math/big: add comment about internal assumptions on nat values

Change-Id: I7ed40507a019c0bf521ba748fc22c03d74bb17b7
Reviewed-on: https://go-review.googlesource.com/100719Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/math/big/nat.go

@@ -5,6 +5,11 @@
 // This file implements unsigned multi-precision integers (natural
 // numbers). They are the building blocks for the implementation
 // of signed integers, rationals, and floating-point numbers.
+//
+// Caution: This implementation relies on the function "alias"
+//          which assumes that (nat) slice capacities are never
+//          changed (no 3-operand slice expressions). If that
+//          changes, alias needs to be updated for correctness.
 
 package big
 
@@ -352,6 +357,10 @@ func karatsuba(z, x, y nat) {
 }
 
 // alias reports whether x and y share the same base array.
+// Note: alias assumes that the capacity of underlying arrays
+//       is never changed for nat values; i.e. that there are
+//       no 3-operand slice expressions in this code (or worse,
+//       reflect-based operations to the same effect).
 func alias(x, y nat) bool {
 	return cap(x) > 0 && cap(y) > 0 && &x[0:cap(x)][cap(x)-1] == &y[0:cap(y)][cap(y)-1]
 }