fix dropck performance regression

src/librustc/middle/infer/region_inference/mod.rs

@@ -18,6 +18,7 @@ pub use self::RegionResolutionError::*;
 pub use self::VarValue::*;
 
 use super::{RegionVariableOrigin, SubregionOrigin, TypeTrace, MiscVariable};
+use super::unify_key;
 
 use rustc_data_structures::graph::{self, Direction, NodeIndex};
 use rustc_data_structures::unify::{self, UnificationTable};
@@ -345,10 +346,13 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
     }
 
     pub fn new_region_var(&self, origin: RegionVariableOrigin) -> RegionVid {
-        let id = self.num_vars();
+        let vid = RegionVid { index: self.num_vars() };
         self.var_origins.borrow_mut().push(origin.clone());
-        let vid = self.unification_table.borrow_mut().new_key(());
-        assert_eq!(vid.index, id);
+
+        let u_vid = self.unification_table.borrow_mut().new_key(
+            unify_key::RegionVidKey { min_vid: vid }
+            );
+        assert_eq!(vid, u_vid);
         if self.in_snapshot() {
             self.undo_log.borrow_mut().push(AddVar(vid));
         }
@@ -581,7 +585,7 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
     }
 
     pub fn opportunistic_resolve_var(&self, rid: RegionVid) -> ty::Region {
-        ty::ReVar(self.unification_table.borrow_mut().find(rid))
+        ty::ReVar(self.unification_table.borrow_mut().find_value(rid).min_vid)
     }
 
     fn combine_map(&self, t: CombineMapType) -> &RefCell<CombineMap> {

src/librustc/middle/infer/unify_key.rs

@@ -10,7 +10,7 @@
 
 use syntax::ast;
 use middle::ty::{self, IntVarValue, Ty};
-use rustc_data_structures::unify::UnifyKey;
+use rustc_data_structures::unify::{Combine, UnifyKey};
 
 pub trait ToType<'tcx> {
     fn to_type(&self, tcx: &ty::ctxt<'tcx>) -> Ty<'tcx>;
@@ -23,8 +23,28 @@ impl UnifyKey for ty::IntVid {
     fn tag(_: Option<ty::IntVid>) -> &'static str { "IntVid" }
 }
 
+#[derive(PartialEq, Copy, Clone, Debug)]
+pub struct RegionVidKey {
+    /// The minimum region vid in the unification set. This is needed
+    /// to have a canonical name for a type to prevent infinite
+    /// recursion.
+    pub min_vid: ty::RegionVid
+}
+
+impl Combine for RegionVidKey {
+    fn combine(&self, other: &RegionVidKey) -> RegionVidKey {
+        let min_vid = if self.min_vid.index < other.min_vid.index {
+            self.min_vid
+        } else {
+            other.min_vid
+        };
+
+        RegionVidKey { min_vid: min_vid }
+    }
+}
+
 impl UnifyKey for ty::RegionVid {
-    type Value = ();
+    type Value = RegionVidKey;
     fn index(&self) -> u32 { self.index }
     fn from_index(i: u32) -> ty::RegionVid { ty::RegionVid { index: i } }
     fn tag(_: Option<ty::RegionVid>) -> &'static str { "RegionVid" }

src/librustc_data_structures/unify/mod.rs

@@ -37,6 +37,16 @@ pub trait UnifyKey : Copy + Clone + Debug + PartialEq {
     fn tag(k: Option<Self>) -> &'static str;
 }
 
+/// This trait is implemented for unify values that can be
+/// combined. This relation should be a monoid.
+pub trait Combine {
+    fn combine(&self, other: &Self) -> Self;
+}
+
+impl Combine for () {
+    fn combine(&self, _other: &()) {}
+}
+
 /// Value of a unification key. We implement Tarjan's union-find
 /// algorithm: when two keys are unified, one of them is converted
 /// into a "redirect" pointing at the other. These redirects form a
@@ -243,23 +253,28 @@ impl<K:UnifyKey> sv::SnapshotVecDelegate for Delegate<K> {
 ///////////////////////////////////////////////////////////////////////////
 // Base union-find algorithm, where we are just making sets
 
