Don't generate a mutation for .with_capacity

[pashadia]

Hi all,

New contributor here, first of all congratulations on a great project!

I've used cargo mutants on a fairly large codebase (12kloc). One of the things that appear regularly on my project is to preallocate memory for some data structures. For instance, we use things like let v = Vec::with_capacity(a+b);. Obviously, cargo-mutants is generating a mutation for the argument, into things like a-b or a*b. However, this won't break any tests, as it doesn't actually introduce a bug in functionality, only the memory allocation footprint may be different.

My proposal is to remove the generation of mutations for the .with_capacity parameter, for all the structs in std::collections, and perhaps not only. I would gladly attempt an implementation, if this proposal is well received.

[sourcefrog]

Sounds like a great idea, and probably not too hard to have.

This change ought to go into visit.rs and to also have a unit test in that module. You can see existing examples there where we skip new and Default: but this is a bit different: we don't want to primarily skip mutations of with_capacity, but rather not recurse into the arguments of any call to functions with that name. (If you get stuck please let me know here.)