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 illegal 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.
225 Initializers for constants and statics are evaluated at compile time.
226 User-defined operators rely on user-defined functions, which cannot be evaluated
236 impl Index<u8> for Foo {
239 fn index<'a>(&'a self, idx: u8) -> &'a u8 { &self.a }
242 const a: Foo = Foo { a: 0u8 };
243 const b: u8 = a[0]; // Index trait is defined by the user, bad!
246 Only operators on builtin types are allowed.
251 const a: &'static [i32] = &[1, 2, 3];
252 const b: i32 = a[0]; // Good!
257 Static and const variables can refer to other const variables. But a const
258 variable cannot refer to a static variable. For example, `Y` cannot refer to `X`
266 To fix this, the value can be extracted as a const and then used:
276 Constants can only be initialized by a constant value or, in a future
277 version of Rust, a call to a const function. This error indicates the use
278 of a path (like a::b, or x) denoting something other than one of these
279 allowed items. Example:
282 const FOO: i32 = { let x = 0; x }; // 'x' isn't a constant nor a function!
285 To avoid it, you have to replace the non-constant value:
288 const FOO: i32 = { const X : i32 = 0; X };
290 const FOO: i32 = { 0 }; // but brackets are useless here
295 The only functions that can be called in static or constant expressions are
296 `const` functions. Rust currently does not support more general compile-time
299 See [RFC 911] for more details on the design of `const fn`s.
301 [RFC 911]: https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md
305 Blocks in constants may only contain items (such as constant, function
306 definition, etc...) and a tail expression. Example:
309 const FOO: i32 = { let x = 0; x }; // 'x' isn't an item!
312 To avoid it, you have to replace the non-item object:
315 const FOO: i32 = { const X : i32 = 0; X };
320 References in statics and constants may only refer to immutable values. Example:
326 // these three are not allowed:
327 const CR: &'static mut i32 = &mut C;
328 static STATIC_REF: &'static mut i32 = &mut X;
329 static CONST_REF: &'static mut i32 = &mut C;
332 Statics are shared everywhere, and if they refer to mutable data one might
333 violate memory safety since holding multiple mutable references to shared data
336 If you really want global mutable state, try using `static mut` or a global
342 The value of static and const variables must be known at compile time. You
343 can't cast a pointer as an integer because we can't know what value the
346 However, pointers to other constants' addresses are allowed in constants,
351 const Y: *const u32 = &X;
354 Therefore, casting one of these non-constant pointers to an integer results
355 in a non-constant integer which lead to this error. Example:
359 const Y: usize = &X as *const u32 as usize;
365 A function call isn't allowed in the const's initialization expression
366 because the expression's value must be known at compile-time. Example of
375 fn test(&self) -> i32 {
381 const FOO: Test = Test::V1;
383 const A: i32 = FOO.test(); // You can't call Test::func() here !
387 Remember: you can't use a function call inside a const's initialization
388 expression! However, you can totally use it anywhere else:
392 const FOO: Test = Test::V1;
394 FOO.func(); // here is good
395 let x = FOO.func(); // or even here!
401 This error indicates that an attempt was made to divide by zero (or take the
402 remainder of a zero divisor) in a static or constant expression.
406 Constant functions are not allowed to mutate anything. Thus, binding to an
407 argument with a mutable pattern is not allowed. For example,
410 const fn foo(mut x: u8) {
415 is bad because the function body may not mutate `x`.
417 Remove any mutable bindings from the argument list to fix this error. In case
418 you need to mutate the argument, try lazily initializing a global variable
419 instead of using a `const fn`, or refactoring the code to a functional style to
420 avoid mutation if possible.
424 When matching against a range, the compiler verifies that the range is
425 non-empty. Range patterns include both end-points, so this is equivalent to
426 requiring the start of the range to be less than or equal to the end of the
433 // This range is ok, albeit pointless.
435 // This range is empty, and the compiler can tell.
442 Trait objects like `Box<Trait>` can only be constructed when certain
443 requirements are satisfied by the trait in question.
445 Trait objects are a form of dynamic dispatch and use a dynamically sized type
446 for the inner type. So, for a given trait `Trait`, when `Trait` is treated as a
447 type, as in `Box<Trait>`, the inner type is 'unsized'. In such cases the boxed
448 pointer is a 'fat pointer' that contains an extra pointer to a table of methods
449 (among other things) for dynamic dispatch. This design mandates some
450 restrictions on the types of traits that are allowed to be used in trait
451 objects, which are collectively termed as 'object safety' rules.
