1 // Copyright 2015 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 register_long_diagnostics! {
17 #### Note: this error code is no longer emitted by the compiler.
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
24 one is too specific or the ordering is incorrect.
26 For example, the following `match` block has too many arms:
30 Some(bar) => {/* ... */}
31 x => {/* ... */} // This handles the `None` case
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 #### Note: this error code is no longer emitted by the compiler.
46 This error indicates that an empty match expression is invalid because the type
47 it is matching on is non-empty (there exist values of this type). In safe code
48 it is impossible to create an instance of an empty type, so empty match
49 expressions are almost never desired. This error is typically fixed by adding
50 one or more cases to the match expression.
52 An example of an empty type is `enum Empty { }`. So, the following will work:
67 fn foo(x: Option<String>) {
76 This error indicates that the compiler cannot guarantee a matching pattern for
77 one or more possible inputs to a match expression. Guaranteed matches are
78 required in order to assign values to match expressions, or alternatively,
79 determine the flow of execution. Erroneous code example:
87 let x = Terminator::HastaLaVistaBaby;
89 match x { // error: non-exhaustive patterns: `HastaLaVistaBaby` not covered
90 Terminator::TalkToMyHand => {}
94 If you encounter this error you must alter your patterns so that every possible
95 value of the input type is matched. For types with a small number of variants
96 (like enums) you should probably cover all cases explicitly. Alternatively, the
97 underscore `_` wildcard pattern can be added after all other patterns to match
98 "anything else". Example:
106 let x = Terminator::HastaLaVistaBaby;
109 Terminator::TalkToMyHand => {}
110 Terminator::HastaLaVistaBaby => {}
116 Terminator::TalkToMyHand => {}
123 Patterns used to bind names must be irrefutable, that is, they must guarantee
124 that a name will be extracted in all cases. Erroneous code example:
126 ```compile_fail,E0005
129 // error: refutable pattern in local binding: `None` not covered
132 If you encounter this error you probably need to use a `match` or `if let` to
133 deal with the possibility of failure. Example:
154 This error indicates that the bindings in a match arm would require a value to
155 be moved into more than one location, thus violating unique ownership. Code
156 like the following is invalid as it requires the entire `Option<String>` to be
157 moved into a variable called `op_string` while simultaneously requiring the
158 inner `String` to be moved into a variable called `s`.
160 ```compile_fail,E0007
161 let x = Some("s".to_string());
164 op_string @ Some(s) => {}, // error: cannot bind by-move with sub-bindings
169 See also the error E0303.
173 Names bound in match arms retain their type in pattern guards. As such, if a
174 name is bound by move in a pattern, it should also be moved to wherever it is
175 referenced in the pattern guard code. Doing so however would prevent the name
176 from being available in the body of the match arm. Consider the following:
178 ```compile_fail,E0008
179 match Some("hi".to_string()) {
180 Some(s) if s.len() == 0 => {}, // use s.
185 The variable `s` has type `String`, and its use in the guard is as a variable of
186 type `String`. The guard code effectively executes in a separate scope to the
187 body of the arm, so the value would be moved into this anonymous scope and
188 therefore becomes unavailable in the body of the arm.
190 The problem above can be solved by using the `ref` keyword.
193 match Some("hi".to_string()) {
194 Some(ref s) if s.len() == 0 => {},
199 Though this example seems innocuous and easy to solve, the problem becomes clear
200 when it encounters functions which consume the value:
202 ```compile_fail,E0008
206 fn consume(self) -> usize {
214 Some(y) if y.consume() > 0 => {}
220 In this situation, even the `ref` keyword cannot solve it, since borrowed
221 content cannot be moved. This problem cannot be solved generally. If the value
222 can be cloned, here is a not-so-specific solution:
229 fn consume(self) -> usize {
237 Some(ref y) if y.clone().consume() > 0 => {}
243 If the value will be consumed in the pattern guard, using its clone will not
244 move its ownership, so the code works.
248 In a pattern, all values that don't implement the `Copy` trait have to be bound
249 the same way. The goal here is to avoid binding simultaneously by-move and
252 This limitation may be removed in a future version of Rust.
254 Erroneous code example:
256 ```compile_fail,E0009
259 let x = Some((X { x: () }, X { x: () }));
261 Some((y, ref z)) => {}, // error: cannot bind by-move and by-ref in the
267 You have two solutions:
269 Solution #1: Bind the pattern's values the same way.
274 let x = Some((X { x: () }, X { x: () }));
276 Some((ref y, ref z)) => {},
277 // or Some((y, z)) => {}
282 Solution #2: Implement the `Copy` trait for the `X` structure.
284 However, please keep in mind that the first solution should be preferred.
287 #[derive(Clone, Copy)]
290 let x = Some((X { x: () }, X { x: () }));
292 Some((y, ref z)) => {},
299 When matching against a range, the compiler verifies that the range is
300 non-empty. Range patterns include both end-points, so this is equivalent to
301 requiring the start of the range to be less than or equal to the end of the
308 // This range is ok, albeit pointless.
310 // This range is empty, and the compiler can tell.
317 `const` and `static` mean different things. A `const` is a compile-time
318 constant, an alias for a literal value. This property means you can match it
319 directly within a pattern.
321 The `static` keyword, on the other hand, guarantees a fixed location in memory.
322 This does not always mean that the value is constant. For example, a global
323 mutex can be declared `static` as well.
325 If you want to match against a `static`, consider using a guard instead:
328 static FORTY_TWO: i32 = 42;
331 Some(x) if x == FORTY_TWO => {}
338 An if-let pattern attempts to match the pattern, and enters the body if the
339 match was successful. If the match is irrefutable (when it cannot fail to
340 match), use a regular `let`-binding instead. For instance:
342 ```compile_fail,E0162
343 struct Irrefutable(i32);
344 let irr = Irrefutable(0);
346 // This fails to compile because the match is irrefutable.
