change string([]byte) to pass array, rather than &a[0],

to string convert.  if the byte array has length 0,
the computation of &a[0] throws an index bounds error.

for fixed size arrays, this ends up invoking arrays2d
unnecessarily, but it works.

R=ken
DELTA=304  (44 added, 28 deleted, 232 changed)
OCL=15674
CL=15678

src/cmd/gc/sys.go

@@ -24,6 +24,7 @@ func	slicestring(string, int32, int32) string;
 func	indexstring(string, int32) byte;
 func	intstring(int64) string;
 func	byteastring(*byte, int32) string;
+func	arraystring(*[]byte) string;
 
 func	ifaceT2I(sigi *byte, sigt *byte, elem any) (ret any);
 func	ifaceI2T(sigt *byte, iface any) (ret any);
@@ -100,6 +101,7 @@ export
 	indexstring
 	intstring
 	byteastring
+	arraystring
 
 	// interface
 	ifaceT2I

src/cmd/gc/walk.c

@@ -1824,25 +1824,11 @@ stringop(Node *n, int top)
 		break;
 
 	case OARRAY:
-		// byteastring(*byte, int32) string;
-		t = n->left->type;
-		l = bytearraysz(t);
-
-		// &a[0]
-		c = nodintconst(0);
-		r = nod(OINDEX, n->left, c);
-		r = nod(OADDR, r, N);
-
-		if(l >= 0) {
-			// static size
-			c = nodintconst(l);
-		} else {
-			// dynamic size
-			c = nod(OLEN, n->left, N);
-		}
-		r = list(r, c);
-
-		on = syslook("byteastring", 0);
+		// arraystring(*[]byte) string;
+		r = n->left;
+		if(!isptr[r->type->etype])
+			r = nod(OADDR, r, N);
+		on = syslook("arraystring", 0);
 		r = nod(OCALL, on, r);
 		break;
 	}

src/runtime/string.c

@@ -164,3 +164,12 @@ sys·byteastring(byte *a, int32 l, string s)
 	mcpy(s->str, a, l);
 	FLUSH(&s);
 }
+
+void
+sys·arraystring(Array *b, string s)
+{
+	s = mal(sizeof(s->len)+b->nel);
+	s->len = b->nel;
+	mcpy(s->str, b->array, s->len);
+	FLUSH(&s);
+}

test/bugs/bug102.go

@@ -9,5 +9,17 @@ package main
 func main() {
 	var b [0]byte;
 	s := string(b);	// out of bounds trap
+	if s != "" {
+		panic("bad convert")
+	}
+	var b1 = [5]byte{'h', 'e', 'l', 'l', 'o'};
+	if string(b1) != "hello" {
+		panic("bad convert 1")
+	}
+	var b2 = new([]byte, 5)
+	for i := 0; i < 5; i++ { b2[i] = b1[i] }
+	if string(b2) != "hello" {
+		panic("bad convert 2")
+	}
 }