gc: better error for non-calling use of unsafe builtins.

Fixes #1951

R=rsc
CC=golang-dev
https://golang.org/cl/5372041

src/cmd/gc/go.h

@@ -1247,6 +1247,7 @@ void	queuemethod(Node *n);
 /*
  *	unsafe.c
  */
+int	isunsafebuiltin(Node *n);
 Node*	unsafenmagic(Node *n);
 
 /*

src/cmd/gc/typecheck.c

@@ -210,6 +210,10 @@ reswitch:
 			}
 			n->used = 1;
 		}
+		if(!(top &Ecall) && isunsafebuiltin(n)) {
+			yyerror("%N is not an expression, must be called", n);
+			goto error;
+		}
 		ok |= Erv;
 		goto ret;
 

src/cmd/gc/unsafe.c

@@ -10,6 +10,7 @@
  * look for
  *	unsafe.Sizeof
  *	unsafe.Offsetof
+ *	unsafe.Alignof
  * rewrite with a constant
  */
 Node*
@@ -97,3 +98,17 @@ ret:
 	n->type = types[TUINTPTR];
 	return n;
 }
+
+int
+isunsafebuiltin(Node *n)
+{
+	if(n == N || n->op != ONAME || n->sym == S || n->sym->pkg != unsafepkg)
+		return 0;
+	if(strcmp(n->sym->name, "Sizeof") == 0)
+		return 1;
+	if(strcmp(n->sym->name, "Offsetof") == 0)
+		return 1;
+	if(strcmp(n->sym->name, "Alignof") == 0)
+		return 1;
+	return 0;
+}

test/fixedbugs/bug376.go

+// errchk $G $D/$F.go
+
+// Copyright 2011 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.
+
+// issue 1951
+package foo
+import "unsafe"
+var v = unsafe.Sizeof  // ERROR "must be called"
+