Fil-C: Garbage In, Memory Safety Out [video]

Ha, looking through the links dang added, I came upon something I wrote back in December

I gave (somewhat arbitrarily) the example that safe Rust is OK with a 64-bit pointer (on a modern PC for example) having the value made by the UTF-8 text "LAUGHING". That's 8 bytes, 8 bytes is 64 bits, it fits perfectly. Of course I point out, unsafe Rust, which is allowed to dereference pointers, must never dereference this LAUGHING pointer, it's not actually pointing at anything, but it is allowed to exist in safe Rust.

Coincidentally, in like February or so this year, a new third-party Rust type "ColdString" was introduced to their reddit by somebody I've never met - I read about it and encouraged its author. ColdString is 64 bits (on modern hardware) because inside it's a pointer, and indeed if the text you wanted to store in a ColdString was "LAUGHING" you do that exactly as I described in the December post - it's just the UTF-8 encoded text, no problem. The clever trick inside ColdString is how it can know when that 64-bit pointer really is actually a pointer for longer strings - yet also allows any 0 to 8 byte UTF-8 text string to just be encoded in the ColdString itself directly.

There are getting to be quite a lot of these so I'll do it CPS style:

Memory Safe Context Switching - https://news.ycombinator.com/item?id=48727177 - June 2026 (30 comments)

Memory Safe Inline Assembly - https://news.ycombinator.com/item?id=48606096 - June 2026 (47 comments)

The Fil-C Optimized Calling Convention - https://news.ycombinator.com/item?id=48162876 - May 2026 (32 comments)

A simplified model of Fil-C - https://news.ycombinator.com/item?id=47810872 - April 2026 (136 comments)

Pizlix: Memory Safe Linux from Scratch - https://news.ycombinator.com/item?id=46260852 - Dec 2025 (30 comments)

Linux Sandboxes and Fil-C - https://news.ycombinator.com/item?id=46259064 - Dec 2025 (156 comments)

Ported freetype, fontconfig, harfbuzz, and graphite to Fil-C - https://news.ycombinator.com/item?id=46090009 - Nov 2025 (56 comments)

A Note on Fil-C - https://news.ycombinator.com/item?id=45842494 - Nov 2025 (210 comments)

Fil-C: A memory-safe C implementation - https://news.ycombinator.com/item?id=45842242 - Nov 2025 (1 comment)

Notes by djb on using Fil-C - https://news.ycombinator.com/item?id=45788040 - Nov 2025 (246 comments)

for more, goto https://news.ycombinator.com/item?id=45792588

He claims that all syscalls are safe because they're implemented by his custom libc, but that libc then calls out to the system libc, where the system calls are unsafe. He then claims that this is unlike rust, where making a syscall is unsafe. If you stick to the rust standard library in the same way that fil-c programs stick to the fil-c standard library, then all of your rust "system calls" are safe, too.

Someone in the audience also pointed out that a tool like this could be used to compile rust programs, not just C programs, in which case it's odd to hear Fil-C repeatedly framed as a language in opposition to rust rather than as a tool which might complement it.

I was just thinking that we were overdue for a Fil-C post on HN.