347 if let Irrefutable(x) = irr {
348 // This body will always be executed.
356 struct Irrefutable(i32);
357 let irr = Irrefutable(0);
359 let Irrefutable(x) = irr;
365 A while-let pattern attempts to match the pattern, and enters the body if the
366 match was successful. If the match is irrefutable (when it cannot fail to
367 match), use a regular `let`-binding inside a `loop` instead. For instance:
369 ```compile_fail,E0165
370 struct Irrefutable(i32);
371 let irr = Irrefutable(0);
373 // This fails to compile because the match is irrefutable.
374 while let Irrefutable(x) = irr {
382 struct Irrefutable(i32);
383 let irr = Irrefutable(0);
386 let Irrefutable(x) = irr;
393 Enum variants are qualified by default. For example, given this type:
402 You would match it using:
418 If you don't qualify the names, the code will bind new variables named "GET" and
419 "POST" instead. This behavior is likely not what you want, so `rustc` warns when
422 Qualified names are good practice, and most code works well with them. But if
423 you prefer them unqualified, you can import the variants into scope:
427 enum Method { GET, POST }
431 If you want others to be able to import variants from your module directly, use
436 pub enum Method { GET, POST }
443 #### Note: this error code is no longer emitted by the compiler.
445 Patterns used to bind names must be irrefutable. That is, they must guarantee
446 that a name will be extracted in all cases. Instead of pattern matching the
447 loop variable, consider using a `match` or `if let` inside the loop body. For
450 ```compile_fail,E0005
451 let xs : Vec<Option<i32>> = vec![Some(1), None];
453 // This fails because `None` is not covered.
459 Match inside the loop instead:
462 let xs : Vec<Option<i32>> = vec![Some(1), None];
475 let xs : Vec<Option<i32>> = vec![Some(1), None];
478 if let Some(x) = item {
486 Mutable borrows are not allowed in pattern guards, because matching cannot have
487 side effects. Side effects could alter the matched object or the environment
488 on which the match depends in such a way, that the match would not be
489 exhaustive. For instance, the following would not match any arm if mutable
490 borrows were allowed:
492 ```compile_fail,E0301
495 option if option.take().is_none() => {
496 /* impossible, option is `Some` */
498 Some(_) => { } // When the previous match failed, the option became `None`.
504 Assignments are not allowed in pattern guards, because matching cannot have
505 side effects. Side effects could alter the matched object or the environment
506 on which the match depends in such a way, that the match would not be
507 exhaustive. For instance, the following would not match any arm if assignments
510 ```compile_fail,E0302
513 option if { option = None; false } => { },
514 Some(_) => { } // When the previous match failed, the option became `None`.
520 In certain cases it is possible for sub-bindings to violate memory safety.
521 Updates to the borrow checker in a future version of Rust may remove this
522 restriction, but for now patterns must be rewritten without sub-bindings.
526 ```compile_fail,E0303
527 match Some("hi".to_string()) {
528 ref op_string_ref @ Some(s) => {},
536 match Some("hi".to_string()) {
538 let op_string_ref = &Some(s);
545 The `op_string_ref` binding has type `&Option<&String>` in both cases.
547 See also https://github.com/rust-lang/rust/issues/14587
551 The value of statics and constants must be known at compile time, and they live
552 for the entire lifetime of a program. Creating a boxed value allocates memory on
553 the heap at runtime, and therefore cannot be done at compile time. Erroneous
556 ```compile_fail,E0010
557 #![feature(box_syntax)]
559 const CON : Box<i32> = box 0;
564 Static and const variables can refer to other const variables. But a const
565 variable cannot refer to a static variable. For example, `Y` cannot refer to
568 ```compile_fail,E0013
573 To fix this, the value can be extracted as a const and then used:
582 // FIXME(#24111) Change the language here when const fn stabilizes
584 The only functions that can be called in static or constant expressions are
585 `const` functions, and struct/enum constructors. `const` functions are only
586 available on a nightly compiler. Rust currently does not support more general
587 compile-time function execution.
590 const FOO: Option<u8> = Some(1); // enum constructor
592 const BAR: Bar = Bar {x: 1}; // struct constructor
595 See [RFC 911] for more details on the design of `const fn`s.
597 [RFC 911]: https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md
601 References in statics and constants may only refer to immutable values.
602 Erroneous code example:
604 ```compile_fail,E0017
608 // these three are not allowed:
609 const CR: &'static mut i32 = &mut C;
610 static STATIC_REF: &'static mut i32 = &mut X;
611 static CONST_REF: &'static mut i32 = &mut C;
614 Statics are shared everywhere, and if they refer to mutable data one might
615 violate memory safety since holding multiple mutable references to shared data
618 If you really want global mutable state, try using `static mut` or a global
624 The value of static and constant integers must be known at compile time. You
625 can't cast a pointer to an integer because the address of a pointer can
628 For example, if you write:
630 ```compile_fail,E0018
631 static MY_STATIC: u32 = 42;
632 static MY_STATIC_ADDR: usize = &MY_STATIC as *const _ as usize;
633 static WHAT: usize = (MY_STATIC_ADDR^17) + MY_STATIC_ADDR;
636 Then `MY_STATIC_ADDR` would contain the address of `MY_STATIC`. However,
637 the address can change when the program is linked, as well as change
638 between different executions due to ASLR, and many linkers would
639 not be able to calculate the value of `WHAT`.
641 On the other hand, static and constant pointers can point either to
642 a known numeric address or to the address of a symbol.
645 static MY_STATIC: u32 = 42;
646 static MY_STATIC_ADDR: &'static u32 = &MY_STATIC;
647 const CONST_ADDR: *const u8 = 0x5f3759df as *const u8;
650 This does not pose a problem by itself because they can't be
655 A function call isn't allowed in the const's initialization expression
656 because the expression's value must be known at compile-time. Erroneous code
665 fn test(&self) -> i32 {
671 const FOO: Test = Test::V1;
673 const A: i32 = FOO.test(); // You can't call Test::func() here!
