fix: Do not ICE when missing match arm with ill-formed subty is met
Fixesrust-lang/rust#148192
The ICE comes from the following line, calling `normalize_erasing_regions` to a projection type whose trait bound is not met:
https://github.com/rust-lang/rust/blob/2fcbda6c1a70606bdb09857e01d01fc6229da712/compiler/rustc_pattern_analysis/src/rustc.rs#L185-L194
The above function is called while trying to lint missing match arms, or scrutinize ctors of missing(not necessary error) match arms.
So, the following code can trigger ICEs.
```rust
trait WhereTrait {
type Type;
}
fn foo(e: Enum) {
match e {
Enum::Map(_) => (), // ICE, while trying to lint missing arms
}
if let Enum::Map(_) = e {} // ICE, while trying to scrutinize missing ctors (even worse)
}
enum Enum {
Map(()),
Map2(<() as WhereTrait>::Type),
}
```
This ICE won't be triggered with the following code, as this is filtered out before `check_match` as the existence of ill-formed type inside the variant marks the body as tainted by error in `hir_typeck`, but for the above code, the `hir_typeck` complains nothing because everything it sees is locally correct.
```rust
fn foo(e: Enum) {
match e {
Enum::Map2(_) => (), // No ICE
}
}
```
I've considered visiting and wf checking for the match scrutinee before entering `check_match`, but that might regress the perf and I think just emitting delayed bug would enough as the normalization failure would be originated by other errors like ill-formdness.
Stuart Cook
17c25e4483