From 1fece3d84b41bf1d544e48b10823f3dade45a120 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <nnethercote@mozilla.com>
Date: Thu, 29 Sep 2016 20:34:28 +1000
Subject: [PATCH 1/2] Optimize plug_leaks.

This commit avoids the `fold_regions` call in `plug_leaks` when
`skol_map` is empty, which is the common case. This gives speed-ups of
up to 1.14x on some of the rustc-benchmarks.
---
 src/librustc/infer/higher_ranked/mod.rs | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/librustc/infer/higher_ranked/mod.rs b/src/librustc/infer/higher_ranked/mod.rs
index 7c02de05d26d5..980c7d3e991b8 100644
--- a/src/librustc/infer/higher_ranked/mod.rs
+++ b/src/librustc/infer/higher_ranked/mod.rs
@@ -756,6 +756,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                skol_map,
                value);
 
+        if skol_map.is_empty() {
+            return self.resolve_type_vars_if_possible(value);
+        }
+
         // Compute a mapping from the "taint set" of each skolemized
         // region back to the `ty::BoundRegion` that it originally
         // represented. Because `leak_check` passed, we know that
@@ -813,9 +817,6 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             }
         });
 
-        debug!("plug_leaks: result={:?}",
-               result);
-
         self.pop_skolemized(skol_map, snapshot);
 
         debug!("plug_leaks: result={:?}", result);

From 3779971dbb397ff8b1668c379812b903a6a907ec Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <nnethercote@mozilla.com>
Date: Fri, 30 Sep 2016 17:44:48 +1000
Subject: [PATCH 2/2] Optimize plug_leaks some more.

This commit avoids the `resolve_type_vars_if_possible` call in
`plug_leaks` when `skol_map` is empty, which is the common case. It also
changes the signature of `plug_leaks` slightly to avoid the need for a
`clone` of `value`. These changes give speed-ups of up a few percent on
some of the rustc-benchmarks.
---
 src/librustc/infer/higher_ranked/mod.rs | 6 +++---
 src/librustc/traits/project.rs          | 2 +-
 src/librustc/traits/select.rs           | 4 ++--
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/src/librustc/infer/higher_ranked/mod.rs b/src/librustc/infer/higher_ranked/mod.rs
index 980c7d3e991b8..c1d9240ba0634 100644
--- a/src/librustc/infer/higher_ranked/mod.rs
+++ b/src/librustc/infer/higher_ranked/mod.rs
@@ -749,7 +749,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
     pub fn plug_leaks<T>(&self,
                          skol_map: SkolemizationMap<'tcx>,
                          snapshot: &CombinedSnapshot,
-                         value: &T) -> T
+                         value: T) -> T
         where T : TypeFoldable<'tcx>
     {
         debug!("plug_leaks(skol_map={:?}, value={:?})",
@@ -757,7 +757,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                value);
 
         if skol_map.is_empty() {
-            return self.resolve_type_vars_if_possible(value);
+            return value;
         }
 
         // Compute a mapping from the "taint set" of each skolemized
@@ -779,7 +779,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 
         // Remove any instantiated type variables from `value`; those can hide
         // references to regions from the `fold_regions` code below.
-        let value = self.resolve_type_vars_if_possible(value);
+        let value = self.resolve_type_vars_if_possible(&value);
 
         // Map any skolemization byproducts back to a late-bound
         // region. Put that late-bound region at whatever the outermost
diff --git a/src/librustc/traits/project.rs b/src/librustc/traits/project.rs
index ea4fc1c25ab42..ddabc53a89a81 100644
--- a/src/librustc/traits/project.rs
+++ b/src/librustc/traits/project.rs
@@ -171,7 +171,7 @@ pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>(
             Ok(result) => {
                 let span = obligation.cause.span;
                 match infcx.leak_check(false, span, &skol_map, snapshot) {
-                    Ok(()) => Ok(infcx.plug_leaks(skol_map, snapshot, &result)),
+                    Ok(()) => Ok(infcx.plug_leaks(skol_map, snapshot, result)),
                     Err(e) => Err(MismatchedProjectionTypes { err: e }),
                 }
             }
diff --git a/src/librustc/traits/select.rs b/src/librustc/traits/select.rs
index 9d7131dc96cc5..666311110971d 100644
--- a/src/librustc/traits/select.rs
+++ b/src/librustc/traits/select.rs
@@ -1980,7 +1980,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                                                   normalized_ty,
                                                   &[]);
                 obligations.push(skol_obligation);
-                this.infcx().plug_leaks(skol_map, snapshot, &obligations)
+                this.infcx().plug_leaks(skol_map, snapshot, obligations)
             })
         }).collect()
     }
@@ -2899,7 +2899,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                     predicate: predicate.value
                 }))
         }).collect();
-        self.infcx().plug_leaks(skol_map, snapshot, &predicates)
+        self.infcx().plug_leaks(skol_map, snapshot, predicates)
     }
 }