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! {
19 This error suggests that the expression arm corresponding to the noted pattern
20 will never be reached as for all possible values of the expression being
21 matched, one of the preceding patterns will match.
23 This means that perhaps some of the preceding patterns are too general, this one
24 is too specific or the ordering is incorrect.
26 For example, the following `match` block has too many arms:
30 Some(bar) => {/* ... */}
32 _ => {/* ... */} // All possible cases have already been handled
36 `match` blocks have their patterns matched in order, so, for example, putting
37 a wildcard arm above a more specific arm will make the latter arm irrelevant.
39 Ensure the ordering of the match arm is correct and remove any superfluous
44 This error indicates that an empty match expression is invalid because the type
45 it is matching on is non-empty (there exist values of this type). In safe code
46 it is impossible to create an instance of an empty type, so empty match
47 expressions are almost never desired. This error is typically fixed by adding
48 one or more cases to the match expression.
50 An example of an empty type is `enum Empty { }`. So, the following will work:
63 fn foo(x: Option<String>) {
72 Not-a-Number (NaN) values cannot be compared for equality and hence can never
73 match the input to a match expression. So, the following will not compile:
76 const NAN: f32 = 0.0 / 0.0;
84 To match against NaN values, you should instead use the `is_nan()` method in a
90 x if x.is_nan() => { /* ... */ }
97 This error indicates that the compiler cannot guarantee a matching pattern for
98 one or more possible inputs to a match expression. Guaranteed matches are
99 required in order to assign values to match expressions, or alternatively,
100 determine the flow of execution.
102 If you encounter this error you must alter your patterns so that every possible
103 value of the input type is matched. For types with a small number of variants
104 (like enums) you should probably cover all cases explicitly. Alternatively, the
105 underscore `_` wildcard pattern can be added after all other patterns to match
110 Patterns used to bind names must be irrefutable, that is, they must guarantee
111 that a name will be extracted in all cases. If you encounter this error you
112 probably need to use a `match` or `if let` to deal with the possibility of
117 This error indicates that the bindings in a match arm would require a value to
118 be moved into more than one location, thus violating unique ownership. Code like
119 the following is invalid as it requires the entire `Option<String>` to be moved
120 into a variable called `op_string` while simultaneously requiring the inner
121 String to be moved into a variable called `s`.
124 let x = Some("s".to_string());
126 op_string @ Some(s) => ...
135 Names bound in match arms retain their type in pattern guards. As such, if a
136 name is bound by move in a pattern, it should also be moved to wherever it is
137 referenced in the pattern guard code. Doing so however would prevent the name
138 from being available in the body of the match arm. Consider the following:
141 match Some("hi".to_string()) {
142 Some(s) if s.len() == 0 => // use s.
147 The variable `s` has type `String`, and its use in the guard is as a variable of
148 type `String`. The guard code effectively executes in a separate scope to the
149 body of the arm, so the value would be moved into this anonymous scope and
150 therefore become unavailable in the body of the arm. Although this example seems
151 innocuous, the problem is most clear when considering functions that take their
155 match Some("hi".to_string()) {
156 Some(s) if { drop(s); false } => (),
162 The value would be dropped in the guard then become unavailable not only in the
163 body of that arm but also in all subsequent arms! The solution is to bind by
164 reference when using guards or refactor the entire expression, perhaps by
165 putting the condition inside the body of the arm.
169 In a pattern, all values that don't implement the `Copy` trait have to be bound
170 the same way. The goal here is to avoid binding simultaneously by-move and
173 This limitation may be removed in a future version of Rust.
180 let x = Some((X { x: () }, X { x: () }));
182 Some((y, ref z)) => {},
187 You have two solutions:
189 Solution #1: Bind the pattern's values the same way.
194 let x = Some((X { x: () }, X { x: () }));
196 Some((ref y, ref z)) => {},
197 // or Some((y, z)) => {}
202 Solution #2: Implement the `Copy` trait for the `X` structure.
204 However, please keep in mind that the first solution should be preferred.
207 #[derive(Clone, Copy)]
210 let x = Some((X { x: () }, X { x: () }));
212 Some((y, ref z)) => {},
219 The value of statics and constants must be known at compile time, and they live
220 for the entire lifetime of a program. Creating a boxed value allocates memory on
221 the heap at runtime, and therefore cannot be done at compile time. Erroneous
225 #![feature(box_syntax)]
227 const CON : Box<i32> = box 0;
232 Initializers for constants and statics are evaluated at compile time.
233 User-defined operators rely on user-defined functions, which cannot be evaluated
243 impl Index<u8> for Foo {
246 fn index<'a>(&'a self, idx: u8) -> &'a u8 { &self.a }
249 const a: Foo = Foo { a: 0u8 };
250 const b: u8 = a[0]; // Index trait is defined by the user, bad!
253 Only operators on builtin types are allowed.
258 const a: &'static [i32] = &[1, 2, 3];
259 const b: i32 = a[0]; // Good!
264 Static and const variables can refer to other const variables. But a const
265 variable cannot refer to a static variable. For example, `Y` cannot refer to `X`
273 To fix this, the value can be extracted as a const and then used:
283 Constants can only be initialized by a constant value or, in a future
284 version of Rust, a call to a const function. This error indicates the use
285 of a path (like a::b, or x) denoting something other than one of these
286 allowed items. Example:
289 const FOO: i32 = { let x = 0; x }; // 'x' isn't a constant nor a function!
292 To avoid it, you have to replace the non-constant value:
295 const FOO: i32 = { const X : i32 = 0; X };
297 const FOO: i32 = { 0 }; // but brackets are useless here
301 // FIXME(#24111) Change the language here when const fn stabilizes
303 The only functions that can be called in static or constant expressions are
304 `const` functions, and struct/enum constructors. `const` functions are only
305 available on a nightly compiler. Rust currently does not support more general
306 compile-time function execution.
309 const FOO: Option<u8> = Some(1); // enum constructor
311 const BAR: Bar = Bar {x: 1}; // struct constructor
314 See [RFC 911] for more details on the design of `const fn`s.
316 [RFC 911]: https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md
320 Blocks in constants may only contain items (such as constant, function
321 definition, etc...) and a tail expression. Example:
324 const FOO: i32 = { let x = 0; x }; // 'x' isn't an item!
