go.oracle: describe: disambiguate 'func' object kinds.

The typechecker uses *types.Func for functions, concrete
methods and interface methods; and *types.Var for variables
and struct fields.  This change makes clear which kind of
function we're describing.  (We can't do it for vars since
go/types doesn't expose enough information, yet.)

Also: add "omitempty" to one JSON field.

R=crawshaw
CC=golang-dev
https://golang.org/cl/13527044

oracle/describe.go

@@ -448,19 +448,36 @@ type describeValueResult struct {
 }
 
 func (r *describeValueResult) display(printf printfFunc) {
-	suffix := ""
+	var prefix, suffix string
 	if r.constVal != nil {
 		suffix = fmt.Sprintf(" of constant value %s", r.constVal)
 	}
+	switch obj := r.obj.(type) {
+	case *types.Func:
+		if recv := obj.Type().(*types.Signature).Recv(); recv != nil {
+			if _, ok := recv.Type().Underlying().(*types.Interface); ok {
+				prefix = "interface method "
+			} else {
+				prefix = "method "
+			}
+		}
+
+	case *types.Var:
+		// TODO(adonovan): go/types should make it simple to
+		// ask: IsStructField(*Var)?
+		if false {
+			prefix = "struct field "
+		}
+	}
 
 	// Describe the expression.
 	if r.obj != nil {
 		if r.obj.Pos() == r.expr.Pos() {
 			// defining ident
-			printf(r.expr, "definition of %s%s", r.obj, suffix)
+			printf(r.expr, "definition of %s%s%s", prefix, r.obj, suffix)
 		} else {
 			// referring ident
-			printf(r.expr, "reference to %s%s", r.obj, suffix)
+			printf(r.expr, "reference to %s%s%s", prefix, r.obj, suffix)
 			if def := r.obj.Pos(); def != token.NoPos {
 				printf(def, "defined here")
 			}

oracle/json/json.go

@@ -116,9 +116,9 @@ type DescribePTALabel struct {
 // set of "labels" it points to.
 //
 type DescribePointer struct {
-	Type    string             `json:"type"`             // (concrete) type of the pointer
-	NamePos string             `json:"namepos"`          // location of type defn, if Named
-	Labels  []DescribePTALabel `json:"labels,omitempty"` // pointed-to objects
+	Type    string             `json:"type"`              // (concrete) type of the pointer
+	NamePos string             `json:"namepos,omitempty"` // location of type defn, if Named
+	Labels  []DescribePTALabel `json:"labels,omitempty"`  // pointed-to objects
 }
 
 // A DescribeValue is the additional result of a 'describe' query

oracle/testdata/src/main/describe-json.golden

@@ -95,7 +95,6 @@
 			"pts": [
 				{
 					"type": "*int",
-					"namepos": "",
 					"labels": [
 						{
 							"pos": "testdata/src/main/describe-json.go:10:6",

oracle/testdata/src/main/describe.golden

@@ -35,15 +35,15 @@ reference to func describe.main()
 defined here
 
 -------- @describe func-ref-*C.f --------
-reference to func (*describe.C).f()
+reference to method func (*describe.C).f()
 defined here
 
 -------- @describe func-ref-D.f --------
-reference to func (describe.D).f()
+reference to method func (describe.D).f()
 defined here
 
 -------- @describe func-ref-I.f --------
-reference to func (describe.I).f()
+reference to interface method func (describe.I).f()
 defined here
 
 -------- @describe type-D --------
@@ -59,11 +59,11 @@ Method set:
 	method (describe.I) f()
 
 -------- @describe func-ref-d.f --------
-reference to func (describe.D).f()
+reference to method func (describe.D).f()
 defined here
 
 -------- @describe func-ref-i.f --------
-reference to func (describe.I).f()
+reference to interface method func (describe.I).f()
 defined here
 
 -------- @describe ref-lexical-d --------

oracle/testdata/src/main/imports.golden

@@ -27,7 +27,7 @@ Method set:
 	method (lib.Type) Method(x *int) *int
 
 -------- @describe ref-method --------
-reference to func (lib.Type).Method(x *int) *int
+reference to method func (lib.Type).Method(x *int) *int
 defined here
 
 -------- @describe p --------