[dev.regabi] cmd/compile: first start towards using Ident

This CL adds Ident, which will eventually replace *Name and *PkgName
within the AST for representing uses of declared names. (Originally, I
intended to call it "IdentExpr", but neither go/ast nor
cmd/compile/internal/syntax include the "Expr" suffix for their
respective types.)

To start, this CL converts two uses of *Name to *Ident: the tag
identifier in a TypeSwitchGuard (which doesn't actually declare a
variable by itself), and the not-yet-known placeholder ONONAME
returned by oldname to stand-in for identifiers that might be declared
later in the package.

The TypeSwitchGuard's Name's Used flag was previously used for
detecting whether none of the per-clause variables were used. To avoid
bloating all Idents for this rare use, a "Used" bool is added to
TypeSwitchGuard instead. Eventually it could maybe be packed into
miniNode.bits, but for now this is good enough.

Passes buildall w/ toolstash -cmp.

Change-Id: I393284d86757cbbebd26e1320c7354e2bdcb30b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/276113
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/compile/internal/gc/dcl.go

@@ -215,7 +215,7 @@ func oldname(s *types.Sym) ir.Node {
 		// Maybe a top-level declaration will come along later to
 		// define s. resolve will check s.Def again once all input
 		// source has been processed.
-		return ir.NewDeclNameAt(base.Pos, s)
+		return ir.NewIdent(base.Pos, s)
 	}
 
 	if Curfn != nil && n.Op() == ir.ONAME && n.Name().Curfn != nil && n.Name().Curfn != Curfn {

src/cmd/compile/internal/gc/iimport.go

@@ -833,13 +833,13 @@ func (r *importReader) node() ir.Node {
 		return ir.TypeNode(r.typ())
 
 	case ir.OTYPESW:
-		n := ir.NodAt(r.pos(), ir.OTYPESW, nil, nil)
+		pos := r.pos()
+		var tag *ir.Ident
 		if s := r.ident(); s != nil {
-			n.SetLeft(ir.NewDeclNameAt(n.Pos(), s))
+			tag = ir.NewIdent(pos, s)
 		}
-		right, _ := r.exprsOrNil()
-		n.SetRight(right)
-		return n
+		expr, _ := r.exprsOrNil()
+		return ir.NewTypeSwitchGuard(pos, tag, expr)
 
 	// case OTARRAY, OTMAP, OTCHAN, OTSTRUCT, OTINTER, OTFUNC:
 	//      unreachable - should have been resolved by typechecking

src/cmd/compile/internal/gc/noder.go

@@ -751,14 +751,14 @@ func (p *noder) expr(expr syntax.Expr) ir.Node {
 			p.typeExpr(expr.Elem), p.chanDir(expr.Dir))
 
 	case *syntax.TypeSwitchGuard:
-		n := p.nod(expr, ir.OTYPESW, nil, p.expr(expr.X))
+		var tag *ir.Ident
 		if expr.Lhs != nil {
-			n.SetLeft(p.declName(expr.Lhs))
-			if ir.IsBlank(n.Left()) {
-				base.Errorf("invalid variable name %v in type switch", n.Left())
+			tag = ir.NewIdent(p.pos(expr.Lhs), p.name(expr.Lhs))
+			if ir.IsBlank(tag) {
+				base.Errorf("invalid variable name %v in type switch", tag)
 			}
 		}
-		return n
+		return ir.NewTypeSwitchGuard(p.pos(expr), tag, p.expr(expr.X))
 	}
 	panic("unhandled Expr")
 }

src/cmd/compile/internal/gc/typecheck.go

@@ -90,12 +90,16 @@ func resolve(n ir.Node) (res ir.Node) {
 		defer tracePrint("resolve", n)(&res)
 	}
 