-impl<'tcx,K> UnificationTable<K>
-    where K : UnifyKey<Value=()>,
+impl<'tcx,K:UnifyKey> UnificationTable<K>
+    where K::Value: Combine
 {
     pub fn union(&mut self, a_id: K, b_id: K) {
         let node_a = self.get(a_id);
         let node_b = self.get(b_id);
         let a_id = node_a.key();
         let b_id = node_b.key();
         if a_id != b_id {
-            self.unify(node_a, node_b, ());
+            let new_value = node_a.value.combine(&node_b.value);
+            self.unify(node_a, node_b, new_value);
         }
     }
 
     pub fn find(&mut self, id: K) -> K {
         self.get(id).key()
     }
 
+    pub fn find_value(&mut self, id: K) -> K::Value {
+        self.get(id).value
+    }
+
     pub fn unioned(&mut self, a_id: K, b_id: K) -> bool {
         self.find(a_id) == self.find(b_id)
     }

src/librustc_typeck/check/dropck.rs

@@ -17,6 +17,7 @@ use middle::region;
 use middle::subst::{self, Subst};
 use middle::traits;
 use middle::ty::{self, Ty};
+use util::nodemap::FnvHashSet;
 
 use syntax::ast;
 use syntax::codemap::{self, Span};
@@ -279,7 +280,7 @@ pub fn check_safety_of_destructor_if_necessary<'a, 'tcx>(rcx: &mut Rcx<'a, 'tcx>
             rcx: rcx,
             span: span,
             parent_scope: parent_scope,
-            breadcrumbs: Vec::new(),
+            breadcrumbs: FnvHashSet()
         },
         TypeContext::Root,
         typ,
@@ -340,7 +341,7 @@ enum TypeContext {
 struct DropckContext<'a, 'b: 'a, 'tcx: 'b> {
     rcx: &'a mut Rcx<'b, 'tcx>,
     /// types that have already been traversed
-    breadcrumbs: Vec<Ty<'tcx>>,
+    breadcrumbs: FnvHashSet<Ty<'tcx>>,
     /// span for error reporting
     span: Span,
     /// the scope reachable dtorck types must outlive
@@ -355,8 +356,6 @@ fn iterate_over_potentially_unsafe_regions_in_type<'a, 'b, 'tcx>(
     depth: usize) -> Result<(), Error<'tcx>>
 {
     let tcx = cx.rcx.tcx();
-    let ty = cx.rcx.infcx().resolve_type_and_region_vars_if_possible(&ty);
-
     // Issue #22443: Watch out for overflow. While we are careful to
     // handle regular types properly, non-regular ones cause problems.
     let recursion_limit = tcx.sess.recursion_limit.get();
@@ -367,19 +366,17 @@ fn iterate_over_potentially_unsafe_regions_in_type<'a, 'b, 'tcx>(
         return Err(Error::Overflow(context, ty))
     }
 
-    for breadcrumb in &mut cx.breadcrumbs {
-        *breadcrumb =
-            cx.rcx.infcx().resolve_type_and_region_vars_if_possible(breadcrumb);
-        if *breadcrumb == ty {
-            debug!("iterate_over_potentially_unsafe_regions_in_type \
-                   {}ty: {} scope: {:?} - cached",
-                   (0..depth).map(|_| ' ').collect::<String>(),
-                   ty, cx.parent_scope);
-            return Ok(()); // we already visited this type
-        }
-    }
-    cx.breadcrumbs.push(ty);
+    // canoncialize the regions in `ty` before inserting - infinitely many
+    // region variables can refer to the same region.
+    let ty = cx.rcx.infcx().resolve_type_and_region_vars_if_possible(&ty);
 
+    if !cx.breadcrumbs.insert(ty) {
+        debug!("iterate_over_potentially_unsafe_regions_in_type \
+               {}ty: {} scope: {:?} - cached",
+               (0..depth).map(|_| ' ').collect::<String>(),
+               ty, cx.parent_scope);
+        return Ok(()); // we already visited this type
+    }
     debug!("iterate_over_potentially_unsafe_regions_in_type \
            {}ty: {} scope: {:?}",
            (0..depth).map(|_| ' ').collect::<String>(),