327 To avoid it, you have to replace the non-item object:
330 const FOO: i32 = { const X : i32 = 0; X };
335 References in statics and constants may only refer to immutable values. Example:
341 // these three are not allowed:
342 const CR: &'static mut i32 = &mut C;
343 static STATIC_REF: &'static mut i32 = &mut X;
344 static CONST_REF: &'static mut i32 = &mut C;
347 Statics are shared everywhere, and if they refer to mutable data one might
348 violate memory safety since holding multiple mutable references to shared data
351 If you really want global mutable state, try using `static mut` or a global
356 The value of static and const variables must be known at compile time. You
357 can't cast a pointer as an integer because we can't know what value the
360 However, pointers to other constants' addresses are allowed in constants,
365 const Y: *const u32 = &X;
368 Therefore, casting one of these non-constant pointers to an integer results
369 in a non-constant integer which lead to this error. Example:
373 const Y: usize = &X as *const u32 as usize;
379 A function call isn't allowed in the const's initialization expression
380 because the expression's value must be known at compile-time. Example of
389 fn test(&self) -> i32 {
395 const FOO: Test = Test::V1;
397 const A: i32 = FOO.test(); // You can't call Test::func() here !
401 Remember: you can't use a function call inside a const's initialization
402 expression! However, you can totally use it anywhere else:
406 const FOO: Test = Test::V1;
408 FOO.func(); // here is good
409 let x = FOO.func(); // or even here!
415 This error indicates that an attempt was made to divide by zero (or take the
416 remainder of a zero divisor) in a static or constant expression. Erroneous
420 const X: i32 = 42 / 0;
421 // error: attempted to divide by zero in a constant expression
426 Constant functions are not allowed to mutate anything. Thus, binding to an
427 argument with a mutable pattern is not allowed. For example,
430 const fn foo(mut x: u8) {
435 is bad because the function body may not mutate `x`.
437 Remove any mutable bindings from the argument list to fix this error. In case
438 you need to mutate the argument, try lazily initializing a global variable
439 instead of using a `const fn`, or refactoring the code to a functional style to
440 avoid mutation if possible.
444 When matching against a range, the compiler verifies that the range is
445 non-empty. Range patterns include both end-points, so this is equivalent to
446 requiring the start of the range to be less than or equal to the end of the
453 // This range is ok, albeit pointless.
455 // This range is empty, and the compiler can tell.
462 Trait objects like `Box<Trait>` can only be constructed when certain
463 requirements are satisfied by the trait in question.
465 Trait objects are a form of dynamic dispatch and use a dynamically sized type
466 for the inner type. So, for a given trait `Trait`, when `Trait` is treated as a
467 type, as in `Box<Trait>`, the inner type is 'unsized'. In such cases the boxed
468 pointer is a 'fat pointer' that contains an extra pointer to a table of methods
469 (among other things) for dynamic dispatch. This design mandates some
470 restrictions on the types of traits that are allowed to be used in trait
471 objects, which are collectively termed as 'object safety' rules.
473 Attempting to create a trait object for a non object-safe trait will trigger
476 There are various rules:
478 ### The trait cannot require `Self: Sized`
480 When `Trait` is treated as a type, the type does not implement the special
481 `Sized` trait, because the type does not have a known size at compile time and
482 can only be accessed behind a pointer. Thus, if we have a trait like the
486 trait Foo where Self: Sized {
491 we cannot create an object of type `Box<Foo>` or `&Foo` since in this case
492 `Self` would not be `Sized`.
494 Generally, `Self : Sized` is used to indicate that the trait should not be used
495 as a trait object. If the trait comes from your own crate, consider removing
498 ### Method references the `Self` type in its arguments or return type
500 This happens when a trait has a method like the following:
504 fn foo(&self) -> Self;
507 impl Trait for String {
508 fn foo(&self) -> Self {
514 fn foo(&self) -> Self {
520 (Note that `&self` and `&mut self` are okay, it's additional `Self` types which
523 In such a case, the compiler cannot predict the return type of `foo()` in a
524 situation like the following:
527 fn call_foo(x: Box<Trait>) {
528 let y = x.foo(); // What type is y?
533 If only some methods aren't object-safe, you can add a `where Self: Sized` bound
534 on them to mark them as explicitly unavailable to trait objects. The
535 functionality will still be available to all other implementers, including
536 `Box<Trait>` which is itself sized (assuming you `impl Trait for Box<Trait>`).
540 fn foo(&self) -> Self where Self: Sized;
545 Now, `foo()` can no longer be called on a trait object, but you will now be
546 allowed to make a trait object, and that will be able to call any object-safe
547 methods". With such a bound, one can still call `foo()` on types implementing
548 that trait that aren't behind trait objects.
550 ### Method has generic type parameters
552 As mentioned before, trait objects contain pointers to method tables. So, if we
559 impl Trait for String {
572 At compile time each implementation of `Trait` will produce a table containing
573 the various methods (and other items) related to the implementation.
575 This works fine, but when the method gains generic parameters, we can have a
578 Usually, generic parameters get _monomorphized_. For example, if I have
586 the machine code for `foo::<u8>()`, `foo::<bool>()`, `foo::<String>()`, or any
587 other type substitution is different. Hence the compiler generates the
588 implementation on-demand. If you call `foo()` with a `bool` parameter, the
589 compiler will only generate code for `foo::<bool>()`. When we have additional
590 type parameters, the number of monomorphized implementations the compiler
591 generates does not grow drastically, since the compiler will only generate an
592 implementation if the function is called with unparametrized substitutions
593 (i.e., substitutions where none of the substituted types are themselves
596 However, with trait objects we have to make a table containing _every_ object
597 that implements the trait. Now, if it has type parameters, we need to add
598 implementations for every type that implements the trait, and there could
599 theoretically be an infinite number of types.
