coverage: Prepare for upcoming changes to counter creation by Zalathar · Pull Request #135873 · rust-lang/rust (original) (raw)

matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request

Jan 23, 2025

…piler-errors

Allow arena_cache queries to return Option<&'tcx T>

Currently, arena_cache queries always have to return &'tcx T[^deref]. This means that if an arena-cached query wants to return an optional value, it has to return &'tcx Option<T>, which has a few negative consequences:

• It goes against normal Rust style, where Option<&T> is preferred over &Option<T>.
• Callers that actually want an Option<&T> have to manually call .as_ref() on the query result.
• When the query result is None, a full-sized Option<T> still needs to be stored in the arena.

This PR solves that problem by introducing a helper trait ArenaCached that is implemented for both &T and Option<&T>, and takes care of bridging between the provided type, the arena-allocated type, and the declared query return type.

To demonstrate that this works, I have converted the two existing arena-cached queries that currently return &Option<T>: mir_coroutine_witnesses and diagnostic_hir_wf_check. Only the query declarations need to be modified; existing providers and callers continue to work with the new query return type.

(My real goal is to apply this to coverage_ids_info, which will return Option as of rust-lang#135873, but that PR hasn't landed yet.)

[^deref]: Technically they could return other types that implement Deref, but it's hard to imagine this working well with anything other than &T.