Document HIR ty lowering

[nikomatsakis]

[!NOTE] The issue description has been rewritten by @fmease.

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See its API docs: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_analysis/hir_ty_lowering/index.html.

The chapter should explain:

• [ ] Meaning & origin of its current name (HIR ty lowering) and of its previous name (astconv).
• [ ] Purpose, incl. bigger picture
• [ ] "Conventions", "restrictions", "self imposed rules" (e.g., https://github.com/rust-lang/rust/pull/125819#issuecomment-2142394608).
• [ ] "Limitations" (cc arbitrary shorthand projection & inherent associated type debates)

[rajcspsg]

@mark-i-m @nikomatsakis Shall I take this issue to work on?

[mark-i-m]

That would be great, but unfortunately I won't be able to offer much guidance, as I don't really know anything about astconv.

[rajcspsg]

The link provided by @nikomatsakis is not available or I don't have permission.

[mark-i-m]

Hmm... unfortunately, I don’t really know what to do here. Sorry about that...

[nikomatsakis]

@rajcspsg I'm not sure what it said at that link anymore either =) Probably it was just a summary of how "astconv" contains some logic for converting the types from the "AST form" to the internal, semantic form

[nikomatsakis]

maybe reach out on the rust-lang zulip and we can discuss more?

[jieyouxu]

Triage: astconv isn't a thing anymore AFAIK, it's probably now called ast_lowering.

[fmease]

Nope, it's called HIR ty lowering

[jieyouxu]

(For easier reference, the PR that renamed it is https://github.com/rust-lang/rust/pull/120926)

[fmease]

Tentatively assigning myself but I invite others to contribute too if I take too long (cuz I prolly won't prioritize this).