1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 #![allow(non_snake_case)]
13 // Error messages for EXXXX errors.
14 // Each message should start and end with a new line, and be wrapped to 80 characters.
15 // In vim you can `:set tw=80` and use `gq` to wrap paragraphs. Use `:set tw=0` to disable.
16 register_long_diagnostics! {
18 This error indicates that an attempt was made to divide by zero (or take the
19 remainder of a zero divisor) in a static or constant expression. Erroneous
25 const X: i32 = 42 / 0;
26 // error: attempt to divide by zero in a constant expression
31 Trait objects like `Box<Trait>` can only be constructed when certain
32 requirements are satisfied by the trait in question.
34 Trait objects are a form of dynamic dispatch and use a dynamically sized type
35 for the inner type. So, for a given trait `Trait`, when `Trait` is treated as a
36 type, as in `Box<Trait>`, the inner type is 'unsized'. In such cases the boxed
37 pointer is a 'fat pointer' that contains an extra pointer to a table of methods
38 (among other things) for dynamic dispatch. This design mandates some
39 restrictions on the types of traits that are allowed to be used in trait
40 objects, which are collectively termed as 'object safety' rules.
42 Attempting to create a trait object for a non object-safe trait will trigger
45 There are various rules:
47 ### The trait cannot require `Self: Sized`
49 When `Trait` is treated as a type, the type does not implement the special
50 `Sized` trait, because the type does not have a known size at compile time and
51 can only be accessed behind a pointer. Thus, if we have a trait like the
55 trait Foo where Self: Sized {
60 We cannot create an object of type `Box<Foo>` or `&Foo` since in this case
61 `Self` would not be `Sized`.
63 Generally, `Self : Sized` is used to indicate that the trait should not be used
64 as a trait object. If the trait comes from your own crate, consider removing
67 ### Method references the `Self` type in its arguments or return type
69 This happens when a trait has a method like the following:
73 fn foo(&self) -> Self;
76 impl Trait for String {
77 fn foo(&self) -> Self {
83 fn foo(&self) -> Self {
89 (Note that `&self` and `&mut self` are okay, it's additional `Self` types which
92 In such a case, the compiler cannot predict the return type of `foo()` in a
93 situation like the following:
97 fn foo(&self) -> Self;
100 fn call_foo(x: Box<Trait>) {
101 let y = x.foo(); // What type is y?
106 If only some methods aren't object-safe, you can add a `where Self: Sized` bound
107 on them to mark them as explicitly unavailable to trait objects. The
108 functionality will still be available to all other implementers, including
109 `Box<Trait>` which is itself sized (assuming you `impl Trait for Box<Trait>`).
113 fn foo(&self) -> Self where Self: Sized;
118 Now, `foo()` can no longer be called on a trait object, but you will now be
119 allowed to make a trait object, and that will be able to call any object-safe
120 methods. With such a bound, one can still call `foo()` on types implementing
121 that trait that aren't behind trait objects.
123 ### Method has generic type parameters
125 As mentioned before, trait objects contain pointers to method tables. So, if we
133 impl Trait for String {
147 At compile time each implementation of `Trait` will produce a table containing
148 the various methods (and other items) related to the implementation.
150 This works fine, but when the method gains generic parameters, we can have a
153 Usually, generic parameters get _monomorphized_. For example, if I have
161 The machine code for `foo::<u8>()`, `foo::<bool>()`, `foo::<String>()`, or any
162 other type substitution is different. Hence the compiler generates the
163 implementation on-demand. If you call `foo()` with a `bool` parameter, the
164 compiler will only generate code for `foo::<bool>()`. When we have additional
165 type parameters, the number of monomorphized implementations the compiler
166 generates does not grow drastically, since the compiler will only generate an
167 implementation if the function is called with unparametrized substitutions
168 (i.e., substitutions where none of the substituted types are themselves
171 However, with trait objects we have to make a table containing _every_ object
172 that implements the trait. Now, if it has type parameters, we need to add
173 implementations for every type that implements the trait, and there could
174 theoretically be an infinite number of types.
180 fn foo<T>(&self, on: T);
184 impl Trait for String {
185 fn foo<T>(&self, on: T) {
191 fn foo<T>(&self, on: T) {
196 // 8 more implementations
199 Now, if we have the following code:
201 ```compile_fail,E0038
202 # trait Trait { fn foo<T>(&self, on: T); }
203 # impl Trait for String { fn foo<T>(&self, on: T) {} }
204 # impl Trait for u8 { fn foo<T>(&self, on: T) {} }
205 # impl Trait for bool { fn foo<T>(&self, on: T) {} }
207 fn call_foo(thing: Box<Trait>) {
208 thing.foo(true); // this could be any one of the 8 types above
214 We don't just need to create a table of all implementations of all methods of
215 `Trait`, we need to create such a table, for each different type fed to
216 `foo()`. In this case this turns out to be (10 types implementing `Trait`)*(3
217 types being fed to `foo()`) = 30 implementations!
219 With real world traits these numbers can grow drastically.
221 To fix this, it is suggested to use a `where Self: Sized` bound similar to the
222 fix for the sub-error above if you do not intend to call the method with type
227 fn foo<T>(&self, on: T) where Self: Sized;
232 If this is not an option, consider replacing the type parameter with another
233 trait object (e.g. if `T: OtherTrait`, use `on: Box<OtherTrait>`). If the number
234 of types you intend to feed to this method is limited, consider manually listing
235 out the methods of different types.
237 ### Method has no receiver
239 Methods that do not take a `self` parameter can't be called since there won't be
240 a way to get a pointer to the method table for them.
248 This could be called as `<Foo as Foo>::foo()`, which would not be able to pick
251 Adding a `Self: Sized` bound to these methods will generally make this compile.
255 fn foo() -> u8 where Self: Sized;
259 ### The trait cannot use `Self` as a type parameter in the supertrait listing
261 This is similar to the second sub-error, but subtler. It happens in situations
267 trait Trait: Super<Self> {
272 impl Super<Foo> for Foo{}
274 impl Trait for Foo {}
277 Here, the supertrait might have methods as follows:
281 fn get_a(&self) -> A; // note that this is object safe!
