rustc: don't resolve Instances which would produce malformed shims.
There are some `InstanceDef` variants (shims and drop "glue") which contain a `Ty`, and that `Ty` is used in generating the shim MIR. But if that `Ty` mentions any generic parameters, the generated shim would refer to them (but they won't match the `Substs` of the `Instance`), or worse, generating the shim would fail because not enough of the type is known.
Ideally we would always produce a "skeleton" of the type, e.g. `(_, _)` for dropping any tuples with two elements, or `Vec<_>` for dropping any `Vec` value, but that's a lot of work, and they would still not match the `Substs` of the `Instance` as it exists today, so `Instance` would probably need to change.
By making `Instance::resolve` return `None` in the still-generic cases, we get behavior similar to specialization, where a default can only be used if there are no more generic parameters which would allow a more specialized `impl` to match.
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This was found while testing the MIR inliner with #68965, because it was trying to inline shims.
cc @rust-lang/wg-mir-opt
`delay_span_bug` when codegen cannot select obligation
Fix#69602, introduced in #60126 by letting the compiler continue past
type checking after encountering errors.
Make associated item collection a query
Before this change, every time associated items were iterated over (which rustc does *a lot* – this can probably be further optimized), there would be N+1 queries to fetch all assoc. items. Now there's just one after they've been computed once.
Tweak obligation error output
- Point at arguments or output when fn obligations come from them, or ident when they don't
- Point at `Sized` bound (fix#47990)
- When object unsafe trait uses itself in associated item suggest using `Self` (fix#66424, fix#33375, partially address #38376, cc #61525)
- Point at reason in object unsafe trait with `Self` in supertraits or `where`-clause (cc #40533, cc #68377)
- On implicit type parameter `Sized` obligations, suggest `?Sized` (fix#57744, fix#46683)