-	if n.Sym().Pkg != types.LocalPkg {
+	// Stub ir.Name left for us by iimport.
+	if n, ok := n.(*ir.Name); ok {
+		if n.Sym().Pkg == types.LocalPkg {
+			base.Fatalf("unexpected Name: %+v", n)
+		}
 		if inimport {
 			base.Fatalf("recursive inimport")
 		}
 		inimport = true
-		expandDecl(n.(*ir.Name))
+		expandDecl(n)
 		inimport = false
 		return n
 	}

src/cmd/compile/internal/gc/walk.go

@@ -44,7 +44,7 @@ func walk(fn *ir.Func) {
 	// Propagate the used flag for typeswitch variables up to the NONAME in its definition.
 	for _, ln := range fn.Dcl {
 		if ln.Op() == ir.ONAME && (ln.Class() == ir.PAUTO || ln.Class() == ir.PAUTOHEAP) && ln.Defn != nil && ln.Defn.Op() == ir.OTYPESW && ln.Used() {
-			ln.Defn.Left().Name().SetUsed(true)
+			ln.Defn.(*ir.TypeSwitchGuard).Used = true
 		}
 	}
 
@@ -52,12 +52,12 @@ func walk(fn *ir.Func) {
 		if ln.Op() != ir.ONAME || (ln.Class() != ir.PAUTO && ln.Class() != ir.PAUTOHEAP) || ln.Sym().Name[0] == '&' || ln.Used() {
 			continue
 		}
-		if defn := ln.Defn; defn != nil && defn.Op() == ir.OTYPESW {
-			if defn.Left().Name().Used() {
+		if defn, ok := ln.Defn.(*ir.TypeSwitchGuard); ok {
+			if defn.Used {
 				continue
 			}
-			base.ErrorfAt(defn.Left().Pos(), "%v declared but not used", ln.Sym())
-			defn.Left().Name().SetUsed(true) // suppress repeats
+			base.ErrorfAt(defn.Tag.Pos(), "%v declared but not used", ln.Sym())
+			defn.Used = true // suppress repeats
 		} else {
 			base.ErrorfAt(ln.Pos(), "%v declared but not used", ln.Sym())
 		}

src/cmd/compile/internal/ir/mknode.go

@@ -39,7 +39,7 @@ func main() {
 	nodesType := lookup("Nodes")
 	ptrFieldType := types.NewPointer(lookup("Field"))
 	slicePtrFieldType := types.NewSlice(ptrFieldType)
-	ptrNameType := types.NewPointer(lookup("Name"))
+	ptrIdentType := types.NewPointer(lookup("Ident"))
 
 	var buf bytes.Buffer
 	fmt.Fprintln(&buf, "// Code generated by mknode.go. DO NOT EDIT.")
@@ -84,7 +84,7 @@ func main() {
 		fmt.Fprintf(&buf, "func (n *%s) doChildren(do func(Node) error) error { var err error\n", name)
 		forNodeFields(typName, typ, func(name string, is func(types.Type) bool) {
 			switch {
-			case is(ptrNameType):
+			case is(ptrIdentType):
 				fmt.Fprintf(&buf, "if n.%s != nil { err = maybeDo(n.%s, err, do) }\n", name, name)
 			case is(nodeType), is(ntypeType):
 				fmt.Fprintf(&buf, "err = maybeDo(n.%s, err, do)\n", name)
@@ -101,8 +101,8 @@ func main() {
 		fmt.Fprintf(&buf, "func (n *%s) editChildren(edit func(Node) Node) {\n", name)
 		forNodeFields(typName, typ, func(name string, is func(types.Type) bool) {
 			switch {
-			case is(ptrNameType):
-				fmt.Fprintf(&buf, "if n.%s != nil { n.%s = edit(n.%s).(*Name) }\n", name, name, name)
+			case is(ptrIdentType):
+				fmt.Fprintf(&buf, "if n.%s != nil { n.%s = edit(n.%s).(*Ident) }\n", name, name, name)
 			case is(nodeType):
 				fmt.Fprintf(&buf, "n.%s = maybeEdit(n.%s, edit)\n", name, name)
 			case is(ntypeType):

src/cmd/compile/internal/ir/name.go

@@ -13,6 +13,25 @@ import (
 	"go/constant"
 )
 
