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 enum Bar { A(u8), B(&bool), } // error
366 enum Bar<'a> { A(u8), B(&'a bool), } // correct
368 type MyStr = &str; // error
369 type MyStr<'a> = &'a str; // correct
372 Lifetime elision is a special, limited kind of inference for lifetimes in
373 function signatures which allows you to leave out lifetimes in certain cases.
374 For more background on lifetime elision see [the book][book-le].
376 The lifetime elision rules require that any function signature with an elided
377 output lifetime must either have
379 - exactly one input lifetime
380 - or, multiple input lifetimes, but the function must also be a method with a
381 `&self` or `&mut self` receiver
383 In the first case, the output lifetime is inferred to be the same as the unique
384 input lifetime. In the second case, the lifetime is instead inferred to be the
385 same as the lifetime on `&self` or `&mut self`.
387 Here are some examples of elision errors:
389 ```compile_fail,E0106
390 // error, no input lifetimes
393 // error, `x` and `y` have distinct lifetimes inferred
394 fn bar(x: &str, y: &str) -> &str { }
396 // error, `y`'s lifetime is inferred to be distinct from `x`'s
397 fn baz<'a>(x: &'a str, y: &str) -> &str { }
400 Here's an example that is currently an error, but may work in a future version
403 ```compile_fail,E0106
404 struct Foo<'a>(&'a str);
407 impl Quux for Foo { }
410 Lifetime elision in implementation headers was part of the lifetime elision
411 RFC. It is, however, [currently unimplemented][iss15872].
413 [book-le]: https://doc.rust-lang.org/nightly/book/first-edition/lifetimes.html#lifetime-elision
414 [iss15872]: https://github.com/rust-lang/rust/issues/15872
418 There are conflicting trait implementations for the same type.
419 Example of erroneous code:
421 ```compile_fail,E0119
423 fn get(&self) -> usize;
426 impl<T> MyTrait for T {
427 fn get(&self) -> usize { 0 }
434 impl MyTrait for Foo { // error: conflicting implementations of trait
435 // `MyTrait` for type `Foo`
436 fn get(&self) -> usize { self.value }
440 When looking for the implementation for the trait, the compiler finds
441 both the `impl<T> MyTrait for T` where T is all types and the `impl
442 MyTrait for Foo`. Since a trait cannot be implemented multiple times,
443 this is an error. So, when you write:
447 fn get(&self) -> usize;
450 impl<T> MyTrait for T {
451 fn get(&self) -> usize { 0 }
455 This makes the trait implemented on all types in the scope. So if you
456 try to implement it on another one after that, the implementations will
461 fn get(&self) -> usize;
464 impl<T> MyTrait for T {
465 fn get(&self) -> usize { 0 }
473 f.get(); // the trait is implemented so we can use it
479 Unsafe code was used outside of an unsafe function or block.
481 Erroneous code example:
483 ```compile_fail,E0133
484 unsafe fn f() { return; } // This is the unsafe code
487 f(); // error: call to unsafe function requires unsafe function or block
491 Using unsafe functionality is potentially dangerous and disallowed by safety
494 * Dereferencing raw pointers
495 * Calling functions via FFI
496 * Calling functions marked unsafe
498 These safety checks can be relaxed for a section of the code by wrapping the
499 unsafe instructions with an `unsafe` block. For instance:
502 unsafe fn f() { return; }
505 unsafe { f(); } // ok!
509 See also https://doc.rust-lang.org/book/first-edition/unsafe.html
512 // This shouldn't really ever trigger since the repeated value error comes first
514 A binary can only have one entry point, and by default that entry point is the
515 function `main()`. If there are multiple such functions, please rename one.
519 More than one function was declared with the `#[main]` attribute.
521 Erroneous code example:
523 ```compile_fail,E0137
530 fn f() {} // error: multiple functions with a #[main] attribute
533 This error indicates that the compiler found multiple functions with the
534 `#[main]` attribute. This is an error because there must be a unique entry
535 point into a Rust program. Example:
546 More than one function was declared with the `#[start]` attribute.
548 Erroneous code example:
550 ```compile_fail,E0138
554 fn foo(argc: isize, argv: *const *const u8) -> isize {}
557 fn f(argc: isize, argv: *const *const u8) -> isize {}
558 // error: multiple 'start' functions
561 This error indicates that the compiler found multiple functions with the
562 `#[start]` attribute. This is an error because there must be a unique entry
563 point into a Rust program. Example:
569 fn foo(argc: isize, argv: *const *const u8) -> isize { 0 } // ok!
574 #### Note: this error code is no longer emitted by the compiler.
576 There are various restrictions on transmuting between types in Rust; for example
577 types being transmuted must have the same size. To apply all these restrictions,
578 the compiler must know the exact types that may be transmuted. When type
579 parameters are involved, this cannot always be done.
