error[E0384]: cannot assign to immutable argument `_x` --> $DIR/immutable-arg.rs:2:5 | LL | fn foo(_x: u32) { | -- help: consider making this binding mutable: `mut _x` LL | _x = 4; | ^^^^^^ cannot assign to immutable argument error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0384`.