Idea: subpatterns and non-trivial patterns affect lifetimes

[QuineDot]

See this issue.

Fails:

fn f(mut x: &String) {
    x = &String::new();
}

Compiles:

fn f(mut x @ _: &String) {
    x = &String::new();
}

fn f((mut x,): (&String,)) {
    x = &String::new();
}

[dtolnay]

This seems to fall in the "does this program compile or does it not compile?" category that https://github.com/dtolnay/rust-quiz#contributing explains, so I don't think this would make a good quiz question. Thanks anyway for the suggestion!