453 Attempting to create a trait object for a non object-safe trait will trigger
456 There are various rules:
458 ### The trait cannot require `Self: Sized`
460 When `Trait` is treated as a type, the type does not implement the special
461 `Sized` trait, because the type does not have a known size at compile time and
462 can only be accessed behind a pointer. Thus, if we have a trait like the
466 trait Foo where Self: Sized {
471 we cannot create an object of type `Box<Foo>` or `&Foo` since in this case
472 `Self` would not be `Sized`.
474 Generally, `Self : Sized` is used to indicate that the trait should not be used
475 as a trait object. If the trait comes from your own crate, consider removing
478 ### Method references the `Self` type in its arguments or return type
480 This happens when a trait has a method like the following:
484 fn foo(&self) -> Self;
487 impl Trait for String {
488 fn foo(&self) -> Self {
494 fn foo(&self) -> Self {
500 (Note that `&self` and `&mut self` are okay, it's additional `Self` types which
503 In such a case, the compiler cannot predict the return type of `foo()` in a
504 situation like the following:
507 fn call_foo(x: Box<Trait>) {
508 let y = x.foo(); // What type is y?
513 If only some methods aren't object-safe, you can add a `where Self: Sized` bound
514 on them to mark them as explicitly unavailable to trait objects. The
515 functionality will still be available to all other implementers, including
516 `Box<Trait>` which is itself sized (assuming you `impl Trait for Box<Trait>`).
520 fn foo(&self) -> Self where Self: Sized;
525 Now, `foo()` can no longer be called on a trait object, but you will now be
526 allowed to make a trait object, and that will be able to call any object-safe
527 methods". With such a bound, one can still call `foo()` on types implementing
528 that trait that aren't behind trait objects.
530 ### Method has generic type parameters
532 As mentioned before, trait objects contain pointers to method tables. So, if we
539 impl Trait for String {
552 At compile time each implementation of `Trait` will produce a table containing
553 the various methods (and other items) related to the implementation.
555 This works fine, but when the method gains generic parameters, we can have a
558 Usually, generic parameters get _monomorphized_. For example, if I have
566 the machine code for `foo::<u8>()`, `foo::<bool>()`, `foo::<String>()`, or any
567 other type substitution is different. Hence the compiler generates the
568 implementation on-demand. If you call `foo()` with a `bool` parameter, the
569 compiler will only generate code for `foo::<bool>()`. When we have additional
570 type parameters, the number of monomorphized implementations the compiler
571 generates does not grow drastically, since the compiler will only generate an
572 implementation if the function is called with unparametrized substitutions
573 (i.e., substitutions where none of the substituted types are themselves
576 However, with trait objects we have to make a table containing _every_ object
577 that implements the trait. Now, if it has type parameters, we need to add
578 implementations for every type that implements the trait, and there could
579 theoretically be an infinite number of types.
585 fn foo<T>(&self, on: T);
588 impl Trait for String {
589 fn foo<T>(&self, on: T) {
594 fn foo<T>(&self, on: T) {
598 // 8 more implementations
601 Now, if we have the following code:
604 fn call_foo(thing: Box<Trait>) {
605 thing.foo(true); // this could be any one of the 8 types above
611 we don't just need to create a table of all implementations of all methods of
612 `Trait`, we need to create such a table, for each different type fed to
613 `foo()`. In this case this turns out to be (10 types implementing `Trait`)*(3
614 types being fed to `foo()`) = 30 implementations!
616 With real world traits these numbers can grow drastically.
618 To fix this, it is suggested to use a `where Self: Sized` bound similar to the
619 fix for the sub-error above if you do not intend to call the method with type
624 fn foo<T>(&self, on: T) where Self: Sized;
629 If this is not an option, consider replacing the type parameter with another
630 trait object (e.g. if `T: OtherTrait`, use `on: Box<OtherTrait>`). If the number
631 of types you intend to feed to this method is limited, consider manually listing
632 out the methods of different types.
634 ### Method has no receiver
636 Methods that do not take a `self` parameter can't be called since there won't be
637 a way to get a pointer to the method table for them
645 This could be called as `<Foo as Foo>::foo()`, which would not be able to pick
648 Adding a `Self: Sized` bound to these methods will generally make this compile.
652 fn foo() -> u8 where Self: Sized;
656 ### The trait cannot use `Self` as a type parameter in the supertrait listing
658 This is similar to the second sub-error, but subtler. It happens in situations
664 trait Trait: Super<Self> {
669 impl Super<Foo> for Foo{}
671 impl Trait for Foo {}
674 Here, the supertrait might have methods as follows:
678 fn get_a(&self) -> A; // note that this is object safe!
682 If the trait `Foo` was deriving from something like `Super<String>` or
683 `Super<T>` (where `Foo` itself is `Foo<T>`), this is okay, because given a type
684 `get_a()` will definitely return an object of that type.
