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A mutable reference was used in a constant.
Erroneous code example:
```compile_fail,E0764
#![feature(const_mut_refs)]
fn main() {
const OH_NO: &'static mut usize = &mut 1; // error!
}
```
Mutable references (`&mut`) can only be used in constant functions, not statics
or constants. This limitation exists to prevent the creation of constants that
have a mutable reference in their final value. If you had a constant of
`&mut i32` type, you could modify the value through that reference, making the
constant essentially mutable.
While there could be a more fine-grained scheme in the future that allows
mutable references if they are not "leaked" to the final value, a more
conservative approach was chosen for now. `const fn` do not have this problem,
as the borrow checker will prevent the `const fn` from returning new mutable
references.
Remember: you cannot use a function call inside a constant or static. However,
you can totally use it in constant functions:
```
#![feature(const_mut_refs)]
const fn foo(x: usize) -> usize {
let mut y = 1;
let z = &mut y;
*z += x;
y
}
fn main() {
const FOO: usize = foo(10); // ok!
}
```
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