cmd/compile: better recovery after := (rather than =) in declarations

Before this fix, a mistaken := in a (const/type/var) declaration
ended that declaration with an error from which the parser didn't
recover well. This low-cost change will provide a better error
message and lets the parser recover perfectly.

Fixes #31092.

Change-Id: Ic4f94dc5e29dd00b7ef6d53a80dded638e3cea80
Reviewed-on: https://go-review.googlesource.com/c/go/+/169958
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/syntax/parser.go

@@ -170,6 +170,20 @@ func (p *parser) want(tok token) {
 	}
 }
 
+// gotAssign is like got(_Assign) but it also accepts ":="
+// (and reports an error) for better parser error recovery.
+func (p *parser) gotAssign() bool {
+	switch p.tok {
+	case _Define:
+		p.syntaxError("expecting =")
+		fallthrough
+	case _Assign:
+		p.next()
+		return true
+	}
+	return false
+}
+
 // ----------------------------------------------------------------------------
 // Error handling
 
@@ -514,7 +528,7 @@ func (p *parser) constDecl(group *Group) Decl {
 	d.NameList = p.nameList(p.name())
 	if p.tok != _EOF && p.tok != _Semi && p.tok != _Rparen {
 		d.Type = p.typeOrNil()
-		if p.got(_Assign) {
+		if p.gotAssign() {
 			d.Values = p.exprList()
 		}
 	}
@@ -533,7 +547,7 @@ func (p *parser) typeDecl(group *Group) Decl {
 	d.pos = p.pos()
 
 	d.Name = p.name()
-	d.Alias = p.got(_Assign)
+	d.Alias = p.gotAssign()
 	d.Type = p.typeOrNil()
 	if d.Type == nil {
 		d.Type = p.bad()
@@ -556,11 +570,11 @@ func (p *parser) varDecl(group *Group) Decl {
 	d.pos = p.pos()
 
 	d.NameList = p.nameList(p.name())
-	if p.got(_Assign) {
+	if p.gotAssign() {
 		d.Values = p.exprList()
 	} else {
 		d.Type = p.type_()
-		if p.got(_Assign) {
+		if p.gotAssign() {
 			d.Values = p.exprList()
 		}
 	}

src/cmd/compile/internal/syntax/testdata/issue31092.src

@@ -0,0 +1,16 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test cases for issue 31092: Better synchronization of
+// parser after seeing an := rather than an = in a const,
+// type, or variable declaration.
+
+package p
+
+const _ /* ERROR unexpected := */ := 0
+type _ /* ERROR unexpected := */ := int
+var _ /* ERROR unexpected := */ := 0
+
+const _ int /* ERROR unexpected := */ := 0
+var _ int /* ERROR unexpected := */ := 0