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Use the zeroedness of the base pointer in alloc.

	Instead of the special case being .len==0, use
	.base==(0 castto(void#)). This means that trimmed
	slices will no longer leak.

--- a/lib/std/alloc.myr

+++ b/lib/std/alloc.myr

@@ -47,6 +47,7 @@

 /* null pointers. only used internally. */

 const Zslab	= 0 castto(slab#)

 const Zchunk	= 0 castto(chunk#)

+const Zsliceptr	= 0 castto(byte#)

 const Slabsz 	= 1*MiB	/* 1 meg slabs */

 const Cachemax	= 16	/* maximum number of slabs in the cache */

@@ -149,7 +150,7 @@

 generic slfree	 = {sl

 	var head

-	if sl.len == 0

+	if sl castto(byte#) == Zsliceptr

 		-> void

 	;;

@@ -161,11 +162,16 @@

 /* Grows a slice */

 generic slgrow = {sl : @a[:], len

-	var n

+	var cap

 	var new

+	var n

 	/* if the slice doesn't need a bigger bucket, we don't need to realloc. */

-	if sl.len > 0 && slcap(sl castto(byte#)) >= allocsz(len*sizeof(@a))

+	cap = 0

+	if sl castto(byte#) != Zsliceptr

+		cap = slcap(sl castto(byte#))

+	;;

+	if cap >= allocsz(len*sizeof(@a))

 		-> (sl castto(@a#))[:len]

 	;;