285 If the trait `Foo` was deriving from something like `Super<String>` or
286 `Super<T>` (where `Foo` itself is `Foo<T>`), this is okay, because given a type
287 `get_a()` will definitely return an object of that type.
289 However, if it derives from `Super<Self>`, even though `Super` is object safe,
290 the method `get_a()` would return an object of unknown type when called on the
291 function. `Self` type parameters let us make object safe traits no longer safe,
292 so they are forbidden when specifying supertraits.
294 There's no easy fix for this, generally code will need to be refactored so that
295 you no longer need to derive from `Super<Self>`.
299 When defining a recursive struct or enum, any use of the type being defined
300 from inside the definition must occur behind a pointer (like `Box` or `&`).
301 This is because structs and enums must have a well-defined size, and without
302 the pointer, the size of the type would need to be unbounded.
304 Consider the following erroneous definition of a type for a list of bytes:
306 ```compile_fail,E0072
307 // error, invalid recursive struct type
310 tail: Option<ListNode>,
314 This type cannot have a well-defined size, because it needs to be arbitrarily
315 large (since we would be able to nest `ListNode`s to any depth). Specifically,
318 size of `ListNode` = 1 byte for `head`
319 + 1 byte for the discriminant of the `Option`
323 One way to fix this is by wrapping `ListNode` in a `Box`, like so:
328 tail: Option<Box<ListNode>>,
332 This works because `Box` is a pointer, so its size is well-known.
336 This error indicates that the compiler was unable to sensibly evaluate an
337 constant expression that had to be evaluated. Attempting to divide by 0
338 or causing integer overflow are two ways to induce this error. For example:
340 ```compile_fail,E0080
347 Ensure that the expressions given can be evaluated as the desired integer type.
348 See the FFI section of the Reference for more information about using a custom
351 https://doc.rust-lang.org/reference.html#ffi-attributes
355 This error indicates that a lifetime is missing from a type. If it is an error
356 inside a function signature, the problem may be with failing to adhere to the
357 lifetime elision rules (see below).
359 Here are some simple examples of where you'll run into this error:
361 ```compile_fail,E0106
362 struct Foo { x: &bool } // error
363 struct Foo<'a> { x: &'a bool } // correct
365 struct Bar { x: Foo }
366 ^^^ expected lifetime parameter
367 struct Bar<'a> { x: Foo<'a> } // correct
369 enum Bar { A(u8), B(&bool), } // error
370 enum Bar<'a> { A(u8), B(&'a bool), } // correct
372 type MyStr = &str; // error
373 type MyStr<'a> = &'a str; // correct
376 Lifetime elision is a special, limited kind of inference for lifetimes in
377 function signatures which allows you to leave out lifetimes in certain cases.
378 For more background on lifetime elision see [the book][book-le].
380 The lifetime elision rules require that any function signature with an elided
381 output lifetime must either have
383 - exactly one input lifetime
384 - or, multiple input lifetimes, but the function must also be a method with a
385 `&self` or `&mut self` receiver
387 In the first case, the output lifetime is inferred to be the same as the unique
388 input lifetime. In the second case, the lifetime is instead inferred to be the
389 same as the lifetime on `&self` or `&mut self`.
391 Here are some examples of elision errors:
393 ```compile_fail,E0106
394 // error, no input lifetimes
397 // error, `x` and `y` have distinct lifetimes inferred
398 fn bar(x: &str, y: &str) -> &str { }
400 // error, `y`'s lifetime is inferred to be distinct from `x`'s
401 fn baz<'a>(x: &'a str, y: &str) -> &str { }
404 Here's an example that is currently an error, but may work in a future version
407 ```compile_fail,E0106
408 struct Foo<'a>(&'a str);
411 impl Quux for Foo { }
414 Lifetime elision in implementation headers was part of the lifetime elision
415 RFC. It is, however, [currently unimplemented][iss15872].
417 [book-le]: https://doc.rust-lang.org/nightly/book/first-edition/lifetimes.html#lifetime-elision
418 [iss15872]: https://github.com/rust-lang/rust/issues/15872
422 There are conflicting trait implementations for the same type.
423 Example of erroneous code:
425 ```compile_fail,E0119
427 fn get(&self) -> usize;
430 impl<T> MyTrait for T {
431 fn get(&self) -> usize { 0 }
438 impl MyTrait for Foo { // error: conflicting implementations of trait
439 // `MyTrait` for type `Foo`
440 fn get(&self) -> usize { self.value }
444 When looking for the implementation for the trait, the compiler finds
445 both the `impl<T> MyTrait for T` where T is all types and the `impl
446 MyTrait for Foo`. Since a trait cannot be implemented multiple times,
447 this is an error. So, when you write:
451 fn get(&self) -> usize;
454 impl<T> MyTrait for T {
455 fn get(&self) -> usize { 0 }
459 This makes the trait implemented on all types in the scope. So if you
460 try to implement it on another one after that, the implementations will
465 fn get(&self) -> usize;
468 impl<T> MyTrait for T {
469 fn get(&self) -> usize { 0 }
477 f.get(); // the trait is implemented so we can use it
483 Unsafe code was used outside of an unsafe function or block.
485 Erroneous code example:
487 ```compile_fail,E0133
488 unsafe fn f() { return; } // This is the unsafe code
491 f(); // error: call to unsafe function requires unsafe function or block
495 Using unsafe functionality is potentially dangerous and disallowed by safety
498 * Dereferencing raw pointers
499 * Calling functions via FFI
500 * Calling functions marked unsafe
502 These safety checks can be relaxed for a section of the code by wrapping the
503 unsafe instructions with an `unsafe` block. For instance:
506 unsafe fn f() { return; }
509 unsafe { f(); } // ok!
513 See also https://doc.rust-lang.org/book/first-edition/unsafe.html
516 // This shouldn't really ever trigger since the repeated value error comes first
518 A binary can only have one entry point, and by default that entry point is the
519 function `main()`. If there are multiple such functions, please rename one.
