cmd/compile: better error message when offending/missing token is a keyword

Prefix keywords (type, default, case, etc.) with "keyword" in error
messages to make them less ambiguous.

Fixes #68589.

Change-Id: I1eb92d1382f621b934167b3a4c335045da26be9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/623819
Auto-Submit: Robert Griesemer <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Tim King <[email protected]>

Author: griesemer

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

@@ -300,7 +300,11 @@ func tokstring(tok token) string {
 	case _Semi:
 		return "semicolon or newline"
 	}
-	return tok.String()
+	s := tok.String()
+	if _Break <= tok && tok <= _Var {
+		return "keyword " + s
+	}
+	return s
 }
 
 // Convenience methods using the current token position.
@@ -2337,7 +2341,7 @@ func (p *parser) header(keyword token) (init SimpleStmt, cond Expr, post SimpleS
 	if p.tok != _Semi {
 		// accept potential varDecl but complain
 		if p.got(_Var) {
-			p.syntaxError(fmt.Sprintf("var declaration not allowed in %s initializer", tokstring(keyword)))
+			p.syntaxError(fmt.Sprintf("var declaration not allowed in %s initializer", keyword.String()))
 		}
 		init = p.simpleStmt(nil, keyword)
 		// If we have a range clause, we are done (can only happen for keyword == _For).

src/cmd/compile/internal/syntax/testdata/issue68589.go

@@ -0,0 +1,7 @@
+// Copyright 2024 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
+
+var _ (/* ERROR unexpected keyword type */ type T)

test/fixedbugs/issue11610.go

@@ -10,7 +10,7 @@
 package a
 var?      // ERROR "invalid character U\+003F '\?'|invalid character 0x3f in input file"
 
-var x int // ERROR "unexpected var|expected identifier|expected type"
+var x int // ERROR "unexpected keyword var|expected identifier|expected type"
 
 func main() {
 }

test/switch2.go

@@ -25,12 +25,12 @@ func f() {
 
 	switch {
 	case 0: f(); case 0:
-	case 0: f() case 0: // ERROR "unexpected case at end of statement"
+	case 0: f() case 0: // ERROR "unexpected keyword case at end of statement"
 	}
 
 	switch {
 	case 0: f(); default:
-	case 0: f() default: // ERROR "unexpected default at end of statement"
+	case 0: f() default: // ERROR "unexpected keyword default at end of statement"
 	}
 
 	switch {

test/syntax/semi7.go

@@ -8,7 +8,7 @@ package main
 
 func main() {
 	if x { }	// GCCGO_ERROR "undefined"
-	else { }	// ERROR "unexpected semicolon or newline before .?else.?|unexpected else"
+	else { }	// ERROR "unexpected semicolon or newline before .?else.?|unexpected keyword else"
 }