Trait solver re-derives `Send` proofs per impl when an outlives where-bound is in scope

One trait, M impls, each wrapping the same Shared struct (150 fields of Arc<Mutex<Vec<…>>> nested 10 deep); every method's future borrows &self. The two files differ only in how the method's lifetimes are spelled:

// outlives_*.rs — the shape #[async_trait] emits for every method
fn check<'life0, 'at>(&'life0 self, x: &'at [u8])
    -> Pin<Box<dyn Future<Output = u64> + Send + 'at>>
where Self: 'at, 'life0: 'at;

// unified_*.rs — same borrows under one lifetime, no outlives clause
fn check<'a>(&'a self, x: &'a [u8]) -> Pin<Box<dyn Future<Output = u64> + Send + 'a>>;

$ git clone https://github.com/ashi009/region-cache-repro && cd region-cache-repro
$ rustc -V
rustc 1.96.0 (ac68faa20 2026-05-25)
$ time rustc --edition 2021 --crate-type=lib --emit=metadata -o /tmp/r.rmeta outlives_150.rs
real	0m3.326s
$ time rustc --edition 2021 --crate-type=lib --emit=metadata -o /tmp/r.rmeta outlives_300.rs
real	0m6.627s
$ time rustc --edition 2021 --crate-type=lib --emit=metadata -o /tmp/r.rmeta unified_150.rs
real	0m0.134s
$ time rustc --edition 2021 --crate-type=lib --emit=metadata -o /tmp/r.rmeta unified_300.rs
real	0m0.180s

$ rustc -V
rustc 1.98.0-nightly (8954863c8 2026-06-05)
$ time rustc --edition 2021 --crate-type=lib --emit=metadata -o /tmp/r.rmeta outlives_300.rs
real	0m6.612s
$ time rustc --edition 2021 --crate-type=lib --emit=metadata -o /tmp/r.rmeta unified_300.rs
real	0m0.188s
$ time rustc --edition 2021 --crate-type=lib --emit=metadata -Znext-solver=globally -o /tmp/r.rmeta outlives_300.rs
real	0m14.539s
$ time rustc --edition 2021 --crate-type=lib --emit=metadata -Znext-solver=globally -o /tmp/r.rmeta unified_300.rs
real	0m0.273s

97% of the outlives time is evaluate_obligation, re-deriving the same Shared: Send proof once per impl. The outlives bound puts a region in caller_bounds. The query canonicalizes the ParamEnv, so that region comes back as an infer var. can_use_global_caches then bails on param_env.has_infer(), and the proof never reaches tcx.evaluation_cache. #[async_trait] emits that bound on every method, so large async codebases pay this per method × impl.

Same diagnosis and fix as #92044 (validated on #87012), closed unmerged over selection soundness (impl<T: 'static> bounds participate in selection); it reproduces unchanged today.

(-Znext-solver=globally not removing the gap is unexpected; cause not investigated.)

Clone region-cache-repro and reproduce the outlives_150.rs, outlives_300.rs, and unified_*.rs timings with rustc. Start at evaluate_obligation and the ParamEnv/canonicalization path, then compare the diagnosis and proposed fix in #92044 and validation in #87012. Done means addressing the repeated Send proof without reopening the selection-soundness concern.