686 However, if it derives from `Super<Self>`, even though `Super` is object safe,
687 the method `get_a()` would return an object of unknown type when called on the
688 function. `Self` type parameters let us make object safe traits no longer safe,
689 so they are forbidden when specifying supertraits.
691 There's no easy fix for this, generally code will need to be refactored so that
692 you no longer need to derive from `Super<Self>`.
696 Enum variants which contain no data can be given a custom integer
697 representation. This error indicates that the value provided is not an integer
698 literal and is therefore invalid.
700 For example, in the following code,
708 we try to set the representation to a string.
710 There's no general fix for this; if you can work with an integer then just set
719 however if you actually wanted a mapping between variants and non-integer
720 objects, it may be preferable to use a method with a match instead:
725 fn get_str(&self) -> &'static str {
735 This error indicates that the compiler was unable to sensibly evaluate an
736 integer expression provided as an enum discriminant. Attempting to divide by 0
737 or causing integer overflow are two ways to induce this error. For example:
746 Ensure that the expressions given can be evaluated as the desired integer type.
747 See the FFI section of the Reference for more information about using a custom
750 https://doc.rust-lang.org/reference.html#ffi-attributes
754 You tried to give a type parameter to a type which doesn't need it. Erroneous
758 type X = u32<i32>; // error: type parameters are not allowed on this type
761 Please check that you used the correct type and recheck its definition. Perhaps
762 it doesn't need the type parameter.
767 type X = u32; // this compiles
772 You tried to give a lifetime parameter to a type which doesn't need it.
773 Erroneous code example:
776 type X = u32<'static>; // error: lifetime parameters are not allowed on
780 Please check that the correct type was used and recheck its definition; perhaps
781 it doesn't need the lifetime parameter. Example:
789 Using unsafe functionality, such as dereferencing raw pointers and calling
790 functions via FFI or marked as unsafe, is potentially dangerous and disallowed
791 by safety checks. These safety checks can be relaxed for a section of the code
792 by wrapping the unsafe instructions with an `unsafe` block. For instance:
795 unsafe fn f() { return; }
802 See also https://doc.rust-lang.org/book/unsafe.html
805 // This shouldn't really ever trigger since the repeated value error comes first
807 A binary can only have one entry point, and by default that entry point is the
808 function `main()`. If there are multiple such functions, please rename one.
812 This error indicates that the compiler found multiple functions with the
813 `#[main]` attribute. This is an error because there must be a unique entry
814 point into a Rust program.
818 This error indicates that the compiler found multiple functions with the
819 `#[start]` attribute. This is an error because there must be a unique entry
820 point into a Rust program.
823 // FIXME link this to the relevant turpl chapters for instilling fear of the
824 // transmute gods in the user
826 There are various restrictions on transmuting between types in Rust; for example
827 types being transmuted must have the same size. To apply all these restrictions,
828 the compiler must know the exact types that may be transmuted. When type
829 parameters are involved, this cannot always be done.
831 So, for example, the following is not allowed:
834 struct Foo<T>(Vec<T>)
836 fn foo<T>(x: Vec<T>) {
837 // we are transmuting between Vec<T> and Foo<T> here
838 let y: Foo<T> = unsafe { transmute(x) };
839 // do something with y
843 In this specific case there's a good chance that the transmute is harmless (but
844 this is not guaranteed by Rust). However, when alignment and enum optimizations
845 come into the picture, it's quite likely that the sizes may or may not match
846 with different type parameter substitutions. It's not possible to check this for
847 _all_ possible types, so `transmute()` simply only accepts types without any
848 unsubstituted type parameters.
850 If you need this, there's a good chance you're doing something wrong. Keep in
851 mind that Rust doesn't guarantee much about the layout of different structs
852 (even two structs with identical declarations may have different layouts). If
853 there is a solution that avoids the transmute entirely, try it instead.
855 If it's possible, hand-monomorphize the code by writing the function for each
856 possible type substitution. It's possible to use traits to do this cleanly,
860 trait MyTransmutableType {
861 fn transmute(Vec<Self>) -> Foo<Self>
864 impl MyTransmutableType for u8 {
865 fn transmute(x: Foo<u8>) -> Vec<u8> {
869 impl MyTransmutableType for String {
870 fn transmute(x: Foo<String>) -> Vec<String> {
874 // ... more impls for the types you intend to transmute
876 fn foo<T: MyTransmutableType>(x: Vec<T>) {
877 let y: Foo<T> = <T as MyTransmutableType>::transmute(x);
878 // do something with y
882 Each impl will be checked for a size match in the transmute as usual, and since
883 there are no unbound type parameters involved, this should compile unless there
884 is a size mismatch in one of the impls.
