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
878 See also https
://doc.rust-lang.org/book/no-stdlib.html
882 `
const` and `
static` mean different things
. A `
const` is a compile
-time
883 constant
, an alias
for a literal value
. This property means you can
match it
884 directly within a pattern
.
886 The `
static` keyword
, on the other hand
, guarantees a fixed location
in memory
.
887 This does not always mean that the value is constant
. For example
, a global
888 mutex can be declared `
static`
as well
.
890 If you want to
match against a `
static`
, consider using a guard instead
:
893 static FORTY_TWO
: i32 = 42;
895 Some(x
) if x
== FORTY_TWO
=> ...
902 In Rust
, you can only
move a value when its size is known at compile time
.
904 To work around this restriction
, consider
"hiding" the value behind a reference
:
905 either `
&x` or `
&mut x`
. Since a reference has a fixed size
, this lets you
move
910 An
if-let pattern attempts to
match the pattern
, and enters the body
if the
911 match was successful
. If the
match is
irrefutable (when it cannot fail to
912 match), use a regular `
let`
-binding instead
. For instance
:
915 struct Irrefutable(i32);
916 let irr
= Irrefutable(0);
918 // This fails to compile because the match is irrefutable.
919 if let Irrefutable(x
) = irr
{
920 // This body will always be executed.
925 let Irrefutable(x
) = irr
;
931 A
while-let pattern attempts to
match the pattern
, and enters the body
if the
932 match was successful
. If the
match is
irrefutable (when it cannot fail to
933 match), use a regular `
let`
-binding inside a `
loop` instead
. For instance
:
936 struct Irrefutable(i32);
937 let irr
= Irrefutable(0);
939 // This fails to compile because the match is irrefutable.
940 while let Irrefutable(x
) = irr
{
946 let Irrefutable(x
) = irr
;
953 Enum variants are qualified by
default. For example
, given this
type:
962 you would
match it using
:
971 If you don't qualify the names
, the code will bind new variables named
"GET" and
972 "POST" instead
. This behavior is likely not what you want
, so `rustc` warns when
975 Qualified names are good practice
, and most code works well with them
. But
if
976 you prefer them unqualified
, you can import the variants into scope
:
980 enum Method { GET, POST }
983 If you want others to be able to import variants from your module directly
, use
988 enum Method { GET, POST }
993 When using a lifetime like `'a`
in a
type, it must be declared before being
996 These two examples illustrate the problem
:
999 // error, use of undeclared lifetime name `'a`
1000 fn foo(x
: &'a
str) { }
1003 // error, use of undeclared lifetime name `'a`
1008 These can be fixed by declaring lifetime parameters
:
1011 fn foo
<'a
>(x
: &'a
str) { }
1020 Declaring certain lifetime names
in parameters is disallowed
. For example
,
1021 because the `'
static` lifetime is a special built
-in lifetime name denoting
1022 the lifetime of the entire program
, this is an error
:
1025 // error, invalid lifetime parameter name `'static`
1026 fn foo
<'
static>(x
: &'
static str) { }
1031 A lifetime name cannot be declared more than once
in the same scope
. For
1035 // error, lifetime name `'a` declared twice in the same scope
1036 fn foo
<'a
, 'b
, 'a
>(x
: &'a
str, y
: &'b
str) { }
1041 An unknown external lang item was used
. Erroneous code example
:
1044 #![feature(lang_items)]
1047 #[lang = "cake"] // error: unknown external lang item: `cake`
1052 A list of available external lang items is available
in
1053 `src
/librustc
/middle
/weak_lang_items
.rs`
. Example
:
1056 #![feature(lang_items)]
1059 #[lang = "panic_fmt"] // ok!
1066 This error indicates that a
static or constant references itself
.
1067 All statics and constants need to resolve to a value
in an acyclic manner
.
