Use DefiningOpaqueTypes::Yes in rustdoc, where the InferCtxt is guaranteed to have no opaque types it can define

[oli-obk]

r? @lcnr

I manually checked there it's always tcx.infer_ctxt().build()

[compiler-errors]

LOL did you check rustdoc tests?

Ah, I missed the "modulo rustdoc, but that doesn't count".

But why doesn't it count? 🤔

[fmease]

(modulo rustdoc, but that doesn't count).

It does count. In practice, AutoTraitFinder only exists to serve rustdoc. In theory, it's available to rustdoc and “other tools” (#49711). I very much dislike that rustdoc's auto trait impl synthesis code was split in two and I'm thinking about moving the rustc part back into rustdoc again.

[fmease]

One such “other tool” was supposed to be rust-lang/rust-semverver (now archived). However, it never actually got to use AutoTraitFinder. See rust-lang/rust-semverver#48.

[fmease]

NB: AutoTraitFinder is one of the most atrocious, abysmal, hairy and hacky pieces of code I've seen so far in rustc (respectfully). I've yet to understand the implementation details. It continuously resets the InferCtxt cache to keep going etc. and does other questionable stuff.

[oli-obk]

But why doesn't it count? 🤔

I assumed it was operating on broken code or sth. Should have checked before assuming. I did not realize auto trait finder exists for Rustdoc only

[lukas-code]

It continuously resets the InferCtxt cache to keep going etc. and does other questionable stuff.

AFAICT it only does that because the projection cache only uses the AliasTy as a key and not the ParamEnv. This makes it so that a single InferCtxt can never be used to normalize in different ParamEnvs without resetting the cache, which also seems rather questionable.

[lcnr]

@bors r+ rollup

[bors]

📌 Commit 6bff7f4 has been approved by lcnr

It is now in the queue for this repository.