This is a pretty naive tool that uses a regular expression for
matching lines. It is based on a Google-internal tool that worked in a
similar way.
For now, the regular expression is hard-coded to attempt to match
single-line Python imports. The only commit I've found in the hg core
repo where the tool helped was commit 9cd6292abfdf. I think that's
because we often use multiple imports per import statement. I think
this tool is still a decent first step (especially once the regex is
made configurable in the next patch). The merging should ideally use a
proper Python parser and do the merge at the AST (or CST?) level, but
that's significantly harder, especially if you want to preserve
comments and whitespace. It's also less generic.
It would be pretty low effort to still support lower versions of Rust, but going to 1.59 allows the use of edition 2021 and makes sense for a contrib file anyway, so I understand the appeal of a newer version.