Fix & test for correct highlighting of strings with shared prefix

[faculerena]

Applies here the fix suggested in this PR.

Original issue.

[epage]

Have to double check with @Muscraft but I suspect this belongs in the rustc code calling annotate-snippets. annotate-snippets is a general library with no knowledge of import paths.

[faculerena]

I'm not sure either, but I sent this PR to talk about that. I agree that the path part of the fix looks like it should be in rustc, but the issue will persist here for other strings (the prefix for origin & suggestion matches then the shared_prefix is "wrong" (not really wrong, but it does not feel right))

Edit: I mean, fixing this in rustc would mean getting a handful of things that were removed in favor of using this crate back there.

[Muscraft]

I took some time to try and find a solution that didn't require adding knowledge of rust syntax and ended up finding one (#345). For good measure, I ran the fix against rust-lang/rust's testsuite, as well as a test for std::sync, and it passed. I'll open a PR updating annotate-snippets in rust-lang/rust once #345 is merged and I have released a new version.

[faculerena]

Amazing man, the other fix will probably stay in rustc stable if annotate-snippets is not stabilized before 1.93, then that solution will work. Thanks!