Uplift outlives components to `rustc_type_ir`
We need this to uplift `push_outlives_components`, since the elaborator uses `push_outlives_components` to elaborate type outlives obligations and I want to uplift elaboration.
This ends up reworking and inlining a fair portion of the `GenericArg::walk_shallow` function, whose only callsite was this one. I believe I got the logic correct, but may be worthwhile to look at it closely just in case. Unfortunately github was too dumb to understand that this is a rename + change -- I could also rework the git history to split the "copy the file over" part from the actual logical changes if that makes this easier to review.
r? lcnr
Automatically taint InferCtxt when errors are emitted
r? `@nnethercote`
Basically `InferCtxt::dcx` now returns a `DiagCtxt` that refers back to the `Cell<Option<ErrorGuaranteed>>` of the `InferCtxt` and thus when invoking `Diag::emit`, and the diagnostic is an error, we taint the `InferCtxt` directly.
That change on its own has no effect at all, because `InferCtxt` already tracks whether errors have been emitted by recording the global error count when it gets opened, and checking at the end whether the count changed. So I removed that error count check, which had a bit of fallout that I immediately fixed by invoking `InferCtxt::dcx` instead of `TyCtxt::dcx` in a bunch of places.
The remaining new errors are because an error was reported in another query, and never bubbled up. I think they are minor enough for this to be ok, and sometimes it actually improves diagnostics, by not silencing useful diagnostics anymore.
fixes#126485 (cc `@olafes)`
There are more improvements we can do (like tainting in hir ty lowering), but I would rather do that in follow up PRs, because it requires some refactorings.
Harmonize `AsyncFn` implementations, make async closures conditionally impl `Fn*` traits
This PR implements several changes to the built-in and libcore-provided implementations of `Fn*` and `AsyncFn*` to address two problems:
1. async closures do not implement the `Fn*` family traits, leading to breakage: https://crater-reports.s3.amazonaws.com/pr-120361/index.html
2. *references* to async closures do not implement `AsyncFn*`, as a consequence of the existing blanket impls of the shape `AsyncFn for F where F: Fn, F::Output: Future`.
In order to fix (1.), we implement `Fn` traits appropriately for async closures. It turns out that async closures can:
* always implement `FnOnce`, meaning that they're drop-in compatible with `FnOnce`-bound combinators like `Option::map`.
* conditionally implement `Fn`/`FnMut` if they have no captures, which means that existing usages of async closures should *probably* work without breakage (crater checking this: https://github.com/rust-lang/rust/pull/120712#issuecomment-1930587805).
In order to fix (2.), we make all of the built-in callables implement `AsyncFn*` via built-in impls, and instead adjust the blanket impls for `AsyncFn*` provided by libcore to match the blanket impls for `Fn*`.