1069 For example
, neither of the following can be sensibly compiled
:
1082 This error indicates the
use of a
loop keyword (`
break` or `
continue`
) inside a
1083 closure but outside of any
loop. Erroneous code example
:
1086 let w
= || { break; }
; // error: `break` inside of a closure
1089 `
break` and `
continue` keywords can be used
as normal inside closures
as long
as
1090 they are also contained within a
loop. To halt the execution of a closure you
1091 should instead
use a
return statement
. Example
:
1105 This error indicates the
use of a
loop keyword (`
break` or `
continue`
) outside
1106 of a
loop. Without a
loop to
break out of or
continue in, no sensible action can
1107 be taken
. Erroneous code example
:
1111 break; // error: `break` outside of loop
1115 Please verify that you are using `
break` and `
continue` only
in loops
. Example
:
1127 Functions must eventually
return a value of their
return type. For example
, in
1128 the following function
1131 fn foo(x
: u8) -> u8 {
1133 x
// alternatively, `return x`
1139 if the condition is
true, the value `x` is returned
, but
if the condition is
1140 false, control exits the `
if` block and reaches a place
where nothing is being
1141 returned
. All possible control paths must eventually
return a `
u8`
, which is not
1144 An easy fix
for this
in a complicated function is to specify a
default return
1148 fn foo(x
: u8) -> u8 {
1150 x
// alternatively, `return x`
1152 // lots of other if branches
1153 0 // return 0 if all else fails
1157 It is advisable to find out what the unhandled cases are and check
for them
,
1158 returning an appropriate value or panicking
if necessary
.
1162 Rust lets you define functions which are known to never
return, i
.e
. are
1163 'diverging'
, by marking its
return type as `
!`
.
1165 For example
, the following functions never
return:
1173 foo() // foo() is diverging, so this will diverge too
1177 panic
!(); // this macro internally expands to a call to a diverging function
1182 Such functions can be used
in a place
where a value is expected without
1183 returning a value of that
type, for instance
:
1189 _
=> foo() // diverging function called here
1194 If the third arm of the
match block is reached
, since `
foo()` doesn't ever
1195 return control to the
match block
, it is fine to
use it
in a place
where an
1196 integer was expected
. The `
match` block will never finish executing
, and any
1197 point
where `y`
(like the print statement
) is needed will not be reached
.
1199 However
, if we had a diverging function that actually does finish execution
1207 then we would have an unknown value
for `y`
in the following code
:
1218 In the previous example
, the print statement was never reached when the wildcard
1219 match arm was hit
, so we were okay with `
foo()` not returning an integer that we
1220 could set to `y`
. But
in this example
, `
foo()` actually does
return control
, so
1221 the print statement will be executed with an uninitialized value
.
1223 Obviously we cannot have functions which are allowed to be used
in such
1224 positions and yet can
return control
. So
, if you are defining a function that
1225 returns `
!`
, make sure that there is no way
for it to actually finish executing
.
1229 This is because of a
type mismatch between the associated
type of some
1230 trait (e
.g
. `T
::Bar`
, where `T` implements `
trait Quux { type Bar; }`
)
1231 and another
type `U` that is required to be equal to `T
::Bar`
, but is not
.
1234 Here is a basic example
:
1237 trait Trait { type AssociatedType; }
1238 fn foo
<T
>(t
: T
) where T
: Trait
<AssociatedType
=u32> {
1241 impl Trait
for i8 { type AssociatedType = &'static str; }
1245 Here is that same example again
, with some explanatory comments
:
1248 trait Trait { type AssociatedType; }
1250 fn foo
<T
>(t
: T
) where T
: Trait
<AssociatedType
=u32> {
1251 // ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
1253 // This says `foo` can |
1254 // only be used with |
1256 // implements `Trait`. |
1258 // This says not only must
1259 // `T` be an impl of `Trait`
1260 // but also that the impl
1261 // must assign the type `u32`
1262 // to the associated type.
1266 impl Trait
for i8 { type AssociatedType = &'static str; }
1267 ~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1272 // ... but it is an implementation
1273 // that assigns `&'static str` to
1274 // the associated type.
1277 // Here, we invoke `foo` with an `i8`, which does not satisfy
1278 // the constraint `<i8 as Trait>::AssociatedType=u32`, and
1279 // therefore the type-checker complains with this error code.
1282 Here is a more subtle instance of the same problem
, that can
1283 arise with
for-loops
in Rust
:
1286 let vs
: Vec
<i32> = vec
![1, 2, 3, 4];
1295 The above fails because of an analogous
type mismatch
,
1296 though may be harder to see
. Again
, here are some
1297 explanatory comments
for the same example
:
1301 let vs
= vec
![1, 2, 3, 4];
1303 // `for`-loops use a protocol based on the `Iterator`
1304 // trait. Each item yielded in a `for` loop has the
1305 // type `Iterator::Item` -- that is,I `Item` is the
1306 // associated type of the concrete iterator impl.
