go.tools/go/types: implement Info.Selections

Should make it easier for clients to deal with
selector expressions.

Removed Field type: The respective information
is now reported in Info.Selections.

R=adonovan
CC=golang-dev
https://golang.org/cl/11942043

go/types/api.go

@@ -115,6 +115,12 @@ type Info struct {
 	//      *ast.Field         anonymous struct field or parameter *Var
 	//
 	Implicits map[ast.Node]Object
+
+	// If Selections != nil, it records the selector expression and corresponding
+	// selection, i.e., package object (qualified identifier), struct field (field
+	// selector), or method (method expression or value) for each selector expression
+	// that is type-checked.
+	Selections map[*ast.SelectorExpr]*Selection
 }
 
 // Check type-checks a package and returns the resulting package object, the first

go/types/builtins.go

@@ -356,8 +356,8 @@ func (check *checker) builtin(x *operand, call *ast.CallExpr, bin *Builtin) {
 			goto Error
 		}
 
-		// TODO(gri) don't create a lookupResult if no Objects map exists
-		check.recordObject(arg.Sel, lookupResult(base, obj, index, indirect))
+		// TODO(gri) Should we pass x.typ instead of base (and indirect report if derefStructPtr indirected)?
+		check.recordSelection(arg, FieldVal, base, obj, index, false)
 
 		offs := check.conf.offsetof(base, index)
 		x.mode = constant

go/types/call.go

@@ -182,7 +182,7 @@ func (check *checker) selector(x *operand, e *ast.SelectorExpr) {
 				check.errorf(e.Pos(), "%s not exported by package %s", sel, ident)
 				goto Error
 			}
-			check.recordObject(e.Sel, exp)
+			check.recordSelection(e, PackageObj, nil, exp, nil, false)
 			// Simplified version of the code for *ast.Idents:
 			// - imported packages use types.Scope and types.Objects
 			// - imported objects are always fully initialized
@@ -225,9 +225,6 @@ func (check *checker) selector(x *operand, e *ast.SelectorExpr) {
 		goto Error
 	}
 
-	// TODO(gri) don't create a lookupResult if no Objects map exists
-	check.recordObject(e.Sel, lookupResult(x.typ, obj, index, indirect))
-
 	if x.mode == typexpr {
 		// method expression
 		m, _ := obj.(*Func)
@@ -242,6 +239,8 @@ func (check *checker) selector(x *operand, e *ast.SelectorExpr) {
 			goto Error
 		}
 
+		check.recordSelection(e, MethodExpr, x.typ, m, index, indirect)
+
 		// the receiver type becomes the type of the first function
 		// argument of the method expression's function type
 		var params []*Var
@@ -260,6 +259,7 @@ func (check *checker) selector(x *operand, e *ast.SelectorExpr) {
 		// regular selector
 		switch obj := obj.(type) {
 		case *Var:
+			check.recordSelection(e, FieldVal, x.typ, obj, index, indirect)
 			x.mode = variable
 			x.typ = obj.typ
 
@@ -276,6 +276,8 @@ func (check *checker) selector(x *operand, e *ast.SelectorExpr) {
 				goto Error
 			}
 
+			check.recordSelection(e, MethodVal, x.typ, obj, index, indirect)
+
 			if debug {
 				// Verify that LookupFieldOrMethod and MethodSet.Lookup agree.
 				typ := x.typ

go/types/check.go

@@ -96,6 +96,15 @@ func (check *checker) recordImplicit(node ast.Node, obj Object) {
 	}
 }
 
+func (check *checker) recordSelection(x *ast.SelectorExpr, kind SelectionKind, recv Type, obj Object, index []int, indirect bool) {
+	assert(obj != nil && (recv == nil || len(index) > 0))
+	check.recordObject(x.Sel, obj)
+	// TODO(gri) Should we also call recordTypeAndValue?
+	if m := check.Selections; m != nil {
+		m[x] = &Selection{kind, obj, selectorPath{recv, index, indirect}}
+	}
+}
+
 // A bailout panic is raised to indicate early termination.
 type bailout struct{}
 

go/types/lookup.go

@@ -11,26 +11,6 @@ package types
 //           types always have only one representation (even when imported
 //           indirectly via different packages.)
 
