Make elaboration generic over input
Combines all the `elaborate_*` family of functions into just one, which is an iterator over the same type that you pass in (e.g. elaborating `Predicate` gives `Predicate`s, elaborating `Obligation`s gives `Obligation`s, etc.)
diagnostics: account for self type when looking for source of unsolved type variable
Fixes#109905.
When searching for the source of an unsolved infer var inside of a list of generic args, we look through the `tcx.generics_of(…).own_substs(…)` which *skips* the self type if present. However, the computed `argument_index` is later[^1] used to index into `tcx.generics_of(…).params` which may still contain the self type. In such case, we are off by one when indexing into the parameters.
From now on, we account for this immediately after calling `own_substs` which keeps things local.
This also fixes the wrong output in the preexisting UI test `inference/need_type_info/concrete-impl.rs` which was overlooked. It used to claim that the *type of type parameter `Self`* couldn't be inferred in `<Struct as Ambiguous<_>>::method()` which of course isn't true: `Self` equals `Struct` here, `A` couldn't be inferred.
`@rustbot` label A-diagnostics
[^1]: f98a271814/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs (L471)
And while doing the updates for that, also uses `FieldIdx` in `ProjectionKind::Field` and `TypeckResults::field_indices`.
There's more places that could use it (like `rustc_const_eval` and `LayoutS`), but I tried to keep this PR from exploding to *even more* places.
Part 2/? of https://github.com/rust-lang/compiler-team/issues/606
stop special-casing `'static` in evaluation
fixes#102360
I have no idea whether this actually removed all places where `'static` matters. Without canonicalization it's very easy to accidentally rely on `'static` again. Blocked on changing the `order_dependent_trait_objects` future-compat lint to a hard error
r? `@nikomatsakis`
Don't elaborate non-obligations into obligations
It's suspicious to elaborate a `PolyTraitRef` or `Predicate` into an `Obligation`, since the former does not have a param-env associated with it, but the latter does. This is a footgun that, while not being misused *currently* in the compiler, easily could be misused by someone less familiar with the elaborator's inner workings.
This PR just changes the API -- ideally, the elaborator wouldn't even have to deal with obligations if we're not elaborating obligations, but that would require a bit more abstraction than I could be bothered with today.