605 fn foo<T>(&self, on: T);
608 impl Trait for String {
609 fn foo<T>(&self, on: T) {
614 fn foo<T>(&self, on: T) {
618 // 8 more implementations
621 Now, if we have the following code:
624 fn call_foo(thing: Box<Trait>) {
625 thing.foo(true); // this could be any one of the 8 types above
631 we don't just need to create a table of all implementations of all methods of
632 `Trait`, we need to create such a table, for each different type fed to
633 `foo()`. In this case this turns out to be (10 types implementing `Trait`)*(3
634 types being fed to `foo()`) = 30 implementations!
636 With real world traits these numbers can grow drastically.
638 To fix this, it is suggested to use a `where Self: Sized` bound similar to the
639 fix for the sub-error above if you do not intend to call the method with type
644 fn foo<T>(&self, on: T) where Self: Sized;
649 If this is not an option, consider replacing the type parameter with another
650 trait object (e.g. if `T: OtherTrait`, use `on: Box<OtherTrait>`). If the number
651 of types you intend to feed to this method is limited, consider manually listing
652 out the methods of different types.
654 ### Method has no receiver
656 Methods that do not take a `self` parameter can't be called since there won't be
657 a way to get a pointer to the method table for them
665 This could be called as `<Foo as Foo>::foo()`, which would not be able to pick
668 Adding a `Self: Sized` bound to these methods will generally make this compile.
672 fn foo() -> u8 where Self: Sized;
676 ### The trait cannot use `Self` as a type parameter in the supertrait listing
678 This is similar to the second sub-error, but subtler. It happens in situations
684 trait Trait: Super<Self> {
689 impl Super<Foo> for Foo{}
691 impl Trait for Foo {}
694 Here, the supertrait might have methods as follows:
698 fn get_a(&self) -> A; // note that this is object safe!
702 If the trait `Foo` was deriving from something like `Super<String>` or
703 `Super<T>` (where `Foo` itself is `Foo<T>`), this is okay, because given a type
704 `get_a()` will definitely return an object of that type.
706 However, if it derives from `Super<Self>`, even though `Super` is object safe,
707 the method `get_a()` would return an object of unknown type when called on the
708 function. `Self` type parameters let us make object safe traits no longer safe,
709 so they are forbidden when specifying supertraits.
711 There's no easy fix for this, generally code will need to be refactored so that
712 you no longer need to derive from `Super<Self>`.
716 You tried to give a type parameter to a type which doesn't need it. Erroneous
720 type X = u32<i32>; // error: type parameters are not allowed on this type
723 Please check that you used the correct type and recheck its definition. Perhaps
724 it doesn't need the type parameter.
729 type X = u32; // this compiles
732 Note that type parameters for enum-variant constructors go after the variant,
733 not after the enum (Option::None::<u32>, not Option::<u32>::None).
737 You tried to give a lifetime parameter to a type which doesn't need it.
738 Erroneous code example:
741 type X = u32<'static>; // error: lifetime parameters are not allowed on
745 Please check that the correct type was used and recheck its definition; perhaps
746 it doesn't need the lifetime parameter. Example:
754 Using unsafe functionality, is potentially dangerous and disallowed
755 by safety checks. Examples:
757 - Dereferencing raw pointers
758 - Calling functions via FFI
759 - Calling functions marked unsafe
761 These safety checks can be relaxed for a section of the code
762 by wrapping the unsafe instructions with an `unsafe` block. For instance:
765 unsafe fn f() { return; }
772 See also https://doc.rust-lang.org/book/unsafe.html
775 // This shouldn't really ever trigger since the repeated value error comes first
777 A binary can only have one entry point, and by default that entry point is the
778 function `main()`. If there are multiple such functions, please rename one.
782 This error indicates that the compiler found multiple functions with the
783 `#[main]` attribute. This is an error because there must be a unique entry
784 point into a Rust program.
788 This error indicates that the compiler found multiple functions with the
789 `#[start]` attribute. This is an error because there must be a unique entry
790 point into a Rust program.
793 // FIXME link this to the relevant turpl chapters for instilling fear of the
794 // transmute gods in the user
796 There are various restrictions on transmuting between types in Rust; for example
797 types being transmuted must have the same size. To apply all these restrictions,
798 the compiler must know the exact types that may be transmuted. When type
799 parameters are involved, this cannot always be done.
801 So, for example, the following is not allowed:
804 struct Foo<T>(Vec<T>)
806 fn foo<T>(x: Vec<T>) {
807 // we are transmuting between Vec<T> and Foo<T> here
808 let y: Foo<T> = unsafe { transmute(x) };
809 // do something with y
813 In this specific case there's a good chance that the transmute is harmless (but
814 this is not guaranteed by Rust). However, when alignment and enum optimizations
815 come into the picture, it's quite likely that the sizes may or may not match
816 with different type parameter substitutions. It's not possible to check this for
817 _all_ possible types, so `transmute()` simply only accepts types without any
818 unsubstituted type parameters.
820 If you need this, there's a good chance you're doing something wrong. Keep in
821 mind that Rust doesn't guarantee much about the layout of different structs
822 (even two structs with identical declarations may have different layouts). If
823 there is a solution that avoids the transmute entirely, try it instead.
825 If it's possible, hand-monomorphize the code by writing the function for each
826 possible type substitution. It's possible to use traits to do this cleanly,
830 trait MyTransmutableType {
831 fn transmute(Vec<Self>) -> Foo<Self>
834 impl MyTransmutableType for u8 {
835 fn transmute(x: Foo<u8>) -> Vec<u8> {
839 impl MyTransmutableType for String {
840 fn transmute(x: Foo<String>) -> Vec<String> {
844 // ... more impls for the types you intend to transmute
846 fn foo<T: MyTransmutableType>(x: Vec<T>) {
847 let y: Foo<T> = <T as MyTransmutableType>::transmute(x);
848 // do something with y
852 Each impl will be checked for a size match in the transmute as usual, and since
853 there are no unbound type parameters involved, this should compile unless there
854 is a size mismatch in one of the impls.
856 It is also possible to manually transmute:
859 ptr::read(&v as *const _ as *const SomeType) // `v` transmuted to `SomeType`
862 Note that this does not move `v` (unlike `transmute`), and may need a
863 call to `mem::forget(v)` in case you want to avoid destructors being called.