677 Remember: you can't use a function call inside a const's initialization
678 expression! However, you can totally use it anywhere else:
686 fn func(&self) -> i32 {
692 const FOO: Test = Test::V1;
694 FOO.func(); // here is good
695 let x = FOO.func(); // or even here!
701 Constant functions are not allowed to mutate anything. Thus, binding to an
702 argument with a mutable pattern is not allowed. For example,
705 const fn foo(mut x: u8) {
710 Is incorrect because the function body may not mutate `x`.
712 Remove any mutable bindings from the argument list to fix this error. In case
713 you need to mutate the argument, try lazily initializing a global variable
714 instead of using a `const fn`, or refactoring the code to a functional style to
715 avoid mutation if possible.
719 Unsafe code was used outside of an unsafe function or block.
721 Erroneous code example:
723 ```compile_fail,E0133
724 unsafe fn f() { return; } // This is the unsafe code
727 f(); // error: call to unsafe function requires unsafe function or block
731 Using unsafe functionality is potentially dangerous and disallowed by safety
734 * Dereferencing raw pointers
735 * Calling functions via FFI
736 * Calling functions marked unsafe
738 These safety checks can be relaxed for a section of the code by wrapping the
739 unsafe instructions with an `unsafe` block. For instance:
742 unsafe fn f() { return; }
745 unsafe { f(); } // ok!
749 See also https://doc.rust-lang.org/book/first-edition/unsafe.html
753 This error occurs when an attempt is made to use data captured by a closure,
754 when that data may no longer exist. It's most commonly seen when attempting to
757 ```compile_fail,E0373
758 fn foo() -> Box<Fn(u32) -> u32> {
764 Notice that `x` is stack-allocated by `foo()`. By default, Rust captures
765 closed-over data by reference. This means that once `foo()` returns, `x` no
766 longer exists. An attempt to access `x` within the closure would thus be
769 Another situation where this might be encountered is when spawning threads:
771 ```compile_fail,E0373
776 let thr = std::thread::spawn(|| {
782 Since our new thread runs in parallel, the stack frame containing `x` and `y`
783 may well have disappeared by the time we try to use them. Even if we call
784 `thr.join()` within foo (which blocks until `thr` has completed, ensuring the
785 stack frame won't disappear), we will not succeed: the compiler cannot prove
786 that this behaviour is safe, and so won't let us do it.
788 The solution to this problem is usually to switch to using a `move` closure.
789 This approach moves (or copies, where possible) data into the closure, rather
790 than taking references to it. For example:
793 fn foo() -> Box<Fn(u32) -> u32> {
795 Box::new(move |y| x + y)
799 Now that the closure has its own copy of the data, there's no need to worry
804 It is not allowed to use or capture an uninitialized variable. For example:
806 ```compile_fail,E0381
809 let y = x; // error, use of possibly uninitialized variable
813 To fix this, ensure that any declared variables are initialized before being
825 This error occurs when an attempt is made to use a variable after its contents
826 have been moved elsewhere. For example:
828 ```compile_fail,E0382
829 struct MyStruct { s: u32 }
832 let mut x = MyStruct{ s: 5u32 };
839 Since `MyStruct` is a type that is not marked `Copy`, the data gets moved out
840 of `x` when we set `y`. This is fundamental to Rust's ownership system: outside
841 of workarounds like `Rc`, a value cannot be owned by more than one variable.
843 Sometimes we don't need to move the value. Using a reference, we can let another
844 function borrow the value without changing its ownership. In the example below,
845 we don't actually have to move our string to `calculate_length`, we can give it
846 a reference to it with `&` instead.
850 let s1 = String::from("hello");
852 let len = calculate_length(&s1);
854 println!("The length of '{}' is {}.", s1, len);
857 fn calculate_length(s: &String) -> usize {
862 A mutable reference can be created with `&mut`.
864 Sometimes we don't want a reference, but a duplicate. All types marked `Clone`
865 can be duplicated by calling `.clone()`. Subsequent changes to a clone do not
866 affect the original variable.
868 Most types in the standard library are marked `Clone`. The example below
869 demonstrates using `clone()` on a string. `s1` is first set to "many", and then
870 copied to `s2`. Then the first character of `s1` is removed, without affecting
871 `s2`. "any many" is printed to the console.
875 let mut s1 = String::from("many");
878 println!("{} {}", s1, s2);
882 If we control the definition of a type, we can implement `Clone` on it ourselves
883 with `#[derive(Clone)]`.
885 Some types have no ownership semantics at all and are trivial to duplicate. An
886 example is `i32` and the other number types. We don't have to call `.clone()` to
887 clone them, because they are marked `Copy` in addition to `Clone`. Implicit
888 cloning is more convenient in this case. We can mark our own types `Copy` if
889 all their members also are marked `Copy`.
891 In the example below, we implement a `Point` type. Because it only stores two
892 integers, we opt-out of ownership semantics with `Copy`. Then we can
893 `let p2 = p1` without `p1` being moved.
896 #[derive(Copy, Clone)]
897 struct Point { x: i32, y: i32 }
900 let mut p1 = Point{ x: -1, y: 2 };
903 println!("p1: {}, {}", p1.x, p1.y);
904 println!("p2: {}, {}", p2.x, p2.y);
908 Alternatively, if we don't control the struct's definition, or mutable shared
909 ownership is truly required, we can use `Rc` and `RefCell`:
912 use std::cell::RefCell;
915 struct MyStruct { s: u32 }
918 let mut x = Rc::new(RefCell::new(MyStruct{ s: 5u32 }));
920 x.borrow_mut().s = 6;
921 println!("{}", x.borrow().s);
925 With this approach, x and y share ownership of the data via the `Rc` (reference
926 count type). `RefCell` essentially performs runtime borrow checking: ensuring
927 that at most one writer or multiple readers can access the data at any one time.