1310 // | We borrow `vs`, iterating over a sequence of
1311 // | *references* of type `&Elem` (where `Elem` is
1312 // | vector's element type). Thus, the associated
1313 // | type `Item` must be a reference `&`-type ...
1315 // ... and `v` has the type `Iterator::Item`, as dictated by
1316 // the `for`-loop protocol ...
1322 // ... but *here*, `v` is forced to have some integral type;
1323 // only types like `u8`,`i8`,`u16`,`i16`, et cetera can
1324 // match the pattern `1` ...
1329 // ... therefore, the compiler complains, because it sees
1330 // an attempt to solve the equations
1331 // `some integral-type` = type-of-`v`
1332 // = `Iterator::Item`
1333 // = `&Elem` (i.e. `some reference type`)
1335 // which cannot possibly all be true.
1341 To avoid those issues
, you have to make the types
match correctly
.
1342 So we can fix the previous examples like this
:
1346 trait Trait { type AssociatedType; }
1347 fn foo
<T
>(t
: T
) where T
: Trait
<AssociatedType
= &'
static str> {
1350 impl Trait
for i8 { type AssociatedType = &'static str; }
1353 // For-Loop Example:
1354 let vs
= vec
![1, 2, 3, 4];
1365 The `
#[rustc_on_unimplemented]` attribute lets you specify a custom error
1366 message
for when a particular
trait isn't implemented on a
type placed
in a
1367 position that needs that
trait. For example
, when the following code is
1371 fn foo
<T
: Index
<u8>>(x
: T
){}
1373 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
1374 trait Index
<Idx
> { ... }
1376 foo(true); // `bool` does not implement `Index<u8>`
1379 there will be an error about `bool` not implementing `Index
<u8>`
, followed by a
1380 note saying
"the type `bool` cannot be indexed by `u8`".
1382 As you can see
, you can specify
type parameters
in curly braces
for substitution
1383 with the actual
types (using the regular format string syntax
) in a given
1384 situation
. Furthermore
, `{Self}` will substitute to the
type (in this case
,
1385 `bool`
) that we tried to
use.
1387 This error appears when the curly braces contain an identifier which doesn't
1388 match with any of the
type parameters or the string `
Self`
. This might happen
if
1389 you misspelled a
type parameter
, or
if you intended to
use literal curly braces
.
1390 If it is the latter
, escape the curly braces with a second curly brace of the
1391 same
type; e
.g
. a literal `
{` is `
{{`
1395 The `
#[rustc_on_unimplemented]` attribute lets you specify a custom error
1396 message
for when a particular
trait isn't implemented on a
type placed
in a
1397 position that needs that
trait. For example
, when the following code is
1401 fn foo
<T
: Index
<u8>>(x
: T
){}
1403 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
1404 trait Index
<Idx
> { ... }
1406 foo(true); // `bool` does not implement `Index<u8>`
1409 there will be an error about `bool` not implementing `Index
<u8>`
, followed by a
1410 note saying
"the type `bool` cannot be indexed by `u8`".
1412 As you can see
, you can specify
type parameters
in curly braces
for substitution
1413 with the actual
types (using the regular format string syntax
) in a given
1414 situation
. Furthermore
, `{Self}` will substitute to the
type (in this case
,
1415 `bool`
) that we tried to
use.
1417 This error appears when the curly braces
do not contain an identifier
. Please
1418 add one of the same name
as a
type parameter
. If you intended to
use literal
1419 braces
, use `{{` and `}
}` to escape them
.
1423 The `
#[rustc_on_unimplemented]` attribute lets you specify a custom error
1424 message
for when a particular
trait isn't implemented on a
type placed
in a
1425 position that needs that
trait. For example
, when the following code is
1429 fn foo
<T
: Index
<u8>>(x
: T
){}
1431 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
1432 trait Index
<Idx
> { ... }
1434 foo(true); // `bool` does not implement `Index<u8>`
1437 there will be an error about `bool` not implementing `Index
<u8>`
, followed by a
1438 note saying
"the type `bool` cannot be indexed by `u8`".
1440 For this to work
, some note must be specified
. An empty attribute will not
do
1441 anything
, please remove the attribute or add some helpful note
for users of the
1446 This error occurs when there was a recursive
trait requirement that overflowed
1447 before it could be evaluated
. Often this means that there is unbounded recursion
1448 in resolving some
type bounds
.
