Exclude based on regex of statements?

[Martin Pool]

cargo-mutants/src/span.rs

Line 103 in b2a4bd1

 let mut r = String::with_capacity(s.len() + replacement.len()); 

The String::with_capacity isn't wrong, and it's probably slightly beneficial to have there, but the side effects are subtle and it's probably asking too much to test it. (Actually, in this specific case we could debug_assert the capacity to shush it, but that's a bit messy.)

We cannot yet add a #[mutants::skip] here, because expression attributes aren't stable yet (rust-lang/rust#15701).

Maybe it would be useful to have config that can match against the whole expression text and then skip; that would probably help in other cases of code that only has unimportant side effects such as trace statements.