Auto merge of #64939 - nnethercote:snapshot-cleanups, r=nikomatsakis

Snapshot clean-ups

Two minor clean-ups involving snapshots.

src/librustc/infer/mod.rs

@@ -814,16 +814,16 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     /// Executes `f` and commit the bindings.
     pub fn commit_unconditionally<R, F>(&self, f: F) -> R
     where
-        F: FnOnce() -> R,
+        F: FnOnce(&CombinedSnapshot<'a, 'tcx>) -> R,
     {
-        debug!("commit()");
+        debug!("commit_unconditionally()");
         let snapshot = self.start_snapshot();
-        let r = f();
+        let r = f(&snapshot);
         self.commit_from(snapshot);
         r
     }
 
-    /// Executes `f` and commit the bindings if closure `f` returns `Ok(_)`.
+    /// Execute `f` and commit the bindings if closure `f` returns `Ok(_)`.
     pub fn commit_if_ok<T, E, F>(&self, f: F) -> Result<T, E>
     where
         F: FnOnce(&CombinedSnapshot<'a, 'tcx>) -> Result<T, E>,
@@ -843,19 +843,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         r
     }
 
-    /// Execute `f` in a snapshot, and commit the bindings it creates.
-    pub fn in_snapshot<T, F>(&self, f: F) -> T
-    where
-        F: FnOnce(&CombinedSnapshot<'a, 'tcx>) -> T,
-    {
-        debug!("in_snapshot()");
-        let snapshot = self.start_snapshot();
-        let r = f(&snapshot);
-        self.commit_from(snapshot);
-        r
-    }
-
-    /// Executes `f` then unroll any bindings it creates.
+    /// Execute `f` then unroll any bindings it creates.
     pub fn probe<R, F>(&self, f: F) -> R
     where
         F: FnOnce(&CombinedSnapshot<'a, 'tcx>) -> R,

src/librustc/infer/region_constraints/leak_check.rs

@@ -14,9 +14,11 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
     /// retain the older (arguably incorrect) behavior of the
     /// compiler.
     ///
-    /// NB. The use of snapshot here is mostly an efficiency thing --
-    /// we could search *all* region constraints, but that'd be a
-    /// bigger set and the data structures are not setup for that. If
+    /// NB. Although `_snapshot` isn't used, it's passed in to prove
+    /// that we are in a snapshot, which guarantees that we can just
+    /// search the "undo log" for edges. This is mostly an efficiency
+    /// thing -- we could search *all* region constraints, but that'd be
+    /// a bigger set and the data structures are not setup for that. If
     /// we wind up keeping some form of this check long term, it would
     /// probably be better to remove the snapshot parameter and to
     /// refactor the constraint set.

src/librustc/traits/select.rs

@@ -2819,7 +2819,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // binder moved -\
                 let ty: ty::Binder<Ty<'tcx>> = ty::Binder::bind(ty); // <----/
 
-                self.infcx.in_snapshot(|_| {
+                self.infcx.commit_unconditionally(|_| {
                     let (skol_ty, _) = self.infcx
                         .replace_bound_vars_with_placeholders(&ty);
                     let Normalized {
@@ -2932,7 +2932,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     }
 
     fn confirm_projection_candidate(&mut self, obligation: &TraitObligation<'tcx>) {
-        self.infcx.in_snapshot(|snapshot| {
+        self.infcx.commit_unconditionally(|snapshot| {
             let result =
                 self.match_projection_obligation_against_definition_bounds(
                     obligation,
@@ -3054,19 +3054,20 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             nested,
         );
 
-        let trait_obligations: Vec<PredicateObligation<'_>> = self.infcx.in_snapshot(|_| {
-            let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
-            let (trait_ref, _) = self.infcx
-                .replace_bound_vars_with_placeholders(&poly_trait_ref);
-            let cause = obligation.derived_cause(ImplDerivedObligation);
-            self.impl_or_trait_obligations(
-                cause,
-                obligation.recursion_depth + 1,
-                obligation.param_env,
-                trait_def_id,
-                &trait_ref.substs,
-            )
-        });
+        let trait_obligations: Vec<PredicateObligation<'_>> =
+            self.infcx.commit_unconditionally(|_| {
+                let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
+                let (trait_ref, _) = self.infcx
+                    .replace_bound_vars_with_placeholders(&poly_trait_ref);
+                let cause = obligation.derived_cause(ImplDerivedObligation);
+                self.impl_or_trait_obligations(
+                    cause,
+                    obligation.recursion_depth + 1,
+                    obligation.param_env,
+                    trait_def_id,
+                    &trait_ref.substs,
+                )
+            });
 
         // Adds the predicates from the trait.  Note that this contains a `Self: Trait`
         // predicate as usual.  It won't have any effect since auto traits are coinductive.
@@ -3089,7 +3090,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         // First, create the substitutions by matching the impl again,
         // this time not in a probe.
-        self.infcx.in_snapshot(|snapshot| {
+        self.infcx.commit_unconditionally(|snapshot| {
             let substs = self.rematch_impl(impl_def_id, obligation, snapshot);
             debug!("confirm_impl_candidate: substs={:?}", substs);
             let cause = obligation.derived_cause(ImplDerivedObligation);
@@ -3253,7 +3254,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             obligation, alias_def_id
         );
 
-        self.infcx.in_snapshot(|_| {
+        self.infcx.commit_unconditionally(|_| {
             let (predicate, _) = self.infcx()
                 .replace_bound_vars_with_placeholders(&obligation.predicate);
             let trait_ref = predicate.trait_ref;