1450 For example
, in the following code
1457 impl<T
> Foo
for T
where Bar
<T
>: Foo {}
1460 to determine
if a `T` is `Foo`
, we need to check
if `Bar
<T
>` is `Foo`
. However
,
1461 to
do this check
, we need to determine that `Bar
<Bar
<T
>>` is `Foo`
. To determine
1462 this
, we check
if `Bar
<Bar
<Bar
<T
>>>` is `Foo`
, and so on
. This is clearly a
1463 recursive requirement that can't be resolved directly
.
1465 Consider changing your
trait bounds so that they're less
self-referential
.
1469 This error occurs when a bound
in an implementation of a
trait does not
match
1470 the bounds specified
in the original
trait. For example
:
1478 fn foo
<T
>(x
: T
) where T
: Copy {}
1482 Here
, all types implementing `Foo` must have a method `foo
<T
>(x
: T
)` which can
1483 take any
type `T`
. However
, in the `
impl`
for `bool`
, we have added an extra
1484 bound that `T` is `Copy`
, which isn't compatible with the original
trait.
1486 Consider removing the bound from the method or adding the bound to the original
1487 method definition
in the
trait.
1491 You tried to
use a
type which doesn't implement some
trait in a place which
1492 expected that
trait. Erroneous code example
:
1495 // here we declare the Foo trait with a bar method
1500 // we now declare a function which takes an object implementing the Foo trait
1501 fn some_func
<T
: Foo
>(foo
: T
) {
1506 // we now call the method with the i32 type, which doesn't implement
1508 some_func(5i32); // error: the trait `Foo` is not implemented for the
1513 In order to fix this error
, verify that the
type you're using does implement
1521 fn some_func
<T
: Foo
>(foo
: T
) {
1522 foo
.bar(); // we can now use this method since i32 implements the
1526 // we implement the trait on the i32 type
1532 some_func(5i32); // ok!
1538 You tried to supply a
type which doesn't implement some
trait in a location
1539 which expected that
trait. This error typically occurs when working with
1540 `Fn`
-based types
. Erroneous code example
:
1543 fn foo
<F
: Fn()>(x
: F
) { }
1546 // type mismatch: the type ... implements the trait `core::ops::Fn<(_,)>`,
1547 // but the trait `core::ops::Fn<()>` is required (expected (), found tuple
1553 The issue
in this case is that `foo` is defined
as accepting a `Fn` with no
1554 arguments
, but the closure we attempted to pass to it requires one argument
.
1558 This error indicates that
type inference did not result
in one unique possible
1559 type, and extra information is required
. In most cases this can be provided
1560 by adding a
type annotation
. Sometimes you need to specify a generic
type
1563 A common example is the `collect` method on `Iterator`
. It has a generic
type
1564 parameter with a `FromIterator` bound
, which
for a `
char` iterator is
1565 implemented by `Vec` and `String` among others
. Consider the following snippet
1566 that reverses the characters of a string
:
1569 let x
= "hello".chars().rev().collect();
1572 In this case
, the compiler cannot infer what the
type of `x` should be
:
1573 `Vec
<char>` and `String` are both suitable candidates
. To specify which
type to
1574 use, you can
use a
type annotation on `x`
:
1577 let x
: Vec
<char> = "hello".chars().rev().collect();
1580 It is not necessary to annotate the full
type. Once the ambiguity is resolved
,
1581 the compiler can infer the rest
:
1584 let x
: Vec
<_
> = "hello".chars().rev().collect();
1587 Another way to provide the compiler with enough information
, is to specify the
1588 generic
type parameter
:
1591 let x
= "hello".chars().rev().collect
::<Vec
<char>>();
1594 Again
, you need not specify the full
type if the compiler can infer it
:
1597 let x
= "hello".chars().rev().collect
::<Vec
<_
>>();
1600 Apart from a method or function with a generic
type parameter
, this error can
1601 occur when a
type parameter of a
struct or
trait cannot be inferred
. In that
1602 case it is not always possible to
use a
type annotation
, because all candidates
1603 have the same
return type. For instance
:
1607 // Some fields omitted.
1616 let number
= Foo
::bar();
1621 This will fail because the compiler does not know which instance of `Foo` to
1622 call `bar` on
. Change `Foo
::bar()` to `Foo
::<T
>::bar()` to resolve the error
.
