rust-index: document safety invariants being upheld for every `unsafe` block We've added a lot of `unsafe` code that shares Rust structs with Python. While this is unfortunate, it is also unavoidable, so let's at least systematically explain why each call to `unsafe` is sound. If any of the unsafe code ends up being wrong (because everyone screws up at some point), this change at least continues the unspoken rule of always explaining the need for `unsafe`, so we at least get a chance to think.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "phabricator.uri": "https://phab.mercurial-scm.org/",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}