523 More than one function was declared with the `#[main]` attribute.
525 Erroneous code example:
527 ```compile_fail,E0137
534 fn f() {} // error: multiple functions with a #[main] attribute
537 This error indicates that the compiler found multiple functions with the
538 `#[main]` attribute. This is an error because there must be a unique entry
539 point into a Rust program. Example:
550 More than one function was declared with the `#[start]` attribute.
552 Erroneous code example:
554 ```compile_fail,E0138
558 fn foo(argc: isize, argv: *const *const u8) -> isize {}
561 fn f(argc: isize, argv: *const *const u8) -> isize {}
562 // error: multiple 'start' functions
565 This error indicates that the compiler found multiple functions with the
566 `#[start]` attribute. This is an error because there must be a unique entry
567 point into a Rust program. Example:
573 fn foo(argc: isize, argv: *const *const u8) -> isize { 0 } // ok!
578 #### Note: this error code is no longer emitted by the compiler.
580 There are various restrictions on transmuting between types in Rust; for example
581 types being transmuted must have the same size. To apply all these restrictions,
582 the compiler must know the exact types that may be transmuted. When type
583 parameters are involved, this cannot always be done.
585 So, for example, the following is not allowed:
588 use std::mem::transmute;
590 struct Foo<T>(Vec<T>);
592 fn foo<T>(x: Vec<T>) {
593 // we are transmuting between Vec<T> and Foo<F> here
594 let y: Foo<T> = unsafe { transmute(x) };
595 // do something with y
599 In this specific case there's a good chance that the transmute is harmless (but
600 this is not guaranteed by Rust). However, when alignment and enum optimizations
601 come into the picture, it's quite likely that the sizes may or may not match
602 with different type parameter substitutions. It's not possible to check this for
603 _all_ possible types, so `transmute()` simply only accepts types without any
604 unsubstituted type parameters.
606 If you need this, there's a good chance you're doing something wrong. Keep in
607 mind that Rust doesn't guarantee much about the layout of different structs
608 (even two structs with identical declarations may have different layouts). If
609 there is a solution that avoids the transmute entirely, try it instead.
611 If it's possible, hand-monomorphize the code by writing the function for each
612 possible type substitution. It's possible to use traits to do this cleanly,
616 use std::mem::transmute;
618 struct Foo<T>(Vec<T>);
620 trait MyTransmutableType: Sized {
621 fn transmute(_: Vec<Self>) -> Foo<Self>;
624 impl MyTransmutableType for u8 {
625 fn transmute(x: Vec<u8>) -> Foo<u8> {
626 unsafe { transmute(x) }
630 impl MyTransmutableType for String {
631 fn transmute(x: Vec<String>) -> Foo<String> {
632 unsafe { transmute(x) }
636 // ... more impls for the types you intend to transmute
638 fn foo<T: MyTransmutableType>(x: Vec<T>) {
639 let y: Foo<T> = <T as MyTransmutableType>::transmute(x);
640 // do something with y
644 Each impl will be checked for a size match in the transmute as usual, and since
645 there are no unbound type parameters involved, this should compile unless there
646 is a size mismatch in one of the impls.
648 It is also possible to manually transmute:
652 # let v = Some("value");
653 # type SomeType = &'static [u8];
655 ptr::read(&v as *const _ as *const SomeType) // `v` transmuted to `SomeType`
660 Note that this does not move `v` (unlike `transmute`), and may need a
661 call to `mem::forget(v)` in case you want to avoid destructors being called.
665 A lang item was redefined.
667 Erroneous code example:
669 ```compile_fail,E0152
670 #![feature(lang_items)]
672 #[lang = "panic_fmt"]
673 struct Foo; // error: duplicate lang item found: `panic_fmt`
676 Lang items are already implemented in the standard library. Unless you are
677 writing a free-standing application (e.g. a kernel), you do not need to provide
680 You can build a free-standing crate by adding `#![no_std]` to the crate
683 ```ignore (only-for-syntax-highlight)
687 See also https://doc.rust-lang.org/book/first-edition/no-stdlib.html
691 A generic type was described using parentheses rather than angle brackets.
694 ```compile_fail,E0214
696 let v: Vec(&str) = vec!["foo"];
700 This is not currently supported: `v` should be defined as `Vec<&str>`.
701 Parentheses are currently only used with generic types when defining parameters
702 for `Fn`-family traits.
706 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
707 message for when a particular trait isn't implemented on a type placed in a
708 position that needs that trait. For example, when the following code is
712 #![feature(on_unimplemented)]
714 fn foo<T: Index<u8>>(x: T){}
716 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
717 trait Index<Idx> { /* ... */ }
719 foo(true); // `bool` does not implement `Index<u8>`
722 There will be an error about `bool` not implementing `Index<u8>`, followed by a
723 note saying "the type `bool` cannot be indexed by `u8`".
725 As you can see, you can specify type parameters in curly braces for
726 substitution with the actual types (using the regular format string syntax) in
727 a given situation. Furthermore, `{Self}` will substitute to the type (in this
728 case, `bool`) that we tried to use.
730 This error appears when the curly braces contain an identifier which doesn't
731 match with any of the type parameters or the string `Self`. This might happen
732 if you misspelled a type parameter, or if you intended to use literal curly
733 braces. If it is the latter, escape the curly braces with a second curly brace
734 of the same type; e.g. a literal `{` is `{{`.
738 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
739 message for when a particular trait isn't implemented on a type placed in a
740 position that needs that trait. For example, when the following code is
744 #![feature(on_unimplemented)]
746 fn foo<T: Index<u8>>(x: T){}
748 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
749 trait Index<Idx> { /* ... */ }
751 foo(true); // `bool` does not implement `Index<u8>`
754 there will be an error about `bool` not implementing `Index<u8>`, followed by a
755 note saying "the type `bool` cannot be indexed by `u8`".
757 As you can see, you can specify type parameters in curly braces for
758 substitution with the actual types (using the regular format string syntax) in
759 a given situation. Furthermore, `{Self}` will substitute to the type (in this
760 case, `bool`) that we tried to use.
762 This error appears when the curly braces do not contain an identifier. Please
763 add one of the same name as a type parameter. If you intended to use literal
764 braces, use `{{` and `}}` to escape them.
