[dev.unified] cmd/compile/internal/noder: add optExpr for optional expressions

Previously, {writer,reader}.expr would allow for nil
expressions (i.e., no expression at all, not a "nil" identifier). But
only a few contexts allow this, and it simplifies some logic if we can
assume the expression is non-nil.

So this CL introduces optExpr as a wrapper method for handling nil
expressions specially.

Change-Id: I438bae7a3191126f7790ec0bf5b77320fe855514
Reviewed-on: https://go-review.googlesource.com/c/go/+/410099
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

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

@@ -38,11 +38,10 @@ func (c codeExpr) Value() int                 { return int(c) }
 
 // TODO(mdempsky): Split expr into addr, for lvalues.
 const (
-	exprNone codeExpr = iota
+	exprConst  codeExpr = iota
-	exprConst
+	exprType            // type expression
-	exprType   // type expression
+	exprLocal           // local variable
-	exprLocal  // local variable
+	exprGlobal          // global variable or function
-	exprGlobal // global variable or function
 	exprBlank
 	exprCompLit
 	exprFuncLit

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

@@ -1380,7 +1380,7 @@ func (r *reader) forStmt(label *types.Sym) ir.Node {
 
 	pos := r.pos()
 	init := r.stmt()
-	cond := r.expr()
+	cond := r.optExpr()
 	post := r.stmt()
 	body := r.blockStmt()
 	r.closeAnotherScope()
@@ -1450,7 +1450,7 @@ func (r *reader) switchStmt(label *types.Sym) ir.Node {
 		iface = x.Type()
 		tag = ir.NewTypeSwitchGuard(pos, ident, x)
 	} else {
-		tag = r.expr()
+		tag = r.optExpr()
 	}
 
 	clauses := make([]*ir.CaseClause, r.Len())
@@ -1551,9 +1551,6 @@ func (r *reader) expr() (res ir.Node) {
 	default:
 		panic("unhandled expression")
 
-	case exprNone:
-		return nil
-
 	case exprBlank:
 		// blank only allowed in LHS of assignments
 		// TODO(mdempsky): Handle directly in assignList instead?
@@ -1622,7 +1619,7 @@ func (r *reader) expr() (res ir.Node) {
 		pos := r.pos()
 		var index [3]ir.Node
 		for i := range index {
-			index[i] = r.expr()
+			index[i] = r.optExpr()
 		}
 		op := ir.OSLICE
 		if index[2] != nil {
@@ -1701,6 +1698,13 @@ func (r *reader) expr() (res ir.Node) {
 	}
 }
 
+func (r *reader) optExpr() ir.Node {
+	if r.Bool() {
+		return r.expr()
+	}
+	return nil
+}
+
 func (r *reader) compLit() ir.Node {
 	r.Sync(pkgbits.SyncCompLit)
 	pos := r.pos()

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

@@ -1077,7 +1077,7 @@ func (w *writer) forStmt(stmt *syntax.ForStmt) {
 	} else {
 		w.pos(stmt)
 		w.stmt(stmt.Init)
-		w.expr(stmt.Cond)
+		w.optExpr(stmt.Cond)
 		w.stmt(stmt.Post)
 	}
 
@@ -1136,7 +1136,7 @@ func (w *writer) switchStmt(stmt *syntax.SwitchStmt) {
 		}
 		w.expr(guard.X)
 	} else {
-		w.expr(stmt.Tag)
+		w.optExpr(stmt.Tag)
 	}
 
 	w.Len(len(stmt.Body))
@@ -1201,6 +1201,8 @@ func (w *writer) optLabel(label *syntax.Name) {
 // @@@ Expressions
 
 func (w *writer) expr(expr syntax.Expr) {
+	base.Assertf(expr != nil, "missing expression")
+
 	expr = unparen(expr) // skip parens; unneeded after typecheck
 
 	obj, inst := lookupObj(w.p.info, expr)
@@ -1254,9 +1256,6 @@ func (w *writer) expr(expr syntax.Expr) {
 	default:
 		w.p.unexpected("expression", expr)
 
-	case nil: // absent slice index, for condition, or switch tag
-		w.Code(exprNone)
-
 	case *syntax.Name:
 		assert(expr.Value == "_")
 		w.Code(exprBlank)
@@ -1292,7 +1291,7 @@ func (w *writer) expr(expr syntax.Expr) {
 		w.expr(expr.X)
 		w.pos(expr)
 		for _, n := range &expr.Index {
-			w.expr(n)
+			w.optExpr(n)
 		}
 
 	case *syntax.AssertExpr:
@@ -1356,6 +1355,12 @@ func (w *writer) expr(expr syntax.Expr) {
 	}
 }
 
+func (w *writer) optExpr(expr syntax.Expr) {
+	if w.Bool(expr != nil) {
+		w.expr(expr)
+	}
+}
+
 func (w *writer) compLit(lit *syntax.CompositeLit) {
 	tv, ok := w.p.info.Types[lit]
 	assert(ok)