rustc: Don't error out early when constructing a boxed trait.

This improves error messages a bit and helps pave the way for "@int as @trait"
(as opposed to "int as @trait").

Author: pcwalton

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

@@ -51,8 +51,13 @@ fn lookup_vtables(fcx: @fn_ctxt,
             match *bound {
               ty::bound_trait(i_ty) => {
                 let i_ty = ty::subst(tcx, substs, i_ty);
-                result.push(lookup_vtable(fcx, expr, *ty, i_ty,
-                                          allow_unsafe, is_early));
+                match lookup_vtable(fcx, expr, *ty, i_ty, allow_unsafe,
+                                    is_early) {
+                    None => {}
+                    Some(vtable) => {
+                        result.push(vtable);
+                    }
+                }
               }
               _ => ()
             }
@@ -91,7 +96,7 @@ fn lookup_vtable(fcx: @fn_ctxt,
                  trait_ty: ty::t,
                  allow_unsafe: bool,
                  is_early: bool)
-    -> vtable_origin
+    -> Option<vtable_origin>
 {
 
     debug!("lookup_vtable(ty=%s, trait_ty=%s)",
@@ -113,7 +118,7 @@ fn lookup_vtable(fcx: @fn_ctxt,
             // The type has unconstrained type variables in it, so we can't
             // do early resolution on it. Return some completely bogus vtable
             // information: we aren't storing it anyways.
-            return vtable_param(0, 0);
+            return Some(vtable_param(0, 0));
         }
     };
 
@@ -135,7 +140,7 @@ fn lookup_vtable(fcx: @fn_ctxt,
                                            idid);
                                     relate_trait_tys(fcx, expr,
                                                      trait_ty, ity);
-                                    return vtable_param(n, n_bound);
+                                    return Some(vtable_param(n, n_bound));
                                 }
                             }
                             _ => tcx.sess.impossible_case(
@@ -170,7 +175,7 @@ fn lookup_vtable(fcx: @fn_ctxt,
                     }
                 }
             }
-            return vtable_trait(did, substs.tps);
+            return Some(vtable_trait(did, substs.tps));
         }
 
         _ => {
@@ -303,7 +308,7 @@ fn lookup_vtable(fcx: @fn_ctxt,
                                 None => {
                                     assert is_early;
                                     // Bail out with a bogus answer
-                                    return vtable_param(0, 0);
+                                    return Some(vtable_param(0, 0));
                                 }
                             };
 
@@ -341,23 +346,20 @@ fn lookup_vtable(fcx: @fn_ctxt,
 
             match found.len() {
                 0 => { /* fallthrough */ }
-                1 => { return found[0]; }
+                1 => { return Some(found[0]); }
                 _ => {
                     if !is_early {
                         fcx.ccx.tcx.sess.span_err(
                             expr.span,
                             ~"multiple applicable methods in scope");
                     }
-                    return found[0];
+                    return Some(found[0]);
                 }
             }
         }
     }
 
-    tcx.sess.span_fatal(
-        expr.span,
-        fmt!("failed to find an implementation of trait %s for %s",
-             ty_to_str(tcx, trait_ty), ty_to_str(tcx, ty)));
+    return None;
 }
 
 fn fixup_ty(fcx: @fn_ctxt,
@@ -459,13 +461,26 @@ fn early_resolve_expr(ex: @ast::expr, &&fcx: @fn_ctxt, is_early: bool) {
             Look up vtables for the type we're casting to,
             passing in the source and target type
             */
-            let vtable = lookup_vtable(fcx, ex, fcx.expr_ty(src),
-                                       target_ty, true, is_early);
-            /*
-            Map this expression to that vtable (that is: "ex has
-            vtable <vtable>")
-            */
-            if !is_early { cx.vtable_map.insert(ex.id, @~[vtable]); }
+            let ty = fcx.expr_ty(src);
+            let vtable_opt = lookup_vtable(fcx, ex, ty, target_ty, true,
+                                           is_early);
+            match vtable_opt {
+                None => {
+                    fcx.tcx().sess.span_err(
+                        ex.span,
+                        fmt!("failed to find an implementation of trait %s \
+                              for %s",
+                             ty_to_str(fcx.tcx(), target_ty),
+                             ty_to_str(fcx.tcx(), ty)));
+                }
+                Some(vtable) => {
+                    /*
+                    Map this expression to that vtable (that is: "ex has
+                    vtable <vtable>")
+                    */
+                    if !is_early { cx.vtable_map.insert(ex.id, @~[vtable]); }
+                }
+            }
           }
           _ => ()
         }