929 If you wish to learn more about ownership in Rust, start with the chapter in the
932 https://doc.rust-lang.org/book/first-edition/ownership.html
936 This error occurs when an attempt is made to partially reinitialize a
937 structure that is currently uninitialized.
939 For example, this can happen when a drop has taken place:
941 ```compile_fail,E0383
946 fn drop(&mut self) { /* ... */ }
949 let mut x = Foo { a: 1 };
950 drop(x); // `x` is now uninitialized
951 x.a = 2; // error, partial reinitialization of uninitialized structure `t`
954 This error can be fixed by fully reinitializing the structure in question:
961 fn drop(&mut self) { /* ... */ }
964 let mut x = Foo { a: 1 };
971 This error occurs when an attempt is made to reassign an immutable variable.
974 ```compile_fail,E0384
977 x = 5; // error, reassignment of immutable variable
981 By default, variables in Rust are immutable. To fix this error, add the keyword
982 `mut` after the keyword `let` when declaring the variable. For example:
993 This error occurs when an attempt is made to mutate the target of a mutable
994 reference stored inside an immutable container.
996 For example, this can happen when storing a `&mut` inside an immutable `Box`:
998 ```compile_fail,E0386
1000 let y: Box<_> = Box::new(&mut x);
1001 **y = 2; // error, cannot assign to data in an immutable container
1004 This error can be fixed by making the container mutable:
1008 let mut y: Box<_> = Box::new(&mut x);
1012 It can also be fixed by using a type with interior mutability, such as `Cell`
1016 use std::cell::Cell;
1019 let y: Box<Cell<_>> = Box::new(Cell::new(x));
1025 This error occurs when an attempt is made to mutate or mutably reference data
1026 that a closure has captured immutably. Examples of this error are shown below:
1028 ```compile_fail,E0387
1029 // Accepts a function or a closure that captures its environment immutably.
1030 // Closures passed to foo will not be able to mutate their closed-over state.
1031 fn foo<F: Fn()>(f: F) { }
1033 // Attempts to mutate closed-over data. Error message reads:
1034 // `cannot assign to data in a captured outer variable...`
1040 // Attempts to take a mutable reference to closed-over data. Error message
1041 // reads: `cannot borrow data mutably in a captured outer variable...`
1044 foo(|| { let y = &mut x; });
1048 The problem here is that foo is defined as accepting a parameter of type `Fn`.
1049 Closures passed into foo will thus be inferred to be of type `Fn`, meaning that
1050 they capture their context immutably.
1052 If the definition of `foo` is under your control, the simplest solution is to
1053 capture the data mutably. This can be done by defining `foo` to take FnMut
1057 fn foo<F: FnMut()>(f: F) { }
1060 Alternatively, we can consider using the `Cell` and `RefCell` types to achieve
1061 interior mutability through a shared reference. Our example's `mutable`
1062 function could be redefined as below:
1065 use std::cell::Cell;
1067 fn foo<F: Fn()>(f: F) { }
1070 let x = Cell::new(0u32);
1075 You can read more about cell types in the API documentation:
1077 https://doc.rust-lang.org/std/cell/
1081 E0388 was removed and is no longer issued.
1085 An attempt was made to mutate data using a non-mutable reference. This
1086 commonly occurs when attempting to assign to a non-mutable reference of a
1087 mutable reference (`&(&mut T)`).
1089 Example of erroneous code:
1091 ```compile_fail,E0389
1097 let mut fancy = FancyNum{ num: 5 };
1098 let fancy_ref = &(&mut fancy);
1099 fancy_ref.num = 6; // error: cannot assign to data in a `&` reference
1100 println!("{}", fancy_ref.num);
1104 Here, `&mut fancy` is mutable, but `&(&mut fancy)` is not. Creating an
1105 immutable reference to a value borrows it immutably. There can be multiple
1106 references of type `&(&mut T)` that point to the same value, so they must be
1107 immutable to prevent multiple mutable references to the same value.
1109 To fix this, either remove the outer reference:
1117 let mut fancy = FancyNum{ num: 5 };
1119 let fancy_ref = &mut fancy;
1120 // `fancy_ref` is now &mut FancyNum, rather than &(&mut FancyNum)
1122 fancy_ref.num = 6; // No error!
1124 println!("{}", fancy_ref.num);
1128 Or make the outer reference mutable:
1136 let mut fancy = FancyNum{ num: 5 };
1138 let fancy_ref = &mut (&mut fancy);
1139 // `fancy_ref` is now &mut(&mut FancyNum), rather than &(&mut FancyNum)
1141 fancy_ref.num = 6; // No error!
1143 println!("{}", fancy_ref.num);
1149 The value assigned to a constant scalar must be known at compile time,
1150 which is not the case when comparing raw pointers.
1152 Erroneous code example:
1154 ```compile_fail,E0395
1155 static FOO: i32 = 42;
1156 static BAR: i32 = 42;
1158 static BAZ: bool = { (&FOO as *const i32) == (&BAR as *const i32) };
1159 // error: raw pointers cannot be compared in statics!
1162 The address assigned by the linker to `FOO` and `BAR` may or may not
1163 be identical, so the value of `BAZ` can't be determined.
1165 If you want to do the comparison, please do it at run-time.
1170 static FOO: i32 = 42;
1171 static BAR: i32 = 42;
1173 let baz: bool = { (&FOO as *const i32) == (&BAR as *const i32) };
1174 // baz isn't a constant expression so it's ok
1179 A value was moved. However, its size was not known at compile time, and only
1180 values of a known size can be moved.