581 So, for example, the following is not allowed:
584 use std::mem::transmute;
586 struct Foo<T>(Vec<T>);
588 fn foo<T>(x: Vec<T>) {
589 // we are transmuting between Vec<T> and Foo<F> here
590 let y: Foo<T> = unsafe { transmute(x) };
591 // do something with y
595 In this specific case there's a good chance that the transmute is harmless (but
596 this is not guaranteed by Rust). However, when alignment and enum optimizations
597 come into the picture, it's quite likely that the sizes may or may not match
598 with different type parameter substitutions. It's not possible to check this for
599 _all_ possible types, so `transmute()` simply only accepts types without any
600 unsubstituted type parameters.
602 If you need this, there's a good chance you're doing something wrong. Keep in
603 mind that Rust doesn't guarantee much about the layout of different structs
604 (even two structs with identical declarations may have different layouts). If
605 there is a solution that avoids the transmute entirely, try it instead.
607 If it's possible, hand-monomorphize the code by writing the function for each
608 possible type substitution. It's possible to use traits to do this cleanly,
612 use std::mem::transmute;
614 struct Foo<T>(Vec<T>);
616 trait MyTransmutableType: Sized {
617 fn transmute(_: Vec<Self>) -> Foo<Self>;
620 impl MyTransmutableType for u8 {
621 fn transmute(x: Vec<u8>) -> Foo<u8> {
622 unsafe { transmute(x) }
626 impl MyTransmutableType for String {
627 fn transmute(x: Vec<String>) -> Foo<String> {
628 unsafe { transmute(x) }
632 // ... more impls for the types you intend to transmute
634 fn foo<T: MyTransmutableType>(x: Vec<T>) {
635 let y: Foo<T> = <T as MyTransmutableType>::transmute(x);
636 // do something with y
640 Each impl will be checked for a size match in the transmute as usual, and since
641 there are no unbound type parameters involved, this should compile unless there
642 is a size mismatch in one of the impls.
644 It is also possible to manually transmute:
648 # let v = Some("value");
649 # type SomeType = &'static [u8];
651 ptr::read(&v as *const _ as *const SomeType) // `v` transmuted to `SomeType`
656 Note that this does not move `v` (unlike `transmute`), and may need a
657 call to `mem::forget(v)` in case you want to avoid destructors being called.
661 A lang item was redefined.
663 Erroneous code example:
665 ```compile_fail,E0152
666 #![feature(lang_items)]
668 #[lang = "panic_fmt"]
669 struct Foo; // error: duplicate lang item found: `panic_fmt`
672 Lang items are already implemented in the standard library. Unless you are
673 writing a free-standing application (e.g. a kernel), you do not need to provide
676 You can build a free-standing crate by adding `#![no_std]` to the crate
679 ```ignore (only-for-syntax-highlight)
683 See also https://doc.rust-lang.org/book/first-edition/no-stdlib.html
687 When using a lifetime like `'a` in a type, it must be declared before being
690 These two examples illustrate the problem:
692 ```compile_fail,E0261
693 // error, use of undeclared lifetime name `'a`
694 fn foo(x: &'a str) { }
697 // error, use of undeclared lifetime name `'a`
702 These can be fixed by declaring lifetime parameters:
705 fn foo<'a>(x: &'a str) {}
714 Declaring certain lifetime names in parameters is disallowed. For example,
715 because the `'static` lifetime is a special built-in lifetime name denoting
716 the lifetime of the entire program, this is an error:
718 ```compile_fail,E0262
719 // error, invalid lifetime parameter name `'static`
720 fn foo<'static>(x: &'static str) { }
725 A lifetime name cannot be declared more than once in the same scope. For
728 ```compile_fail,E0263
729 // error, lifetime name `'a` declared twice in the same scope
730 fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str) { }
735 An unknown external lang item was used. Erroneous code example:
737 ```compile_fail,E0264
738 #![feature(lang_items)]
741 #[lang = "cake"] // error: unknown external lang item: `cake`
746 A list of available external lang items is available in
747 `src/librustc/middle/weak_lang_items.rs`. Example:
750 #![feature(lang_items)]
753 #[lang = "panic_fmt"] // ok!
760 This is because of a type mismatch between the associated type of some
761 trait (e.g. `T::Bar`, where `T` implements `trait Quux { type Bar; }`)
762 and another type `U` that is required to be equal to `T::Bar`, but is not.
765 Here is a basic example:
767 ```compile_fail,E0271
768 trait Trait { type AssociatedType; }
770 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
774 impl Trait for i8 { type AssociatedType = &'static str; }
779 Here is that same example again, with some explanatory comments:
781 ```compile_fail,E0271
782 trait Trait { type AssociatedType; }
784 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
785 // ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
787 // This says `foo` can |
788 // only be used with |
790 // implements `Trait`. |
792 // This says not only must
793 // `T` be an impl of `Trait`
794 // but also that the impl
795 // must assign the type `u32`
796 // to the associated type.
800 impl Trait for i8 { type AssociatedType = &'static str; }
801 //~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
806 // ... but it is an implementation
807 // that assigns `&'static str` to
808 // the associated type.
811 // Here, we invoke `foo` with an `i8`, which does not satisfy
812 // the constraint `<i8 as Trait>::AssociatedType=u32`, and
813 // therefore the type-checker complains with this error code.
