src/test/ui/match/match-ref-mut-invariance.rs

// Check that when making a ref mut binding with type `&mut T`, the
// type `T` must match precisely the type `U` of the value being
// matched, and in particular cannot be some supertype of `U`. Issue
// #23116. This test focuses on a `match`.

#![allow(dead_code)]
struct S<'b>(&'b i32);
impl<'b> S<'b> {
    fn bar<'a>(&'a mut self) -> &'a mut &'a i32 {
        match self.0 { ref mut x => x } //~ ERROR mismatched types
    }
}

fn main() {}