867 Lang items are already implemented in the standard library. Unless you are
868 writing a free-standing application (e.g. a kernel), you do not need to provide
871 You can build a free-standing crate by adding `#![no_std]` to the crate
879 See also https://doc.rust-lang.org/book/no-stdlib.html
883 `const` and `static` mean different things. A `const` is a compile-time
884 constant, an alias for a literal value. This property means you can match it
885 directly within a pattern.
887 The `static` keyword, on the other hand, guarantees a fixed location in memory.
888 This does not always mean that the value is constant. For example, a global
889 mutex can be declared `static` as well.
891 If you want to match against a `static`, consider using a guard instead:
894 static FORTY_TWO: i32 = 42;
896 Some(x) if x == FORTY_TWO => ...
903 In Rust, you can only move a value when its size is known at compile time.
905 To work around this restriction, consider "hiding" the value behind a reference:
906 either `&x` or `&mut x`. Since a reference has a fixed size, this lets you move
911 An if-let pattern attempts to match the pattern, and enters the body if the
912 match was successful. If the match is irrefutable (when it cannot fail to
913 match), use a regular `let`-binding instead. For instance:
916 struct Irrefutable(i32);
917 let irr = Irrefutable(0);
919 // This fails to compile because the match is irrefutable.
920 if let Irrefutable(x) = irr {
921 // This body will always be executed.
926 let Irrefutable(x) = irr;
932 A while-let pattern attempts to match the pattern, and enters the body if the
933 match was successful. If the match is irrefutable (when it cannot fail to
934 match), use a regular `let`-binding inside a `loop` instead. For instance:
937 struct Irrefutable(i32);
938 let irr = Irrefutable(0);
940 // This fails to compile because the match is irrefutable.
941 while let Irrefutable(x) = irr {
947 let Irrefutable(x) = irr;
954 Enum variants are qualified by default. For example, given this type:
963 you would match it using:
972 If you don't qualify the names, the code will bind new variables named "GET" and
973 "POST" instead. This behavior is likely not what you want, so `rustc` warns when
976 Qualified names are good practice, and most code works well with them. But if
977 you prefer them unqualified, you can import the variants into scope:
981 enum Method { GET, POST }
984 If you want others to be able to import variants from your module directly, use
989 enum Method { GET, POST }
994 When using a lifetime like `'a` in a type, it must be declared before being
997 These two examples illustrate the problem:
1000 // error, use of undeclared lifetime name `'a`
1001 fn foo(x: &'a str) { }
1004 // error, use of undeclared lifetime name `'a`
1009 These can be fixed by declaring lifetime parameters:
1012 fn foo<'a>(x: &'a str) { }
1021 Declaring certain lifetime names in parameters is disallowed. For example,
1022 because the `'static` lifetime is a special built-in lifetime name denoting
1023 the lifetime of the entire program, this is an error:
1026 // error, invalid lifetime parameter name `'static`
1027 fn foo<'static>(x: &'static str) { }
1032 A lifetime name cannot be declared more than once in the same scope. For
1036 // error, lifetime name `'a` declared twice in the same scope
1037 fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str) { }
1042 This error indicates that a static or constant references itself.
1043 All statics and constants need to resolve to a value in an acyclic manner.
1045 For example, neither of the following can be sensibly compiled:
1058 This error indicates the use of a loop keyword (`break` or `continue`) inside a
1059 closure but outside of any loop. Erroneous code example:
1062 let w = || { break; }; // error: `break` inside of a closure
1065 `break` and `continue` keywords can be used as normal inside closures as long as
1066 they are also contained within a loop. To halt the execution of a closure you
1067 should instead use a return statement. Example:
1081 This error indicates the use of a loop keyword (`break` or `continue`) outside
1082 of a loop. Without a loop to break out of or continue in, no sensible action can
1083 be taken. Erroneous code example:
1087 break; // error: `break` outside of loop
1091 Please verify that you are using `break` and `continue` only in loops. Example:
1103 Functions must eventually return a value of their return type. For example, in
1104 the following function
1107 fn foo(x: u8) -> u8 {
1109 x // alternatively, `return x`
1115 if the condition is true, the value `x` is returned, but if the condition is
1116 false, control exits the `if` block and reaches a place where nothing is being
1117 returned. All possible control paths must eventually return a `u8`, which is not
1120 An easy fix for this in a complicated function is to specify a default return
1124 fn foo(x: u8) -> u8 {
1126 x // alternatively, `return x`
1128 // lots of other if branches
1129 0 // return 0 if all else fails
1133 It is advisable to find out what the unhandled cases are and check for them,
1134 returning an appropriate value or panicking if necessary.
1138 Rust lets you define functions which are known to never return, i.e. are
1139 'diverging', by marking its return type as `!`.
1141 For example, the following functions never return:
1149 foo() // foo() is diverging, so this will diverge too
1153 panic!(); // this macro internally expands to a call to a diverging function
1158 Such functions can be used in a place where a value is expected without
1159 returning a value of that type, for instance:
1165 _ => foo() // diverging function called here
1170 If the third arm of the match block is reached, since `foo()` doesn't ever
1171 return control to the match block, it is fine to use it in a place where an
1172 integer was expected. The `match` block will never finish executing, and any
1173 point where `y` (like the print statement) is needed will not be reached.
1175 However, if we had a diverging function that actually does finish execution
1183 then we would have an unknown value for `y` in the following code:
1194 In the previous example, the print statement was never reached when the wildcard
1195 match arm was hit, so we were okay with `foo()` not returning an integer that we
1196 could set to `y`. But in this example, `foo()` actually does return control, so
1197 the print statement will be executed with an uninitialized value.
1199 Obviously we cannot have functions which are allowed to be used in such
1200 positions and yet can return control. So, if you are defining a function that
1201 returns `!`, make sure that there is no way for it to actually finish executing.
1205 This is because of a type mismatch between the associated type of some
1206 trait (e.g. `T::Bar`, where `T` implements `trait Quux { type Bar; }`)
1207 and another type `U` that is required to be equal to `T::Bar`, but is not.
