2 files changed, 49 insertions, 12 deletions
diff --git a/src/librustc/infer/mod.rs b/src/librustc/infer/mod.rs
index a886c44a479..c5712cc9941 100644
--- a/src/librustc/infer/mod.rs
+++ b/src/librustc/infer/mod.rs
@@ -1562,11 +1562,7 @@ impl<'a, 'tcx> ShallowResolver<'a, 'tcx> {
         ShallowResolver { infcx }
     }
 
-    // We have this force-inlined variant of `shallow_resolve` for the one
-    // callsite that is extremely hot. All other callsites use the normal
-    // variant.
-    #[inline(always)]
-    pub fn inlined_shallow_resolve(&mut self, typ: Ty<'tcx>) -> Ty<'tcx> {
+    pub fn shallow_resolve(&mut self, typ: Ty<'tcx>) -> Ty<'tcx> {
         match typ.sty {
             ty::Infer(ty::TyVar(v)) => {
                 // Not entirely obvious: if `typ` is a type variable,
@@ -1601,6 +1597,42 @@ impl<'a, 'tcx> ShallowResolver<'a, 'tcx> {
             _ => typ,
         }
     }
+
+    // `resolver.shallow_resolve_changed(ty)` is equivalent to
+    // `resolver.shallow_resolve(ty) != ty`, but more efficient. It's always
+    // inlined, despite being large, because it has a single call site that is
+    // extremely hot.
+    #[inline(always)]
+    pub fn shallow_resolve_changed(&mut self, typ: Ty<'tcx>) -> bool {
+        match typ.sty {
+            ty::Infer(ty::TyVar(v)) => {
+                use self::type_variable::TypeVariableValue;
+
+                // See the comment in `shallow_resolve()`.
+                match self.infcx.type_variables.borrow_mut().probe(v) {
+                    TypeVariableValue::Known { value: t } => self.fold_ty(t) != typ,
+                    TypeVariableValue::Unknown { .. } => false,
+                }
+            }
+
+            ty::Infer(ty::IntVar(v)) => {
+                match self.infcx.int_unification_table.borrow_mut().probe_value(v) {
+                    Some(v) => v.to_type(self.infcx.tcx) != typ,
+                    None => false,
+                }
+            }
+
+            ty::Infer(ty::FloatVar(v)) => {
+                match self.infcx.float_unification_table.borrow_mut().probe_value(v) {
+                    Some(v) => v.to_type(self.infcx.tcx) != typ,
+                    None => false,
+                }
+            }
+
+            _ => false,
+        }
+    }
+
 }
 
 impl<'a, 'tcx> TypeFolder<'tcx> for ShallowResolver<'a, 'tcx> {
@@ -1609,7 +1641,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for ShallowResolver<'a, 'tcx> {
     }
 
     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
-        self.inlined_shallow_resolve(ty)
+        self.shallow_resolve(ty)
     }
 
     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
diff --git a/src/librustc/traits/fulfill.rs b/src/librustc/traits/fulfill.rs
index 4494c034d51..805727b6ce0 100644
--- a/src/librustc/traits/fulfill.rs
+++ b/src/librustc/traits/fulfill.rs
@@ -256,15 +256,20 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
         &mut self,
         pending_obligation: &mut Self::Obligation,
     ) -> ProcessResult<Self::Obligation, Self::Error> {
-        // if we were stalled on some unresolved variables, first check
+        // If we were stalled on some unresolved variables, first check
         // whether any of them have been resolved; if not, don't bother
         // doing more work yet
         if !pending_obligation.stalled_on.is_empty() {
-            if pending_obligation.stalled_on.iter().all(|&ty| {
-                // Use the force-inlined variant of shallow_resolve() because this code is hot.
-                let resolved = ShallowResolver::new(self.selcx.infcx()).inlined_shallow_resolve(ty);
-                resolved == ty // nothing changed here
-            }) {
+            let mut changed = false;
+            // This `for` loop was once a call to `all()`, but this lower-level
+            // form was a perf win. See #64545 for details.
+            for &ty in &pending_obligation.stalled_on {
+                if ShallowResolver::new(self.selcx.infcx()).shallow_resolve_changed(ty) {
+                    changed = true;
+                    break;
+                }
+            }
+            if !changed {
                 debug!("process_predicate: pending obligation {:?} still stalled on {:?}",
                        self.selcx.infcx()
                            .resolve_vars_if_possible(&pending_obligation.obligation),