar();
1192 This will fail because the compiler does not know which instance of `Foo` to
1193 call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.
1197 This error occurs when the compiler doesn't have enough information
1198 to unambiguously choose an implementation.
1202 ```compile_fail,E0283
1209 impl Generator for Impl {
1210 fn create() -> u32 { 1 }
1215 impl Generator for AnotherImpl {
1216 fn create() -> u32 { 2 }
1220 let cont: u32 = Generator::create();
1221 // error, impossible to choose one of Generator trait implementation
1222 // Impl or AnotherImpl? Maybe anything else?
1226 To resolve this error use the concrete type:
1235 impl Generator for AnotherImpl {
1236 fn create() -> u32 { 2 }
1240 let gen1 = AnotherImpl::create();
1242 // if there are multiple methods with same name (different traits)
1243 let gen2 = <AnotherImpl as Generator>::create();
1249 This error indicates that the given recursion limit could not be parsed. Ensure
1250 that the value provided is a positive integer between quotes.
1252 Erroneous code example:
1254 ```compile_fail,E0296
1260 And a working example:
1263 #![recursion_limit="1000"]
1270 This error occurs when the compiler was unable to infer the concrete type of a
1271 variable. It can occur for several cases, the most common of which is a
1272 mismatch in the expected type that the compiler inferred for a variable's
1273 initializing expression, and the actual type explicitly assigned to the
1278 ```compile_fail,E0308
1279 let x: i32 = "I am not a number!";
1280 // ~~~ ~~~~~~~~~~~~~~~~~~~~
1282 // | initializing expression;
1283 // | compiler infers type `&str`
1285 // type `i32` assigned to variable `x`
1290 Types in type definitions have lifetimes associated with them that represent
1291 how long the data stored within them is guaranteed to be live. This lifetime
1292 must be as long as the data needs to be alive, and missing the constraint that
1293 denotes this will cause this error.
1295 ```compile_fail,E0309
1296 // This won't compile because T is not constrained, meaning the data
1297 // stored in it is not guaranteed to last as long as the reference
1303 This will compile, because it has the constraint on the type parameter:
1306 struct Foo<'a, T: 'a> {
1311 To see why this is important, consider the case where `T` is itself a reference
1312 (e.g., `T = &str`). If we don't include the restriction that `T: 'a`, the
1313 following code would be perfectly legal:
1315 ```compile_fail,E0309
1321 let v = "42".to_string();
1322 let f = Foo{foo: &v};
1324 println!("{}", f.foo); // but we've already dropped v!
1330 Types in type definitions have lifetimes associated with them that represent
1331 how long the data stored within them is guaranteed to be live. This lifetime
1332 must be as long as the data needs to be alive, and missing the constraint that
1333 denotes this will cause this error.
1335 ```compile_fail,E0310
1336 // This won't compile because T is not constrained to the static lifetime
1337 // the reference needs
1343 This will compile, because it has the constraint on the type parameter:
1346 struct Foo<T: 'static> {
1353 A lifetime of reference outlives lifetime of borrowed content.
1355 Erroneous code example:
1357 ```compile_fail,E0312
1358 fn make_child<'human, 'elve>(x: &mut &'human isize, y: &mut &'elve isize) {
1360 // error: lifetime of reference outlives lifetime of borrowed content
1364 The compiler cannot determine if the `human` lifetime will live long enough
1365 to keep up on the elve one. To solve this error, you have to give an
1366 explicit lifetime hierarchy:
1369 fn make_child<'human, 'elve: 'human>(x: &mut &'human isize,
1370 y: &mut &'elve isize) {
1375 Or use the same lifetime for every variable:
1378 fn make_child<'elve>(x: &mut &'elve isize, y: &mut &'elve isize) {
1385 This error occurs when an `if` expression without an `else` block is used in a
1386 context where a type other than `()` is expected, for example a `let`
1389 ```compile_fail,E0317
1392 let a = if x == 5 { 1 };
1396 An `if` expression without an `else` block has the type `()`, so this is a type
1397 error. To resolve it, add an `else` block having the same type as the `if`
1402 This error indicates that some types or traits depend on each other
1403 and therefore cannot be constructed.
1405 The following example contains a circular dependency between two traits:
1407 ```compile_fail,E0391
1408 trait FirstTrait : SecondTrait {
1412 trait SecondTrait : FirstTrait {
1419 In Rust 1.3, the default object lifetime bounds are expected to change, as
1420 described in [RFC 1156]. You are getting a warning because the compiler
1421 thinks it is possible that this change will cause a compilation error in your
1422 code. It is possible, though unlikely, that this is a false alarm.