1182 Erroneous code example:
1185 #![feature(box_syntax)]
1188 let array: &[isize] = &[1, 2, 3];
1189 let _x: Box<[isize]> = box *array;
1190 // error: cannot move a value of type [isize]: the size of [isize] cannot
1191 // be statically determined
1195 In Rust, you can only move a value when its size is known at compile time.
1197 To work around this restriction, consider "hiding" the value behind a reference:
1198 either `&x` or `&mut x`. Since a reference has a fixed size, this lets you move
1199 it around as usual. Example:
1202 #![feature(box_syntax)]
1205 let array: &[isize] = &[1, 2, 3];
1206 let _x: Box<&[isize]> = box array; // ok!
1212 The value behind a raw pointer can't be determined at compile-time
1213 (or even link-time), which means it can't be used in a constant
1214 expression. Erroneous code example:
1216 ```compile_fail,E0396
1217 const REG_ADDR: *const u8 = 0x5f3759df as *const u8;
1219 const VALUE: u8 = unsafe { *REG_ADDR };
1220 // error: raw pointers cannot be dereferenced in constants
1223 A possible fix is to dereference your pointer at some point in run-time.
1228 const REG_ADDR: *const u8 = 0x5f3759df as *const u8;
1230 let reg_value = unsafe { *REG_ADDR };
1235 A borrow of a constant containing interior mutability was attempted. Erroneous
1238 ```compile_fail,E0492
1239 use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT};
1241 const A: AtomicUsize = ATOMIC_USIZE_INIT;
1242 static B: &'static AtomicUsize = &A;
1243 // error: cannot borrow a constant which may contain interior mutability,
1244 // create a static instead
1247 A `const` represents a constant value that should never change. If one takes
1248 a `&` reference to the constant, then one is taking a pointer to some memory
1249 location containing the value. Normally this is perfectly fine: most values
1250 can't be changed via a shared `&` pointer, but interior mutability would allow
1251 it. That is, a constant value could be mutated. On the other hand, a `static` is
1252 explicitly a single memory location, which can be mutated at will.
1254 So, in order to solve this error, either use statics which are `Sync`:
1257 use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT};
1259 static A: AtomicUsize = ATOMIC_USIZE_INIT;
1260 static B: &'static AtomicUsize = &A; // ok!
1263 You can also have this error while using a cell type:
1265 ```compile_fail,E0492
1266 use std::cell::Cell;
1268 const A: Cell<usize> = Cell::new(1);
1269 const B: &'static Cell<usize> = &A;
1270 // error: cannot borrow a constant which may contain interior mutability,
1271 // create a static instead
1274 struct C { a: Cell<usize> }
1276 const D: C = C { a: Cell::new(1) };
1277 const E: &'static Cell<usize> = &D.a; // error
1280 const F: &'static C = &D; // error
1283 This is because cell types do operations that are not thread-safe. Due to this,
1284 they don't implement Sync and thus can't be placed in statics. In this
1285 case, `StaticMutex` would work just fine, but it isn't stable yet:
1286 https://doc.rust-lang.org/nightly/std/sync/struct.StaticMutex.html
1288 However, if you still wish to use these types, you can achieve this by an unsafe
1292 use std::cell::Cell;
1293 use std::marker::Sync;
1295 struct NotThreadSafe<T> {
1299 unsafe impl<T> Sync for NotThreadSafe<T> {}
1301 static A: NotThreadSafe<usize> = NotThreadSafe { value : Cell::new(1) };
1302 static B: &'static NotThreadSafe<usize> = &A; // ok!
1305 Remember this solution is unsafe! You will have to ensure that accesses to the
1306 cell are synchronized.
1310 A variable was borrowed as mutable more than once. Erroneous code example:
1312 ```compile_fail,E0499
1316 // error: cannot borrow `i` as mutable more than once at a time
1319 Please note that in rust, you can either have many immutable references, or one
1320 mutable reference. Take a look at
1321 https://doc.rust-lang.org/stable/book/references-and-borrowing.html for more
1322 information. Example:
1327 let mut x = &mut i; // ok!
1332 let b = &i; // still ok!
1333 let c = &i; // still ok!
1338 A borrowed variable was used in another closure. Example of erroneous code:
1341 fn you_know_nothing(jon_snow: &mut i32) {
1342 let nights_watch = || {
1346 *jon_snow = 3; // error: closure requires unique access to `jon_snow`
1347 // but it is already borrowed
1352 In here, `jon_snow` is already borrowed by the `nights_watch` closure, so it
1353 cannot be borrowed by the `starks` closure at the same time. To fix this issue,
1354 you can put the closure in its own scope:
1357 fn you_know_nothing(jon_snow: &mut i32) {
1359 let nights_watch = || {
1362 } // At this point, `jon_snow` is free.
1369 Or, if the type implements the `Clone` trait, you can clone it between
1373 fn you_know_nothing(jon_snow: &mut i32) {
1374 let mut jon_copy = jon_snow.clone();
1375 let nights_watch = || {
1386 This error indicates that a mutable variable is being used while it is still
1387 captured by a closure. Because the closure has borrowed the variable, it is not
1388 available for use until the closure goes out of scope.
1390 Note that a capture will either move or borrow a variable, but in this
1391 situation, the closure is borrowing the variable. Take a look at
1392 http://rustbyexample.com/fn/closures/capture.html for more information about
1395 Example of erroneous code:
1397 ```compile_fail,E0501
1398 fn inside_closure(x: &mut i32) {
1399 // Actions which require unique access
1402 fn outside_closure(x: &mut i32) {
1403 // Actions which require unique access
1406 fn foo(a: &mut i32) {
1410 outside_closure(a); // error: cannot borrow `*a` as mutable because previous
1411 // closure requires unique access.
1415 To fix this error, you can place the closure in its own scope:
1418 fn inside_closure(x: &mut i32) {}
1419 fn outside_closure(x: &mut i32) {}
1421 fn foo(a: &mut i32) {
1426 } // borrow on `a` ends.
1427 outside_closure(a); // ok!
