Error code E0764
A mutable reference was used in a constant.
Erroneous code example:
#![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! }