Auto merge of #123936 - Mark-Simulacrum:zst-no-alloc, r=oli-obk

Codegen ZSTs without an allocation

This makes sure that &[] is equivalent to unsafe code (from_raw_parts(dangling, 0)). No new stable guarantee is intended about whether or not we do this, this is just an optimization.

This regressed in #67000 (no comments I can see about that regression in the PR, though it did change the test modified here). We had previously performed this optimization since #63635.

Author: bors