matching is always considered a use of the place, even with `_` patterns

[nikomatsakis]

In this example playground:

    let foo = [1, 2, 3];
    let c = || match foo { _ => () };

the match foo winds up registering a read of foo in the ExprUseVisitor. Note that the behavior here is different from let _ = foo. This is also somewhat inconsistent with the borrow checker, which considers match foo { _ => () } to not be a use (i.e., this code compiles, playground):

let mut x = 1;
let y = &x;
match x { _ => () }
drop(y);

[arora-aman]

• From a back-compat perspective, inside closures do we want to stop capturing the place that is being matched on?
• If yes, the fix is kind of simple for capture analysis. Clippy uses ExprUseVisitor which makes this a little tricky.
• For capture analysis, we just remove this line
• The reason it works is:
  • When capture_disjoint_fields is enabled, the case described above falls under Handle patterns within closures with the feature gate enabled #24
  • When it's not enabled, since we capture everything that is mentioned inside the closure, we will end up capturing
    the discriminant as well, similar to how we have been doing things so far.

What remains is to see if this breaks clippy. If it does, how they use it will define how we end up handling the problem.