768 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
769 message for when a particular trait isn't implemented on a type placed in a
770 position that needs that trait. For example, when the following code is
774 #![feature(on_unimplemented)]
776 fn foo<T: Index<u8>>(x: T){}
778 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
779 trait Index<Idx> { /* ... */ }
781 foo(true); // `bool` does not implement `Index<u8>`
784 there will be an error about `bool` not implementing `Index<u8>`, followed by a
785 note saying "the type `bool` cannot be indexed by `u8`".
787 For this to work, some note must be specified. An empty attribute will not do
788 anything, please remove the attribute or add some helpful note for users of the
793 When using a lifetime like `'a` in a type, it must be declared before being
796 These two examples illustrate the problem:
798 ```compile_fail,E0261
799 // error, use of undeclared lifetime name `'a`
800 fn foo(x: &'a str) { }
803 // error, use of undeclared lifetime name `'a`
808 These can be fixed by declaring lifetime parameters:
811 fn foo<'a>(x: &'a str) {}
820 Declaring certain lifetime names in parameters is disallowed. For example,
821 because the `'static` lifetime is a special built-in lifetime name denoting
822 the lifetime of the entire program, this is an error:
824 ```compile_fail,E0262
825 // error, invalid lifetime parameter name `'static`
826 fn foo<'static>(x: &'static str) { }
831 A lifetime name cannot be declared more than once in the same scope. For
834 ```compile_fail,E0263
835 // error, lifetime name `'a` declared twice in the same scope
836 fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str) { }
841 An unknown external lang item was used. Erroneous code example:
843 ```compile_fail,E0264
844 #![feature(lang_items)]
847 #[lang = "cake"] // error: unknown external lang item: `cake`
852 A list of available external lang items is available in
853 `src/librustc/middle/weak_lang_items.rs`. Example:
856 #![feature(lang_items)]
859 #[lang = "panic_fmt"] // ok!
866 This is because of a type mismatch between the associated type of some
867 trait (e.g. `T::Bar`, where `T` implements `trait Quux { type Bar; }`)
868 and another type `U` that is required to be equal to `T::Bar`, but is not.
871 Here is a basic example:
873 ```compile_fail,E0271
874 trait Trait { type AssociatedType; }
876 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
880 impl Trait for i8 { type AssociatedType = &'static str; }
885 Here is that same example again, with some explanatory comments:
887 ```compile_fail,E0271
888 trait Trait { type AssociatedType; }
890 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
891 // ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
893 // This says `foo` can |
894 // only be used with |
896 // implements `Trait`. |
898 // This says not only must
899 // `T` be an impl of `Trait`
900 // but also that the impl
901 // must assign the type `u32`
902 // to the associated type.
906 impl Trait for i8 { type AssociatedType = &'static str; }
907 //~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
912 // ... but it is an implementation
913 // that assigns `&'static str` to
914 // the associated type.
917 // Here, we invoke `foo` with an `i8`, which does not satisfy
918 // the constraint `<i8 as Trait>::AssociatedType=u32`, and
919 // therefore the type-checker complains with this error code.
922 Here is a more subtle instance of the same problem, that can
923 arise with for-loops in Rust:
926 let vs: Vec<i32> = vec![1, 2, 3, 4];
935 The above fails because of an analogous type mismatch,
936 though may be harder to see. Again, here are some
937 explanatory comments for the same example:
941 let vs = vec![1, 2, 3, 4];
943 // `for`-loops use a protocol based on the `Iterator`
944 // trait. Each item yielded in a `for` loop has the
945 // type `Iterator::Item` -- that is, `Item` is the
946 // associated type of the concrete iterator impl.
950 // | We borrow `vs`, iterating over a sequence of
951 // | *references* of type `&Elem` (where `Elem` is
952 // | vector's element type). Thus, the associated
953 // | type `Item` must be a reference `&`-type ...
955 // ... and `v` has the type `Iterator::Item`, as dictated by
956 // the `for`-loop protocol ...
962 // ... but *here*, `v` is forced to have some integral type;
963 // only types like `u8`,`i8`,`u16`,`i16`, et cetera can
964 // match the pattern `1` ...
969 // ... therefore, the compiler complains, because it sees
970 // an attempt to solve the equations
971 // `some integral-type` = type-of-`v`
972 // = `Iterator::Item`
973 // = `&Elem` (i.e. `some reference type`)
975 // which cannot possibly all be true.
981 To avoid those issues, you have to make the types match correctly.
982 So we can fix the previous examples like this:
986 trait Trait { type AssociatedType; }
988 fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
992 impl Trait for i8 { type AssociatedType = &'static str; }
997 let vs = vec![1, 2, 3, 4];
1009 This error occurs when there was a recursive trait requirement that overflowed
1010 before it could be evaluated. Often this means that there is unbounded
1011 recursion in resolving some type bounds.
1013 For example, in the following code:
1015 ```compile_fail,E0275
1020 impl<T> Foo for T where Bar<T>: Foo {}
1023 To determine if a `T` is `Foo`, we need to check if `Bar<T>` is `Foo`. However,
1024 to do this check, we need to determine that `Bar<Bar<T>>` is `Foo`. To
1025 determine this, we check if `Bar<Bar<Bar<T>>>` is `Foo`, and so on. This is
1026 clearly a recursive requirement that can't be resolved directly.
1028 Consider changing your trait bounds so that they're less self-referential.
1032 This error occurs when a bound in an implementation of a trait does not match
1033 the bounds specified in the original trait. For example:
1035 ```compile_fail,E0276
1041 fn foo<T>(x: T) where T: Copy {}
1045 Here, all types implementing `Foo` must have a method `foo<T>(x: T)` which can
1046 take any type `T`. However, in the `impl` for `bool`, we have added an extra
1047 bound that `T` is `Copy`, which isn't compatible with the original trait.
1049 Consider removing the bound from the method or adding the bound to the original
1050 method definition in the trait.
