gc: preserve original expression for errors

Fixes #1722.

R=ken2
CC=golang-dev
https://golang.org/cl/4442099

src/cmd/gc/go.h

@@ -225,6 +225,7 @@ struct	Node
 	Type*	realtype;	// as determined by typecheck
 	NodeList*	list;
 	NodeList*	rlist;
+	Node*	orig;		// original form, for printing
 
 	// for-body
 	NodeList*	ninit;

src/cmd/gc/subr.c

@@ -1454,6 +1454,8 @@ Nconv(Fmt *fp)
 	}
 
 	if(fp->flags & FmtSharp) {
+		if(n->orig != N)
+			n = n->orig;
 		exprfmt(fp, n, 0);
 		goto out;
 	}

src/cmd/gc/typecheck.c

@@ -894,12 +894,20 @@ reswitch:
 		// might be constant
 		switch(t->etype) {
 		case TSTRING:
-			if(isconst(l, CTSTR))
-				nodconst(n, types[TINT], l->val.u.sval->len);
+			if(isconst(l, CTSTR)) {
+				r = nod(OXXX, N, N);
+				nodconst(r, types[TINT], l->val.u.sval->len);
+				r->orig = n;
+				n = r;
+			}
 			break;
 		case TARRAY:
-			if(t->bound >= 0 && l->op == ONAME)
-				nodconst(n, types[TINT], t->bound);
+			if(t->bound >= 0 && l->op == ONAME) {
+				r = nod(OXXX, N, N);
+				nodconst(r, types[TINT], t->bound);
+				r->orig = n;
+				n = r;
+			}
 			break;
 		}
 		n->type = types[TINT];
@@ -1357,7 +1365,10 @@ ret:
 		goto error;
 	}
 	if((top & Etop) && !(top & (Ecall|Erv|Etype)) && !(ok & Etop)) {
+		if(n->diag == 0) {
 			yyerror("%#N not used", n);
+			n->diag = 1;
+		}
 		goto error;
 	}
 

test/fixedbugs/bug337.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 1722.
+
+// Check that the error messages says 
+//	bug337.go:16: len("foo") not used
+// and not
+//	bug337.go:16: 3 not used
+
+package main
+
+func main() {
+	len("foo")	// ERROR "len"
+}
+