1431 Or you can pass the variable as a parameter to the closure:
1434 fn inside_closure(x: &mut i32) {}
1435 fn outside_closure(x: &mut i32) {}
1437 fn foo(a: &mut i32) {
1438 let bar = |s: &mut i32| {
1446 It may be possible to define the closure later:
1449 fn inside_closure(x: &mut i32) {}
1450 fn outside_closure(x: &mut i32) {}
1452 fn foo(a: &mut i32) {
1462 This error indicates that you are trying to borrow a variable as mutable when it
1463 has already been borrowed as immutable.
1465 Example of erroneous code:
1467 ```compile_fail,E0502
1468 fn bar(x: &mut i32) {}
1469 fn foo(a: &mut i32) {
1470 let ref y = a; // a is borrowed as immutable.
1471 bar(a); // error: cannot borrow `*a` as mutable because `a` is also borrowed
1476 To fix this error, ensure that you don't have any other references to the
1477 variable before trying to access it mutably:
1480 fn bar(x: &mut i32) {}
1481 fn foo(a: &mut i32) {
1483 let ref y = a; // ok!
1487 For more information on the rust ownership system, take a look at
1488 https://doc.rust-lang.org/stable/book/references-and-borrowing.html.
1492 A value was used after it was mutably borrowed.
1494 Example of erroneous code:
1496 ```compile_fail,E0503
1499 // Create a mutable borrow of `value`. This borrow
1500 // lives until the end of this function.
1501 let _borrow = &mut value;
1502 let _sum = value + 1; // error: cannot use `value` because
1503 // it was mutably borrowed
1507 In this example, `value` is mutably borrowed by `borrow` and cannot be
1508 used to calculate `sum`. This is not possible because this would violate
1509 Rust's mutability rules.
1511 You can fix this error by limiting the scope of the borrow:
1516 // By creating a new block, you can limit the scope
1517 // of the reference.
1519 let _borrow = &mut value; // Use `_borrow` inside this block.
1521 // The block has ended and with it the borrow.
1522 // You can now use `value` again.
1523 let _sum = value + 1;
1527 Or by cloning `value` before borrowing it:
1532 // We clone `value`, creating a copy.
1533 let value_cloned = value.clone();
1534 // The mutable borrow is a reference to `value` and
1535 // not to `value_cloned`...
1536 let _borrow = &mut value;
1537 // ... which means we can still use `value_cloned`,
1538 let _sum = value_cloned + 1;
1539 // even though the borrow only ends here.
1543 You can find more information about borrowing in the rust-book:
1544 http://doc.rust-lang.org/stable/book/references-and-borrowing.html
1548 This error occurs when an attempt is made to move a borrowed variable into a
1551 Example of erroneous code:
1553 ```compile_fail,E0504
1559 let fancy_num = FancyNum { num: 5 };
1560 let fancy_ref = &fancy_num;
1563 println!("child function: {}", fancy_num.num);
1564 // error: cannot move `fancy_num` into closure because it is borrowed
1568 println!("main function: {}", fancy_ref.num);
1572 Here, `fancy_num` is borrowed by `fancy_ref` and so cannot be moved into
1573 the closure `x`. There is no way to move a value into a closure while it is
1574 borrowed, as that would invalidate the borrow.
1576 If the closure can't outlive the value being moved, try using a reference
1585 let fancy_num = FancyNum { num: 5 };
1586 let fancy_ref = &fancy_num;
1589 // fancy_ref is usable here because it doesn't move `fancy_num`
1590 println!("child function: {}", fancy_ref.num);
1595 println!("main function: {}", fancy_num.num);
1599 If the value has to be borrowed and then moved, try limiting the lifetime of
1600 the borrow using a scoped block:
1608 let fancy_num = FancyNum { num: 5 };
1611 let fancy_ref = &fancy_num;
1612 println!("main function: {}", fancy_ref.num);
1613 // `fancy_ref` goes out of scope here
1617 // `fancy_num` can be moved now (no more references exist)
1618 println!("child function: {}", fancy_num.num);
1625 If the lifetime of a reference isn't enough, such as in the case of threading,
1626 consider using an `Arc` to create a reference-counted value:
1637 let fancy_ref1 = Arc::new(FancyNum { num: 5 });
1638 let fancy_ref2 = fancy_ref1.clone();
1640 let x = thread::spawn(move || {
1641 // `fancy_ref1` can be moved and has a `'static` lifetime
1642 println!("child thread: {}", fancy_ref1.num);
1645 x.join().expect("child thread should finish");
1646 println!("main thread: {}", fancy_ref2.num);
1652 A value was moved out while it was still borrowed.
1654 Erroneous code example:
1656 ```compile_fail,E0505
1659 fn eat(val: Value) {}
1664 let _ref_to_val: &Value = &x;
1670 Here, the function `eat` takes the ownership of `x`. However,
1671 `x` cannot be moved because it was borrowed to `_ref_to_val`.
1672 To fix that you can do few different things:
1674 * Try to avoid moving the variable.
1675 * Release borrow before move.
1676 * Implement the `Copy` trait on the type.
1683 fn eat(val: &Value) {}
1688 let _ref_to_val: &Value = &x;
1689 eat(&x); // pass by reference, if it's possible
1699 fn eat(val: Value) {}
1704 let _ref_to_val: &Value = &x;
1706 eat(x); // release borrow and then move it.
1713 #[derive(Clone, Copy)] // implement Copy trait
1716 fn eat(val: Value) {}
1721 let _ref_to_val: &Value = &x;
1722 eat(x); // it will be copied here.
1727 You can find more information about borrowing in the rust-book:
1728 http://doc.rust-lang.org/stable/book/references-and-borrowing.html
1732 This error occurs when an attempt is made to assign to a borrowed value.
1734 Example of erroneous code:
1736 ```compile_fail,E0506
1742 let mut fancy_num = FancyNum { num: 5 };
1743 let fancy_ref = &fancy_num;
1744 fancy_num = FancyNum { num: 6 };
1745 // error: cannot assign to `fancy_num` because it is borrowed
1747 println!("Num: {}, Ref: {}", fancy_num.num, fancy_ref.num);
1751 Because `fancy_ref` still holds a reference to `fancy_num`, `fancy_num` can't
1752 be assigned to a new value as it would invalidate the reference.