1054 You tried to use a type which doesn't implement some trait in a place which
1055 expected that trait. Erroneous code example:
1057 ```compile_fail,E0277
1058 // here we declare the Foo trait with a bar method
1063 // we now declare a function which takes an object implementing the Foo trait
1064 fn some_func<T: Foo>(foo: T) {
1069 // we now call the method with the i32 type, which doesn't implement
1071 some_func(5i32); // error: the trait bound `i32 : Foo` is not satisfied
1075 In order to fix this error, verify that the type you're using does implement
1083 fn some_func<T: Foo>(foo: T) {
1084 foo.bar(); // we can now use this method since i32 implements the
1088 // we implement the trait on the i32 type
1094 some_func(5i32); // ok!
1098 Or in a generic context, an erroneous code example would look like:
1100 ```compile_fail,E0277
1101 fn some_func<T>(foo: T) {
1102 println!("{:?}", foo); // error: the trait `core::fmt::Debug` is not
1103 // implemented for the type `T`
1107 // We now call the method with the i32 type,
1108 // which *does* implement the Debug trait.
1113 Note that the error here is in the definition of the generic function: Although
1114 we only call it with a parameter that does implement `Debug`, the compiler
1115 still rejects the function: It must work with all possible input types. In
1116 order to make this example compile, we need to restrict the generic type we're
1122 // Restrict the input type to types that implement Debug.
1123 fn some_func<T: fmt::Debug>(foo: T) {
1124 println!("{:?}", foo);
1128 // Calling the method is still fine, as i32 implements Debug.
1131 // This would fail to compile now:
1132 // struct WithoutDebug;
1133 // some_func(WithoutDebug);
1137 Rust only looks at the signature of the called function, as such it must
1138 already specify all requirements that will be used for every type parameter.
1142 You tried to supply a type which doesn't implement some trait in a location
1143 which expected that trait. This error typically occurs when working with
1144 `Fn`-based types. Erroneous code example:
1146 ```compile_fail,E0281
1147 fn foo<F: Fn(usize)>(x: F) { }
1150 // type mismatch: ... implements the trait `core::ops::Fn<(String,)>`,
1151 // but the trait `core::ops::Fn<(usize,)>` is required
1153 foo(|y: String| { });
1157 The issue in this case is that `foo` is defined as accepting a `Fn` with one
1158 argument of type `String`, but the closure we attempted to pass to it requires
1159 one arguments of type `usize`.
1163 This error indicates that type inference did not result in one unique possible
1164 type, and extra information is required. In most cases this can be provided
1165 by adding a type annotation. Sometimes you need to specify a generic type
1168 A common example is the `collect` method on `Iterator`. It has a generic type
1169 parameter with a `FromIterator` bound, which for a `char` iterator is
1170 implemented by `Vec` and `String` among others. Consider the following snippet
1171 that reverses the characters of a string:
1173 ```compile_fail,E0282
1174 let x = "hello".chars().rev().collect();
1177 In this case, the compiler cannot infer what the type of `x` should be:
1178 `Vec<char>` and `String` are both suitable candidates. To specify which type to
1179 use, you can use a type annotation on `x`:
1182 let x: Vec<char> = "hello".chars().rev().collect();
1185 It is not necessary to annotate the full type. Once the ambiguity is resolved,
1186 the compiler can infer the rest:
1189 let x: Vec<_> = "hello".chars().rev().collect();
1192 Another way to provide the compiler with enough information, is to specify the
1193 generic type parameter:
1196 let x = "hello".chars().rev().collect::<Vec<char>>();
1199 Again, you need not specify the full type if the compiler can infer it:
1202 let x = "hello".chars().rev().collect::<Vec<_>>();
1205 Apart from a method or function with a generic type parameter, this error can
1206 occur when a type parameter of a struct or trait cannot be inferred. In that
1207 case it is not always possible to use a type annotation, because all candidates
1208 have the same return type. For instance:
1210 ```compile_fail,E0282
1221 let number = Foo::bar();
1226 This will fail because the compiler does not know which instance of `Foo` to
1227 call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.
1231 This error occurs when the compiler doesn't have enough information
1232 to unambiguously choose an implementation.
1236 ```compile_fail,E0283
1243 impl Generator for Impl {
1244 fn create() -> u32 { 1 }
1249 impl Generator for AnotherImpl {
1250 fn create() -> u32 { 2 }
1254 let cont: u32 = Generator::create();
1255 // error, impossible to choose one of Generator trait implementation
1256 // Impl or AnotherImpl? Maybe anything else?
1260 To resolve this error use the concrete type:
1269 impl Generator for AnotherImpl {
1270 fn create() -> u32 { 2 }
1274 let gen1 = AnotherImpl::create();
1276 // if there are multiple methods with same name (different traits)
1277 let gen2 = <AnotherImpl as Generator>::create();
1283 This error indicates that the given recursion limit could not be parsed. Ensure
1284 that the value provided is a positive integer between quotes.
1286 Erroneous code example:
1288 ```compile_fail,E0296
1294 And a working example:
1297 #![recursion_limit="1000"]
1304 This error occurs when the compiler was unable to infer the concrete type of a
1305 variable. It can occur for several cases, the most common of which is a
1306 mismatch in the expected type that the compiler inferred for a variable's
1307 initializing expression, and the actual type explicitly assigned to the
1312 ```compile_fail,E0308
1313 let x: i32 = "I am not a number!";
1314 // ~~~ ~~~~~~~~~~~~~~~~~~~~
1316 // | initializing expression;
1317 // | compiler infers type `&str`
1319 // type `i32` assigned to variable `x`
1324 Types in type definitions have lifetimes associated with them that represent
1325 how long the data stored within them is guaranteed to be live. This lifetime
1326 must be as long as the data needs to be alive, and missing the constraint that
1327 denotes this will cause this error.
1329 ```compile_fail,E0309
1330 // This won't compile because T is not constrained, meaning the data
1331 // stored in it is not guaranteed to last as long as the reference
1337 This will compile, because it has the constraint on the type parameter:
1340 struct Foo<'a, T: 'a> {
1345 To see why this is important, consider the case where `T` is itself a reference
1346 (e.g., `T = &str`). If we don't include the restriction that `T: 'a`, the
1347 following code would be perfectly legal:
1349 ```compile_fail,E0309
1355 let v = "42".to_string();
1356 let f = Foo{foo: &v};
1358 println!("{}", f.foo); // but we've already dropped v!