1210 Here is a basic example:
1213 trait Trait { type AssociatedType; }
1214 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
1217 impl Trait for i8 { type AssociatedType = &'static str; }
1221 Here is that same example again, with some explanatory comments:
1224 trait Trait { type AssociatedType; }
1226 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
1227 // ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
1229 // This says `foo` can |
1230 // only be used with |
1232 // implements `Trait`. |
1234 // This says not only must
1235 // `T` be an impl of `Trait`
1236 // but also that the impl
1237 // must assign the type `u32`
1238 // to the associated type.
1242 impl Trait for i8 { type AssociatedType = &'static str; }
1243 ~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1248 // ... but it is an implementation
1249 // that assigns `&'static str` to
1250 // the associated type.
1253 // Here, we invoke `foo` with an `i8`, which does not satisfy
1254 // the constraint `<i8 as Trait>::AssociatedType=u32`, and
1255 // therefore the type-checker complains with this error code.
1258 Here is a more subtle instance of the same problem, that can
1259 arise with for-loops in Rust:
1262 let vs: Vec<i32> = vec![1, 2, 3, 4];
1271 The above fails because of an analogous type mismatch,
1272 though may be harder to see. Again, here are some
1273 explanatory comments for the same example:
1277 let vs = vec![1, 2, 3, 4];
1279 // `for`-loops use a protocol based on the `Iterator`
1280 // trait. Each item yielded in a `for` loop has the
1281 // type `Iterator::Item` -- that is,I `Item` is the
1282 // associated type of the concrete iterator impl.
1286 // | We borrow `vs`, iterating over a sequence of
1287 // | *references* of type `&Elem` (where `Elem` is
1288 // | vector's element type). Thus, the associated
1289 // | type `Item` must be a reference `&`-type ...
1291 // ... and `v` has the type `Iterator::Item`, as dictated by
1292 // the `for`-loop protocol ...
1298 // ... but *here*, `v` is forced to have some integral type;
1299 // only types like `u8`,`i8`,`u16`,`i16`, et cetera can
1300 // match the pattern `1` ...
1305 // ... therefore, the compiler complains, because it sees
1306 // an attempt to solve the equations
1307 // `some integral-type` = type-of-`v`
1308 // = `Iterator::Item`
1309 // = `&Elem` (i.e. `some reference type`)
1311 // which cannot possibly all be true.
1317 To avoid those issues, you have to make the types match correctly.
1318 So we can fix the previous examples like this:
1322 trait Trait { type AssociatedType; }
1323 fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
1326 impl Trait for i8 { type AssociatedType = &'static str; }
1329 // For-Loop Example:
1330 let vs = vec![1, 2, 3, 4];
1341 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
1342 message for when a particular trait isn't implemented on a type placed in a
1343 position that needs that trait. For example, when the following code is
1347 fn foo<T: Index<u8>>(x: T){}
1349 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
1350 trait Index<Idx> { ... }
1352 foo(true); // `bool` does not implement `Index<u8>`
1355 there will be an error about `bool` not implementing `Index<u8>`, followed by a
1356 note saying "the type `bool` cannot be indexed by `u8`".
1358 As you can see, you can specify type parameters in curly braces for substitution
1359 with the actual types (using the regular format string syntax) in a given
1360 situation. Furthermore, `{Self}` will substitute to the type (in this case,
1361 `bool`) that we tried to use.
1363 This error appears when the curly braces contain an identifier which doesn't
1364 match with any of the type parameters or the string `Self`. This might happen if
1365 you misspelled a type parameter, or if you intended to use literal curly braces.
1366 If it is the latter, escape the curly braces with a second curly brace of the
1367 same type; e.g. a literal `{` is `{{`
1371 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
1372 message for when a particular trait isn't implemented on a type placed in a
1373 position that needs that trait. For example, when the following code is
1377 fn foo<T: Index<u8>>(x: T){}
1379 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
1380 trait Index<Idx> { ... }
1382 foo(true); // `bool` does not implement `Index<u8>`
1385 there will be an error about `bool` not implementing `Index<u8>`, followed by a
1386 note saying "the type `bool` cannot be indexed by `u8`".
1388 As you can see, you can specify type parameters in curly braces for substitution
1389 with the actual types (using the regular format string syntax) in a given
1390 situation. Furthermore, `{Self}` will substitute to the type (in this case,
1391 `bool`) that we tried to use.
1393 This error appears when the curly braces do not contain an identifier. Please
1394 add one of the same name as a type parameter. If you intended to use literal
1395 braces, use `{{` and `}}` to escape them.
1399 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
1400 message for when a particular trait isn't implemented on a type placed in a
1401 position that needs that trait. For example, when the following code is
1405 fn foo<T: Index<u8>>(x: T){}
1407 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
1408 trait Index<Idx> { ... }
1410 foo(true); // `bool` does not implement `Index<u8>`
1413 there will be an error about `bool` not implementing `Index<u8>`, followed by a
1414 note saying "the type `bool` cannot be indexed by `u8`".
1416 For this to work, some note must be specified. An empty attribute will not do
1417 anything, please remove the attribute or add some helpful note for users of the
1422 This error occurs when there was a recursive trait requirement that overflowed
1423 before it could be evaluated. Often this means that there is unbounded recursion
1424 in resolving some type bounds.
1426 For example, in the following code
1433 impl<T> Foo for T where Bar<T>: Foo {}
1436 to determine if a `T` is `Foo`, we need to check if `Bar<T>` is `Foo`. However,
1437 to do this check, we need to determine that `Bar<Bar<T>>` is `Foo`. To determine
1438 this, we check if `Bar<Bar<Bar<T>>>` is `Foo`, and so on. This is clearly a
1439 recursive requirement that can't be resolved directly.
1441 Consider changing your trait bounds so that they're less self-referential.
1445 This error occurs when a bound in an implementation of a trait does not match
1446 the bounds specified in the original trait. For example:
1454 fn foo<T>(x: T) where T: Copy {}
1458 Here, all types implementing `Foo` must have a method `foo<T>(x: T)` which can
1459 take any type `T`. However, in the `impl` for `bool`, we have added an extra
1460 bound that `T` is `Copy`, which isn't compatible with the original trait.