1754 Alternatively, we can move out of `fancy_num` into a second `fancy_num`:
1762 let mut fancy_num = FancyNum { num: 5 };
1763 let moved_num = fancy_num;
1764 fancy_num = FancyNum { num: 6 };
1766 println!("Num: {}, Moved num: {}", fancy_num.num, moved_num.num);
1770 If the value has to be borrowed, try limiting the lifetime of the borrow using
1779 let mut fancy_num = FancyNum { num: 5 };
1782 let fancy_ref = &fancy_num;
1783 println!("Ref: {}", fancy_ref.num);
1786 // Works because `fancy_ref` is no longer in scope
1787 fancy_num = FancyNum { num: 6 };
1788 println!("Num: {}", fancy_num.num);
1792 Or by moving the reference into a function:
1800 let mut fancy_num = FancyNum { num: 5 };
1802 print_fancy_ref(&fancy_num);
1804 // Works because function borrow has ended
1805 fancy_num = FancyNum { num: 6 };
1806 println!("Num: {}", fancy_num.num);
1809 fn print_fancy_ref(fancy_ref: &FancyNum){
1810 println!("Ref: {}", fancy_ref.num);
1816 You tried to move out of a value which was borrowed. Erroneous code example:
1818 ```compile_fail,E0507
1819 use std::cell::RefCell;
1821 struct TheDarkKnight;
1823 impl TheDarkKnight {
1824 fn nothing_is_true(self) {}
1828 let x = RefCell::new(TheDarkKnight);
1830 x.borrow().nothing_is_true(); // error: cannot move out of borrowed content
1834 Here, the `nothing_is_true` method takes the ownership of `self`. However,
1835 `self` cannot be moved because `.borrow()` only provides an `&TheDarkKnight`,
1836 which is a borrow of the content owned by the `RefCell`. To fix this error,
1837 you have three choices:
1839 * Try to avoid moving the variable.
1840 * Somehow reclaim the ownership.
1841 * Implement the `Copy` trait on the type.
1846 use std::cell::RefCell;
1848 struct TheDarkKnight;
1850 impl TheDarkKnight {
1851 fn nothing_is_true(&self) {} // First case, we don't take ownership
1855 let x = RefCell::new(TheDarkKnight);
1857 x.borrow().nothing_is_true(); // ok!
1864 use std::cell::RefCell;
1866 struct TheDarkKnight;
1868 impl TheDarkKnight {
1869 fn nothing_is_true(self) {}
1873 let x = RefCell::new(TheDarkKnight);
1874 let x = x.into_inner(); // we get back ownership
1876 x.nothing_is_true(); // ok!
1883 use std::cell::RefCell;
1885 #[derive(Clone, Copy)] // we implement the Copy trait
1886 struct TheDarkKnight;
1888 impl TheDarkKnight {
1889 fn nothing_is_true(self) {}
1893 let x = RefCell::new(TheDarkKnight);
1895 x.borrow().nothing_is_true(); // ok!
1899 Moving a member out of a mutably borrowed struct will also cause E0507 error:
1901 ```compile_fail,E0507
1902 struct TheDarkKnight;
1904 impl TheDarkKnight {
1905 fn nothing_is_true(self) {}
1909 knight: TheDarkKnight
1913 let mut cave = Batcave {
1914 knight: TheDarkKnight
1916 let borrowed = &mut cave;
1918 borrowed.knight.nothing_is_true(); // E0507
1922 It is fine only if you put something back. `mem::replace` can be used for that:
1925 # struct TheDarkKnight;
1926 # impl TheDarkKnight { fn nothing_is_true(self) {} }
1927 # struct Batcave { knight: TheDarkKnight }
1930 let mut cave = Batcave {
1931 knight: TheDarkKnight
1933 let borrowed = &mut cave;
1935 mem::replace(&mut borrowed.knight, TheDarkKnight).nothing_is_true(); // ok!
1938 You can find more information about borrowing in the rust-book:
1939 http://doc.rust-lang.org/book/first-edition/references-and-borrowing.html
1943 A value was moved out of a non-copy fixed-size array.
1945 Example of erroneous code:
1947 ```compile_fail,E0508
1951 let array = [NonCopy; 1];
1952 let _value = array[0]; // error: cannot move out of type `[NonCopy; 1]`,
1953 // a non-copy fixed-size array
1957 The first element was moved out of the array, but this is not
1958 possible because `NonCopy` does not implement the `Copy` trait.
1960 Consider borrowing the element instead of moving it:
1966 let array = [NonCopy; 1];
1967 let _value = &array[0]; // Borrowing is allowed, unlike moving.
1971 Alternatively, if your type implements `Clone` and you need to own the value,
1972 consider borrowing and then cloning:
1979 let array = [NonCopy; 1];
1980 // Now you can clone the array element.
1981 let _value = array[0].clone();
1987 This error occurs when an attempt is made to move out of a value whose type
1988 implements the `Drop` trait.
1990 Example of erroneous code:
1992 ```compile_fail,E0509
2001 impl Drop for DropStruct {
2002 fn drop(&mut self) {
2003 // Destruct DropStruct, possibly using FancyNum
2008 let drop_struct = DropStruct{fancy: FancyNum{num: 5}};
2009 let fancy_field = drop_struct.fancy; // Error E0509
2010 println!("Fancy: {}", fancy_field.num);
2011 // implicit call to `drop_struct.drop()` as drop_struct goes out of scope
2015 Here, we tried to move a field out of a struct of type `DropStruct` which
2016 implements the `Drop` trait. However, a struct cannot be dropped if one or
2017 more of its fields have been moved.