1364 Types in type definitions have lifetimes associated with them that represent
1365 how long the data stored within them is guaranteed to be live. This lifetime
1366 must be as long as the data needs to be alive, and missing the constraint that
1367 denotes this will cause this error.
1369 ```compile_fail,E0310
1370 // This won't compile because T is not constrained to the static lifetime
1371 // the reference needs
1377 This will compile, because it has the constraint on the type parameter:
1380 struct Foo<T: 'static> {
1387 A lifetime of reference outlives lifetime of borrowed content.
1389 Erroneous code example:
1391 ```compile_fail,E0312
1392 fn make_child<'human, 'elve>(x: &mut &'human isize, y: &mut &'elve isize) {
1394 // error: lifetime of reference outlives lifetime of borrowed content
1398 The compiler cannot determine if the `human` lifetime will live long enough
1399 to keep up on the elve one. To solve this error, you have to give an
1400 explicit lifetime hierarchy:
1403 fn make_child<'human, 'elve: 'human>(x: &mut &'human isize,
1404 y: &mut &'elve isize) {
1409 Or use the same lifetime for every variable:
1412 fn make_child<'elve>(x: &mut &'elve isize, y: &mut &'elve isize) {
1419 This error occurs when an `if` expression without an `else` block is used in a
1420 context where a type other than `()` is expected, for example a `let`
1423 ```compile_fail,E0317
1426 let a = if x == 5 { 1 };
1430 An `if` expression without an `else` block has the type `()`, so this is a type
1431 error. To resolve it, add an `else` block having the same type as the `if`
1436 This error indicates that some types or traits depend on each other
1437 and therefore cannot be constructed.
1439 The following example contains a circular dependency between two traits:
1441 ```compile_fail,E0391
1442 trait FirstTrait : SecondTrait {
1446 trait SecondTrait : FirstTrait {
1453 #### Note: this error code is no longer emitted by the compiler.
1455 In Rust 1.3, the default object lifetime bounds are expected to change, as
1456 described in [RFC 1156]. You are getting a warning because the compiler
1457 thinks it is possible that this change will cause a compilation error in your
1458 code. It is possible, though unlikely, that this is a false alarm.
1460 The heart of the change is that where `&'a Box<SomeTrait>` used to default to
1461 `&'a Box<SomeTrait+'a>`, it now defaults to `&'a Box<SomeTrait+'static>` (here,
1462 `SomeTrait` is the name of some trait type). Note that the only types which are
1463 affected are references to boxes, like `&Box<SomeTrait>` or
1464 `&[Box<SomeTrait>]`. More common types like `&SomeTrait` or `Box<SomeTrait>`
1467 To silence this warning, edit your code to use an explicit bound. Most of the
1468 time, this means that you will want to change the signature of a function that
1469 you are calling. For example, if the error is reported on a call like `foo(x)`,
1470 and `foo` is defined as follows:
1473 # trait SomeTrait {}
1474 fn foo(arg: &Box<SomeTrait>) { /* ... */ }
1477 You might change it to:
1480 # trait SomeTrait {}
1481 fn foo<'a>(arg: &'a Box<SomeTrait+'a>) { /* ... */ }
1484 This explicitly states that you expect the trait object `SomeTrait` to contain
1485 references (with a maximum lifetime of `'a`).
1487 [RFC 1156]: https://github.com/rust-lang/rfcs/blob/master/text/1156-adjust-default-object-bounds.md
1491 An invalid lint attribute has been given. Erroneous code example:
1493 ```compile_fail,E0452
1494 #![allow(foo = "")] // error: malformed lint attribute
1497 Lint attributes only accept a list of identifiers (where each identifier is a
1498 lint name). Ensure the attribute is of this form:
1501 #![allow(foo)] // ok!
1503 #![allow(foo, foo2)] // ok!
1508 A lint check attribute was overruled by a `forbid` directive set as an
1509 attribute on an enclosing scope, or on the command line with the `-F` option.
1511 Example of erroneous code:
1513 ```compile_fail,E0453
1514 #![forbid(non_snake_case)]
1516 #[allow(non_snake_case)]
1518 let MyNumber = 2; // error: allow(non_snake_case) overruled by outer
1519 // forbid(non_snake_case)
1523 The `forbid` lint setting, like `deny`, turns the corresponding compiler
1524 warning into a hard error. Unlike `deny`, `forbid` prevents itself from being
1525 overridden by inner attributes.
1527 If you're sure you want to override the lint check, you can change `forbid` to
1528 `deny` (or use `-D` instead of `-F` if the `forbid` setting was given as a
1529 command-line option) to allow the inner lint check attribute:
1532 #![deny(non_snake_case)]
1534 #[allow(non_snake_case)]
1536 let MyNumber = 2; // ok!
1540 Otherwise, edit the code to pass the lint check, and remove the overruled
1544 #![forbid(non_snake_case)]
1553 A lifetime bound was not satisfied.
1555 Erroneous code example:
1557 ```compile_fail,E0478
1558 // Check that the explicit lifetime bound (`'SnowWhite`, in this example) must
1559 // outlive all the superbounds from the trait (`'kiss`, in this example).
1561 trait Wedding<'t>: 't { }
1563 struct Prince<'kiss, 'SnowWhite> {
1564 child: Box<Wedding<'kiss> + 'SnowWhite>,
1565 // error: lifetime bound not satisfied
1569 In this example, the `'SnowWhite` lifetime is supposed to outlive the `'kiss`
1570 lifetime but the declaration of the `Prince` struct doesn't enforce it. To fix
1571 this issue, you need to specify it:
1574 trait Wedding<'t>: 't { }
1576 struct Prince<'kiss, 'SnowWhite: 'kiss> { // You say here that 'kiss must live
1577 // longer than 'SnowWhite.
1578 child: Box<Wedding<'kiss> + 'SnowWhite>, // And now it's all good!
1584 A reference has a longer lifetime than the data it references.
1586 Erroneous code example:
1588 ```compile_fail,E0491
1589 // struct containing a reference requires a lifetime parameter,
1590 // because the data the reference points to must outlive the struct (see E0106)
1595 // However, a nested struct like this, the signature itself does not tell
1596 // whether 'a outlives 'b or the other way around.
