gcc/testsuite/rust/compile/reference1.rs

fn main() {
    let a = &123;
    let b: &mut i32 = a;
    // { dg-error "mismatched mutability" "" { target *-*-* } .-1 }
    // { dg-error "mismatched types, expected .&mut i32. but got .& <integer>." "" { target *-*-* } .-2 }
}