rustc: Combine and unify regions

Author: pcwalton

src/rustc/middle/infer.rs

@@ -409,39 +409,9 @@ impl unify_methods for infer_ctxt {
     }
 
     fn regions(a: ty::region, b: ty::region) -> ures {
-        alt (a, b) {
-          (ty::re_var(_), _) | (_, ty::re_var(_)) {
-            self.uok()  // FIXME: We need region variables!
-          }
-          (ty::re_inferred, _) | (_, ty::re_inferred) {
-            fail "tried to unify inferred regions"
-          }
-          (ty::re_param(_), ty::re_param(_)) |
-          (ty::re_self, ty::re_self) {
-            if a == b {
-                self.uok()
-            } else {
-                self.uerr(ty::terr_regions_differ(false, a, b))
-            }
-          }
-          (ty::re_param(_), ty::re_block(_)) |
-          (ty::re_self, ty::re_block(_)) {
-            self.uok()
-          }
-          (ty::re_block(_), ty::re_param(_)) |
-          (ty::re_block(_), ty::re_self) {
-            self.uerr(ty::terr_regions_differ(false, a, b))
-          }
-          (ty::re_block(superblock), ty::re_block(subblock)) {
-            // The region corresponding to an outer block is a subtype of the
-            // region corresponding to an inner block.
-            let rm = self.tcx.region_map;
-            if region::scope_contains(rm, subblock, superblock) {
-                self.uok()
-            } else {
-                self.uerr(ty::terr_regions_differ(false, a, b))
-            }
-          }
+        alt combine_or_unify_regions(self.tcx, a, b, false) {
+            ok(_)   { self.uok()   }
+            err(e)  { self.uerr(e) }
         }
     }
 
@@ -1214,6 +1184,47 @@ fn c_tys<C:combine>(
     }
 }
 
+fn combine_or_unify_regions(tcx: ty::ctxt,
+                            a: ty::region,
+                            b: ty::region,
+                            contravariant_combine: bool) -> cres<ty::region> {
+    alt (a, b) {
+      (ty::re_var(_), _) | (_, ty::re_var(_)) {
+        ok(a)   // FIXME: We need region variables!
+      }
+      (ty::re_inferred, _) | (_, ty::re_inferred) {
+        fail "tried to combine or unify inferred regions"
+      }
+      (ty::re_param(_), ty::re_param(_)) |
+      (ty::re_self, ty::re_self) {
+        if a == b {
+            ok(a)
+        } else {
+            err(ty::terr_regions_differ(false, a, b))
+        }
+      }
+      (ty::re_param(_), ty::re_block(_)) |
+      (ty::re_self, ty::re_block(_)) {
+        ok(a)
+      }
+      (ty::re_block(_), ty::re_param(_)) |
+      (ty::re_block(_), ty::re_self) {
+        err(ty::terr_regions_differ(false, a, b))
+      }
+      (ty::re_block(block_a), ty::re_block(block_b)) {
+        // The region corresponding to an outer block is a subtype of the
+        // region corresponding to an inner block.
+        let rm = tcx.region_map;
+        let nca_opt = region::nearest_common_ancestor(rm, block_a, block_b);
+        alt nca_opt {
+            some(nca) if nca == block_b { ok(a) }
+            some(nca) if contravariant_combine { ok(ty::re_block(nca)) }
+            _ { err(ty::terr_regions_differ(false, a, b)) }
+        }
+      }
+    }
+}
+
 impl of combine for lub {
     fn infcx() -> infer_ctxt { *self }
 
@@ -1232,10 +1243,6 @@ impl of combine for lub {
         ok(b)
     }
 
-    fn c_regions(a: ty::region, _b: ty::region) -> cres<ty::region> {
-        ok(a) // FIXME
-    }
-
     fn c_mts(a: ty::mt, b: ty::mt) -> cres<ty::mt> {
         let tcx = self.infcx().tcx;
 
@@ -1302,6 +1309,10 @@ impl of combine for lub {
           }
         }
     }
+
+    fn c_regions(a: ty::region, b: ty::region) -> cres<ty::region> {
+        ret combine_or_unify_regions(self.tcx, a, b, true);
+    }
 }
 
 impl of combine for glb {
@@ -1410,4 +1421,8 @@ impl of combine for glb {
           }
         }
     }
+
+    fn c_regions(a: ty::region, b: ty::region) -> cres<ty::region> {
+        ret combine_or_unify_regions(self.tcx, a, b, false);
+    }
 }

src/rustc/middle/region.rs

@@ -84,6 +84,42 @@ fn scope_contains(region_map: @region_map, superscope: ast::node_id,
     ret true;
 }
 
+fn nearest_common_ancestor(region_map: @region_map, scope_a: ast::node_id,
+                           scope_b: ast::node_id) -> option<ast::node_id> {
+
+    fn ancestors_of(region_map: @region_map, scope: ast::node_id)
+                    -> [ast::node_id] {
+        let mut result = [scope];
+        let mut scope = scope;
+        loop {
+            alt region_map.parents.find(scope) {
+                none { ret result; }
+                some(superscope) {
+                    result += [superscope];
+                    scope = superscope;
+                }
+            }
+        }
+    }
+
+    if scope_a == scope_b { ret some(scope_a); }
+
+    let a_ancestors = ancestors_of(region_map, scope_a);
+    let b_ancestors = ancestors_of(region_map, scope_b);
+    let mut a_index = vec::len(a_ancestors) - 1u;
+    let mut b_index = vec::len(b_ancestors) - 1u;
+    while a_ancestors[a_index] == b_ancestors[b_index] {
+        a_index -= 1u;
+        b_index -= 1u;
+    }
+
+    if a_index == vec::len(a_ancestors) {
+        ret none;
+    }
+
+    ret some(a_ancestors[a_index + 1u]);
+}
+
 fn get_inferred_region(cx: ctxt, sp: syntax::codemap::span) -> ty::region {
     // We infer to the caller region if we're at item scope
     // and to the block region if we're at block scope.