Error code E0764

A mutable reference was used in a constant.

Erroneous code example:

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:

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! }