gc: implement new slice spec

Fixes #382.

R=gri, cw, r, rsc
CC=golang-dev
https://golang.org/cl/1957045

src/cmd/gc/go.y

@@ -845,10 +845,6 @@ pexpr_no_paren:
 	}
 |	pexpr '[' oexpr ':' oexpr ']'
 	{
-		if($3 == N) {
-			yyerror("missing lower bound in slice expression");
-			$3 = nodintconst(0);
-		}
 		$$ = nod(OSLICE, $1, nod(OKEY, $3, $5));
 	}
 |	pseudocall

src/cmd/gc/typecheck.c

@@ -654,16 +654,14 @@ reswitch:
 			typecheck(&n->left, top);
 		}
 		implicitstar(&n->left);
-		if(n->right->left == N) {
-			yyerror("missing slice bounds?");
-			goto error;
-		}
+		if(n->right->left != N) {
 			if((t = n->right->left->type) == T)
 				goto error;
 			if(!isint[t->etype]) {
 				yyerror("invalid slice index %#N (type %T)", n->right->left, t);
 				goto error;
 		}
+		}
 		if(n->right->right != N) {
 			if((t = n->right->right->type) == T)
 				goto error;

src/cmd/gc/walk.c

@@ -1095,13 +1095,17 @@ walkexpr(Node **np, NodeList **init)
 		// sliceslice(old []any, lb uint64, hb uint64, width uint64) (ary []any)
 		// sliceslice1(old []any, lb uint64, width uint64) (ary []any)
 		t = n->type;
+		if(n->right->left == N)
+			l = nodintconst(0);
+		else
+			l = conv(n->right->left, types[TUINT64]);
 		if(n->right->right != N) {
 			fn = syslook("sliceslice", 1);
 			argtype(fn, t->type);			// any-1
 			argtype(fn, t->type);			// any-2
 			n = mkcall1(fn, t, init,
 				n->left,
-				conv(n->right->left, types[TUINT64]),
+				l,
 				conv(n->right->right, types[TUINT64]),
 				nodintconst(t->type->width));
 		} else {
@@ -1110,7 +1114,7 @@ walkexpr(Node **np, NodeList **init)
 			argtype(fn, t->type);			// any-2
 			n = mkcall1(fn, t, init,
 				n->left,
-				conv(n->right->left, types[TUINT64]),
+				l,
 				nodintconst(t->type->width));
 		}
 		goto ret;
@@ -1122,13 +1126,17 @@ walkexpr(Node **np, NodeList **init)
 		fn = syslook("slicearray", 1);
 		argtype(fn, n->left->type);	// any-1
 		argtype(fn, t->type);			// any-2
+		if(n->right->left == N)
+			l = nodintconst(0);
+		else
+			l = conv(n->right->left, types[TUINT64]);
 		if(n->right->right == N)
 			r = nodintconst(n->left->type->bound);
 		else
 			r = conv(n->right->right, types[TUINT64]);
 		n = mkcall1(fn, t, init,
 			nod(OADDR, n->left, N), nodintconst(n->left->type->bound),
-			conv(n->right->left, types[TUINT64]),
+			l,
 			r,
 			nodintconst(t->type->width));
 		goto ret;
@@ -1213,15 +1221,19 @@ walkexpr(Node **np, NodeList **init)
 
 	case OSLICESTR:
 		// sys_slicestring(s, lb, hb)
+		if(n->right->left == N)
+			l = nodintconst(0);
+		else
+			l = conv(n->right->left, types[TINT]);
 		if(n->right->right) {
 			n = mkcall("slicestring", n->type, init,
 				conv(n->left, types[TSTRING]),
-				conv(n->right->left, types[TINT]),
+				l,
 				conv(n->right->right, types[TINT]));
 		} else {
 			n = mkcall("slicestring1", n->type, init,
 				conv(n->left, types[TSTRING]),
-				conv(n->right->left, types[TINT]));
+				l);
 		}
 		goto ret;