Teach project to unify the return type even if a precise match is not

possible.  There is some amount of duplication as a result (similar to
select) -- I am not happy about this but not sure how to fix it
without deeper rewrites.

Author: nikomatsakis

src/librustc/middle/traits/project.rs

@@ -80,37 +80,23 @@ pub fn poly_project_and_unify_type<'cx,'tcx>(
            obligation.repr(selcx.tcx()));
 
     let infcx = selcx.infcx();
-    let result = infcx.try(|snapshot| {
+    infcx.try(|snapshot| {
         let (skol_predicate, skol_map) =
             infcx.skolemize_late_bound_regions(&obligation.predicate, snapshot);
 
         let skol_obligation = obligation.with(skol_predicate);
         match project_and_unify_type(selcx, &skol_obligation) {
-            Ok(Some(obligations)) => {
+            Ok(result) => {
                 match infcx.leak_check(&skol_map, snapshot) {
-                    Ok(()) => Ok(infcx.plug_leaks(skol_map, snapshot, &obligations)),
-                    Err(e) => Err(Some(MismatchedProjectionTypes { err: e })),
+                    Ok(()) => Ok(infcx.plug_leaks(skol_map, snapshot, &result)),
+                    Err(e) => Err(MismatchedProjectionTypes { err: e }),
                 }
             }
-            Ok(None) => {
-                // Signal ambiguity using Err just so that infcx.try()
-                // rolls back the snapshot. We adapt below.
-                Err(None)
-            }
             Err(e) => {
-                Err(Some(e))
+                Err(e)
             }
         }
-    });
-
-    // Above, we use Err(None) to signal ambiguity so that the
-    // snapshot will be rolled back. But here, we want to translate to
-    // Ok(None). Kind of weird.
-    match result {
-        Ok(obligations) => Ok(Some(obligations)),
-        Err(None) => Ok(None),
-        Err(Some(e)) => Err(e),
-    }
+    })
 }
 
 /// Evaluates constraints of the form:
@@ -132,7 +118,10 @@ fn project_and_unify_type<'cx,'tcx>(
                                             obligation.cause.clone(),
                                             obligation.recursion_depth) {
             Some(n) => n,
-            None => { return Ok(None); }
+            None => {
+                consider_unification_despite_ambiguity(selcx, obligation);
+                return Ok(None);
+            }
         };
 
     debug!("project_and_unify_type: normalized_ty={} obligations={}",
@@ -147,6 +136,50 @@ fn project_and_unify_type<'cx,'tcx>(
     }
 }
 
+fn consider_unification_despite_ambiguity<'cx,'tcx>(selcx: &mut SelectionContext<'cx,'tcx>,
+                                                    obligation: &ProjectionObligation<'tcx>) {
+    debug!("consider_unification_despite_ambiguity(obligation={})",
+           obligation.repr(selcx.tcx()));
+
+    let def_id = obligation.predicate.projection_ty.trait_ref.def_id;
+    match selcx.tcx().lang_items.fn_trait_kind(def_id) {
+        Some(_) => { }
+        None => { return; }
+    }
+
+    let infcx = selcx.infcx();
+    let self_ty = obligation.predicate.projection_ty.trait_ref.self_ty();
+    let self_ty = infcx.shallow_resolve(self_ty);
+    debug!("consider_unification_despite_ambiguity: self_ty.sty={:?}",
+           self_ty.sty);
+    match self_ty.sty {
+        ty::ty_closure(closure_def_id, _, substs) => {
+            let closure_typer = selcx.closure_typer();
+            let closure_type = closure_typer.closure_type(closure_def_id, substs);
+            let ty::Binder((_, ret_type)) =
+                util::closure_trait_ref_and_return_type(infcx.tcx,
+                                                        def_id,
+                                                        self_ty,
+                                                        &closure_type.sig,
+                                                        util::TupleArgumentsFlag::No);
+            let (ret_type, _) =
+                infcx.replace_late_bound_regions_with_fresh_var(
+                    obligation.cause.span,
+                    infer::AssocTypeProjection(obligation.predicate.projection_ty.item_name),
+                    &ty::Binder(ret_type));
+            debug!("consider_unification_despite_ambiguity: ret_type={:?}",
+                   ret_type.repr(selcx.tcx()));
+            let origin = infer::RelateOutputImplTypes(obligation.cause.span);
+            let obligation_ty = obligation.predicate.ty;
+            match infer::mk_eqty(infcx, true, origin, obligation_ty, ret_type) {
+                Ok(()) => { }
+                Err(_) => { /* ignore errors */ }
+            }
+        }
+        _ => { }
+    }
+}
+
 /// Normalizes any associated type projections in `value`, replacing
 /// them with a fully resolved type where possible. The return value
 /// combines the normalized result and any additional obligations that

src/librustc/middle/traits/select.rs

@@ -277,8 +277,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// the argument types can only be helpful to the user, because
     /// once they patch up the kind of closure that is expected, the
     /// argment types won't really change.
-    fn consider_unification_despite_ambiguity(&mut self, obligation: &TraitObligation<'tcx>)
-    {
+    fn consider_unification_despite_ambiguity(&mut self, obligation: &TraitObligation<'tcx>) {
         // Is this a `C : FnFoo(...)` trait reference for some trait binding `FnFoo`?
         match self.tcx().lang_items.fn_trait_kind(obligation.predicate.0.def_id()) {
             Some(_) => { }

src/test/run-pass/closure-inference.rs

@@ -9,7 +9,7 @@
 // except according to those terms.
 
 
-fn foo(i: int) -> int { i + 1 }
+fn foo(i: isize) -> isize { i + 1 }
 
 fn apply<A, F>(f: F, v: A) -> A where F: FnOnce(A) -> A { f(v) }
 

src/test/run-pass/last-use-in-cap-clause.rs

@@ -14,9 +14,9 @@
 #![feature(box_syntax)]
 #![feature(unboxed_closures)]
 
-struct A { a: Box<int> }
+struct A { a: Box<isize> }
 
-fn foo() -> Box<FnMut() -> int + 'static> {
+fn foo() -> Box<FnMut() -> isize + 'static> {
     let k = box 22;
     let _u = A {a: k.clone()};
     let result  = |&mut:| 22;

src/test/run-pass/unboxed-closures-zero-args.rs

@@ -11,7 +11,7 @@
 #![feature(unboxed_closures)]
 
 fn main() {
-    let mut zero = |&mut:| {};
-    let () = zero.call_mut(());
+    let mut zero = || {};
+    let () = zero();
 }