auto merge of #6385 : huonw/rust/rustc-dtor-struct-match, r=nikomatsakis

auto merge of #6385 : huonw/rust/rustc-dtor-struct-match, r=nikomatsakis

**Caveat**: With the current commit, this check only works for `match`s, the checks (incorrectly) do not run for patterns in `let`s, and invalid/unsafe code compiles.

I don't know how to fix this, I experimented with some things to try to make let patterns and match patterns run on the same code (since this would presumably fix many of the other unsoundness issues of let-patterns, e.g. #6225), but I don't understand enough of the code. (I think I heard someone talking about a fix for `let` being in progress?)

Fixes #6344 and #6341.