886 It is also possible to manually transmute:
889 let result: SomeType = mem::uninitialized();
890 unsafe { copy_nonoverlapping(&v, &result) };
891 result // `v` transmuted to type `SomeType`
896 Lang items are already implemented in the standard library. Unless you are
897 writing a free-standing application (e.g. a kernel), you do not need to provide
900 You can build a free-standing crate by adding `#![no_std]` to the crate
908 See also https://doc.rust-lang.org/book/no-stdlib.html
912 `const` and `static` mean different things. A `const` is a compile-time
913 constant, an alias for a literal value. This property means you can match it
914 directly within a pattern.
916 The `static` keyword, on the other hand, guarantees a fixed location in memory.
917 This does not always mean that the value is constant. For example, a global
918 mutex can be declared `static` as well.
920 If you want to match against a `static`, consider using a guard instead:
923 static FORTY_TWO: i32 = 42;
925 Some(x) if x == FORTY_TWO => ...
932 In Rust, you can only move a value when its size is known at compile time.
934 To work around this restriction, consider "hiding" the value behind a reference:
935 either `&x` or `&mut x`. Since a reference has a fixed size, this lets you move
940 An if-let pattern attempts to match the pattern, and enters the body if the
941 match was successful. If the match is irrefutable (when it cannot fail to
942 match), use a regular `let`-binding instead. For instance:
945 struct Irrefutable(i32);
946 let irr = Irrefutable(0);
948 // This fails to compile because the match is irrefutable.
949 if let Irrefutable(x) = irr {
950 // This body will always be executed.
955 let Irrefutable(x) = irr;
961 A while-let pattern attempts to match the pattern, and enters the body if the
962 match was successful. If the match is irrefutable (when it cannot fail to
963 match), use a regular `let`-binding inside a `loop` instead. For instance:
966 struct Irrefutable(i32);
967 let irr = Irrefutable(0);
969 // This fails to compile because the match is irrefutable.
970 while let Irrefutable(x) = irr {
976 let Irrefutable(x) = irr;
983 Enum variants are qualified by default. For example, given this type:
992 you would match it using:
1001 If you don't qualify the names, the code will bind new variables named "GET" and
1002 "POST" instead. This behavior is likely not what you want, so `rustc` warns when
1005 Qualified names are good practice, and most code works well with them. But if
1006 you prefer them unqualified, you can import the variants into scope:
1010 enum Method { GET, POST }
1013 If you want others to be able to import variants from your module directly, use
1018 enum Method { GET, POST }
1023 When using a lifetime like `'a` in a type, it must be declared before being
1026 These two examples illustrate the problem:
1029 // error, use of undeclared lifetime name `'a`
1030 fn foo(x: &'a str) { }
1033 // error, use of undeclared lifetime name `'a`
1038 These can be fixed by declaring lifetime parameters:
1041 fn foo<'a>(x: &'a str) { }
1050 Declaring certain lifetime names in parameters is disallowed. For example,
1051 because the `'static` lifetime is a special built-in lifetime name denoting
1052 the lifetime of the entire program, this is an error:
1055 // error, illegal lifetime parameter name `'static`
1056 fn foo<'static>(x: &'static str) { }
1061 A lifetime name cannot be declared more than once in the same scope. For
1065 // error, lifetime name `'a` declared twice in the same scope
1066 fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str) { }
1071 This error indicates that a static or constant references itself.
1072 All statics and constants need to resolve to a value in an acyclic manner.
1074 For example, neither of the following can be sensibly compiled:
1087 This error indicates the use of a loop keyword (`break` or `continue`) inside a
1088 closure but outside of any loop. Erroneous code example:
1091 let w = || { break; }; // error: `break` inside of a closure
1094 `break` and `continue` keywords can be used as normal inside closures as long as
1095 they are also contained within a loop. To halt the execution of a closure you
1096 should instead use a return statement. Example:
1110 This error indicates the use of a loop keyword (`break` or `continue`) outside
1111 of a loop. Without a loop to break out of or continue in, no sensible action can
1112 be taken. Erroneous code example:
1116 break; // error: `break` outside of loop
1120 Please verify that you are using `break` and `continue` only in loops. Example:
1132 Functions must eventually return a value of their return type. For example, in
1133 the following function
1136 fn foo(x: u8) -> u8 {
1138 x // alternatively, `return x`
1144 if the condition is true, the value `x` is returned, but if the condition is
1145 false, control exits the `if` block and reaches a place where nothing is being
1146 returned. All possible control paths must eventually return a `u8`, which is not
1149 An easy fix for this in a complicated function is to specify a default return
1153 fn foo(x: u8) -> u8 {
1155 x // alternatively, `return x`
1157 // lots of other if branches
1158 0 // return 0 if all else fails
1162 It is advisable to find out what the unhandled cases are and check for them,
1163 returning an appropriate value or panicking if necessary.
