error[E0658]: mutable references are not allowed in constant functions --> $DIR/feature-gate-const_mut_refs.rs:5:14 | LL | const fn foo(x: &mut i32) -> i32 { | ^ | = note: see issue #57349 for more information = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable error: aborting due to previous error For more information about this error, try `rustc --explain E0658`.