2019 Structs implementing the `Drop` trait have an implicit destructor that gets
2020 called when they go out of scope. This destructor may use the fields of the
2021 struct, so moving out of the struct could make it impossible to run the
2022 destructor. Therefore, we must think of all values whose type implements the
2023 `Drop` trait as single units whose fields cannot be moved.
2025 This error can be fixed by creating a reference to the fields of a struct,
2026 enum, or tuple using the `ref` keyword:
2037 impl Drop for DropStruct {
2038 fn drop(&mut self) {
2039 // Destruct DropStruct, possibly using FancyNum
2044 let drop_struct = DropStruct{fancy: FancyNum{num: 5}};
2045 let ref fancy_field = drop_struct.fancy; // No more errors!
2046 println!("Fancy: {}", fancy_field.num);
2047 // implicit call to `drop_struct.drop()` as drop_struct goes out of scope
2051 Note that this technique can also be used in the arms of a match expression:
2062 impl Drop for DropEnum {
2063 fn drop(&mut self) {
2064 // Destruct DropEnum, possibly using FancyNum
2069 // Creates and enum of type `DropEnum`, which implements `Drop`
2070 let drop_enum = DropEnum::Fancy(FancyNum{num: 10});
2072 // Creates a reference to the inside of `DropEnum::Fancy`
2073 DropEnum::Fancy(ref fancy_field) => // No error!
2074 println!("It was fancy-- {}!", fancy_field.num),
2076 // implicit call to `drop_enum.drop()` as drop_enum goes out of scope
2082 When matching against an exclusive range, the compiler verifies that the range
2083 is non-empty. Exclusive range patterns include the start point but not the end
2084 point, so this is equivalent to requiring the start of the range to be less
2085 than the end of the range.
2091 // This range is ok, albeit pointless.
2093 // This range is empty, and the compiler can tell.
2100 Closures cannot mutate immutable captured variables.
2102 Erroneous code example:
2104 ```compile_fail,E0595
2105 let x = 3; // error: closure cannot assign to immutable local variable `x`
2106 let mut c = || { x += 1 };
2109 Make the variable binding mutable:
2112 let mut x = 3; // ok!
2113 let mut c = || { x += 1 };
2118 This error occurs because you tried to mutably borrow a non-mutable variable.
2120 Example of erroneous code:
2122 ```compile_fail,E0596
2124 let y = &mut x; // error: cannot borrow mutably
2127 In here, `x` isn't mutable, so when we try to mutably borrow it in `y`, it
2128 fails. To fix this error, you need to make `x` mutable:
2132 let y = &mut x; // ok!
2137 This error occurs because a borrow was made inside a variable which has a
2138 greater lifetime than the borrowed one.
2140 Example of erroneous code:
2142 ```compile_fail,E0597
2147 let mut x = Foo { x: None };
2149 x.x = Some(&y); // error: `y` does not live long enough
2152 In here, `x` is created before `y` and therefore has a greater lifetime. Always
2153 keep in mind that values in a scope are dropped in the opposite order they are
2154 created. So to fix the previous example, just make the `y` lifetime greater than
2163 let mut x = Foo { x: None };
2169 This error occurs because a borrow in a generator persists across a
2172 ```compile_fail,E0626
2173 # #![feature(generators, generator_trait)]
2174 # use std::ops::Generator;
2176 let a = &String::new(); // <-- This borrow...
2177 yield (); // ...is still in scope here, when the yield occurs.
2180 unsafe { b.resume() };
2183 At present, it is not permitted to have a yield that occurs while a
2184 borrow is still in scope. To resolve this error, the borrow must
2185 either be "contained" to a smaller scope that does not overlap the
2186 yield or else eliminated in another way. So, for example, we might
2187 resolve the previous example by removing the borrow and just storing
2188 the integer by value:
2191 # #![feature(generators, generator_trait)]
2192 # use std::ops::Generator;
2198 unsafe { b.resume() };
2201 This is a very simple case, of course. In more complex cases, we may
2202 wish to have more than one reference to the value that was borrowed --
2203 in those cases, something like the `Rc` or `Arc` types may be useful.
2205 This error also frequently arises with iteration:
2207 ```compile_fail,E0626
2208 # #![feature(generators, generator_trait)]
2209 # use std::ops::Generator;
2211 let v = vec![1,2,3];
2212 for &x in &v { // <-- borrow of `v` is still in scope...
2213 yield x; // ...when this yield occurs.
2216 unsafe { b.resume() };
2219 Such cases can sometimes be resolved by iterating "by value" (or using
2220 `into_iter()`) to avoid borrowing:
2223 # #![feature(generators, generator_trait)]
2224 # use std::ops::Generator;
2226 let v = vec![1,2,3];
2227 for x in v { // <-- Take ownership of the values instead!
2228 yield x; // <-- Now yield is OK.
2231 unsafe { b.resume() };
2234 If taking ownership is not an option, using indices can work too:
2237 # #![feature(generators, generator_trait)]
2238 # use std::ops::Generator;
2240 let v = vec![1,2,3];
2241 let len = v.len(); // (*)
2243 let x = v[i]; // (*)
2244 yield x; // <-- Now yield is OK.
2247 unsafe { b.resume() };
2249 // (*) -- Unfortunately, these temporaries are currently required.
2250 // See <https://github.com/rust-lang/rust/issues/43122>.
2256 register_diagnostics! {
2257 // E0298, // cannot compare constants
2258 // E0299, // mismatched types between arms
2259 // E0471, // constant evaluation error (in pattern)
2260 // E0385, // {} in an aliasable location
2261 E0493, // destructors cannot be evaluated at compile-time
2262 E0524, // two closures require unique access to `..` at the same time
2263 E0526, // shuffle indices are not constant
2264 E0594, // cannot assign to {}
2265 E0598, // lifetime of {} is too short to guarantee its contents can be...
2266 E0625, // thread-local statics cannot be accessed at compile-time