1462 Consider removing the bound from the method or adding the bound to the original
1463 method definition in the trait.
1467 You tried to use a type which doesn't implement some trait in a place which
1468 expected that trait. Erroneous code example:
1471 // here we declare the Foo trait with a bar method
1476 // we now declare a function which takes an object implementing the Foo trait
1477 fn some_func<T: Foo>(foo: T) {
1482 // we now call the method with the i32 type, which doesn't implement
1484 some_func(5i32); // error: the trait `Foo` is not implemented for the
1489 In order to fix this error, verify that the type you're using does implement
1497 fn some_func<T: Foo>(foo: T) {
1498 foo.bar(); // we can now use this method since i32 implements the
1502 // we implement the trait on the i32 type
1508 some_func(5i32); // ok!
1514 You tried to supply a type which doesn't implement some trait in a location
1515 which expected that trait. This error typically occurs when working with
1516 `Fn`-based types. Erroneous code example:
1519 fn foo<F: Fn()>(x: F) { }
1522 // type mismatch: the type ... implements the trait `core::ops::Fn<(_,)>`,
1523 // but the trait `core::ops::Fn<()>` is required (expected (), found tuple
1529 The issue in this case is that `foo` is defined as accepting a `Fn` with no
1530 arguments, but the closure we attempted to pass to it requires one argument.
1534 This error indicates that type inference did not result in one unique possible
1535 type, and extra information is required. In most cases this can be provided
1536 by adding a type annotation. Sometimes you need to specify a generic type
1539 A common example is the `collect` method on `Iterator`. It has a generic type
1540 parameter with a `FromIterator` bound, which for a `char` iterator is
1541 implemented by `Vec` and `String` among others. Consider the following snippet
1542 that reverses the characters of a string:
1545 let x = "hello".chars().rev().collect();
1548 In this case, the compiler cannot infer what the type of `x` should be:
1549 `Vec<char>` and `String` are both suitable candidates. To specify which type to
1550 use, you can use a type annotation on `x`:
1553 let x: Vec<char> = "hello".chars().rev().collect();
1556 It is not necessary to annotate the full type. Once the ambiguity is resolved,
1557 the compiler can infer the rest:
1560 let x: Vec<_> = "hello".chars().rev().collect();
1563 Another way to provide the compiler with enough information, is to specify the
1564 generic type parameter:
1567 let x = "hello".chars().rev().collect::<Vec<char>>();
1570 Again, you need not specify the full type if the compiler can infer it:
1573 let x = "hello".chars().rev().collect::<Vec<_>>();
1576 Apart from a method or function with a generic type parameter, this error can
1577 occur when a type parameter of a struct or trait cannot be inferred. In that
1578 case it is not always possible to use a type annotation, because all candidates
1579 have the same return type. For instance:
1583 // Some fields omitted.
1592 let number = Foo::bar();
1597 This will fail because the compiler does not know which instance of `Foo` to
1598 call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.
1602 This error indicates that the given recursion limit could not be parsed. Ensure
1603 that the value provided is a positive integer between quotes, like so:
1606 #![recursion_limit="1000"]
1611 Patterns used to bind names must be irrefutable. That is, they must guarantee
1612 that a name will be extracted in all cases. Instead of pattern matching the
1613 loop variable, consider using a `match` or `if let` inside the loop body. For
1617 // This fails because `None` is not covered.
1622 // Match inside the loop instead:
1632 if let Some(x) = item {
1640 Mutable borrows are not allowed in pattern guards, because matching cannot have
1641 side effects. Side effects could alter the matched object or the environment
1642 on which the match depends in such a way, that the match would not be
1643 exhaustive. For instance, the following would not match any arm if mutable
1644 borrows were allowed:
1649 option if option.take().is_none() => { /* impossible, option is `Some` */ },
1650 Some(_) => { } // When the previous match failed, the option became `None`.
1656 Assignments are not allowed in pattern guards, because matching cannot have
1657 side effects. Side effects could alter the matched object or the environment
1658 on which the match depends in such a way, that the match would not be
1659 exhaustive. For instance, the following would not match any arm if assignments
1665 option if { option = None; false } { },
1666 Some(_) => { } // When the previous match failed, the option became `None`.
1672 In certain cases it is possible for sub-bindings to violate memory safety.
1673 Updates to the borrow checker in a future version of Rust may remove this
1674 restriction, but for now patterns must be rewritten without sub-bindings.
1678 match Some("hi".to_string()) {
1679 ref op_string_ref @ Some(ref s) => ...
1684 match Some("hi".to_string()) {
1686 let op_string_ref = &Some(s);
1693 The `op_string_ref` binding has type `&Option<&String>` in both cases.
1695 See also https://github.com/rust-lang/rust/issues/14587
1699 In an array literal `[x; N]`, `N` is the number of elements in the array. This
1700 number cannot be negative.
1704 The length of an array is part of its type. For this reason, this length must be
1705 a compile-time constant.
1709 This error occurs when the compiler was unable to infer the concrete type of a
1710 variable. It can occur for several cases, the most common of which is a
1711 mismatch in the expected type that the compiler inferred for a variable's
1712 initializing expression, and the actual type explicitly assigned to the
1718 let x: i32 = "I am not a number!";
1719 // ~~~ ~~~~~~~~~~~~~~~~~~~~
1721 // | initializing expression;
1722 // | compiler infers type `&str`
1724 // type `i32` assigned to variable `x`
1729 Types in type definitions have lifetimes associated with them that represent
1730 how long the data stored within them is guaranteed to be live. This lifetime
1731 must be as long as the data needs to be alive, and missing the constraint that
1732 denotes this will cause this error.
1735 // This won't compile because T is not constrained, meaning the data
1736 // stored in it is not guaranteed to last as long as the reference
1741 // This will compile, because it has the constraint on the type parameter
1742 struct Foo<'a, T: 'a> {
1749 Types in type definitions have lifetimes associated with them that represent
1750 how long the data stored within them is guaranteed to be live. This lifetime
1751 must be as long as the data needs to be alive, and missing the constraint that
1752 denotes this will cause this error.
