cmd/compile/internal/syntax: better errors for syntax errors in lists

For syntax errors in various (syntactic) lists, instead of reporting
a set of "expected" tokens (which may be incomplete), provide context
and mention "possibly missing" tokens. The result is a friendlier and
more accurate error message.

Fixes #49205.

Change-Id: I38ae7bf62febfe790075e62deb33ec8c17d64476
Reviewed-on: https://go-review.googlesource.com/c/go/+/396914
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -472,7 +472,7 @@ func isEmptyFuncDecl(dcl Decl) bool {
 //
 // list = [ f { sep f } [sep] ] close .
 //
-func (p *parser) list(sep, close token, f func() bool) Pos {
+func (p *parser) list(context string, sep, close token, f func() bool) Pos {
 	if debug && (sep != _Comma && sep != _Semi || close != _Rparen && close != _Rbrace && close != _Rbrack) {
 		panic("invalid sep or close argument for list")
 	}
@@ -482,7 +482,7 @@ func (p *parser) list(sep, close token, f func() bool) Pos {
 		done = f()
 		// sep is optional before close
 		if !p.got(sep) && p.tok != close {
-			p.syntaxError(fmt.Sprintf("expecting %s or %s", tokstring(sep), tokstring(close)))
+			p.syntaxError(fmt.Sprintf("in %s; possibly missing %s or %s", context, tokstring(sep), tokstring(close)))
 			p.advance(_Rparen, _Rbrack, _Rbrace)
 			if p.tok != close {
 				// position could be better but we had an error so we don't care
@@ -502,7 +502,7 @@ func (p *parser) appendGroup(list []Decl, f func(*Group) Decl) []Decl {
 		g := new(Group)
 		p.clearPragma()
 		p.next() // must consume "(" after calling clearPragma!
-		p.list(_Semi, _Rparen, func() bool {
+		p.list("grouped declaration", _Semi, _Rparen, func() bool {
 			if x := f(g); x != nil {
 				list = append(list, x)
 			}
@@ -1233,7 +1233,7 @@ func (p *parser) complitexpr() *CompositeLit {
 
 	p.xnest++
 	p.want(_Lbrace)
-	x.Rbrace = p.list(_Comma, _Rbrace, func() bool {
+	x.Rbrace = p.list("composite literal", _Comma, _Rbrace, func() bool {
 		// value
 		e := p.bare_complitexpr()
 		if p.tok == _Colon {
@@ -1477,7 +1477,7 @@ func (p *parser) structType() *StructType {
 
 	p.want(_Struct)
 	p.want(_Lbrace)
-	p.list(_Semi, _Rbrace, func() bool {
+	p.list("struct type", _Semi, _Rbrace, func() bool {
 		p.fieldDecl(typ)
 		return false
 	})
@@ -1497,7 +1497,7 @@ func (p *parser) interfaceType() *InterfaceType {
 
 	p.want(_Interface)
 	p.want(_Lbrace)
-	p.list(_Semi, _Rbrace, func() bool {
+	p.list("interface type", _Semi, _Rbrace, func() bool {
 		switch p.tok {
 		case _Name:
 			f := p.methodDecl()
@@ -1980,7 +1980,7 @@ func (p *parser) paramList(name *Name, typ Expr, close token, requireNames bool)
 
 	var named int // number of parameters that have an explicit name and type
 	var typed int // number of parameters that have an explicit type
-	end := p.list(_Comma, close, func() bool {
+	end := p.list("parameter list", _Comma, close, func() bool {
 		var par *Field
 		if typ != nil {
 			if debug && name == nil {
@@ -2660,7 +2660,7 @@ func (p *parser) argList() (list []Expr, hasDots bool) {
 	}
 
 	p.xnest++
-	p.list(_Comma, _Rparen, func() bool {
+	p.list("argument list", _Comma, _Rparen, func() bool {
 		list = append(list, p.expr())
 		hasDots = p.got(_DotDotDot)
 		return hasDots

src/cmd/compile/internal/syntax/testdata/issue49205.go

@@ -0,0 +1,27 @@
+// Copyright 2022 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.
+
+package p
+
+// test case from issue
+
+type _ interface{
+	m /* ERROR unexpected int in interface type; possibly missing semicolon or newline or } */ int
+}
+
+// other cases where the fix for this issue affects the error message
+
+const (
+	x int = 10 /* ERROR unexpected literal "foo" in grouped declaration; possibly missing semicolon or newline or \) */ "foo"
+)
+
+var _ = []int{1, 2, 3 /* ERROR unexpected int in composite literal; possibly missing comma or } */ int }
+
+type _ struct {
+	x y /* ERROR syntax error: unexpected comma in struct type; possibly missing semicolon or newline or } */ ,
+}
+
+func f(a, b c /* ERROR unexpected d in parameter list; possibly missing comma or \) */ d) {
+	f(a, b, c /* ERROR unexpected d in argument list; possibly missing comma or \) */ d)
+}

test/fixedbugs/issue13319.go

@@ -9,10 +9,10 @@ package main
 func f(int, int) {
     switch x {
     case 1:
-        f(1, g()   // ERROR "expecting \)|expecting comma or \)"
+        f(1, g()   // ERROR "expecting \)|possibly missing comma or \)"
     case 2:
         f()
     case 3:
-        f(1, g()   // ERROR "expecting \)|expecting comma or \)"
+        f(1, g()   // ERROR "expecting \)|possibly missing comma or \)"
     }
 }

test/syntax/composite.go

@@ -7,5 +7,5 @@
 package main
 
 var a = []int{
-	3 // ERROR "need trailing comma before newline in composite literal|expecting comma or }"
+	3 // ERROR "need trailing comma before newline in composite literal|possibly missing comma or }"
 }