1424 The heart of the change is that where `&'a Box<SomeTrait>` used to default to
1425 `&'a Box<SomeTrait+'a>`, it now defaults to `&'a Box<SomeTrait+'static>` (here,
1426 `SomeTrait` is the name of some trait type). Note that the only types which are
1427 affected are references to boxes, like `&Box<SomeTrait>` or
1428 `&[Box<SomeTrait>]`. More common types like `&SomeTrait` or `Box<SomeTrait>`
1431 To silence this warning, edit your code to use an explicit bound. Most of the
1432 time, this means that you will want to change the signature of a function that
1433 you are calling. For example, if the error is reported on a call like `foo(x)`,
1434 and `foo` is defined as follows:
1437 fn foo(arg: &Box<SomeTrait>) { ... }
1440 You might change it to:
1443 fn foo<'a>(arg: &Box<SomeTrait+'a>) { ... }
1446 This explicitly states that you expect the trait object `SomeTrait` to contain
1447 references (with a maximum lifetime of `'a`).
1449 [RFC 1156]: https://github.com/rust-lang/rfcs/blob/master/text/1156-adjust-default-object-bounds.md
1453 An invalid lint attribute has been given. Erroneous code example:
1455 ```compile_fail,E0452
1456 #![allow(foo = "")] // error: malformed lint attribute
1459 Lint attributes only accept a list of identifiers (where each identifier is a
1460 lint name). Ensure the attribute is of this form:
1463 #![allow(foo)] // ok!
1465 #![allow(foo, foo2)] // ok!
1470 A lint check attribute was overruled by a `forbid` directive set as an
1471 attribute on an enclosing scope, or on the command line with the `-F` option.
1473 Example of erroneous code:
1475 ```compile_fail,E0453
1476 #![forbid(non_snake_case)]
1478 #[allow(non_snake_case)]
1480 let MyNumber = 2; // error: allow(non_snake_case) overruled by outer
1481 // forbid(non_snake_case)
1485 The `forbid` lint setting, like `deny`, turns the corresponding compiler
1486 warning into a hard error. Unlike `deny`, `forbid` prevents itself from being
1487 overridden by inner attributes.
1489 If you're sure you want to override the lint check, you can change `forbid` to
1490 `deny` (or use `-D` instead of `-F` if the `forbid` setting was given as a
1491 command-line option) to allow the inner lint check attribute:
1494 #![deny(non_snake_case)]
1496 #[allow(non_snake_case)]
1498 let MyNumber = 2; // ok!
1502 Otherwise, edit the code to pass the lint check, and remove the overruled
1506 #![forbid(non_snake_case)]
1515 A lifetime bound was not satisfied.
1517 Erroneous code example:
1519 ```compile_fail,E0478
1520 // Check that the explicit lifetime bound (`'SnowWhite`, in this example) must
1521 // outlive all the superbounds from the trait (`'kiss`, in this example).
1523 trait Wedding<'t>: 't { }
1525 struct Prince<'kiss, 'SnowWhite> {
1526 child: Box<Wedding<'kiss> + 'SnowWhite>,
1527 // error: lifetime bound not satisfied
1531 In this example, the `'SnowWhite` lifetime is supposed to outlive the `'kiss`
1532 lifetime but the declaration of the `Prince` struct doesn't enforce it. To fix
1533 this issue, you need to specify it:
1536 trait Wedding<'t>: 't { }
1538 struct Prince<'kiss, 'SnowWhite: 'kiss> { // You say here that 'kiss must live
1539 // longer than 'SnowWhite.
1540 child: Box<Wedding<'kiss> + 'SnowWhite>, // And now it's all good!
1546 A reference has a longer lifetime than the data it references.
1548 Erroneous code example:
1550 ```compile_fail,E0491
1551 // struct containing a reference requires a lifetime parameter,
1552 // because the data the reference points to must outlive the struct (see E0106)
1557 // However, a nested struct like this, the signature itself does not tell
1558 // whether 'a outlives 'b or the other way around.
1559 // So it could be possible that 'b of reference outlives 'a of the data.
1560 struct Nested<'a, 'b> {
1561 ref_struct: &'b Struct<'a>, // compile error E0491
1565 To fix this issue, you can specify a bound to the lifetime like below:
1572 // 'a: 'b means 'a outlives 'b
1573 struct Nested<'a: 'b, 'b> {
1574 ref_struct: &'b Struct<'a>,
1580 A lifetime name is shadowing another lifetime name. Erroneous code example:
1582 ```compile_fail,E0496
1588 fn f<'a>(x: &'a i32) { // error: lifetime name `'a` shadows a lifetime
1589 // name that is already in scope
1594 Please change the name of one of the lifetimes to remove this error. Example:
1602 fn f<'b>(x: &'b i32) { // ok!
