Auto merge of #96020 - martingms:optimize-relate_substs, r=nnethercote

Micro-optimize `ty::relate::relate_substs` by avoiding `match`

Was a top-20 hot function in a callgrind profile of compiling `bitmaps-3.1.0`.

Yields some small speedups on that crate and some others according to local benching:

Benchmark | Profile | Scenario | % Change | Significance Factor?
-- | -- | -- | -- | --
bitmaps-3.1.0 | check | full | -1.88% | 9.42x
bitmaps-3.1.0 | debug | full | -1.80% | 8.99x
bitmaps-3.1.0 | opt | full | -1.70% | 8.49x
bitmaps-3.1.0 | check | incr-full | -1.54% | 7.68x
deep-vector | debug | full | 1.52% | 7.61x
bitmaps-3.1.0 | debug | incr-full | -1.45% | 7.26x
bitmaps-3.1.0 | opt | incr-full | -1.39% | 6.95x
nalgebra-0.30.1 | check | full | -0.68% | 3.42x
nalgebra-0.30.1 | debug | full | -0.64% | 3.22x
nalgebra-0.30.1 | opt | full | -0.64% | 3.20x
projection-caching | check | full | -0.61% | 3.05x
nalgebra-0.30.1 | check | incr-full | -0.56% | 2.78x
nalgebra-0.30.1 | opt | incr-full | -0.54% | 2.72x
nalgebra-0.30.1 | debug | incr-full | -0.54% | 2.69x
projection-caching | check | incr-full | -0.50% | 2.51x
tt-muncher | opt | full | -0.48% | 2.42x
projection-caching | opt | full | -0.47% | 2.37x
projection-caching | debug | full | -0.47% | 2.35x
projection-caching | opt | incr-full | -0.44% | 2.21x
projection-caching | debug | incr-full | -0.42% | 2.08x
deeply-nested-multi | check | incr-full | 0.37% | 1.87x
wf-projection-stress-65510 | opt | full | -0.37% | 1.84x
deep-vector | debug | incr-patched: add vec item | -0.32% | 1.61x
projection-caching | debug | incr-unchanged | -0.32% | 1.60x
wf-projection-stress-65510 | check | full | -0.31% | 1.55x
projection-caching | opt | incr-unchanged | -0.31% | 1.53x
wf-projection-stress-65510 | debug | incr-full | -0.30% | 1.51x
wf-projection-stress-65510 | opt | incr-full | -0.30% | 1.51x

r? `@nnethercote`

compiler/rustc_infer/src/infer/combine.rs

@@ -567,11 +567,17 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
             // Avoid fetching the variance if we are in an invariant
             // context; no need, and it can induce dependency cycles
             // (e.g., #41849).
-            relate::relate_substs(self, None, a_subst, b_subst)
+            relate::relate_substs(self, a_subst, b_subst)
         } else {
             let tcx = self.tcx();
             let opt_variances = tcx.variances_of(item_def_id);
-            relate::relate_substs(self, Some((item_def_id, &opt_variances)), a_subst, b_subst)
+            relate::relate_substs_with_variances(
+                self,
+                item_def_id,
+                &opt_variances,
+                a_subst,
+                b_subst,
+            )
         }
     }
 

compiler/rustc_infer/src/infer/equate.rs

@@ -53,7 +53,7 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
         // performing trait matching (which then performs equality
         // unification).
 
-        relate::relate_substs(self, None, a_subst, b_subst)
+        relate::relate_substs(self, a_subst, b_subst)
     }
 
     fn relate_with_variance<T: Relate<'tcx>>(