+// An Ident is an identifier, possibly qualified.
+type Ident struct {
+	miniExpr
+	sym  *types.Sym
+	Used bool
+}
+
+func NewIdent(pos src.XPos, sym *types.Sym) *Ident {
+	n := new(Ident)
+	n.op = ONONAME
+	n.pos = pos
+	n.sym = sym
+	return n
+}
+
+func (n *Ident) Sym() *types.Sym { return n.sym }
+
+func (*Ident) CanBeNtype() {}
+
 // Name holds Node fields used only by named nodes (ONAME, OTYPE, some OLITERAL).
 type Name struct {
 	miniExpr

src/cmd/compile/internal/ir/node.go

@@ -794,8 +794,6 @@ func NodAt(pos src.XPos, op Op, nleft, nright Node) Node {
 		return NewSliceHeaderExpr(pos, nil, nleft, nil, nil)
 	case OSWITCH:
 		return NewSwitchStmt(pos, nleft, nil)
-	case OTYPESW:
-		return NewTypeSwitchGuard(pos, nleft, nright)
 	case OINLCALL:
 		return NewInlinedCallExpr(pos, nil, nil)
 	}

src/cmd/compile/internal/ir/node_gen.go

@@ -450,6 +450,22 @@ func (n *GoDeferStmt) editChildren(edit func(Node) Node) {
 	n.Call = maybeEdit(n.Call, edit)
 }
 
+func (n *Ident) String() string                { return fmt.Sprint(n) }
+func (n *Ident) Format(s fmt.State, verb rune) { FmtNode(n, s, verb) }
+func (n *Ident) copy() Node {
+	c := *n
+	c.init = c.init.Copy()
+	return &c
+}
+func (n *Ident) doChildren(do func(Node) error) error {
+	var err error
+	err = maybeDoList(n.init, err, do)
+	return err
+}
+func (n *Ident) editChildren(edit func(Node) Node) {
+	editList(n.init, edit)
+}
+
 func (n *IfStmt) String() string                { return fmt.Sprint(n) }
 func (n *IfStmt) Format(s fmt.State, verb rune) { FmtNode(n, s, verb) }
 func (n *IfStmt) copy() Node {
@@ -1004,15 +1020,15 @@ func (n *TypeSwitchGuard) copy() Node {
 }
 func (n *TypeSwitchGuard) doChildren(do func(Node) error) error {
 	var err error
-	if n.Name_ != nil {
-		err = maybeDo(n.Name_, err, do)
+	if n.Tag != nil {
+		err = maybeDo(n.Tag, err, do)
 	}
 	err = maybeDo(n.X, err, do)
 	return err
 }
 func (n *TypeSwitchGuard) editChildren(edit func(Node) Node) {
-	if n.Name_ != nil {
-		n.Name_ = edit(n.Name_).(*Name)
+	if n.Tag != nil {
+		n.Tag = edit(n.Tag).(*Ident)
 	}
 	n.X = maybeEdit(n.X, edit)
 }

src/cmd/compile/internal/ir/stmt.go

@@ -512,32 +512,30 @@ func (n *SwitchStmt) SetHasBreak(x bool)  { n.HasBreak_ = x }
 // A TypeSwitchGuard is the [Name :=] X.(type) in a type switch.
 type TypeSwitchGuard struct {
 	miniNode
-	Name_ *Name
-	X     Node
+	Tag  *Ident
+	X    Node
+	Used bool
 }
 
-func NewTypeSwitchGuard(pos src.XPos, name, x Node) *TypeSwitchGuard {
-	n := &TypeSwitchGuard{X: x}
-	if name != nil {
-		n.Name_ = name.(*Name)
-	}
+func NewTypeSwitchGuard(pos src.XPos, tag *Ident, x Node) *TypeSwitchGuard {
+	n := &TypeSwitchGuard{Tag: tag, X: x}
 	n.pos = pos
 	n.op = OTYPESW
 	return n
 }
 
 func (n *TypeSwitchGuard) Left() Node {
-	if n.Name_ == nil {
+	if n.Tag == nil {
 		return nil
 	}
-	return n.Name_
+	return n.Tag
 }
 func (n *TypeSwitchGuard) SetLeft(x Node) {
 	if x == nil {
-		n.Name_ = nil
+		n.Tag = nil
 		return
 	}
-	n.Name_ = x.(*Name)
+	n.Tag = x.(*Ident)
 }
 func (n *TypeSwitchGuard) Right() Node     { return n.X }
 func (n *TypeSwitchGuard) SetRight(x Node) { n.X = x }