816 Here is a more subtle instance of the same problem, that can
817 arise with for-loops in Rust:
820 let vs: Vec<i32> = vec![1, 2, 3, 4];
829 The above fails because of an analogous type mismatch,
830 though may be harder to see. Again, here are some
831 explanatory comments for the same example:
835 let vs = vec![1, 2, 3, 4];
837 // `for`-loops use a protocol based on the `Iterator`
838 // trait. Each item yielded in a `for` loop has the
839 // type `Iterator::Item` -- that is, `Item` is the
840 // associated type of the concrete iterator impl.
844 // | We borrow `vs`, iterating over a sequence of
845 // | *references* of type `&Elem` (where `Elem` is
846 // | vector's element type). Thus, the associated
847 // | type `Item` must be a reference `&`-type ...
849 // ... and `v` has the type `Iterator::Item`, as dictated by
850 // the `for`-loop protocol ...
856 // ... but *here*, `v` is forced to have some integral type;
857 // only types like `u8`,`i8`,`u16`,`i16`, et cetera can
858 // match the pattern `1` ...
863 // ... therefore, the compiler complains, because it sees
864 // an attempt to solve the equations
865 // `some integral-type` = type-of-`v`
866 // = `Iterator::Item`
867 // = `&Elem` (i.e. `some reference type`)
869 // which cannot possibly all be true.
875 To avoid those issues, you have to make the types match correctly.
876 So we can fix the previous examples like this:
880 trait Trait { type AssociatedType; }
882 fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
886 impl Trait for i8 { type AssociatedType = &'static str; }
891 let vs = vec![1, 2, 3, 4];
902 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
903 message for when a particular trait isn't implemented on a type placed in a
904 position that needs that trait. For example, when the following code is
908 #![feature(on_unimplemented)]
910 fn foo<T: Index<u8>>(x: T){}
912 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
913 trait Index<Idx> { /* ... */ }
915 foo(true); // `bool` does not implement `Index<u8>`
918 There will be an error about `bool` not implementing `Index<u8>`, followed by a
919 note saying "the type `bool` cannot be indexed by `u8`".
921 As you can see, you can specify type parameters in curly braces for
922 substitution with the actual types (using the regular format string syntax) in
923 a given situation. Furthermore, `{Self}` will substitute to the type (in this
924 case, `bool`) that we tried to use.
926 This error appears when the curly braces contain an identifier which doesn't
927 match with any of the type parameters or the string `Self`. This might happen
928 if you misspelled a type parameter, or if you intended to use literal curly
929 braces. If it is the latter, escape the curly braces with a second curly brace
930 of the same type; e.g. a literal `{` is `{{`.
934 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
935 message for when a particular trait isn't implemented on a type placed in a
936 position that needs that trait. For example, when the following code is
940 #![feature(on_unimplemented)]
942 fn foo<T: Index<u8>>(x: T){}
944 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
945 trait Index<Idx> { /* ... */ }
947 foo(true); // `bool` does not implement `Index<u8>`
950 there will be an error about `bool` not implementing `Index<u8>`, followed by a
951 note saying "the type `bool` cannot be indexed by `u8`".
953 As you can see, you can specify type parameters in curly braces for
954 substitution with the actual types (using the regular format string syntax) in
955 a given situation. Furthermore, `{Self}` will substitute to the type (in this
956 case, `bool`) that we tried to use.
958 This error appears when the curly braces do not contain an identifier. Please
959 add one of the same name as a type parameter. If you intended to use literal
960 braces, use `{{` and `}}` to escape them.
964 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
965 message for when a particular trait isn't implemented on a type placed in a
966 position that needs that trait. For example, when the following code is
970 #![feature(on_unimplemented)]
972 fn foo<T: Index<u8>>(x: T){}
974 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
975 trait Index<Idx> { /* ... */ }
977 foo(true); // `bool` does not implement `Index<u8>`
980 there will be an error about `bool` not implementing `Index<u8>`, followed by a
981 note saying "the type `bool` cannot be indexed by `u8`".
983 For this to work, some note must be specified. An empty attribute will not do
984 anything, please remove the attribute or add some helpful note for users of the
989 This error occurs when there was a recursive trait requirement that overflowed
990 before it could be evaluated. Often this means that there is unbounded
991 recursion in resolving some type bounds.
993 For example, in the following code:
995 ```compile_fail,E0275
1000 impl<T> Foo for T where Bar<T>: Foo {}
1003 To determine if a `T` is `Foo`, we need to check if `Bar<T>` is `Foo`. However,
1004 to do this check, we need to determine that `Bar<Bar<T>>` is `Foo`. To
1005 determine this, we check if `Bar<Bar<Bar<T>>>` is `Foo`, and so on. This is
1006 clearly a recursive requirement that can't be resolved directly.
