Be less eager about implicit borrowing when doing method resolution. Closes #2796.

Author: msullivan

src/rustc/middle/typeck/check/method.rs

@@ -77,16 +77,20 @@ class lookup {
             // loop for impls in scope.  Note: I don't love these
             // semantics, but that's what we had so I am preserving
             // it.
-            if self.candidates.len() > 0u {
-                break;
-            }
+            if self.candidates.len() > 0u { break; }
+
+            // now look for impls in scope, but don't look for impls that
+            // would require doing an implicit borrow of the lhs.
+            self.add_candidates_from_scope(false);
 
-            self.add_candidates_from_scope();
+            // if we found anything, stop before trying borrows
+            if self.candidates.len() > 0u { break; }
+
+            // now look for impls in scope that might require a borrow
+            self.add_candidates_from_scope(true);
 
             // if we found anything, stop before attempting auto-deref.
-            if self.candidates.len() > 0u {
-                break;
-            }
+            if self.candidates.len() > 0u { break; }
 
             // check whether we can autoderef and if so loop around again.
             alt ty::deref(self.tcx(), self.self_ty, false) {
@@ -290,7 +294,7 @@ class lookup {
         */
     }
 
-    fn add_candidates_from_scope() {
+    fn add_candidates_from_scope(use_assignability: bool) {
         let impls_vecs = self.fcx.ccx.impl_map.get(self.expr.id);
         let mut added_any = false;
 
@@ -306,13 +310,18 @@ class lookup {
                     let {substs: impl_substs, ty: impl_ty} =
                         impl_self_ty(self.fcx, im.did);
 
-                    // if we can assign the caller to the callee, that's a
-                    // potential match.  Collect those in the vector.
-                    let can_assign = self.fcx.can_mk_assignty(
-                        self.self_expr, self.borrow_scope,
-                        self.self_ty, impl_ty);
-                    #debug["can_assign = %?", can_assign];
-                    alt can_assign {
+                    // Depending on our argument, we find potential
+                    // matches either by checking subtypability or
+                    // type assignability. Collect the matches.
+                    let matches = if use_assignability {
+                        self.fcx.can_mk_assignty(
+                            self.self_expr, self.borrow_scope,
+                            self.self_ty, impl_ty)
+                    } else {
+                        self.fcx.can_mk_subty(self.self_ty, impl_ty)
+                    };
+                    #debug["matches = %?", matches];
+                    alt matches {
                       result::err(_) { /* keep looking */ }
                       result::ok(_) {
                         if !self.candidate_impls.contains_key(im.did) {

src/test/run-pass/assignability-iface.rs

@@ -0,0 +1,43 @@
+// Tests that type assignability is used to search for instances when
+// making method calls, but only if there aren't any matches without
+// it.
+
+iface iterable<A> {
+    fn iterate(blk: fn(A) -> bool);
+}
+
+impl vec/&<A> of iterable<A> for &[const A] {
+    fn iterate(f: fn(A) -> bool) {
+        vec::each(self, f);
+    }
+}
+
+impl vec<A> of iterable<A> for ~[const A] {
+    fn iterate(f: fn(A) -> bool) {
+        vec::each(self, f);
+    }
+}
+
+fn length<A, T: iterable<A>>(x: T) -> uint {
+    let mut len = 0;
+    for x.iterate() |_y| { len += 1 }
+    ret len;
+}
+
+fn main() {
+    let x = ~[0,1,2,3];
+    // Call a method
+    for x.iterate() |y| { assert x[y] == y; }
+    // Call a parameterized function
+    assert length(x) == vec::len(x);
+    // Call a parameterized function, with type arguments that require
+    // a borrow
+    assert length::<int, &[int]>(x) == vec::len(x);
+
+    // Now try it with a type that *needs* to be borrowed
+    let z = [0,1,2,3]/_;
+    // Call a method
+    for z.iterate() |y| { assert z[y] == y; }
+    // Call a parameterized function
+    assert length::<int, &[int]>(z) == vec::len(z);
+}