1167 Rust lets you define functions which are known to never return, i.e. are
1168 'diverging', by marking its return type as `!`.
1170 For example, the following functions never return:
1178 foo() // foo() is diverging, so this will diverge too
1182 panic!(); // this macro internally expands to a call to a diverging function
1187 Such functions can be used in a place where a value is expected without
1188 returning a value of that type, for instance:
1194 _ => foo() // diverging function called here
1199 If the third arm of the match block is reached, since `foo()` doesn't ever
1200 return control to the match block, it is fine to use it in a place where an
1201 integer was expected. The `match` block will never finish executing, and any
1202 point where `y` (like the print statement) is needed will not be reached.
1204 However, if we had a diverging function that actually does finish execution
1212 then we would have an unknown value for `y` in the following code:
1223 In the previous example, the print statement was never reached when the wildcard
1224 match arm was hit, so we were okay with `foo()` not returning an integer that we
1225 could set to `y`. But in this example, `foo()` actually does return control, so
1226 the print statement will be executed with an uninitialized value.
1228 Obviously we cannot have functions which are allowed to be used in such
1229 positions and yet can return control. So, if you are defining a function that
1230 returns `!`, make sure that there is no way for it to actually finish executing.
1234 This is because of a type mismatch between the associated type of some
1235 trait (e.g. `T::Bar`, where `T` implements `trait Quux { type Bar; }`)
1236 and another type `U` that is required to be equal to `T::Bar`, but is not.
1239 Here is a basic example:
1242 trait Trait { type AssociatedType; }
1243 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
1246 impl Trait for i8 { type AssociatedType = &'static str; }
1250 Here is that same example again, with some explanatory comments:
1253 trait Trait { type AssociatedType; }
1255 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
1256 // ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
1258 // This says `foo` can |
1259 // only be used with |
1261 // implements `Trait`. |
1263 // This says not only must
1264 // `T` be an impl of `Trait`
1265 // but also that the impl
1266 // must assign the type `u32`
1267 // to the associated type.
1271 impl Trait for i8 { type AssociatedType = &'static str; }
1272 ~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1277 // ... but it is an implementation
1278 // that assigns `&'static str` to
1279 // the associated type.
1282 // Here, we invoke `foo` with an `i8`, which does not satisfy
1283 // the constraint `<i8 as Trait>::AssociatedType=u32`, and
1284 // therefore the type-checker complains with this error code.
1287 Here is a more subtle instance of the same problem, that can
1288 arise with for-loops in Rust:
1291 let vs: Vec<i32> = vec![1, 2, 3, 4];
1300 The above fails because of an analogous type mismatch,
1301 though may be harder to see. Again, here are some
1302 explanatory comments for the same example:
1306 let vs = vec![1, 2, 3, 4];
1308 // `for`-loops use a protocol based on the `Iterator`
1309 // trait. Each item yielded in a `for` loop has the
1310 // type `Iterator::Item` -- that is,I `Item` is the
1311 // associated type of the concrete iterator impl.
1315 // | We borrow `vs`, iterating over a sequence of
1316 // | *references* of type `&Elem` (where `Elem` is
1317 // | vector's element type). Thus, the associated
1318 // | type `Item` must be a reference `&`-type ...
1320 // ... and `v` has the type `Iterator::Item`, as dictated by
1321 // the `for`-loop protocol ...
1327 // ... but *here*, `v` is forced to have some integral type;
1328 // only types like `u8`,`i8`,`u16`,`i16`, et cetera can
1329 // match the pattern `1` ...
1334 // ... therefore, the compiler complains, because it sees
1335 // an attempt to solve the equations
1336 // `some integral-type` = type-of-`v`
1337 // = `Iterator::Item`
1338 // = `&Elem` (i.e. `some reference type`)
1340 // which cannot possibly all be true.
1346 To avoid those issues, you have to make the types match correctly.
1347 So we can fix the previous examples like this:
1351 trait Trait { type AssociatedType; }
1352 fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
1355 impl Trait for i8 { type AssociatedType = &'static str; }
1358 // For-Loop Example:
1359 let vs = vec![1, 2, 3, 4];
1370 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
1371 message for when a particular trait isn't implemented on a type placed in a
1372 position that needs that trait. For example, when the following code is
1376 fn foo<T: Index<u8>>(x: T){}
1378 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
1379 trait Index<Idx> { ... }
1381 foo(true); // `bool` does not implement `Index<u8>`
1384 there will be an error about `bool` not implementing `Index<u8>`, followed by a
1385 note saying "the type `bool` cannot be indexed by `u8`".