1008 Consider changing your trait bounds so that they're less self-referential.
1012 This error occurs when a bound in an implementation of a trait does not match
1013 the bounds specified in the original trait. For example:
1015 ```compile_fail,E0276
1021 fn foo<T>(x: T) where T: Copy {}
1025 Here, all types implementing `Foo` must have a method `foo<T>(x: T)` which can
1026 take any type `T`. However, in the `impl` for `bool`, we have added an extra
1027 bound that `T` is `Copy`, which isn't compatible with the original trait.
1029 Consider removing the bound from the method or adding the bound to the original
1030 method definition in the trait.
1034 You tried to use a type which doesn't implement some trait in a place which
1035 expected that trait. Erroneous code example:
1037 ```compile_fail,E0277
1038 // here we declare the Foo trait with a bar method
1043 // we now declare a function which takes an object implementing the Foo trait
1044 fn some_func<T: Foo>(foo: T) {
1049 // we now call the method with the i32 type, which doesn't implement
1051 some_func(5i32); // error: the trait bound `i32 : Foo` is not satisfied
1055 In order to fix this error, verify that the type you're using does implement
1063 fn some_func<T: Foo>(foo: T) {
1064 foo.bar(); // we can now use this method since i32 implements the
1068 // we implement the trait on the i32 type
1074 some_func(5i32); // ok!
1078 Or in a generic context, an erroneous code example would look like:
1080 ```compile_fail,E0277
1081 fn some_func<T>(foo: T) {
1082 println!("{:?}", foo); // error: the trait `core::fmt::Debug` is not
1083 // implemented for the type `T`
1087 // We now call the method with the i32 type,
1088 // which *does* implement the Debug trait.
1093 Note that the error here is in the definition of the generic function: Although
1094 we only call it with a parameter that does implement `Debug`, the compiler
1095 still rejects the function: It must work with all possible input types. In
1096 order to make this example compile, we need to restrict the generic type we're
1102 // Restrict the input type to types that implement Debug.
1103 fn some_func<T: fmt::Debug>(foo: T) {
1104 println!("{:?}", foo);
1108 // Calling the method is still fine, as i32 implements Debug.
1111 // This would fail to compile now:
1112 // struct WithoutDebug;
1113 // some_func(WithoutDebug);
1117 Rust only looks at the signature of the called function, as such it must
1118 already specify all requirements that will be used for every type parameter.
1122 You tried to supply a type which doesn't implement some trait in a location
1123 which expected that trait. This error typically occurs when working with
1124 `Fn`-based types. Erroneous code example:
1126 ```compile_fail,E0281
1127 fn foo<F: Fn(usize)>(x: F) { }
1130 // type mismatch: ... implements the trait `core::ops::Fn<(String,)>`,
1131 // but the trait `core::ops::Fn<(usize,)>` is required
1133 foo(|y: String| { });
1137 The issue in this case is that `foo` is defined as accepting a `Fn` with one
1138 argument of type `String`, but the closure we attempted to pass to it requires
1139 one arguments of type `usize`.
1143 This error indicates that type inference did not result in one unique possible
1144 type, and extra information is required. In most cases this can be provided
1145 by adding a type annotation. Sometimes you need to specify a generic type
1148 A common example is the `collect` method on `Iterator`. It has a generic type
1149 parameter with a `FromIterator` bound, which for a `char` iterator is
1150 implemented by `Vec` and `String` among others. Consider the following snippet
1151 that reverses the characters of a string:
1153 ```compile_fail,E0282
1154 let x = "hello".chars().rev().collect();
1157 In this case, the compiler cannot infer what the type of `x` should be:
1158 `Vec<char>` and `String` are both suitable candidates. To specify which type to
1159 use, you can use a type annotation on `x`:
1162 let x: Vec<char> = "hello".chars().rev().collect();
1165 It is not necessary to annotate the full type. Once the ambiguity is resolved,
1166 the compiler can infer the rest:
1169 let x: Vec<_> = "hello".chars().rev().collect();
1172 Another way to provide the compiler with enough information, is to specify the
1173 generic type parameter:
1176 let x = "hello".chars().rev().collect::<Vec<char>>();
1179 Again, you need not specify the full type if the compiler can infer it:
1182 let x = "hello".chars().rev().collect::<Vec<_>>();
1185 Apart from a method or function with a generic type parameter, this error can
1186 occur when a type parameter of a struct or trait cannot be inferred. In that
1187 case it is not always possible to use a type annotation, because all candidates
1188 have the same return type. For instance:
1190 ```compile_fail,E0282
1201 let number = Foo::bar();
1206 This will fail because the compiler does not know which instance of `Foo` to
1207 call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.
1211 This error occurs when the compiler doesn't have enough information
1212 to unambiguously choose an implementation.
1216 ```compile_fail,E0283
1223 impl Generator for Impl {
1224 fn create() -> u32 { 1 }
1229 impl Generator for AnotherImpl {
1230 fn create() -> u32 { 2 }
1234 let cont: u32 = Generator::create();
1235 // error, impossible to choose one of Generator trait implementation
1236 // Impl or AnotherImpl? Maybe anything else?