1597 // So it could be possible that 'b of reference outlives 'a of the data.
1598 struct Nested<'a, 'b> {
1599 ref_struct: &'b Struct<'a>, // compile error E0491
1603 To fix this issue, you can specify a bound to the lifetime like below:
1610 // 'a: 'b means 'a outlives 'b
1611 struct Nested<'a: 'b, 'b> {
1612 ref_struct: &'b Struct<'a>,
1618 A lifetime name is shadowing another lifetime name. Erroneous code example:
1620 ```compile_fail,E0496
1626 fn f<'a>(x: &'a i32) { // error: lifetime name `'a` shadows a lifetime
1627 // name that is already in scope
1632 Please change the name of one of the lifetimes to remove this error. Example:
1640 fn f<'b>(x: &'b i32) { // ok!
1650 A stability attribute was used outside of the standard library. Erroneous code
1654 #[stable] // error: stability attributes may not be used outside of the
1659 It is not possible to use stability attributes outside of the standard library.
1660 Also, for now, it is not possible to write deprecation messages either.
1664 Transmute with two differently sized types was attempted. Erroneous code
1667 ```compile_fail,E0512
1668 fn takes_u8(_: u8) {}
1671 unsafe { takes_u8(::std::mem::transmute(0u16)); }
1672 // error: transmute called with types of different sizes
1676 Please use types with same size or use the expected type directly. Example:
1679 fn takes_u8(_: u8) {}
1682 unsafe { takes_u8(::std::mem::transmute(0i8)); } // ok!
1684 unsafe { takes_u8(0u8); } // ok!
1690 This error indicates that a `#[repr(..)]` attribute was placed on an
1693 Examples of erroneous code:
1695 ```compile_fail,E0517
1703 struct Foo {bar: bool, baz: bool}
1711 * The `#[repr(C)]` attribute can only be placed on structs and enums.
1712 * The `#[repr(packed)]` and `#[repr(simd)]` attributes only work on structs.
1713 * The `#[repr(u8)]`, `#[repr(i16)]`, etc attributes only work on enums.
1715 These attributes do not work on typedefs, since typedefs are just aliases.
1717 Representations like `#[repr(u8)]`, `#[repr(i64)]` are for selecting the
1718 discriminant size for C-like enums (when there is no associated data, e.g.
1719 `enum Color {Red, Blue, Green}`), effectively setting the size of the enum to
1720 the size of the provided type. Such an enum can be cast to a value of the same
1721 type as well. In short, `#[repr(u8)]` makes the enum behave like an integer
1722 with a constrained set of allowed values.
1724 Only C-like enums can be cast to numerical primitives, so this attribute will
1725 not apply to structs.
1727 `#[repr(packed)]` reduces padding to make the struct size smaller. The
1728 representation of enums isn't strictly defined in Rust, and this attribute
1729 won't work on enums.
1731 `#[repr(simd)]` will give a struct consisting of a homogeneous series of machine
1732 types (i.e. `u8`, `i32`, etc) a representation that permits vectorization via
1733 SIMD. This doesn't make much sense for enums since they don't consist of a
1734 single list of data.
1738 This error indicates that an `#[inline(..)]` attribute was incorrectly placed
1739 on something other than a function or method.
1741 Examples of erroneous code:
1743 ```compile_fail,E0518
1753 `#[inline]` hints the compiler whether or not to attempt to inline a method or
1754 function. By default, the compiler does a pretty good job of figuring this out
1755 itself, but if you feel the need for annotations, `#[inline(always)]` and
1756 `#[inline(never)]` can override or force the compiler's decision.
1758 If you wish to apply this attribute to all methods in an impl, manually annotate
1759 each method; it is not possible to annotate the entire impl with an `#[inline]`
1764 The lang attribute is intended for marking special items that are built-in to
1765 Rust itself. This includes special traits (like `Copy` and `Sized`) that affect
1766 how the compiler behaves, as well as special functions that may be automatically
1767 invoked (such as the handler for out-of-bounds accesses when indexing a slice).
1768 Erroneous code example:
1770 ```compile_fail,E0522
1771 #![feature(lang_items)]
1774 fn cookie() -> ! { // error: definition of an unknown language item: `cookie`
1781 A closure was used but didn't implement the expected trait.
1783 Erroneous code example:
1785 ```compile_fail,E0525
1789 fn bar<T: Fn(u32)>(_: T) {}
1793 let closure = |_| foo(x); // error: expected a closure that implements
1794 // the `Fn` trait, but this closure only
1795 // implements `FnOnce`
1800 In the example above, `closure` is an `FnOnce` closure whereas the `bar`
1801 function expected an `Fn` closure. In this case, it's simple to fix the issue,
1802 you just have to implement `Copy` and `Clone` traits on `struct X` and it'll
1806 #[derive(Clone, Copy)] // We implement `Clone` and `Copy` traits.
1810 fn bar<T: Fn(u32)>(_: T) {}
1814 let closure = |_| foo(x);
1815 bar(closure); // ok!
1819 To understand better how closures work in Rust, read:
1820 https://doc.rust-lang.org/book/first-edition/closures.html
1824 The `main` function was incorrectly declared.
1826 Erroneous code example:
1828 ```compile_fail,E0580
1829 fn main() -> i32 { // error: main function has wrong type
1834 The `main` function prototype should never take arguments or return type.
1843 If you want to get command-line arguments, use `std::env::args`. To exit with a
1844 specified exit code, use `std::process::exit`.
1848 Per [RFC 401][rfc401], if you have a function declaration `foo`:
1851 // For the purposes of this explanation, all of these
1852 // different kinds of `fn` declarations are equivalent:
1854 fn foo(x: S) { /* ... */ }
1855 # #[cfg(for_demonstration_only)]
1856 extern "C" { fn foo(x: S); }
1857 # #[cfg(for_demonstration_only)]
1858 impl S { fn foo(self) { /* ... */ } }
1861 the type of `foo` is **not** `fn(S)`, as one might expect.
