Meeting 2025-08-19

[adamgreig]

This is the agenda for the next meeting. Please add anything you'd like to discuss below!

• Meetings: Tuesday 8pm Europe/Berlin time
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• IRC/Matrix logs

[jamesmunns]

We've been asked by the build-std team to review their RFC. It is a fairly lengthy read, but very relevant to embedded folks.

We've been asked to limit direct feedback to members of the WG for now, but we may want to curate any community feedback here (or in a tracking issue) if necessary.

[mathias-arm]

hmmm, removing the cli flag (-Zbuild-std) because poor ux doesn't make much sense to me.

I think he is saying that only doing the Cargo command-line instead of doing build-std in Cargo.toml would be a poorer UX, and I agree with that. I don't want to have to use a wrapper tool (like make) to populate all cargo command-line arguments or environment variables.

[kaspar030]

I don't want to have to use a wrapper tool (like make) to populate all cargo command-line arguments or environment variables.

With Ariel OS, we don't want to either, but are forced to. Right now in our Ariel workspace, we have 34 targets across eleven rustc targets. Combine that with ~50 test applications that can be built for most of the targets, we currently have >1000 build configurations in our matrix, each needing more than --target FOO. Cargo on its own doesn't handle this well, so we wrap it (in laze).

Where possible, we select things using features and dependencies, but are using --cfg foo to trigger these, like with [target.'cfg(context = "cortex-m")'.dependencies]. We pass that from laze to Cargo via RUSTFLAGS, among other build configuration specific Cargo settings.

What that means is that Cargo's config, e.g., .cargo/config.toml, is not fine-grained enough to set build settings - e.g., only applications using alloc might want to build-std with alloc, so globally setting it for any target is too static. We need to be able to set that per-build.
Also, that configuration is supposed to be a user configuration (and in a hidden directory or even messed with by a user's global ~/.cargo/config.toml), so if anything in there is needed for a build to succeed, we need to take control over it.

Regular dependencies can be triggered using the combination of RUSTFLAGS=--cfg foo at cargo invocation time, and #[target.'cfg("foo").dependencies] anywhere in the crate graph.

We need the option to trigger build-std from the Cargo cli. What I meant is that a clear cli arg (-Cbuild-std[=core,alloc]) is better UX than translating cargo config toml values to --config ..., as that is IMO difficult to get right.

[davidtwco]

@kaspar030: We intend build-std-crate in .cargo/config to be replaced over time (though not removed, just used less) by being able to explicitly declare your dependencies on the standard library in your Cargo.tomls which would determine which standard library crates Cargo will build for your project. You could then change those per-target in the manifest just like any other dependency, e.g. [target.'cfg(context = "cortex-m")'.dependencies] alloc = { builtin = true }. Would that better scale to your use case?

[kaspar030]

Would that better scale to your use case?

Yes it would! ... if this is not restricted to the main application crate, but can be specified by any crate in the dependency tree.

[adamgemmell]

That's the plan with stage 1b 👍

As for build-std, you can put it in the build section, so CARGO_BUILD_BUILD_STD=always would work (though it's a bit hard to read actually), as would --config "build.build-std=always". However this would be overridden by the target triple/cfg options in lower-precedence config locations which is quite unintuitive.

[sgued]

Release of heapless 0.9.1

Thanks edit @zeenix and I joining the libs team heapless is ready for a 0.9.1 release. I think we can release it this evening.

[jamesmunns]

Ergot - Call for Experimentation

I wrote up a "call for experimentation" for Ergot, my messaging library. Check it out here: jamesmunns/ergot#76, I would love to have more embedded rust folks try it out, especially if you're someone who has/plans to hand roll some kind of communication based on postcard :D

[adamgreig]

• svd2rust 0.37 was released
• Add mutex-traits-based implementations of SpiDevice and I2c embedded-hal#683
• Add a macro to make initialization easier embedded-alloc#103

[jamesmunns]

I can look into the rust-embedded/embedded-hal#683 impl!