From 685e15dcc11bacb4861bdd8934245d4f4bdbe688 Mon Sep 17 00:00:00 2001
From: scalexm <alexandre@scalexm.fr>
Date: Mon, 26 Nov 2018 13:44:40 +0100
Subject: [PATCH] Create multiple universes at once

---
 src/librustc/infer/canonical/mod.rs | 11 ++++-------
 src/librustc/infer/mod.rs           | 11 +++++++++++
 src/librustc/ty/mod.rs              |  5 +++++
 3 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/src/librustc/infer/canonical/mod.rs b/src/librustc/infer/canonical/mod.rs
index 230f8958b3385..3a8ff30faddf6 100644
--- a/src/librustc/infer/canonical/mod.rs
+++ b/src/librustc/infer/canonical/mod.rs
@@ -315,14 +315,11 @@ impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
         T: TypeFoldable<'tcx>,
     {
         // For each universe that is referred to in the incoming
-        // query, create a universe in our local inference context. In
-        // practice, as of this writing, all queries have no universes
-        // in them, so this code has no effect, but it is looking
-        // forward to the day when we *do* want to carry universes
-        // through into queries.
+        // query, create a universe in our local inference context.
         let universes: IndexVec<ty::UniverseIndex, _> = std::iter::once(ty::UniverseIndex::ROOT)
-            .chain((0..canonical.max_universe.as_u32()).map(|_| self.create_next_universe()))
-            .collect();
+            .chain(
+                self.create_next_universes(canonical.max_universe.as_u32())
+            ).collect();
 
         let canonical_inference_vars =
             self.instantiate_canonical_vars(span, canonical.variables, |ui| universes[ui]);
diff --git a/src/librustc/infer/mod.rs b/src/librustc/infer/mod.rs
index d8beae45b0ad4..8ba1ef2e865b1 100644
--- a/src/librustc/infer/mod.rs
+++ b/src/librustc/infer/mod.rs
@@ -1519,6 +1519,17 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         self.universe.set(u);
         u
     }
+
+    /// Creates `amount` new universes at once and returns all the created universes
+    /// in ascending order.
+    pub fn create_next_universes(&self, amount: u32) -> impl Iterator<Item = ty::UniverseIndex> {
+        let start = self.universe.get();
+        let end = start.advance_by(amount);
+        self.universe.set(end);
+
+        // Empty range if `amount == 0`
+        (start.next_universe() ..= end).into_iter()
+    }
 }
 
 impl<'a, 'gcx, 'tcx> TypeTrace<'tcx> {
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index b371f4532e5fa..fa6b1d7f18c2c 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -1568,6 +1568,11 @@ impl UniverseIndex {
         UniverseIndex::from_u32(self.private.checked_add(1).unwrap())
     }
 
+    /// Increases the universe index by `amount`.
+    pub fn advance_by(self, amount: u32) -> UniverseIndex {
+        UniverseIndex::from_u32(self.private.checked_add(amount).unwrap())
+    }
+
     /// Returns `true` if `self` can name a name from `other` -- in other words,
     /// if the set of names in `self` is a superset of those in
     /// `other` (`self >= other`).