message: document -Zmiri-symbolic-alignment-check
diff: @@ -189,6 +189,17 @@ Miri adds its own set of `-Z` flags:
  entropy. The default seed is 0. **NOTE**: This entropy is not good enough
  for cryptographic use! Do not generate secret keys in Miri or perform other
  kinds of cryptographic operations that rely on proper random numbers.
  +* `-Zmiri-symbolic-alignment-check` makes the alignment check more strict. By
  + default, alignment is checked by casting the pointer to an integer, and making
  + sure that is a multiple of the alignment. This can lead to cases where a
  + program passes the alignment check by pure chance, because things "happened to
  + be" sufficiently aligned. To avoid such cases, the symbolic alignment check
  + only takes into account the requested alignment of the relevant allocation,
  + and the offset into that allocation. This avoids such false negatives, but it
  + also incurs some false positives when the code does manual integer arithmetic
  + to ensure alignment. (The standard library `align_to` method works fine in
  + both modes; under symbolic alignment it only fills the middle slice when the
  + allocation guarantees sufficient alignment.)
  * `-Zmiri-track-alloc-id=<id>` shows a backtrace when the given allocation is
  being allocated or freed. This helps in debugging memory leaks and
  use after free bugs.
  