1240 To resolve this error use the concrete type:
1249 impl Generator for AnotherImpl {
1250 fn create() -> u32 { 2 }
1254 let gen1 = AnotherImpl::create();
1256 // if there are multiple methods with same name (different traits)
1257 let gen2 = <AnotherImpl as Generator>::create();
1263 This error indicates that the given recursion limit could not be parsed. Ensure
1264 that the value provided is a positive integer between quotes.
1266 Erroneous code example:
1268 ```compile_fail,E0296
1274 And a working example:
1277 #![recursion_limit="1000"]
1284 This error occurs when the compiler was unable to infer the concrete type of a
1285 variable. It can occur for several cases, the most common of which is a
1286 mismatch in the expected type that the compiler inferred for a variable's
1287 initializing expression, and the actual type explicitly assigned to the
1292 ```compile_fail,E0308
1293 let x: i32 = "I am not a number!";
1294 // ~~~ ~~~~~~~~~~~~~~~~~~~~
1296 // | initializing expression;
1297 // | compiler infers type `&str`
1299 // type `i32` assigned to variable `x`
1304 Types in type definitions have lifetimes associated with them that represent
1305 how long the data stored within them is guaranteed to be live. This lifetime
1306 must be as long as the data needs to be alive, and missing the constraint that
1307 denotes this will cause this error.
1309 ```compile_fail,E0309
1310 // This won't compile because T is not constrained, meaning the data
1311 // stored in it is not guaranteed to last as long as the reference
1317 This will compile, because it has the constraint on the type parameter:
1320 struct Foo<'a, T: 'a> {
1325 To see why this is important, consider the case where `T` is itself a reference
1326 (e.g., `T = &str`). If we don't include the restriction that `T: 'a`, the
1327 following code would be perfectly legal:
1329 ```compile_fail,E0309
1335 let v = "42".to_string();
1336 let f = Foo{foo: &v};
1338 println!("{}", f.foo); // but we've already dropped v!
1344 Types in type definitions have lifetimes associated with them that represent
1345 how long the data stored within them is guaranteed to be live. This lifetime
1346 must be as long as the data needs to be alive, and missing the constraint that
1347 denotes this will cause this error.
1349 ```compile_fail,E0310
1350 // This won't compile because T is not constrained to the static lifetime
1351 // the reference needs
1357 This will compile, because it has the constraint on the type parameter:
1360 struct Foo<T: 'static> {
1367 A lifetime of reference outlives lifetime of borrowed content.
1369 Erroneous code example:
1371 ```compile_fail,E0312
1372 fn make_child<'human, 'elve>(x: &mut &'human isize, y: &mut &'elve isize) {
1374 // error: lifetime of reference outlives lifetime of borrowed content
1378 The compiler cannot determine if the `human` lifetime will live long enough
1379 to keep up on the elve one. To solve this error, you have to give an
1380 explicit lifetime hierarchy:
1383 fn make_child<'human, 'elve: 'human>(x: &mut &'human isize,
1384 y: &mut &'elve isize) {
1389 Or use the same lifetime for every variable:
1392 fn make_child<'elve>(x: &mut &'elve isize, y: &mut &'elve isize) {
1399 This error occurs when an `if` expression without an `else` block is used in a
1400 context where a type other than `()` is expected, for example a `let`
1403 ```compile_fail,E0317
1406 let a = if x == 5 { 1 };
1410 An `if` expression without an `else` block has the type `()`, so this is a type
1411 error. To resolve it, add an `else` block having the same type as the `if`
1416 This error indicates that some types or traits depend on each other
1417 and therefore cannot be constructed.
1419 The following example contains a circular dependency between two traits:
1421 ```compile_fail,E0391
1422 trait FirstTrait : SecondTrait {
1426 trait SecondTrait : FirstTrait {
1433 #### Note: this error code is no longer emitted by the compiler.
1435 In Rust 1.3, the default object lifetime bounds are expected to change, as
1436 described in [RFC 1156]. You are getting a warning because the compiler
1437 thinks it is possible that this change will cause a compilation error in your
1438 code. It is possible, though unlikely, that this is a false alarm.
1440 The heart of the change is that where `&'a Box<SomeTrait>` used to default to
1441 `&'a Box<SomeTrait+'a>`, it now defaults to `&'a Box<SomeTrait+'static>` (here,
1442 `SomeTrait` is the name of some trait type). Note that the only types which are
1443 affected are references to boxes, like `&Box<SomeTrait>` or
1444 `&[Box<SomeTrait>]`. More common types like `&SomeTrait` or `Box<SomeTrait>`
1447 To silence this warning, edit your code to use an explicit bound. Most of the
1448 time, this means that you will want to change the signature of a function that
1449 you are calling. For example, if the error is reported on a call like `foo(x)`,
1450 and `foo` is defined as follows:
1453 # trait SomeTrait {}
1454 fn foo(arg: &Box<SomeTrait>) { /* ... */ }
1457 You might change it to:
1460 # trait SomeTrait {}
1461 fn foo<'a>(arg: &'a Box<SomeTrait+'a>) { /* ... */ }
1464 This explicitly states that you expect the trait object `SomeTrait` to contain
1465 references (with a maximum lifetime of `'a`).
