Redesign comment system

[kdy1]

It seems like a misdesign.

Problems

Convenience

API is tricky because of comments.

Performance

The current design makes fixer slow because it requires moving comments in hashmap again and again.
Moving comments involves acquiring / releasing lock and rehashing.

It can be improved using buffer, but maybe there is a fundamental problem.

cc @dsherret as you may be interested in this.

[dsherret]

Yeah, this would be good. I also still think there are race conditions in with_leading and with_trailing where two threads might remove one after the other and one thread would end up with different comments.

swc/common/src/comments.rs

Line 291 in f44e25c

fn with_leading<F, Ret>(&self, pos: BytePos, op: F) -> Ret

Edit: I have opened #2001 which fixes this race condition until the redesign.