Auto merge of #64627 - nnethercote:ObligForest-even-more, r=nikomatsakis

Even more `ObligationForest` improvements

Following on from #64545, more speed and readability improvements.

r? @nikomatsakis

src/librustc/infer/mod.rs

@@ -1600,8 +1600,8 @@ impl<'a, 'tcx> ShallowResolver<'a, 'tcx> {
 
     // `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.
+    // inlined, despite being large, because it has only two call sites that
+    // are extremely hot.
     #[inline(always)]
     pub fn shallow_resolve_changed(&mut self, typ: Ty<'tcx>) -> bool {
         match typ.sty {
@@ -1609,20 +1609,21 @@ impl<'a, 'tcx> ShallowResolver<'a, 'tcx> {
                 use self::type_variable::TypeVariableValue;
 
                 // See the comment in `shallow_resolve()`.
-                match self.infcx.type_variables.borrow_mut().probe(v) {
+                match self.infcx.type_variables.borrow_mut().inlined_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) {
+                match self.infcx.int_unification_table.borrow_mut().inlined_probe_value(v) {
                     Some(v) => v.to_type(self.infcx.tcx) != typ,
                     None => false,
                 }
             }
 
             ty::Infer(ty::FloatVar(v)) => {
+                // Not `inlined_probe_value(v)` because this call site is colder.
                 match self.infcx.float_unification_table.borrow_mut().probe_value(v) {
                     Some(v) => v.to_type(self.infcx.tcx) != typ,
                     None => false,

src/librustc/infer/type_variable.rs

@@ -234,7 +234,13 @@ impl<'tcx> TypeVariableTable<'tcx> {
     /// Retrieves the type to which `vid` has been instantiated, if
     /// any.
     pub fn probe(&mut self, vid: ty::TyVid) -> TypeVariableValue<'tcx> {
-        self.eq_relations.probe_value(vid)
+        self.inlined_probe(vid)
+    }
+
+    /// An always-inlined variant of `probe`, for very hot call sites.
+    #[inline(always)]
+    pub fn inlined_probe(&mut self, vid: ty::TyVid) -> TypeVariableValue<'tcx> {
+        self.eq_relations.inlined_probe_value(vid)
     }
 
     /// If `t` is a type-inference variable, and it has been

src/librustc/traits/fulfill.rs

@@ -256,29 +256,46 @@ 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
-        // whether any of them have been resolved; if not, don't bother
-        // doing more work yet
-        if !pending_obligation.stalled_on.is_empty() {
-            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 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.
+        let change = match pending_obligation.stalled_on.len() {
+            // Match arms are in order of frequency, which matters because this
+            // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
+            1 => {
+                let ty = pending_obligation.stalled_on[0];
+                ShallowResolver::new(self.selcx.infcx()).shallow_resolve_changed(ty)
             }
-            if !changed {
-                debug!("process_predicate: pending obligation {:?} still stalled on {:?}",
-                       self.selcx.infcx()
-                           .resolve_vars_if_possible(&pending_obligation.obligation),
-                       pending_obligation.stalled_on);
-                return ProcessResult::Unchanged;
+            0 => {
+                // In this case we haven't changed, but wish to make a change.
+                true
             }
-            pending_obligation.stalled_on = vec![];
+            _ => {
+                // 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) {
+                            return true;
+                        }
+                    }
+                    false
+                })()
+            }
+        };
+
+        if !change {
+            debug!("process_predicate: pending obligation {:?} still stalled on {:?}",
+                   self.selcx.infcx()
+                       .resolve_vars_if_possible(&pending_obligation.obligation),
+                   pending_obligation.stalled_on);
+            return ProcessResult::Unchanged;
         }
 
+        // This part of the code is much colder.
+
+        pending_obligation.stalled_on.truncate(0);
+
         let obligation = &mut pending_obligation.obligation;
 
         if obligation.predicate.has_infer_types() {