1467 [RFC 1156]: https://github.com/rust-lang/rfcs/blob/master/text/1156-adjust-default-object-bounds.md
1471 An invalid lint attribute has been given. Erroneous code example:
1473 ```compile_fail,E0452
1474 #![allow(foo = "")] // error: malformed lint attribute
1477 Lint attributes only accept a list of identifiers (where each identifier is a
1478 lint name). Ensure the attribute is of this form:
1481 #![allow(foo)] // ok!
1483 #![allow(foo, foo2)] // ok!
1488 A lint check attribute was overruled by a `forbid` directive set as an
1489 attribute on an enclosing scope, or on the command line with the `-F` option.
1491 Example of erroneous code:
1493 ```compile_fail,E0453
1494 #![forbid(non_snake_case)]
1496 #[allow(non_snake_case)]
1498 let MyNumber = 2; // error: allow(non_snake_case) overruled by outer
1499 // forbid(non_snake_case)
1503 The `forbid` lint setting, like `deny`, turns the corresponding compiler
1504 warning into a hard error. Unlike `deny`, `forbid` prevents itself from being
1505 overridden by inner attributes.
1507 If you're sure you want to override the lint check, you can change `forbid` to
1508 `deny` (or use `-D` instead of `-F` if the `forbid` setting was given as a
1509 command-line option) to allow the inner lint check attribute:
1512 #![deny(non_snake_case)]
1514 #[allow(non_snake_case)]
1516 let MyNumber = 2; // ok!
1520 Otherwise, edit the code to pass the lint check, and remove the overruled
1524 #![forbid(non_snake_case)]
1533 A lifetime bound was not satisfied.
1535 Erroneous code example:
1537 ```compile_fail,E0478
1538 // Check that the explicit lifetime bound (`'SnowWhite`, in this example) must
1539 // outlive all the superbounds from the trait (`'kiss`, in this example).
1541 trait Wedding<'t>: 't { }
1543 struct Prince<'kiss, 'SnowWhite> {
1544 child: Box<Wedding<'kiss> + 'SnowWhite>,
1545 // error: lifetime bound not satisfied
1549 In this example, the `'SnowWhite` lifetime is supposed to outlive the `'kiss`
1550 lifetime but the declaration of the `Prince` struct doesn't enforce it. To fix
1551 this issue, you need to specify it:
1554 trait Wedding<'t>: 't { }
1556 struct Prince<'kiss, 'SnowWhite: 'kiss> { // You say here that 'kiss must live
1557 // longer than 'SnowWhite.
1558 child: Box<Wedding<'kiss> + 'SnowWhite>, // And now it's all good!
1564 A reference has a longer lifetime than the data it references.
1566 Erroneous code example:
1568 ```compile_fail,E0491
1569 // struct containing a reference requires a lifetime parameter,
1570 // because the data the reference points to must outlive the struct (see E0106)
1575 // However, a nested struct like this, the signature itself does not tell
1576 // whether 'a outlives 'b or the other way around.
1577 // So it could be possible that 'b of reference outlives 'a of the data.
1578 struct Nested<'a, 'b> {
1579 ref_struct: &'b Struct<'a>, // compile error E0491
1583 To fix this issue, you can specify a bound to the lifetime like below:
1590 // 'a: 'b means 'a outlives 'b
1591 struct Nested<'a: 'b, 'b> {
1592 ref_struct: &'b Struct<'a>,
1598 A lifetime name is shadowing another lifetime name. Erroneous code example:
1600 ```compile_fail,E0496
1606 fn f<'a>(x: &'a i32) { // error: lifetime name `'a` shadows a lifetime
1607 // name that is already in scope
1612 Please change the name of one of the lifetimes to remove this error. Example:
1620 fn f<'b>(x: &'b i32) { // ok!
1630 A stability attribute was used outside of the standard library. Erroneous code
1634 #[stable] // error: stability attributes may not be used outside of the
1639 It is not possible to use stability attributes outside of the standard library.
1640 Also, for now, it is not possible to write deprecation messages either.
1644 Transmute with two differently sized types was attempted. Erroneous code
1647 ```compile_fail,E0512
1648 fn takes_u8(_: u8) {}
1651 unsafe { takes_u8(::std::mem::transmute(0u16)); }
1652 // error: transmute called with types of different sizes
1656 Please use types with same size or use the expected type directly. Example:
1659 fn takes_u8(_: u8) {}
1662 unsafe { takes_u8(::std::mem::transmute(0i8)); } // ok!
1664 unsafe { takes_u8(0u8); } // ok!
1670 This error indicates that a `#[repr(..)]` attribute was placed on an
1673 Examples of erroneous code:
1675 ```compile_fail,E0517
1683 struct Foo {bar: bool, baz: bool}
1691 * The `#[repr(C)]` attribute can only be placed on structs and enums.
1692 * The `#[repr(packed)]` and `#[repr(simd)]` attributes only work on structs.