1612 A stability attribute was used outside of the standard library. Erroneous code
1616 #[stable] // error: stability attributes may not be used outside of the
1621 It is not possible to use stability attributes outside of the standard library.
1622 Also, for now, it is not possible to write deprecation messages either.
1626 Transmute with two differently sized types was attempted. Erroneous code
1629 ```compile_fail,E0512
1630 fn takes_u8(_: u8) {}
1633 unsafe { takes_u8(::std::mem::transmute(0u16)); }
1634 // error: transmute called with differently sized types
1638 Please use types with same size or use the expected type directly. Example:
1641 fn takes_u8(_: u8) {}
1644 unsafe { takes_u8(::std::mem::transmute(0i8)); } // ok!
1646 unsafe { takes_u8(0u8); } // ok!
1652 This error indicates that a `#[repr(..)]` attribute was placed on an
1655 Examples of erroneous code:
1657 ```compile_fail,E0517
1665 struct Foo {bar: bool, baz: bool}
1673 * The `#[repr(C)]` attribute can only be placed on structs and enums.
1674 * The `#[repr(packed)]` and `#[repr(simd)]` attributes only work on structs.
1675 * The `#[repr(u8)]`, `#[repr(i16)]`, etc attributes only work on enums.
1677 These attributes do not work on typedefs, since typedefs are just aliases.
1679 Representations like `#[repr(u8)]`, `#[repr(i64)]` are for selecting the
1680 discriminant size for C-like enums (when there is no associated data, e.g.
1681 `enum Color {Red, Blue, Green}`), effectively setting the size of the enum to
1682 the size of the provided type. Such an enum can be cast to a value of the same
1683 type as well. In short, `#[repr(u8)]` makes the enum behave like an integer
1684 with a constrained set of allowed values.
1686 Only C-like enums can be cast to numerical primitives, so this attribute will
1687 not apply to structs.
1689 `#[repr(packed)]` reduces padding to make the struct size smaller. The
1690 representation of enums isn't strictly defined in Rust, and this attribute
1691 won't work on enums.
1693 `#[repr(simd)]` will give a struct consisting of a homogenous series of machine
1694 types (i.e. `u8`, `i32`, etc) a representation that permits vectorization via
1695 SIMD. This doesn't make much sense for enums since they don't consist of a
1696 single list of data.
1700 This error indicates that an `#[inline(..)]` attribute was incorrectly placed
1701 on something other than a function or method.
1703 Examples of erroneous code:
1705 ```compile_fail,E0518
1715 `#[inline]` hints the compiler whether or not to attempt to inline a method or
1716 function. By default, the compiler does a pretty good job of figuring this out
1717 itself, but if you feel the need for annotations, `#[inline(always)]` and
1718 `#[inline(never)]` can override or force the compiler's decision.
1720 If you wish to apply this attribute to all methods in an impl, manually annotate
1721 each method; it is not possible to annotate the entire impl with an `#[inline]`
1726 The lang attribute is intended for marking special items that are built-in to
1727 Rust itself. This includes special traits (like `Copy` and `Sized`) that affect
1728 how the compiler behaves, as well as special functions that may be automatically
1729 invoked (such as the handler for out-of-bounds accesses when indexing a slice).
1730 Erroneous code example:
1732 ```compile_fail,E0522
1733 #![feature(lang_items)]
1736 fn cookie() -> ! { // error: definition of an unknown language item: `cookie`
1743 A closure was used but didn't implement the expected trait.
1745 Erroneous code example:
1747 ```compile_fail,E0525
1751 fn bar<T: Fn(u32)>(_: T) {}
1755 let closure = |_| foo(x); // error: expected a closure that implements
1756 // the `Fn` trait, but this closure only
1757 // implements `FnOnce`
1762 In the example above, `closure` is an `FnOnce` closure whereas the `bar`
1763 function expected an `Fn` closure. In this case, it's simple to fix the issue,
1764 you just have to implement `Copy` and `Clone` traits on `struct X` and it'll
1768 #[derive(Clone, Copy)] // We implement `Clone` and `Copy` traits.
1772 fn bar<T: Fn(u32)>(_: T) {}
1776 let closure = |_| foo(x);
1777 bar(closure); // ok!
1781 To understand better how closures work in Rust, read:
1782 https://doc.rust-lang.org/book/closures.html
1786 The `main` function was incorrectly declared.
1788 Erroneous code example:
1790 ```compile_fail,E0580
1791 fn main() -> i32 { // error: main function has wrong type
1796 The `main` function prototype should never take arguments or return type.
1805 If you want to get command-line arguments, use `std::env::args`. To exit with a
1806 specified exit code, use `std::process::exit`.
1810 Per [RFC 401][rfc401], if you have a function declaration `foo`:
1813 // For the purposes of this explanation, all of these
1814 // different kinds of `fn` declarations are equivalent:
1815 fn foo(x: i32) { ... }
1816 extern "C" fn foo(x: i32);
1817 impl i32 { fn foo(x: self) { ... } }
1820 the type of `foo` is **not** `fn(i32)`, as one might expect.