1755 // This won't compile because T is not constrained to the static lifetime
1756 // the reference needs
1761 // This will compile, because it has the constraint on the type parameter
1762 struct Foo<T: 'static> {
1769 Method calls that aren't calls to inherent `const` methods are disallowed
1770 in statics, constants, and constant functions.
1775 const BAZ: i32 = Foo(25).bar(); // error, `bar` isn't `const`
1780 const fn foo(&self) -> i32 {
1781 self.bar() // error, `bar` isn't `const`
1784 fn bar(&self) -> i32 { self.0 }
1788 For more information about `const fn`'s, see [RFC 911].
1790 [RFC 911]: https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md
1796 > It is invalid for a static to reference another static by value. It is
1797 > required that all references be borrowed.
1799 [RFC 246]: https://github.com/rust-lang/rfcs/pull/246
1803 The value assigned to a constant expression must be known at compile time,
1804 which is not the case when comparing raw pointers. Erroneous code example:
1807 static foo: i32 = 42;
1808 static bar: i32 = 43;
1810 static baz: bool = { (&foo as *const i32) == (&bar as *const i32) };
1811 // error: raw pointers cannot be compared in statics!
1814 Please check that the result of the comparison can be determined at compile time
1815 or isn't assigned to a constant expression. Example:
1818 static foo: i32 = 42;
1819 static bar: i32 = 43;
1821 let baz: bool = { (&foo as *const i32) == (&bar as *const i32) };
1822 // baz isn't a constant expression so it's ok
1827 The value assigned to a constant expression must be known at compile time,
1828 which is not the case when dereferencing raw pointers. Erroneous code
1832 const foo: i32 = 42;
1833 const baz: *const i32 = (&foo as *const i32);
1835 const deref: i32 = *baz;
1836 // error: raw pointers cannot be dereferenced in constants
1839 To fix this error, please do not assign this value to a constant expression.
1843 const foo: i32 = 42;
1844 const baz: *const i32 = (&foo as *const i32);
1846 unsafe { let deref: i32 = *baz; }
1847 // baz isn't a constant expression so it's ok
1850 You'll also note that this assignment must be done in an unsafe block!
1854 It is not allowed for a mutable static to allocate or have destructors. For
1858 // error: mutable statics are not allowed to have boxes
1859 static mut FOO: Option<Box<usize>> = None;
1861 // error: mutable statics are not allowed to have destructors
1862 static mut BAR: Option<Vec<i32>> = None;
1867 In Rust 1.3, the default object lifetime bounds are expected to
1868 change, as described in RFC #1156 [1]. You are getting a warning
1869 because the compiler thinks it is possible that this change will cause
1870 a compilation error in your code. It is possible, though unlikely,
1871 that this is a false alarm.
1873 The heart of the change is that where `&'a Box<SomeTrait>` used to
1874 default to `&'a Box<SomeTrait+'a>`, it now defaults to `&'a
1875 Box<SomeTrait+'static>` (here, `SomeTrait` is the name of some trait
1876 type). Note that the only types which are affected are references to
1877 boxes, like `&Box<SomeTrait>` or `&[Box<SomeTrait>]`. More common
1878 types like `&SomeTrait` or `Box<SomeTrait>` are unaffected.
1880 To silence this warning, edit your code to use an explicit bound.
1881 Most of the time, this means that you will want to change the
1882 signature of a function that you are calling. For example, if
1883 the error is reported on a call like `foo(x)`, and `foo` is
1887 fn foo(arg: &Box<SomeTrait>) { ... }
1890 you might change it to:
1893 fn foo<'a>(arg: &Box<SomeTrait+'a>) { ... }
1896 This explicitly states that you expect the trait object `SomeTrait` to
1897 contain references (with a maximum lifetime of `'a`).
1899 [1]: https://github.com/rust-lang/rfcs/pull/1156
1903 A borrow of a constant containing interior mutability was attempted. Erroneous
1907 use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT};
1909 const A: AtomicUsize = ATOMIC_USIZE_INIT;
1910 static B: &'static AtomicUsize = &A;
1911 // error: cannot borrow a constant which contains interior mutability, create a
1915 A `const` represents a constant value that should never change. If one takes
1916 a `&` reference to the constant, then one is taking a pointer to some memory
1917 location containing the value. Normally this is perfectly fine: most values
1918 can't be changed via a shared `&` pointer, but interior mutability would allow
1919 it. That is, a constant value could be mutated. On the other hand, a `static` is
1920 explicitly a single memory location, which can be mutated at will.
1922 So, in order to solve this error, either use statics which are `Sync`:
1925 use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT};
1927 static A: AtomicUsize = ATOMIC_USIZE_INIT;
1928 static B: &'static AtomicUsize = &A; // ok!
1931 You can also have this error while using a cell type:
1934 #![feature(const_fn)]
1936 use std::cell::Cell;
1938 const A: Cell<usize> = Cell::new(1);
1939 const B: &'static Cell<usize> = &A;
1940 // error: cannot borrow a constant which contains interior mutability, create
1944 struct C { a: Cell<usize> }
1946 const D: C = C { a: Cell::new(1) };
1947 const E: &'static Cell<usize> = &D.a; // error
1950 const F: &'static C = &D; // error
1953 This is because cell types do operations that are not thread-safe. Due to this,
1954 they don't implement Sync and thus can't be placed in statics. In this
1955 case, `StaticMutex` would work just fine, but it isn't stable yet:
1956 https://doc.rust-lang.org/nightly/std/sync/struct.StaticMutex.html
1958 However, if you still wish to use these types, you can achieve this by an unsafe
1962 #![feature(const_fn)]
1964 use std::cell::Cell;
1965 use std::marker::Sync;
1967 struct NotThreadSafe<T> {
1971 unsafe impl<T> Sync for NotThreadSafe<T> {}
1973 static A: NotThreadSafe<usize> = NotThreadSafe { value : Cell::new(1) };
1974 static B: &'static NotThreadSafe<usize> = &A; // ok!
