gc: keep pointer to original node in constant rewrites.

This allows printing meaningful expressions in error messages
instead of evaluated constants.
Fixes #2276.

R=golang-dev, rsc
CC=golang-dev, remy
https://golang.org/cl/5432082

src/cmd/gc/const.c

@@ -842,8 +842,12 @@ unary:
 	}
 
 ret:
-	norig = n->orig;
-	// rewrite n in place.
+	if(n == n->orig) {
+		// duplicate node for n->orig.
+		norig = nod(OLITERAL, N, N);
+		*norig = *n;
+	} else
+		norig = n->orig;
 	*n = *nl;
 	// restore value of n->orig.
 	n->orig = norig;

src/cmd/gc/unsafe.c

@@ -94,8 +94,10 @@ ret:
 	val.u.xval = mal(sizeof(*n->val.u.xval));
 	mpmovecfix(val.u.xval, v);
 	n = nod(OLITERAL, N, N);
+	n->orig = nn;
 	n->val = val;
 	n->type = types[TUINTPTR];
+	nn->type = types[TUINTPTR];
 	return n;
 }
 

test/fixedbugs/bug379.go

@@ -7,12 +7,12 @@
 // Issue 2452.
 
 // Check that the error messages says 
-//	bug378.go:17: 3 not used
+//	bug378.go:17: 1 + 2 not used
 // and not
 //	bug378.go:17: 1 not used
 
 package main
 
 func main() {
-	1 + 2 // ERROR "3 not used|value computed is not used"
+	1 + 2 // ERROR "1 \+ 2 not used|value computed is not used"
 }

test/fixedbugs/bug381.go

@@ -0,0 +1,20 @@
+// 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 2276.
+
+// Check that the error messages says 
+//	bug378.go:19: unsafe.Alignof(0) not used
+// and not
+//	bug378.go:19: 4 not used
+
+package main
+
+import "unsafe"
+
+func main() {
+	unsafe.Alignof(0) // ERROR "unsafe\.Alignof|value computed is not used"
+}