Use for<'tcx> fn pointers in Providers, instead of having Providers<'tcx>.
In order to work around normalization-under-HRTB (for `provide!` in `rustc_metadata`), we ended up with this:
```rust
struct Providers<'tcx> {
type_of: fn(TyCtxt<'tcx>, DefId) -> Ty<'tcx>,
// ...
}
```
But what I initially wanted to do, IIRC, was this:
```rust
struct Providers {
type_of: for<'tcx> fn(TyCtxt<'tcx>, DefId) -> Ty<'tcx>,
// ...
}
```
This PR moves to the latter, for the simple reason that only the latter allows keeping a `Providers` value, or a subset of its `fn` pointer fields, around in a `static` or `thread_local!`, which can be really useful for custom drivers that override queries.
(@jyn514 and I came across a concrete usecase of that in `rustdoc`)
The `provide!` macro in `rustc_metadata` is fixed by making the query key/value types available as type aliases under `ty::query::query_{keys,values}`, not just associated types (this is the first commit).
r? @nikomatsakis
65ac3948ae