1821 Rather, it is a unique, zero-sized marker type written here as `typeof(foo)`.
1822 However, `typeof(foo)` can be _coerced_ to a function pointer `fn(i32)`,
1823 so you rarely notice this:
1826 let x: fn(i32) = foo; // OK, coerces
1829 The reason that this matter is that the type `fn(i32)` is not specific to
1830 any particular function: it's a function _pointer_. So calling `x()` results
1831 in a virtual call, whereas `foo()` is statically dispatched, because the type
1832 of `foo` tells us precisely what function is being called.
1834 As noted above, coercions mean that most code doesn't have to be
1835 concerned with this distinction. However, you can tell the difference
1836 when using **transmute** to convert a fn item into a fn pointer.
1838 This is sometimes done as part of an FFI:
1841 extern "C" fn foo(userdata: Box<i32>) {
1845 let f: extern "C" fn(*mut i32) = transmute(foo);
1850 Here, transmute is being used to convert the types of the fn arguments.
1851 This pattern is incorrect because, because the type of `foo` is a function
1852 **item** (`typeof(foo)`), which is zero-sized, and the target type (`fn()`)
1853 is a function pointer, which is not zero-sized.
1854 This pattern should be rewritten. There are a few possible ways to do this:
1856 - change the original fn declaration to match the expected signature,
1857 and do the cast in the fn body (the prefered option)
1858 - cast the fn item fo a fn pointer before calling transmute, as shown here:
1859 - `let f: extern "C" fn(*mut i32) = transmute(foo as extern "C" fn(_))`
1860 - `let f: extern "C" fn(*mut i32) = transmute(foo as usize) /* works too */`
1862 The same applies to transmutes to `*mut fn()`, which were observedin practice.
1863 Note though that use of this type is generally incorrect.
1864 The intention is typically to describe a function pointer, but just `fn()`
1865 alone suffices for that. `*mut fn()` is a pointer to a fn pointer.
1866 (Since these values are typically just passed to C code, however, this rarely
1867 makes a difference in practice.)
1869 [rfc401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md
1873 You tried to supply an `Fn`-based type with an incorrect number of arguments
1874 than what was expected.
1876 Erroneous code example:
1878 ```compile_fail,E0593
1879 fn foo<F: Fn()>(x: F) { }
1882 // [E0593] closure takes 1 argument but 0 arguments are required
1889 No `main` function was found in a binary crate. To fix this error, just add a
1890 `main` function. For example:
1894 // Your program will start here.
1895 println!("Hello world!");
1899 If you don't know the basics of Rust, you can go look to the Rust Book to get
1900 started: https://doc.rust-lang.org/book/
1906 register_diagnostics
! {
1907 // E0006 // merged with E0005
1908 // E0101, // replaced with E0282
1909 // E0102, // replaced with E0282
1912 E0278
, // requirement is not satisfied
1913 E0279
, // requirement is not satisfied
1914 E0280
, // requirement is not satisfied
1915 E0284
, // cannot resolve type
1916 // E0285, // overflow evaluation builtin bounds
1917 // E0300, // unexpanded macro
1918 // E0304, // expected signed integer constant
1919 // E0305, // expected constant
1920 E0311
, // thing may not live long enough
1921 E0313
, // lifetime of borrowed pointer outlives lifetime of captured variable
1922 E0314
, // closure outlives stack frame
1923 E0315
, // cannot invoke closure outside of its lifetime
1924 E0316
, // nested quantification of lifetimes
1925 E0320
, // recursive overflow during dropck
1926 E0473
, // dereference of reference outside its lifetime
1927 E0474
, // captured variable `..` does not outlive the enclosing closure
1928 E0475
, // index of slice outside its lifetime
1929 E0476
, // lifetime of the source pointer does not outlive lifetime bound...
1930 E0477
, // the type `..` does not fulfill the required lifetime...
1931 E0479
, // the type `..` (provided as the value of a type parameter) is...
1932 E0480
, // lifetime of method receiver does not outlive the method call
1933 E0481
, // lifetime of function argument does not outlive the function call
1934 E0482
, // lifetime of return value does not outlive the function call
1935 E0483
, // lifetime of operand does not outlive the operation
1936 E0484
, // reference is not valid at the time of borrow
1937 E0485
, // automatically reference is not valid at the time of borrow
1938 E0486
, // type of expression contains references that are not valid during...
1939 E0487
, // unsafe use of destructor: destructor might be called while...
1940 E0488
, // lifetime of variable does not enclose its declaration
1941 E0489
, // type/lifetime parameter not in scope here
1942 E0490
, // a value of type `..` is borrowed for too long
1943 E0495
, // cannot infer an appropriate lifetime due to conflicting requirements
1944 E0566
, // conflicting representation hints
1945 E0587
, // conflicting packed and align representation hints