1387 As you can see, you can specify type parameters in curly braces for substitution
1388 with the actual types (using the regular format string syntax) in a given
1389 situation. Furthermore, `{Self}` will substitute to the type (in this case,
1390 `bool`) that we tried to use.
1392 This error appears when the curly braces contain an identifier which doesn't
1393 match with any of the type parameters or the string `Self`. This might happen if
1394 you misspelled a type parameter, or if you intended to use literal curly braces.
1395 If it is the latter, escape the curly braces with a second curly brace of the
1396 same type; e.g. a literal `{` is `{{`
1400 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
1401 message for when a particular trait isn't implemented on a type placed in a
1402 position that needs that trait. For example, when the following code is
1406 fn foo<T: Index<u8>>(x: T){}
1408 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
1409 trait Index<Idx> { ... }
1411 foo(true); // `bool` does not implement `Index<u8>`
1414 there will be an error about `bool` not implementing `Index<u8>`, followed by a
1415 note saying "the type `bool` cannot be indexed by `u8`".
1417 As you can see, you can specify type parameters in curly braces for substitution
1418 with the actual types (using the regular format string syntax) in a given
1419 situation. Furthermore, `{Self}` will substitute to the type (in this case,
1420 `bool`) that we tried to use.
1422 This error appears when the curly braces do not contain an identifier. Please
1423 add one of the same name as a type parameter. If you intended to use literal
1424 braces, use `{{` and `}}` to escape them.
1428 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
1429 message for when a particular trait isn't implemented on a type placed in a
1430 position that needs that trait. For example, when the following code is
1434 fn foo<T: Index<u8>>(x: T){}
1436 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
1437 trait Index<Idx> { ... }
1439 foo(true); // `bool` does not implement `Index<u8>`
1442 there will be an error about `bool` not implementing `Index<u8>`, followed by a
1443 note saying "the type `bool` cannot be indexed by `u8`".
1445 For this to work, some note must be specified. An empty attribute will not do
1446 anything, please remove the attribute or add some helpful note for users of the
1451 This error occurs when there was a recursive trait requirement that overflowed
1452 before it could be evaluated. Often this means that there is unbounded recursion
1453 in resolving some type bounds.
1455 For example, in the following code
1462 impl<T> Foo for T where Bar<T>: Foo {}
1465 to determine if a `T` is `Foo`, we need to check if `Bar<T>` is `Foo`. However,
1466 to do this check, we need to determine that `Bar<Bar<T>>` is `Foo`. To determine
1467 this, we check if `Bar<Bar<Bar<T>>>` is `Foo`, and so on. This is clearly a
1468 recursive requirement that can't be resolved directly.
1470 Consider changing your trait bounds so that they're less self-referential.
1474 This error occurs when a bound in an implementation of a trait does not match
1475 the bounds specified in the original trait. For example:
1483 fn foo<T>(x: T) where T: Copy {}
1487 Here, all types implementing `Foo` must have a method `foo<T>(x: T)` which can
1488 take any type `T`. However, in the `impl` for `bool`, we have added an extra
1489 bound that `T` is `Copy`, which isn't compatible with the original trait.
1491 Consider removing the bound from the method or adding the bound to the original
1492 method definition in the trait.
1496 You tried to use a type which doesn't implement some trait in a place which
1497 expected that trait. Erroneous code example:
1500 // here we declare the Foo trait with a bar method
1505 // we now declare a function which takes an object implementing the Foo trait
1506 fn some_func<T: Foo>(foo: T) {
1511 // we now call the method with the i32 type, which doesn't implement
1513 some_func(5i32); // error: the trait `Foo` is not implemented for the
1518 In order to fix this error, verify that the type you're using does implement
1526 fn some_func<T: Foo>(foo: T) {
1527 foo.bar(); // we can now use this method since i32 implements the
1531 // we implement the trait on the i32 type
1537 some_func(5i32); // ok!
