I'm mostly just opening this for myself since I want somewhere to track the work. Look away unless you care about completely compiler-internal stuff.

The motivation for doing this is that error reporting in rustc_infer is fraught with challenges, like the lack of trait selection. This became evident in #127172, which demonstrates that the new trait solver needs trait selection in order to actually even prove that things can be equal or not. We also suffer from the lack of normalization, and to a lesser extent autoderef (though the latter isn't yet uplifted into trait selection, it very well could be, or at least replicated in some way for the purposes of diagnostics, but that requires trait selection lol)

• Create top level rustc_trait_selection::error_reporting module (Move trait selection error reporting to its own top-level module #127493)
• Move trait error reporting to error_reporting::traits (from traits::error_reporting) (Move trait selection error reporting to its own top-level module #127493)
  • Split out overflow error reporting from other error reporting (More trait error reworking #127495)
  • Split out ambiguity error reporting from other error reporting (More trait error reworking #127495)
  • flatten module structure of error_reporting::traits (e.g. no need to have type_err_ctxt_ext module) (Get rid of InferCtxtExt from error_reporting::traits #128076)
• Move rustc_infer's error reporting to rustc_trait_selection::error_reporting::infer (Uplift most type-system related error reporting from rustc_infer to rustc_trait_selection #128041)
  • Move all the diagnostics from infer -> trait selection ftl file (Uplift most type-system related error reporting from rustc_infer to rustc_trait_selection #128041)
  • Split out region error reporting from type error reporting (Consolidate region error reporting in rustc_infer #127500)
• Pull TypeErrCtxt into rustc_trait_selection, and delete all the extension traits (since they can be inherent now) (Uplift most type-system related error reporting from rustc_infer to rustc_trait_selection #128041)
  • Delete can_eq_shallow (Get rid of can_eq_shallow #128067)
  • Delete normalization helper from TypeErrCtxt since normalization is defined in rustc_trait_selection
• Audit code for anything that we can now use since error reporting lives in trait selection