1977 Remember this solution is unsafe! You will have to ensure that accesses to the
1978 cell are synchronized.
1982 A type with a destructor was assigned to an invalid type of variable. Erroneous
1991 fn drop(&mut self) {}
1994 const F : Foo = Foo { a : 0 };
1995 // error: constants are not allowed to have destructors
1996 static S : Foo = Foo { a : 0 };
1997 // error: statics are not allowed to have destructors
2000 To solve this issue, please use a type which does allow the usage of type with
2005 A reference of an interior static was assigned to another const/static.
2006 Erroneous code example:
2013 static S : Foo = Foo { a : 0 };
2014 static A : &'static u32 = &S.a;
2015 // error: cannot refer to the interior of another static, use a
2019 The "base" variable has to be a const if you want another static/const variable
2020 to refer to one of its fields. Example:
2027 const S : Foo = Foo { a : 0 };
2028 static A : &'static u32 = &S.a; // ok!
2033 A lifetime name is shadowing another lifetime name. Erroneous code example:
2041 fn f<'a>(x: &'a i32) { // error: lifetime name `'a` shadows a lifetime
2042 // name that is already in scope
2047 Please change the name of one of the lifetimes to remove this error. Example:
2055 fn f<'b>(x: &'b i32) { // ok!
2065 A stability attribute was used outside of the standard library. Erroneous code
2069 #[stable] // error: stability attributes may not be used outside of the
2074 It is not possible to use stability attributes outside of the standard library.
2075 Also, for now, it is not possible to write deprecation messages either.
2079 This error indicates that a `#[repr(..)]` attribute was placed on an unsupported
2082 Examples of erroneous code:
2092 struct Foo {bar: bool, baz: bool}
2100 - The `#[repr(C)]` attribute can only be placed on structs and enums
2101 - The `#[repr(packed)]` and `#[repr(simd)]` attributes only work on structs
2102 - The `#[repr(u8)]`, `#[repr(i16)]`, etc attributes only work on enums
2104 These attributes do not work on typedefs, since typedefs are just aliases.
2106 Representations like `#[repr(u8)]`, `#[repr(i64)]` are for selecting the
2107 discriminant size for C-like enums (when there is no associated data, e.g. `enum
2108 Color {Red, Blue, Green}`), effectively setting the size of the enum to the size
2109 of the provided type. Such an enum can be cast to a value of the same type as
2110 well. In short, `#[repr(u8)]` makes the enum behave like an integer with a
2111 constrained set of allowed values.
2113 Only C-like enums can be cast to numerical primitives, so this attribute will
2114 not apply to structs.
2116 `#[repr(packed)]` reduces padding to make the struct size smaller. The
2117 representation of enums isn't strictly defined in Rust, and this attribute won't
2120 `#[repr(simd)]` will give a struct consisting of a homogenous series of machine
2121 types (i.e. `u8`, `i32`, etc) a representation that permits vectorization via
2122 SIMD. This doesn't make much sense for enums since they don't consist of a
2123 single list of data.
2127 This error indicates that an `#[inline(..)]` attribute was incorrectly placed on
2128 something other than a function or method.
2130 Examples of erroneous code:
2142 `#[inline]` hints the compiler whether or not to attempt to inline a method or
2143 function. By default, the compiler does a pretty good job of figuring this out
2144 itself, but if you feel the need for annotations, `#[inline(always)]` and
2145 `#[inline(never)]` can override or force the compiler's decision.
2147 If you wish to apply this attribute to all methods in an impl, manually annotate
2148 each method; it is not possible to annotate the entire impl with an `#[inline]`
2155 register_diagnostics! {
2156 // E0006 // merged with E0005
2159 E0229, // associated type bindings are not allowed here
2160 E0264, // unknown external lang item
2161 E0278, // requirement is not satisfied
2162 E0279, // requirement is not satisfied
2163 E0280, // requirement is not satisfied
2164 E0283, // cannot resolve type
2165 E0284, // cannot resolve type
2166 E0285, // overflow evaluation builtin bounds
2167 E0298, // mismatched types between arms
2168 E0299, // mismatched types between arms
2169 E0300, // unexpanded macro
2170 E0304, // expected signed integer constant
2171 E0305, // expected constant
2172 E0311, // thing may not live long enough
2173 E0312, // lifetime of reference outlives lifetime of borrowed content
2174 E0313, // lifetime of borrowed pointer outlives lifetime of captured variable
2175 E0314, // closure outlives stack frame
2176 E0315, // cannot invoke closure outside of its lifetime
2177 E0316, // nested quantification of lifetimes
2178 E0400, // overloaded derefs are not allowed in constants
2179 E0452, // malformed lint attribute
2180 E0453, // overruled by outer forbid
2181 E0471, // constant evaluation error: ..
2182 E0472, // asm! is unsupported on this target
2183 E0473, // dereference of reference outside its lifetime
2184 E0474, // captured variable `..` does not outlive the enclosing closure
2185 E0475, // index of slice outside its lifetime
2186 E0476, // lifetime of the source pointer does not outlive lifetime bound...
2187 E0477, // the type `..` does not fulfill the required lifetime...
2188 E0478, // lifetime bound not satisfied
2189 E0479, // the type `..` (provided as the value of a type parameter) is...
2190 E0480, // lifetime of method receiver does not outlive the method call
2191 E0481, // lifetime of function argument does not outlive the function call
2192 E0482, // lifetime of return value does not outlive the function call
2193 E0483, // lifetime of operand does not outlive the operation
2194 E0484, // reference is not valid at the time of borrow
2195 E0485, // automatically reference is not valid at the time of borrow
2196 E0486, // type of expression contains references that are not valid during...
2197 E0487, // unsafe use of destructor: destructor might be called while...
2198 E0488, // lifetime of variable does not enclose its declaration
2199 E0489, // type/lifetime parameter not in scope here
2200 E0490, // a value of type `..` is borrowed for too long
2201 E0491, // in type `..`, reference has a longer lifetime than the data it...
2202 E0495, // cannot infer an appropriate lifetime due to conflicting requirements