1543 This error indicates that type inference did not result in one unique possible
1544 type, and extra information is required. In most cases this can be provided
1545 by adding a type annotation. Sometimes you need to specify a generic type
1548 A common example is the `collect` method on `Iterator`. It has a generic type
1549 parameter with a `FromIterator` bound, which for a `char` iterator is
1550 implemented by `Vec` and `String` among others. Consider the following snippet
1551 that reverses the characters of a string:
1554 let x = "hello".chars().rev().collect();
1557 In this case, the compiler cannot infer what the type of `x` should be:
1558 `Vec<char>` and `String` are both suitable candidates. To specify which type to
1559 use, you can use a type annotation on `x`:
1562 let x: Vec<char> = "hello".chars().rev().collect();
1565 It is not necessary to annotate the full type. Once the ambiguity is resolved,
1566 the compiler can infer the rest:
1569 let x: Vec<_> = "hello".chars().rev().collect();
1572 Another way to provide the compiler with enough information, is to specify the
1573 generic type parameter:
1576 let x = "hello".chars().rev().collect::<Vec<char>>();
1579 Again, you need not specify the full type if the compiler can infer it:
1582 let x = "hello".chars().rev().collect::<Vec<_>>();
1585 Apart from a method or function with a generic type parameter, this error can
1586 occur when a type parameter of a struct or trait cannot be inferred. In that
1587 case it is not always possible to use a type annotation, because all candidates
1588 have the same return type. For instance:
1592 // Some fields omitted.
1601 let number = Foo::bar();
1606 This will fail because the compiler does not know which instance of `Foo` to
1607 call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.
1611 This error indicates that the given recursion limit could not be parsed. Ensure
1612 that the value provided is a positive integer between quotes, like so:
1615 #![recursion_limit="1000"]
1620 Patterns used to bind names must be irrefutable. That is, they must guarantee
1621 that a name will be extracted in all cases. Instead of pattern matching the
1622 loop variable, consider using a `match` or `if let` inside the loop body. For
1626 // This fails because `None` is not covered.
1631 // Match inside the loop instead:
1641 if let Some(x) = item {
1649 Mutable borrows are not allowed in pattern guards, because matching cannot have
1650 side effects. Side effects could alter the matched object or the environment
1651 on which the match depends in such a way, that the match would not be
1652 exhaustive. For instance, the following would not match any arm if mutable
1653 borrows were allowed:
1658 option if option.take().is_none() => { /* impossible, option is `Some` */ },
1659 Some(_) => { } // When the previous match failed, the option became `None`.
1665 Assignments are not allowed in pattern guards, because matching cannot have
1666 side effects. Side effects could alter the matched object or the environment
1667 on which the match depends in such a way, that the match would not be
1668 exhaustive. For instance, the following would not match any arm if assignments
1674 option if { option = None; false } { },
1675 Some(_) => { } // When the previous match failed, the option became `None`.
1681 In certain cases it is possible for sub-bindings to violate memory safety.
1682 Updates to the borrow checker in a future version of Rust may remove this
1683 restriction, but for now patterns must be rewritten without sub-bindings.
1687 match Some("hi".to_string()) {
1688 ref op_string_ref @ Some(ref s) => ...
1693 match Some("hi".to_string()) {
1695 let op_string_ref = &Some(s);
1702 The `op_string_ref` binding has type `&Option<&String>` in both cases.
1704 See also https://github.com/rust-lang/rust/issues/14587
1708 In an array literal `[x; N]`, `N` is the number of elements in the array. This
1709 number cannot be negative.
1713 The length of an array is part of its type. For this reason, this length must be
1714 a compile-time constant.
1718 This error occurs when the compiler was unable to infer the concrete type of a
1719 variable. It can occur for several cases, the most common of which is a
1720 mismatch in the expected type that the compiler inferred for a variable's
1721 initializing expression, and the actual type explicitly assigned to the
1727 let x: i32 = "I am not a number!";
1728 // ~~~ ~~~~~~~~~~~~~~~~~~~~
1730 // | initializing expression;
1731 // | compiler infers type `&str`
1733 // type `i32` assigned to variable `x`
1738 Types in type definitions have lifetimes associated with them that represent
1739 how long the data stored within them is guaranteed to be live. This lifetime
1740 must be as long as the data needs to be alive, and missing the constraint that
1741 denotes this will cause this error.
1744 // This won't compile because T is not constrained, meaning the data
1745 // stored in it is not guaranteed to last as long as the reference
1750 // This will compile, because it has the constraint on the type parameter
1751 struct Foo<'a, T: 'a> {
1758 Types in type definitions have lifetimes associated with them that represent
1759 how long the data stored within them is guaranteed to be live. This lifetime
1760 must be as long as the data needs to be alive, and missing the constraint that
1761 denotes this will cause this error.
1764 // This won't compile because T is not constrained to the static lifetime
1765 // the reference needs
1770 // This will compile, because it has the constraint on the type parameter
1771 struct Foo<T: 'static> {
1778 Method calls that aren't calls to inherent `const` methods are disallowed
1779 in statics, constants, and constant functions.