1862 Rather, it is a unique, zero-sized marker type written here as `typeof(foo)`.
1863 However, `typeof(foo)` can be _coerced_ to a function pointer `fn(S)`,
1864 so you rarely notice this:
1869 let x: fn(S) = foo; // OK, coerces
1872 The reason that this matter is that the type `fn(S)` is not specific to
1873 any particular function: it's a function _pointer_. So calling `x()` results
1874 in a virtual call, whereas `foo()` is statically dispatched, because the type
1875 of `foo` tells us precisely what function is being called.
1877 As noted above, coercions mean that most code doesn't have to be
1878 concerned with this distinction. However, you can tell the difference
1879 when using **transmute** to convert a fn item into a fn pointer.
1881 This is sometimes done as part of an FFI:
1883 ```compile_fail,E0591
1884 extern "C" fn foo(userdata: Box<i32>) {
1888 # fn callback(_: extern "C" fn(*mut i32)) {}
1889 # use std::mem::transmute;
1891 let f: extern "C" fn(*mut i32) = transmute(foo);
1896 Here, transmute is being used to convert the types of the fn arguments.
1897 This pattern is incorrect because, because the type of `foo` is a function
1898 **item** (`typeof(foo)`), which is zero-sized, and the target type (`fn()`)
1899 is a function pointer, which is not zero-sized.
1900 This pattern should be rewritten. There are a few possible ways to do this:
1902 - change the original fn declaration to match the expected signature,
1903 and do the cast in the fn body (the prefered option)
1904 - cast the fn item fo a fn pointer before calling transmute, as shown here:
1907 # extern "C" fn foo(_: Box<i32>) {}
1908 # use std::mem::transmute;
1910 let f: extern "C" fn(*mut i32) = transmute(foo as extern "C" fn(_));
1911 let f: extern "C" fn(*mut i32) = transmute(foo as usize); // works too
1915 The same applies to transmutes to `*mut fn()`, which were observedin practice.
1916 Note though that use of this type is generally incorrect.
1917 The intention is typically to describe a function pointer, but just `fn()`
1918 alone suffices for that. `*mut fn()` is a pointer to a fn pointer.
1919 (Since these values are typically just passed to C code, however, this rarely
1920 makes a difference in practice.)
1922 [rfc401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md
1926 You tried to supply an `Fn`-based type with an incorrect number of arguments
1927 than what was expected.
1929 Erroneous code example:
1931 ```compile_fail,E0593
1932 fn foo<F: Fn()>(x: F) { }
1935 // [E0593] closure takes 1 argument but 0 arguments are required
1942 No `main` function was found in a binary crate. To fix this error, just add a
1943 `main` function. For example:
1947 // Your program will start here.
1948 println!("Hello world!");
1952 If you don't know the basics of Rust, you can go look to the Rust Book to get
1953 started: https://doc.rust-lang.org/book/
1957 An unknown lint was used on the command line.
1962 rustc -D bogus omse_file.rs
1965 Maybe you just misspelled the lint name or the lint doesn't exist anymore.
1966 Either way, try to update/remove it in order to fix the error.
1970 This error code indicates a mismatch between the lifetimes appearing in the
1971 function signature (i.e., the parameter types and the return type) and the
1972 data-flow found in the function body.
1974 Erroneous code example:
1976 ```compile_fail,E0621
1977 fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 { // error: explicit lifetime
1978 // required in the type of
1980 if x > y { x } else { y }
1984 In the code above, the function is returning data borrowed from either `x` or
1985 `y`, but the `'a` annotation indicates that it is returning data only from `x`.
1986 To fix the error, the signature and the body must be made to match. Typically,
1987 this is done by updating the function signature. So, in this case, we change
1988 the type of `y` to `&'a i32`, like so:
1991 fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {
1992 if x > y { x } else { y }
1996 Now the signature indicates that the function data borrowed from either `x` or
1997 `y`. Alternatively, you could change the body to not return data from `y`:
2000 fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 {
2009 register_diagnostics! {
2010 // E0006 // merged with E0005
2011 // E0101, // replaced with E0282
2012 // E0102, // replaced with E0282
2015 // E0272, // on_unimplemented #0
2016 // E0273, // on_unimplemented #1
2017 // E0274, // on_unimplemented #2
2018 E0278, // requirement is not satisfied
2019 E0279, // requirement is not satisfied
2020 E0280, // requirement is not satisfied
2021 E0284, // cannot resolve type
2022 // E0285, // overflow evaluation builtin bounds
2023 // E0300, // unexpanded macro
2024 // E0304, // expected signed integer constant
2025 // E0305, // expected constant
2026 E0311, // thing may not live long enough
2027 E0313, // lifetime of borrowed pointer outlives lifetime of captured variable
2028 E0314, // closure outlives stack frame
2029 E0315, // cannot invoke closure outside of its lifetime
2030 E0316, // nested quantification of lifetimes
2031 E0320, // recursive overflow during dropck
2032 E0473, // dereference of reference outside its lifetime
2033 E0474, // captured variable `..` does not outlive the enclosing closure
2034 E0475, // index of slice outside its lifetime
2035 E0476, // lifetime of the source pointer does not outlive lifetime bound...
2036 E0477, // the type `..` does not fulfill the required lifetime...
2037 E0479, // the type `..` (provided as the value of a type parameter) is...
2038 E0480, // lifetime of method receiver does not outlive the method call
2039 E0481, // lifetime of function argument does not outlive the function call
2040 E0482, // lifetime of return value does not outlive the function call
2041 E0483, // lifetime of operand does not outlive the operation
2042 E0484, // reference is not valid at the time of borrow
2043 E0485, // automatically reference is not valid at the time of borrow
2044 E0486, // type of expression contains references that are not valid during...
2045 E0487, // unsafe use of destructor: destructor might be called while...
2046 E0488, // lifetime of variable does not enclose its declaration
2047 E0489, // type/lifetime parameter not in scope here
2048 E0490, // a value of type `..` is borrowed for too long
2049 E0495, // cannot infer an appropriate lifetime due to conflicting requirements
2050 E0566, // conflicting representation hints
2051 E0623, // lifetime mismatch where both parameters are anonymous regions
2052 E0628, // generators cannot have explicit arguments