cmd/guru: implements: if selected expr is a value, use its type

...as the basis of the query, instead of reporting an error.

+ test

Change-Id: Ie5defa98cd8dfc8e200e296c2aa02c88893cf9ac
Reviewed-on: https://go-review.googlesource.com/22117
Reviewed-by: Michael Matloob <matloob@golang.org>

cmd/guru/implements.go

@@ -88,11 +88,17 @@ func implements(q *Query) error {
 				T = recv.Type()
 			}
 		}
+
+		// If not a method, use the expression's type.
+		if T == nil {
+			T = qpos.info.TypeOf(path[0].(ast.Expr))
+		}
+
 	case actionType:
 		T = qpos.info.TypeOf(path[0].(ast.Expr))
 	}
 	if T == nil {
-		return fmt.Errorf("no type or method here")
+		return fmt.Errorf("not a type, method, or value")
 	}
 
 	// Find all named types, even local types (which can have

cmd/guru/testdata/src/implements/main.go

@@ -37,3 +37,8 @@ func (sorter) Swap(i, j int)      {}
 type I interface { // @implements I "I"
 	Method(*int) *int
 }
+
+func _() {
+	var d D
+	_ = d // @implements var_d "d"
+}

cmd/guru/testdata/src/implements/main.golden

@@ -42,3 +42,9 @@ slice type sorter
 interface type I
 	is implemented by basic type lib.Type
 
+-------- @implements var_d --------
+struct type D
+	implements F
+pointer type *D
+	implements FG
+