…used type parameter

The user wrote the bound, so it's obvious they want a type.

…const-with-bounds-unused-generic-param, r=jackh726

Do not suggest using a const parameter when there are bounds on an unused type parameter

The user wrote the bound, so it's obvious they want a type.

…ackh726

Provide extra note if synthetic type args are specified

Implement the unresolved question in rust-lang#83701 as suggested in rust-lang#86176 (comment).

r? ``@jackh726``

…=jackh726

Remove unused `unsound_ignore_borrow_on_drop`

…r=jackh726

chalk: Fix wrong debrujin index in opaque type handling.

A folder in opaque type lowering was substituting all opaque type references with a variable with debrujin index 0 ignoring how many binders deep we are.

This caused an ICE with `Not enough bound vars: ^0 not found in []` ([full logs](https://gist.github.com/Dirbaio/2b9374ff4fce37afb9d665dc9f0000df)) with the following code.

```rust
fn main() -> () {}
async fn foo(x: u32) -> u32 {
    x
}
```

With the fix, it no longer ICEs. It still doesn't typecheck due to generator issues. I've added a "known-bug" test so that at least it doesn't regress back to ICEing.

r? ``@jackh726``

Avoid exhausting stack space in dominator compression

Doesn't add a test case -- I ended up running into this while playing with the generated example from rust-lang#43578, which we could do with a run-make test (to avoid checking a large code snippet into tree), but I suspect we don't want to wait for it to compile (locally it takes ~14s -- not terrible, but doesn't seem worth it to me). In practice stack space exhaustion is difficult to test for, too, since if we set the bound too low a different call structure above us (e.g., a nearer ensure_sufficient_stack call) would let the test pass even with the old impl, most likely.

Locally it seems like this manages to perform approximately equivalently to the recursion, but will run perf to confirm.