misc coercion cleanups and handle safety correctly
r? lcnr
### "remove normalize call"
Fixesrust-lang/rust#132765
If the normalization fails we would sometimes get a `TypeError` containing inference variables created inside of the probe used by coercion. These would then get leaked out causing ICEs in diagnostics logic
### "leak check and lub for closure<->closure coerce-lubs of same defids"
Fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/233
```rust
fn peculiar() -> impl Fn(u8) -> u8 {
return |x| x + 1
}
```
the `|x| x + 1` expr has a type of `Closure(?31t)` which we wind up inferring the RPIT to. The `CoerceMany` `ret_coercion` for the whole `peculiar` typeck has an expected type of `RPIT` (unnormalized). When we type check the `return |x| x + 1` expr we go from the never type to `Closure(?31t)` which then participates in the `ret_coercion` giving us a `coerce-lub(RPIT, Closure(?31t))`.
Normalizing `RPIT` gives us some `Closure(?50t)` where `?31t` and `?50t` have been unified with `?31t` as the root var. `resolve_vars_if_possible` doesn't resolve infer vars to their roots so these wind up with different structural identities so the fast path doesn't apply and we fall back to coercing to a `fn` ptr. cc rust-lang/rust#147193 which also fixes this
New solver probably just gets more inference variables here because canonicalization + generally different approach to normalization of opaques. Idk :3
### FCP worthy stuffy
there are some other FCP worthy things but they're in my FCP comment which also contains some analysis of the breaking nature of the previously listed changes in this PR: https://github.com/rust-lang/rust/pull/148602#issuecomment-3503497467
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Also please add a doc comment at the top of the file explaining why
this test exists. :)
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