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> "As of Rust 2018, this still can cause undefined behavior."
This is referring to the fact that you have to be careful when accessing the fields of a packed struct that everything is aligned correctly. Normally anything where "you have to be careful" in order to uphold memory safety requires use of the `unsafe` keyword, but due to an oversight Rust doesn't currently require it in this instance. So, in typical Rust fashion, this isn't referring to anything sinister like a compiler miscompilation, it's just an error that Rust isn't being as paranoid as it strives to be. :P
The patch to require this `unsafe` annotation was actually just filed last week; it will become a warning first, then might become a hard error for users who opt-in to the upcoming 2021 edition: https://github.com/rust-lang/rust/pull/82525
In answer to what appears to be a misunderstanding about Rust:
> Its foreign function interface seems particularly poorly implemented. The official Rust documentation suggests the use of the external third-party libc library (called a 'crate' in Rust parlance) to provide the type definitions necessary to interface with C programs. As of the time of writing, this crate has had 95 releases. Contrast this with Ada’s Interfaces.C package, which was added the language in Ada 95 and hasn’t needed to change in any fundamental way since.
Rust's libc crate isn't third-party, it's first-party, developed by the Rust project itself: https://github.com/rust-lang/libc/ . It's also not just for "type definitions necessary to interface with C programs"; here's the first heading and first paragraph of its README:
"libc - Raw FFI bindings to platforms' system libraries"
libc provides all of the definitions necessary to easily interoperate with C code (or "C-like" code) on each of the platforms that Rust supports. This includes type definitions (e.g. c_int), constants (e.g. EINVAL) as well as function headers (e.g. malloc).
The fact that this library contains low-level type definitions for every platform that Rust supports explains why it's had more than one release: new platforms get added, platforms add new interfaces, and platforms change the definitions of existing interfaces.
> It lacks basic features necessary for the task, like bitfields, and data structure packing.
The latter is achieved via the built-in `repr(packed)` attribute (https://doc.rust-lang.org/nomicon/other-reprs.html#reprpacke...) and the former is provided by the bitflags crate: https://crates.io/crates/bitflags (while unlike libc this does not live under the rust-lang org on Github, it does live under its own org which appears to be populated exclusively by Rust project team members).