1693 * The `#[repr(u8)]`, `#[repr(i16)]`, etc attributes only work on enums.
1695 These attributes do not work on typedefs, since typedefs are just aliases.
1697 Representations like `#[repr(u8)]`, `#[repr(i64)]` are for selecting the
1698 discriminant size for C-like enums (when there is no associated data, e.g.
1699 `enum Color {Red, Blue, Green}`), effectively setting the size of the enum to
1700 the size of the provided type. Such an enum can be cast to a value of the same
1701 type as well. In short, `#[repr(u8)]` makes the enum behave like an integer
1702 with a constrained set of allowed values.
1704 Only C-like enums can be cast to numerical primitives, so this attribute will
1705 not apply to structs.
1707 `#[repr(packed)]` reduces padding to make the struct size smaller. The
1708 representation of enums isn't strictly defined in Rust, and this attribute
1709 won't work on enums.
1711 `#[repr(simd)]` will give a struct consisting of a homogeneous series of machine
1712 types (i.e. `u8`, `i32`, etc) a representation that permits vectorization via
1713 SIMD. This doesn't make much sense for enums since they don't consist of a
1714 single list of data.
1718 This error indicates that an `#[inline(..)]` attribute was incorrectly placed
1719 on something other than a function or method.
1721 Examples of erroneous code:
1723 ```compile_fail,E0518
1733 `#[inline]` hints the compiler whether or not to attempt to inline a method or
1734 function. By default, the compiler does a pretty good job of figuring this out
1735 itself, but if you feel the need for annotations, `#[inline(always)]` and
1736 `#[inline(never)]` can override or force the compiler's decision.
1738 If you wish to apply this attribute to all methods in an impl, manually annotate
1739 each method; it is not possible to annotate the entire impl with an `#[inline]`
1744 The lang attribute is intended for marking special items that are built-in to
1745 Rust itself. This includes special traits (like `Copy` and `Sized`) that affect
1746 how the compiler behaves, as well as special functions that may be automatically
1747 invoked (such as the handler for out-of-bounds accesses when indexing a slice).
1748 Erroneous code example:
1750 ```compile_fail,E0522
1751 #![feature(lang_items)]
1754 fn cookie() -> ! { // error: definition of an unknown language item: `cookie`
1761 A closure was used but didn't implement the expected trait.
1763 Erroneous code example:
1765 ```compile_fail,E0525
1769 fn bar<T: Fn(u32)>(_: T) {}
1773 let closure = |_| foo(x); // error: expected a closure that implements
1774 // the `Fn` trait, but this closure only
1775 // implements `FnOnce`
1780 In the example above, `closure` is an `FnOnce` closure whereas the `bar`
1781 function expected an `Fn` closure. In this case, it's simple to fix the issue,
1782 you just have to implement `Copy` and `Clone` traits on `struct X` and it'll
1786 #[derive(Clone, Copy)] // We implement `Clone` and `Copy` traits.
1790 fn bar<T: Fn(u32)>(_: T) {}
1794 let closure = |_| foo(x);
1795 bar(closure); // ok!
1799 To understand better how closures work in Rust, read:
1800 https://doc.rust-lang.org/book/first-edition/closures.html
1804 The `main` function was incorrectly declared.
1806 Erroneous code example:
1808 ```compile_fail,E0580
1809 fn main() -> i32 { // error: main function has wrong type
1814 The `main` function prototype should never take arguments or return type.
1823 If you want to get command-line arguments, use `std::env::args`. To exit with a
1824 specified exit code, use `std::process::exit`.
1828 Per [RFC 401][rfc401], if you have a function declaration `foo`:
1831 // For the purposes of this explanation, all of these
1832 // different kinds of `fn` declarations are equivalent:
1834 fn foo(x: S) { /* ... */ }
1835 # #[cfg(for_demonstration_only)]
1836 extern "C" { fn foo(x: S); }
1837 # #[cfg(for_demonstration_only)]
1838 impl S { fn foo(self) { /* ... */ } }
1841 the type of `foo` is **not** `fn(S)`, as one might expect.
1842 Rather, it is a unique, zero-sized marker type written here as `typeof(foo)`.
1843 However, `typeof(foo)` can be _coerced_ to a function pointer `fn(S)`,
1844 so you rarely notice this:
1849 let x: fn(S) = foo; // OK, coerces
1852 The reason that this matter is that the type `fn(S)` is not specific to
1853 any particular function: it's a function _pointer_. So calling `x()` results
1854 in a virtual call, whereas `foo()` is statically dispatched, because the type
1855 of `foo` tells us precisely what function is being called.
1857 As noted above, coercions mean that most code doesn't have to be
1858 concerned with this distinction. However, you can tell the difference
1859 when using **transmute** to convert a fn item into a fn pointer.