1626 This error indicates that the given recursion limit could not be parsed
. Ensure
1627 that the value provided is a positive integer between quotes
, like so
:
1630 #![recursion_limit="1000"]
1635 Patterns used to bind names must be irrefutable
. That is
, they must guarantee
1636 that a name will be extracted
in all cases
. Instead of pattern matching the
1637 loop variable
, consider using a `
match` or `
if let` inside the
loop body
. For
1641 // This fails because `None` is not covered.
1646 // Match inside the loop instead:
1656 if let Some(x
) = item
{
1664 Mutable borrows are not allowed
in pattern guards
, because matching cannot have
1665 side effects
. Side effects could alter the matched object or the environment
1666 on which the
match depends
in such a way
, that the
match would not be
1667 exhaustive
. For instance
, the following would not
match any arm
if mutable
1668 borrows were allowed
:
1673 option
if option
.take().is_none() => { /* impossible, option is `Some` */ }
,
1674 Some(_
) => { }
// When the previous match failed, the option became `None`.
1680 Assignments are not allowed
in pattern guards
, because matching cannot have
1681 side effects
. Side effects could alter the matched object or the environment
1682 on which the
match depends
in such a way
, that the
match would not be
1683 exhaustive
. For instance
, the following would not
match any arm
if assignments
1689 option
if { option = None; false } { }
,
1690 Some(_
) => { }
// When the previous match failed, the option became `None`.
1696 In certain cases it is possible
for sub
-bindings to violate memory safety
.
1697 Updates to the borrow checker
in a future version of Rust may remove this
1698 restriction
, but
for now patterns must be rewritten without sub
-bindings
.
1702 match Some("hi".to_string()) {
1703 ref op_string_ref @
Some(ref s
) => ...
1708 match Some("hi".to_string()) {
1710 let op_string_ref
= &Some(s
);
1717 The `op_string_ref` binding has
type `
&Option
<&String
>`
in both cases
.
1719 See also https
://github.com/rust-lang/rust/issues/14587
1723 In an array literal `
[x
; N
]`
, `N` is the number of elements
in the array
. This
1724 number cannot be negative
.
1728 The length of an array is part of its
type. For this reason
, this length must be
1729 a compile
-time constant
.
1733 This error occurs when the compiler was unable to infer the concrete
type of a
1734 variable
. It can occur
for several cases
, the most common of which is a
1735 mismatch
in the expected
type that the compiler inferred
for a variable's
1736 initializing expression
, and the actual
type explicitly assigned to the
1742 let x
: i32 = "I am not a number!";
1743 // ~~~ ~~~~~~~~~~~~~~~~~~~~
1745 // | initializing expression;
1746 // | compiler infers type `&str`
1748 // type `i32` assigned to variable `x`
1753 Types
in type definitions have lifetimes associated with them that represent
1754 how long the data stored within them is guaranteed to be live
. This lifetime
1755 must be
as long
as the data needs to be alive
, and missing the constraint that
1756 denotes this will cause this error
.
1759 // This won't compile because T is not constrained, meaning the data
1760 // stored in it is not guaranteed to last as long as the reference
1765 // This will compile, because it has the constraint on the type parameter
1766 struct Foo
<'a
, T
: 'a
> {
1773 Types
in type definitions have lifetimes associated with them that represent
1774 how long the data stored within them is guaranteed to be live
. This lifetime
1775 must be
as long
as the data needs to be alive
, and missing the constraint that
1776 denotes this will cause this error
.
1779 // This won't compile because T is not constrained to the static lifetime
1780 // the reference needs
1785 // This will compile, because it has the constraint on the type parameter
1786 struct Foo
<T
: '
static> {
1793 Method calls that aren't calls to inherent `
const` methods are disallowed
1794 in statics
, constants
, and constant functions
.
1799 const BAZ
: i32 = Foo(25).bar(); // error, `bar` isn't `const`
1804 const fn foo(&self) -> i32 {
1805 self.bar() // error, `bar` isn't `const`
1808 fn bar(&self) -> i32 { self.0 }
1812 For more information about `
const fn`'s
, see
[RFC
911].
1814 [RFC
911]: https
://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md
1820 > It is invalid
for a
static to reference another
static by value
. It is
1821 > required that all references be borrowed
.
