lub instead of relying on structural identity in coerce lub

[BoxyUwU]

Fixes rust-lang/trait-system-refactor-initiative#233

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 #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

In theory I feel like using a lub here ought to cause other code to work that currently doesn't 🤔 E.g. if the two types only differ in normalization. I'm not certain about how to construct such an example though :3

r? lcnr

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling stable_deref_trait v1.2.0
error[E0277]: the trait bound `!: FromStr` is not satisfied
    --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libc-0.2.174/build.rs:242:36
     |
 242 |     let minor = otry!(otry!(minor).parse().ok());
     |                                    ^^^^^ the trait `FromStr` is not implemented for `!`
     |
     = help: the following other types implement trait `FromStr`:
               ByteString
               CString
---
             and 32 others
note: required by a bound in `core::str::<impl str>::parse`
    --> /checkout/library/core/src/str/mod.rs:2701:21
     |
2701 |     pub fn parse<F: FromStr>(&self) -> Result<F, F::Err> {
     |                     ^^^^^^^ required by this bound in `core::str::<impl str>::parse`

[RUSTC-TIMING] stable_deref_trait test:false 0.043
   Compiling memchr v2.7.5
For more information about this error, try `rustc --explain E0277`.

[BoxyUwU]

hell yeah

[bors]

☔ The latest upstream changes (presumably #147111) made this pull request unmergeable. Please resolve the merge conflicts.