remove Panic variant from InterpError
The interpreter engine itself does not raise `Panic` errors any more, so remove them from the error enum. Instead, const-prop and const-eval have to do their own handling of panics.
I used the opportunity to refactor the const-eval error handling a bit to use the `MachineStop` variant.
Also, in const-prop I could do some cleanup as now, no more lints are being reported in `use_ecx`. However, I am quite puzzled by why exactly the linting there works the way it does -- the code can certainly be cleaned up more, but I don't know enough of the intent to do that. I left some questions for the most confusing parts, but for now behavior should be unchanged by this PR (so, all that weirdness I am asking about is pre-existing and merely maintained here). Cc @wesleywiser
Fixes https://github.com/rust-lang/rust/issues/66902
r? @oli-obk
parse: unify function front matter parsing
Part of https://github.com/rust-lang/rust/pull/68728.
- `const extern fn` feature gating is now done post-expansion such that we do not have conditional compatibilities of function qualifiers *in parsing*.
- The `FnFrontMatter` grammar becomes:
```rust
Extern = "extern" StringLit ;
FnQual = "const"? "async"? "unsafe"? Extern? ;
FnFrontMatter = FnQual "fn" ;
```
That is, all item contexts now *syntactically* allow `const async unsafe extern "C" fn` and use semantic restrictions to rule out combinations previously prevented syntactically. The semantic restrictions include in particular:
- `fn`s in `extern { ... }` can have no qualifiers.
- `const` and `async` cannot be combined.
- We change `ast::{Unsafety, Spanned<Constness>}>` into `enum ast::{Unsafe, Const} { Yes(Span), No }` respectively. This change in formulation allow us to exclude `Span` in the case of `No`, which facilitates parsing. Moreover, we also add a `Span` to `IsAsync` which is renamed to `Async`. The new `Span`s in `Unsafety` and `Async` are then taken advantage of for better diagnostics. A reason this change was made is to have a more uniform and clear naming scheme.
The HIR keeps the structures in AST (with those definitions moved into HIR) for now to avoid regressing perf.
r? @petrochenkov
