cmd/compile: return assignment mismatch error in var declarations

Some var declarations return "extra expression" or "missing expression"
errors when they should return “assignment mismatch” instead. Change
the returned error messages to exhibit the desired behavior.

Fixes #30085.

Change-Id: I7189355fbb0f976d70100779db4f81a9ae64fb11
Reviewed-on: https://go-review.googlesource.com/c/go/+/161558Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>

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

@@ -166,11 +166,12 @@ func variter(vl []*Node, t *Node, el []*Node) []*Node {
 		return append(init, as2)
 	}
 
+	nel := len(el)
 	for _, v := range vl {
 		var e *Node
 		if doexpr {
 			if len(el) == 0 {
-				yyerror("missing expression in var declaration")
+				yyerror("assignment mismatch: %d variables but %d values", len(vl), nel)
 				break
 			}
 			e = el[0]
@@ -194,7 +195,7 @@ func variter(vl []*Node, t *Node, el []*Node) []*Node {
 	}
 
 	if len(el) != 0 {
-		yyerror("extra expression in var declaration")
+		yyerror("assignment mismatch: %d variables but %d values", len(vl), nel)
 	}
 	return init
 }

test/fixedbugs/issue30085.go

+// errorcheck
+
+// Copyright 2019 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 main
+
+func main() {
+	var c, d = 1, 2, 3 // ERROR "assignment mismatch: 2 variables but 3 values"
+	var e, f, g = 1, 2 // ERROR "assignment mismatch: 3 variables but 2 values"
+}