-// TODO(gri) Move Field to objects.go?
-
-// A Field represents a struct field x.f and corresponding path.
-type Field struct {
-	*Var
-	selectorPath
-}
-
-func lookupResult(typ Type, obj Object, index []int, indirect bool) Object {
-	switch obj := obj.(type) {
-	case nil:
-		return nil
-	case *Var:
-		return &Field{obj, selectorPath{typ, index, indirect}}
-	case *Func:
-		return &Method{obj, selectorPath{typ, index, indirect}}
-	}
-	panic("unreachable")
-}
-
 // LookupFieldOrMethod looks up a field or method with given package and name
 // in typ and returns the corresponding *Var or *Func, an index sequence,
 // and a bool indicating if there were any pointer indirections on the path

go/types/methodset.go

@@ -13,8 +13,78 @@ import (
 	"sync"
 )
 
-// TODO(gri) Move Method to objects.go?
-// TODO(gri) Method.Type() returns the wrong receiver type.
+// SelectorKind describes the kind of a selector expression recv.obj.
+type SelectionKind int
+
+const (
+	FieldVal   SelectionKind = iota // recv.obj is a struct field selector
+	MethodVal                       // recv.obj is a method selector
+	MethodExpr                      // recv.obj is a method expression
+	PackageObj                      // recv.obj is a qualified identifier
+)
+
+// A Selection describes a selector expression recv.obj.
+type Selection struct {
+	kind         SelectionKind
+	obj          Object
+	selectorPath // not valid for PackageObj
+}
+
+func (s *Selection) Kind() SelectionKind { return s.kind }
+func (s *Selection) Obj() Object         { return s.obj }
+
+// Type returns the type of the selector recv.obj which may be different
+// from the type of the selected object obj.
+//
+// Given the example declarations:
+//
+// 	type T struct{ x int; E }
+// 	type E struct{}
+// 	func (e E) m() {}
+// 	var p *T
+//
+// the following relationships exist:
+//
+//	Kind          Selector    Recv type    Selection type
+//
+//      FieldVal      p.x         T            int (type of x)
+//      MethodVal     p.m         *T           func (e *T) m()
+//      MethodExpr    T.m         T            func m(_ T)
+//      PackageObj    math.Pi     nil          untyped numeric constant (type of Pi)
+//
+func (s *Selection) Type() Type {
+	switch s.kind {
+	case MethodVal:
+		// The type of recv.obj is a method with its receiver type
+		// changed to s.recv.
+		// TODO(gri) Similar code is already in Method.Type below.
+		//           Method may go away in favor of Selection.
+		sig := *s.obj.(*Func).typ.(*Signature)
+		recv := *sig.recv
+		recv.typ = s.recv
+		sig.recv = &recv
+		return &sig
+
+	case MethodExpr:
+		// The type of recv.obj is a function (without receiver)
+		// and an additional first argument of type recv.
+		// TODO(gri) Similar code is already in call.go - factor!
+		sig := *s.obj.(*Func).typ.(*Signature)
+		arg0 := *sig.recv
+		arg0.typ = s.recv
+		var params []*Var
+		if sig.params != nil {
+			params = sig.params.vars
+		}
+		sig.params = NewTuple(append([]*Var{&arg0}, params...)...)
+		return &sig
+	}
+
+	// In all other cases, the type of recv.obj is the type of obj.
+	return s.obj.Type()
+}
+
+// TODO(gri) Replace Method in favor of Selection (make Method an interface?).
 
 // A Method represents a concrete or abstract (interface) method x.m
 // and corresponding path. The method belongs to the method set of x.