1784 const BAZ: i32 = Foo(25).bar(); // error, `bar` isn't `const`
1789 const fn foo(&self) -> i32 {
1790 self.bar() // error, `bar` isn't `const`
1793 fn bar(&self) -> i32 { self.0 }
1797 For more information about `const fn`'s, see [RFC 911].
1799 [RFC 911]: https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md
1805 > It is illegal for a static to reference another static by value. It is
1806 > required that all references be borrowed.
1808 [RFC 246]: https://github.com/rust-lang/rfcs/pull/246
1812 The value assigned to a constant expression must be known at compile time,
1813 which is not the case when comparing raw pointers. Erroneous code example:
1816 static foo: i32 = 42;
1817 static bar: i32 = 43;
1819 static baz: bool = { (&foo as *const i32) == (&bar as *const i32) };
1820 // error: raw pointers cannot be compared in statics!
1823 Please check that the result of the comparison can be determined at compile time
1824 or isn't assigned to a constant expression. Example:
1827 static foo: i32 = 42;
1828 static bar: i32 = 43;
1830 let baz: bool = { (&foo as *const i32) == (&bar as *const i32) };
1831 // baz isn't a constant expression so it's ok
1836 The value assigned to a constant expression must be known at compile time,
1837 which is not the case when dereferencing raw pointers. Erroneous code
1841 const foo: i32 = 42;
1842 const baz: *const i32 = (&foo as *const i32);
1844 const deref: i32 = *baz;
1845 // error: raw pointers cannot be dereferenced in constants
1848 To fix this error, please do not assign this value to a constant expression.
1852 const foo: i32 = 42;
1853 const baz: *const i32 = (&foo as *const i32);
1855 unsafe { let deref: i32 = *baz; }
1856 // baz isn't a constant expression so it's ok
1859 You'll also note that this assignment must be done in an unsafe block!
1863 It is not allowed for a mutable static to allocate or have destructors. For
1867 // error: mutable statics are not allowed to have boxes
1868 static mut FOO: Option<Box<usize>> = None;
1870 // error: mutable statics are not allowed to have destructors
1871 static mut BAR: Option<Vec<i32>> = None;
1876 In Rust 1.3, the default object lifetime bounds are expected to
1877 change, as described in RFC #1156 [1]. You are getting a warning
1878 because the compiler thinks it is possible that this change will cause
1879 a compilation error in your code. It is possible, though unlikely,
1880 that this is a false alarm.
1882 The heart of the change is that where `&'a Box<SomeTrait>` used to
1883 default to `&'a Box<SomeTrait+'a>`, it now defaults to `&'a
1884 Box<SomeTrait+'static>` (here, `SomeTrait` is the name of some trait
1885 type). Note that the only types which are affected are references to
1886 boxes, like `&Box<SomeTrait>` or `&[Box<SomeTrait>]`. More common
1887 types like `&SomeTrait` or `Box<SomeTrait>` are unaffected.
1889 To silence this warning, edit your code to use an explicit bound.
1890 Most of the time, this means that you will want to change the
1891 signature of a function that you are calling. For example, if
1892 the error is reported on a call like `foo(x)`, and `foo` is
1896 fn foo(arg: &Box<SomeTrait>) { ... }
1899 you might change it to:
1902 fn foo<'a>(arg: &Box<SomeTrait+'a>) { ... }
1905 This explicitly states that you expect the trait object `SomeTrait` to
1906 contain references (with a maximum lifetime of `'a`).
1908 [1]: https://github.com/rust-lang/rfcs/pull/1156
1914 register_diagnostics! {
1915 // E0006 // merged with E0005
1918 E0264, // unknown external lang item
1919 E0278, // requirement is not satisfied
1920 E0279, // requirement is not satisfied
1921 E0280, // requirement is not satisfied
1922 E0281, // type implements trait but other trait is required
1923 E0283, // cannot resolve type
1924 E0284, // cannot resolve type
1925 E0285, // overflow evaluation builtin bounds
1926 E0298, // mismatched types between arms
1927 E0299, // mismatched types between arms
1928 E0300, // unexpanded macro
1929 E0304, // expected signed integer constant
1930 E0305, // expected constant
1931 E0311, // thing may not live long enough
1932 E0312, // lifetime of reference outlives lifetime of borrowed content
1933 E0313, // lifetime of borrowed pointer outlives lifetime of captured variable
1934 E0314, // closure outlives stack frame
1935 E0315, // cannot invoke closure outside of its lifetime
1936 E0316, // nested quantification of lifetimes
1937 E0370, // discriminant overflow
1938 E0400 // overloaded derefs are not allowed in constants