Make allsorts run on no_std

[fschutt]

This is necessary to have a font shaper for embedded environments.

NOTE: WIP PR, some dependencies do not yet have no_std support, also compiling fails currently

TODO LIST:

• enable no_std on brotli decoder (possible, but the documentation is poor)
• enable no_std on zlib decoder (possible using miniz_oxide?)
• get rid of std::io::Cursor traits, use either Vec<u8> or &[u8] where possible
• wait for encoding-rs to merge no_std support (in progress)
• remove rental crate (rental is not maintained anymore, also no no_std support!)

[fschutt]

Okay, now everything should at least compile on no_std - I am still not 100% sure about the brotli decoder and the zlib decoder - they compile, but they probably don't work yet (i.e. they'll just emit a ParseError). As far as I can see, the brotli decoder needs a custom memory allocator and the API for that is very cumbersome.

Since rental is not maintained anymore, I've published a fork with a custom feature that allows rental to run on no_std. I've not removed it because it's good for what it does.

[Aaron1011]

EDIT - I mistakenly thought that @fschutt was part of the yeslogic organization. cc @adrianwong @brendanzab @wezm - the comment below concerns the allsorts-rental crate.

@fschutt Would it be possible for you to make a new release of allsorts-rental that include this commit from rental: jpernst/rental@61188eb. Doing so will allow allsorts-rental to continue to compile in future versions of Rust (after a backwards-compatibility hack is removed: rust-lang/rust#83125).

Background: It looks like this PR (or something based off of it) was published to crates.io as allsorts_no_std. It was then added as a dependency by https://github.com/fschutt/rust-fontconfig, which is used by https://github.com/LtPeriwinkle/mist.

Unfortunately, once the backwards-compatibility hack rust-lang/rust#83125 is removed from rustc, these crates will stop compiling. The simplest way to keep everything compiling would be to make a new point release of allsorts-rental that includes jpernst/rental@61188eb. Specifically, all of the commas added by that commit need to present (they were previously 'generated' by rustc itself, and the rental proc-macro expects them to exist).

Feel free to ask me if you have any questions about this. Thank you for your time.

[wezm]

Hi @Aaron1011 we (YesLogic) don't own the allsorts-rental and allsorts_no_std crates. It would be great if @fschutt could edit the READMEs and crate metadata to make this clearer but they seem to be busy.

[fschutt]

Yeah, this PR is pretty old now, I think this can be closed for now. My original effort to make allsorts run on no_std was relatively pointless anyway, I wanted to reduce the binary size of azul, but in the end it's easier to use the upstream allsorts in order to not miss out on bugfixes. I could maybe rebase + re-publish (most of the PR is just replacing std::String with alloc::String), but I'm extremely busy atm.

The reason I like no_std crates is not just because of size, but also because of security. I can be relatively certain that allsorts doesn't contain malicious code if it runs on no_std and it just does font parsing. The more no_std, the better.

Yes, I did publish it as allsorts-no-std, since I needed an intermediate solution, but right now I am back to using the regular allsorts crate since I didn't manage to make azul run on no-std anyway (it needs access to the file system to load fonts + access to the windowing system).

The allsorts-rental is just a fork of the rental crate, since that crate was (is?) unmaintained.

[wezm]

Yes, I did publish it as allsorts-no-std, since I needed an intermediate solution

Any chance you could publish a new point release that changes the repo links etc. in the Cargo.toml and README to not point at our repo? A note about maintenance status and that it's fork would be super helpful for avoiding confusion too.

[Aaron1011]

@fschutt Would you be able to publish that point release? It would be a big help in removing the back-compat hack from rustc.

[fschutt]

What back-compat hack? And yeah, sorry about not publishing a release, I forgot, sorry.

[wezm]

What back-compat hack?

I think it was this one from Aaron1011's comment:

the backwards-compatibility hack rust-lang/rust#83125

[Aaron1011]

@fschutt Sorry to keep bothering you - would you be able to publish that release? It would be a huge help in removing the backward-compatibility hack from rustc.