1823 [RFC
246]: https
://github.com/rust-lang/rfcs/pull/246
1827 The value assigned to a constant expression must be known at compile time
,
1828 which is not the case when comparing raw pointers
. Erroneous code example
:
1831 static foo
: i32 = 42;
1832 static bar
: i32 = 43;
1834 static baz
: bool
= { (&foo as *const i32) == (&bar as *const i32) }
;
1835 // error: raw pointers cannot be compared in statics!
1838 Please check that the result of the comparison can be determined at compile time
1839 or isn't assigned to a constant expression
. Example
:
1842 static foo
: i32 = 42;
1843 static bar
: i32 = 43;
1845 let baz
: bool
= { (&foo as *const i32) == (&bar as *const i32) }
;
1846 // baz isn't a constant expression so it's ok
1851 The value assigned to a constant expression must be known at compile time
,
1852 which is not the case when dereferencing raw pointers
. Erroneous code
1856 const foo
: i32 = 42;
1857 const baz
: *const i32 = (&foo
as *const i32);
1859 const deref
: i32 = *baz
;
1860 // error: raw pointers cannot be dereferenced in constants
1863 To fix this error
, please
do not assign this value to a constant expression
.
1867 const foo
: i32 = 42;
1868 const baz
: *const i32 = (&foo
as *const i32);
1870 unsafe { let deref: i32 = *baz; }
1871 // baz isn't a constant expression so it's ok
1874 You'll also note that this assignment must be done
in an
unsafe block
!
1878 It is not allowed
for a mutable
static to allocate or have destructors
. For
1882 // error: mutable statics are not allowed to have boxes
1883 static mut FOO
: Option
<Box
<usize>> = None
;
1885 // error: mutable statics are not allowed to have destructors
1886 static mut BAR
: Option
<Vec
<i32>> = None
;
1891 In Rust
1.3, the
default object lifetime bounds are expected to
1892 change
, as described
in RFC
#1156 [1]. You are getting a warning
1893 because the compiler thinks it is possible that this change will cause
1894 a compilation error
in your code
. It is possible
, though unlikely
,
1895 that this is a
false alarm
.
1897 The heart of the change is that
where `
&'a Box
<SomeTrait
>` used to
1898 default to `
&'a Box
<SomeTrait
+'a
>`
, it now defaults to `
&'a
1899 Box
<SomeTrait
+'
static>`
(here
, `SomeTrait` is the name of some
trait
1900 type). Note that the only types which are affected are references to
1901 boxes
, like `
&Box
<SomeTrait
>` or `
&[Box
<SomeTrait
>]`
. More common
1902 types like `
&SomeTrait` or `Box
<SomeTrait
>` are unaffected
.
1904 To silence this warning
, edit your code to
use an explicit bound
.
1905 Most of the time
, this means that you will want to change the
1906 signature of a function that you are calling
. For example
, if
1907 the error is reported on a call like `
foo(x
)`
, and `foo` is
1911 fn foo(arg
: &Box
<SomeTrait
>) { ... }
1914 you might change it to
:
1917 fn foo
<'a
>(arg
: &Box
<SomeTrait
+'a
>) { ... }
1920 This explicitly states that you expect the
trait object `SomeTrait` to
1921 contain
references (with a maximum lifetime of `'a`
).
1923 [1]: https
://github.com/rust-lang/rfcs/pull/1156
1927 A user
-defined dereference was attempted
in an invalid context
. Erroneous
1931 use std
::ops
::Deref
;
1938 fn deref(&self)-> &str { "foo" }
1941 const S
: &'
static str = &A
;
1942 // error: user-defined dereference operators are not allowed in constants
1949 You cannot directly
use a dereference operation whilst initializing a constant
1950 or a
static. To fix this error
, restructure your code to avoid this dereference
,
1951 perhaps moving it inline
:
1954 use std
::ops
::Deref
;
1961 fn deref(&self)-> &str { "foo" }
1965 let foo
: &str = &A
;
1971 An invalid lint attribute has been given
. Erroneous code example
:
1974 #![allow(foo = "")] // error: malformed lint attribute
1977 Lint attributes only accept a list of
identifiers (where each identifier is a
1978 lint name
). Ensure the attribute is of this form
:
1981 #![allow(foo)] // ok!
1983 #![allow(foo, foo2)] // ok!
