Fast-path some relations via strict equality

compiler/rustc_hir_analysis/src/check/dropck.rs

@@ -248,6 +248,11 @@ impl<'tcx> TypeRelation<'tcx> for SimpleEqRelation<'tcx> {
         self.param_env
     }
 
+    #[inline]
+    fn fast_equate(&self) -> bool {
+        true
+    }
+
     fn tag(&self) -> &'static str {
         "dropck::SimpleEqRelation"
     }

compiler/rustc_infer/src/infer/combine.rs

@@ -525,6 +525,11 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
         self.param_env
     }
 
+    #[inline]
+    fn fast_equate(&self) -> bool {
+        false
+    }
+
     fn tag(&self) -> &'static str {
         "Generalizer"
     }
@@ -802,6 +807,11 @@ impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
         self.param_env
     }
 
+    #[inline]
+    fn fast_equate(&self) -> bool {
+        false
+    }
+
     fn tag(&self) -> &'static str {
         "ConstInferUnifier"
     }

compiler/rustc_infer/src/infer/equate.rs

@@ -32,6 +32,11 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
         self.fields.tcx()
     }
 
+    #[inline]
+    fn fast_equate(&self) -> bool {
+        true
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.fields.param_env
     }

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2945,6 +2945,11 @@ impl<'tcx> TypeRelation<'tcx> for SameTypeModuloInfer<'_, 'tcx> {
         "SameTypeModuloInfer"
     }
 
+    #[inline]
+    fn fast_equate(&self) -> bool {
+        true
+    }
+
     fn a_is_expected(&self) -> bool {
         true
     }

compiler/rustc_infer/src/infer/glb.rs

@@ -34,6 +34,11 @@ impl<'tcx> TypeRelation<'tcx> for Glb<'_, '_, 'tcx> {
         self.fields.tcx()
     }
 
+    #[inline]
+    fn fast_equate(&self) -> bool {
+        true
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.fields.param_env
     }

compiler/rustc_infer/src/infer/lub.rs

@@ -34,6 +34,11 @@ impl<'tcx> TypeRelation<'tcx> for Lub<'_, '_, 'tcx> {
         self.fields.tcx()
     }
 
+    #[inline]
+    fn fast_equate(&self) -> bool {
+        true
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.fields.param_env
     }

compiler/rustc_infer/src/infer/nll_relate/mod.rs

@@ -535,6 +535,11 @@ where
         self.delegate.param_env()
     }
 
+    #[inline]
+    fn fast_equate(&self) -> bool {
+        false
+    }
+
     fn tag(&self) -> &'static str {
         "nll::subtype"
     }
@@ -901,6 +906,11 @@ where
         self.delegate.param_env()
     }
 
+    #[inline]
+    fn fast_equate(&self) -> bool {
+        false
+    }
+
     fn tag(&self) -> &'static str {
         "nll::generalizer"
     }

compiler/rustc_infer/src/infer/outlives/test_type_match.rs

@@ -142,6 +142,10 @@ impl<'tcx> TypeRelation<'tcx> for Match<'tcx> {
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.param_env
     }
+    #[inline]
+    fn fast_equate(&self) -> bool {
+        false
+    }
     fn a_is_expected(&self) -> bool {
         true
     } // irrelevant

compiler/rustc_infer/src/infer/sub.rs

@@ -43,6 +43,11 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
         self.fields.param_env
     }
 
+    #[inline]
+    fn fast_equate(&self) -> bool {
+        true
+    }
+
     fn a_is_expected(&self) -> bool {
         self.a_is_expected
     }

compiler/rustc_middle/src/ty/_match.rs

@@ -39,6 +39,10 @@ impl<'tcx> TypeRelation<'tcx> for Match<'tcx> {
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.param_env
     }
+    #[inline]
+    fn fast_equate(&self) -> bool {
+        false
+    }
     fn a_is_expected(&self) -> bool {
         true
     } // irrelevant

compiler/rustc_middle/src/ty/relate.rs

@@ -28,6 +28,9 @@ pub trait TypeRelation<'tcx>: Sized {
     /// Returns a static string we can use for printouts.
     fn tag(&self) -> &'static str;
 
+    /// Returns whether or not structural equality can be used to fast-path this relation
+    fn fast_equate(&self) -> bool;
+
     /// Returns `true` if the value `a` is the "expected" type in the
     /// relation. Just affects error messages.
     fn a_is_expected(&self) -> bool;
@@ -140,6 +143,10 @@ pub fn relate_substs<'tcx, R: TypeRelation<'tcx>>(
     a_subst: SubstsRef<'tcx>,
     b_subst: SubstsRef<'tcx>,
 ) -> RelateResult<'tcx, SubstsRef<'tcx>> {
+    if relation.fast_equate() && a_subst == b_subst {
+        return Ok(a_subst);
+    }
+
     relation.tcx().mk_substs(iter::zip(a_subst, b_subst).map(|(a, b)| {
         relation.relate_with_variance(ty::Invariant, ty::VarianceDiagInfo::default(), a, b)
     }))
@@ -152,6 +159,10 @@ pub fn relate_substs_with_variances<'tcx, R: TypeRelation<'tcx>>(
     a_subst: SubstsRef<'tcx>,
     b_subst: SubstsRef<'tcx>,
 ) -> RelateResult<'tcx, SubstsRef<'tcx>> {
+    if relation.fast_equate() && a_subst == b_subst {
+        return Ok(a_subst);
+    }
+
     let tcx = relation.tcx();
 
     let mut cached_ty = None;
@@ -345,7 +356,7 @@ impl<'tcx> Relate<'tcx> for ty::ExistentialTraitRef<'tcx> {
     }
 }
 
-#[derive(Copy, Debug, Clone, TypeFoldable, TypeVisitable)]
+#[derive(Copy, Debug, Clone, PartialEq, Eq, TypeFoldable, TypeVisitable)]
 struct GeneratorWitness<'tcx>(&'tcx ty::List<Ty<'tcx>>);
 
 impl<'tcx> Relate<'tcx> for GeneratorWitness<'tcx> {
@@ -354,6 +365,10 @@ impl<'tcx> Relate<'tcx> for GeneratorWitness<'tcx> {
         a: GeneratorWitness<'tcx>,
         b: GeneratorWitness<'tcx>,
     ) -> RelateResult<'tcx, GeneratorWitness<'tcx>> {
+        if relation.fast_equate() && a == b {
+            return Ok(a);
+        }
+
         assert_eq!(a.0.len(), b.0.len());
         let tcx = relation.tcx();
         let types = tcx.mk_type_list(iter::zip(a.0, b.0).map(|(a, b)| relation.relate(a, b)))?;
@@ -655,6 +670,10 @@ impl<'tcx> Relate<'tcx> for &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredi
         a: Self,
         b: Self,
     ) -> RelateResult<'tcx, Self> {
+        if relation.fast_equate() && a == b {
+            return Ok(a);
+        }
+
         let tcx = relation.tcx();
 
         // FIXME: this is wasteful, but want to do a perf run to see how slow it is.
@@ -799,9 +818,9 @@ impl<'tcx> Relate<'tcx> for ty::TraitPredicate<'tcx> {
         b: ty::TraitPredicate<'tcx>,
     ) -> RelateResult<'tcx, ty::TraitPredicate<'tcx>> {
         Ok(ty::TraitPredicate {
-            trait_ref: relation.relate(a.trait_ref, b.trait_ref)?,
             constness: relation.relate(a.constness, b.constness)?,
             polarity: relation.relate(a.polarity, b.polarity)?,
+            trait_ref: relation.relate(a.trait_ref, b.trait_ref)?,
         })
     }
 }