1861 This is sometimes done as part of an FFI:
1863 ```compile_fail,E0591
1864 extern "C" fn foo(userdata: Box<i32>) {
1868 # fn callback(_: extern "C" fn(*mut i32)) {}
1869 # use std::mem::transmute;
1871 let f: extern "C" fn(*mut i32) = transmute(foo);
1876 Here, transmute is being used to convert the types of the fn arguments.
1877 This pattern is incorrect because, because the type of `foo` is a function
1878 **item** (`typeof(foo)`), which is zero-sized, and the target type (`fn()`)
1879 is a function pointer, which is not zero-sized.
1880 This pattern should be rewritten. There are a few possible ways to do this:
1882 - change the original fn declaration to match the expected signature,
1883 and do the cast in the fn body (the prefered option)
1884 - cast the fn item fo a fn pointer before calling transmute, as shown here:
1887 # extern "C" fn foo(_: Box<i32>) {}
1888 # use std::mem::transmute;
1890 let f: extern "C" fn(*mut i32) = transmute(foo as extern "C" fn(_));
1891 let f: extern "C" fn(*mut i32) = transmute(foo as usize); // works too
1895 The same applies to transmutes to `*mut fn()`, which were observedin practice.
1896 Note though that use of this type is generally incorrect.
1897 The intention is typically to describe a function pointer, but just `fn()`
1898 alone suffices for that. `*mut fn()` is a pointer to a fn pointer.
1899 (Since these values are typically just passed to C code, however, this rarely
1900 makes a difference in practice.)
1902 [rfc401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md
1906 You tried to supply an `Fn`-based type with an incorrect number of arguments
1907 than what was expected.
1909 Erroneous code example:
1911 ```compile_fail,E0593
1912 fn foo<F: Fn()>(x: F) { }
1915 // [E0593] closure takes 1 argument but 0 arguments are required
1922 No `main` function was found in a binary crate. To fix this error, just add a
1923 `main` function. For example:
1927 // Your program will start here.
1928 println!("Hello world!");
1932 If you don't know the basics of Rust, you can go look to the Rust Book to get
1933 started: https://doc.rust-lang.org/book/
1937 An unknown lint was used on the command line.
1942 rustc -D bogus omse_file.rs
1945 Maybe you just misspelled the lint name or the lint doesn't exist anymore.
1946 Either way, try to update/remove it in order to fix the error.
1950 This error code indicates a mismatch between the lifetimes appearing in the
1951 function signature (i.e., the parameter types and the return type) and the
1952 data-flow found in the function body.
1954 Erroneous code example:
1956 ```compile_fail,E0621
1957 fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 { // error: explicit lifetime
1958 // required in the type of
1960 if x > y { x } else { y }
1964 In the code above, the function is returning data borrowed from either `x` or
1965 `y`, but the `'a` annotation indicates that it is returning data only from `x`.
1966 To fix the error, the signature and the body must be made to match. Typically,
1967 this is done by updating the function signature. So, in this case, we change
1968 the type of `y` to `&'a i32`, like so:
1971 fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {
1972 if x > y { x } else { y }
1976 Now the signature indicates that the function data borrowed from either `x` or
1977 `y`. Alternatively, you could change the body to not return data from `y`:
1980 fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 {
1989 register_diagnostics! {
1990 // E0006 // merged with E0005
1991 // E0101, // replaced with E0282
1992 // E0102, // replaced with E0282
1995 E0278, // requirement is not satisfied
1996 E0279, // requirement is not satisfied
1997 E0280, // requirement is not satisfied
1998 E0284, // cannot resolve type
1999 // E0285, // overflow evaluation builtin bounds
2000 // E0300, // unexpanded macro
2001 // E0304, // expected signed integer constant
2002 // E0305, // expected constant
2003 E0311, // thing may not live long enough
2004 E0313, // lifetime of borrowed pointer outlives lifetime of captured variable
2005 E0314, // closure outlives stack frame
2006 E0315, // cannot invoke closure outside of its lifetime
2007 E0316, // nested quantification of lifetimes
2008 E0320, // recursive overflow during dropck
2009 E0473, // dereference of reference outside its lifetime
2010 E0474, // captured variable `..` does not outlive the enclosing closure
2011 E0475, // index of slice outside its lifetime
2012 E0476, // lifetime of the source pointer does not outlive lifetime bound...
2013 E0477, // the type `..` does not fulfill the required lifetime...
2014 E0479, // the type `..` (provided as the value of a type parameter) is...
2015 E0480, // lifetime of method receiver does not outlive the method call
2016 E0481, // lifetime of function argument does not outlive the function call
2017 E0482, // lifetime of return value does not outlive the function call
2018 E0483, // lifetime of operand does not outlive the operation
2019 E0484, // reference is not valid at the time of borrow
2020 E0485, // automatically reference is not valid at the time of borrow
2021 E0486, // type of expression contains references that are not valid during...
2022 E0487, // unsafe use of destructor: destructor might be called while...
2023 E0488, // lifetime of variable does not enclose its declaration
2024 E0489, // type/lifetime parameter not in scope here
2025 E0490, // a value of type `..` is borrowed for too long
2026 E0495, // cannot infer an appropriate lifetime due to conflicting requirements
2027 E0566, // conflicting representation hints