1988 A borrow of a constant containing interior mutability was attempted
. Erroneous
1992 use std
::sync
::atomic
::{AtomicUsize, ATOMIC_USIZE_INIT}
;
1994 const A
: AtomicUsize
= ATOMIC_USIZE_INIT
;
1995 static B
: &'
static AtomicUsize
= &A
;
1996 // error: cannot borrow a constant which contains interior mutability, create a
2000 A `
const` represents a constant value that should never change
. If one takes
2001 a `
&` reference to the constant
, then one is taking a pointer to some memory
2002 location containing the value
. Normally this is perfectly fine
: most values
2003 can't be changed via a shared `
&` pointer
, but interior mutability would allow
2004 it
. That is
, a constant value could be mutated
. On the other hand
, a `
static` is
2005 explicitly a single memory location
, which can be mutated at will
.
2007 So
, in order to solve this error
, either
use statics which are `Sync`
:
2010 use std
::sync
::atomic
::{AtomicUsize, ATOMIC_USIZE_INIT}
;
2012 static A
: AtomicUsize
= ATOMIC_USIZE_INIT
;
2013 static B
: &'
static AtomicUsize
= &A
; // ok!
2016 You can also have this error
while using a cell
type:
2019 #![feature(const_fn)]
2021 use std
::cell
::Cell
;
2023 const A
: Cell
<usize> = Cell
::new(1);
2024 const B
: &'
static Cell
<usize> = &A
;
2025 // error: cannot borrow a constant which contains interior mutability, create
2029 struct C { a: Cell<usize> }
2031 const D
: C
= C { a: Cell::new(1) }
;
2032 const E
: &'
static Cell
<usize> = &D
.a
; // error
2035 const F
: &'
static C
= &D
; // error
2038 This is because cell types
do operations that are not thread
-safe
. Due to this
,
2039 they don't implement Sync and thus can't be placed
in statics
. In this
2040 case
, `StaticMutex` would work just fine
, but it isn't stable yet
:
2041 https
://doc.rust-lang.org/nightly/std/sync/struct.StaticMutex.html
2043 However
, if you still wish to
use these types
, you can achieve this by an
unsafe
2047 #![feature(const_fn)]
2049 use std
::cell
::Cell
;
2050 use std
::marker
::Sync
;
2052 struct NotThreadSafe
<T
> {
2056 unsafe impl<T
> Sync
for NotThreadSafe
<T
> {}
2058 static A
: NotThreadSafe
<usize> = NotThreadSafe { value : Cell::new(1) }
;
2059 static B
: &'
static NotThreadSafe
<usize> = &A
; // ok!
2062 Remember this solution is
unsafe! You will have to ensure that accesses to the
2063 cell are synchronized
.
2067 A
type with a destructor was assigned to an invalid
type of variable
. Erroneous
2076 fn drop(&mut self) {}
2079 const F
: Foo
= Foo { a : 0 }
;
2080 // error: constants are not allowed to have destructors
2081 static S
: Foo
= Foo { a : 0 }
;
2082 // error: statics are not allowed to have destructors
2085 To solve this issue
, please
use a
type which does allow the usage of
type with
2090 A reference of an interior
static was assigned to another
const/static.
2091 Erroneous code example
:
2098 static S
: Foo
= Foo { a : 0 }
;
2099 static A
: &'
static u32 = &S
.a
;
2100 // error: cannot refer to the interior of another static, use a
2104 The
"base" variable has to be a
const if you want another
static/const variable
2105 to refer to one of its fields
. Example
:
2112 const S
: Foo
= Foo { a : 0 }
;
2113 static A
: &'
static u32 = &S
.a
; // ok!
2118 A lifetime name is shadowing another lifetime name
. Erroneous code example
:
2126 fn f
<'a
>(x
: &'a
i32) { // error: lifetime name `'a` shadows a lifetime
2127 // name that is already in scope
2132 Please change the name of one of the lifetimes to remove this error
. Example
:
2140 fn f
<'b
>(x
: &'b
i32) { // ok!
2150 A stability attribute was used outside of the standard library
. Erroneous code
2154 #[stable] // error: stability attributes may not be used outside of the
2159 It is not possible to
use stability attributes outside of the standard library
.
2160 Also
, for now
, it is not possible to write deprecation messages either
.
2164 This error indicates that a `
#[repr(..)]` attribute was placed on an unsupported
2167 Examples of erroneous code
:
2177 struct Foo {bar: bool, baz: bool}
2185 - The `
#[repr(C)]` attribute can only be placed on structs and enums
2186 - The `
#[repr(packed)]` and `#[repr(simd)]` attributes only work on structs
2187 - The `
#[repr(u8)]`, `#[repr(i16)]`, etc attributes only work on enums
2189 These attributes
do not work on typedefs
, since typedefs are just aliases
.
2191 Representations like `
#[repr(u8)]`, `#[repr(i64)]` are for selecting the
2192 discriminant size
for C
-like
enums (when there is no associated data
, e
.g
. `
enum
2193 Color {Red, Blue, Green}`
), effectively setting the size of the
enum to the size
2194 of the provided
type. Such an
enum can be cast to a value of the same
type as
2195 well
. In short
, `
#[repr(u8)]` makes the enum behave like an integer with a
2196 constrained set of allowed values
.
2198 Only C
-like enums can be cast to numerical primitives
, so this attribute will
2199 not apply to structs
.
2201 `
#[repr(packed)]` reduces padding to make the struct size smaller. The
2202 representation of enums isn't strictly defined
in Rust
, and this attribute won't
2205 `
#[repr(simd)]` will give a struct consisting of a homogenous series of machine
2206 types (i
.e
. `
u8`
, `
i32`
, etc
) a representation that permits vectorization via
2207 SIMD
. This doesn't make much sense
for enums since they don't consist of a
2208 single list of data
.
2212 This error indicates that an `
#[inline(..)]` attribute was incorrectly placed on
2213 something other than a function or method
.
2215 Examples of erroneous code
:
2227 `
#[inline]` hints the compiler whether or not to attempt to inline a method or
2228 function
. By
default, the compiler does a pretty good job of figuring this out
2229 itself
, but
if you feel the need
for annotations
, `
#[inline(always)]` and
2230 `
#[inline(never)]` can override or force the compiler's decision.
2232 If you wish to apply this attribute to all methods
in an
impl, manually annotate
2233 each method
; it is not possible to annotate the entire
impl with an `
#[inline]`
2240 register_diagnostics! {
2241 // E0006 // merged with E0005
2244 E0229, // associated type bindings are not allowed here
2245 E0278, // requirement is not satisfied
2246 E0279, // requirement is not satisfied
2247 E0280, // requirement is not satisfied
2248 E0283, // cannot resolve type
2249 E0284, // cannot resolve type
2250 E0285, // overflow evaluation builtin bounds
2251 E0298, // mismatched types between arms
2252 E0299, // mismatched types between arms
2253 E0300, // unexpanded macro
2254 E0304, // expected signed integer constant
2255 E0305, // expected constant
2256 E0311, // thing may not live long enough
2257 E0312, // lifetime of reference outlives lifetime of borrowed content
2258 E0313, // lifetime of borrowed pointer outlives lifetime of captured variable
2259 E0314, // closure outlives stack frame
2260 E0315, // cannot invoke closure outside of its lifetime
2261 E0316, // nested quantification of lifetimes
2262 E0453, // overruled by outer forbid
2263 E0471, // constant evaluation error: ..
2264 E0472, // asm! is unsupported on this target
2265 E0473, // dereference of reference outside its lifetime
2266 E0474, // captured variable `..` does not outlive the enclosing closure
2267 E0475, // index of slice outside its lifetime
2268 E0476, // lifetime of the source pointer does not outlive lifetime bound...
2269 E0477, // the type `..` does not fulfill the required lifetime...
2270 E0478, // lifetime bound not satisfied
2271 E0479, // the type `..` (provided as the value of a type parameter) is...
2272 E0480, // lifetime of method receiver does not outlive the method call
2273 E0481, // lifetime of function argument does not outlive the function call
2274 E0482, // lifetime of return value does not outlive the function call
2275 E0483, // lifetime of operand does not outlive the operation
2276 E0484, // reference is not valid at the time of borrow
2277 E0485, // automatically reference is not valid at the time of borrow
2278 E0486, // type of expression contains references that are not valid during...
2279 E0487, // unsafe use of destructor: destructor might be called while...
2280 E0488, // lifetime of variable does not enclose its declaration
2281 E0489, // type/lifetime parameter not in scope here
2282 E0490, // a value of type `..` is borrowed for too long
2283 E0491, // in type `..`, reference has a longer lifetime than the data it...
2284 E0495, // cannot infer an appropriate lifetime due to conflicting requirements