Writing a #[no_std] compatible crate in Rust

I’ve been toying around with Rust during Easter. It has been a while since I last had a go at it for UEFI binaries. Turns out that the uefi-rs crate has gotten tons of love in terms of usability, stability and built-in protocol interfaces.

Now, no_std is useful for a myriad of use cases like embedded platforms, it can even work in environments with no memory allocation. You can find an example UEFI app here, it is nothing fancy, crawls the directory tree of the main EFI System Partition.

For the purpose of my pet project I wanted to add a Boot Loader Spec parser to my project that I could also link in a library with std:: as well as no_std. This introduces the requirement that your no_std environment needs to be hooked up with an allocator, at which point you can consume the alloc crate.

Otherwise you can only use the data types in libcore (as well as crates that only depend on libcore), which is a small subset of std stuff. uefi-rs sets this up for you but this documentation might come handy.

This would be your minimal no_std lib.rs:

#[no_std]
fn my_function () -> bool {
    true
}

to std or no to std

Now, the first problem we have here is that we want to also be able to compile this module with the standard library. How can we do both? Well, turns out there is a convention for this, enter cargo features:

[features]
default = ["std"]
std = []

At which point you can use #![cfg_attr] to make no_std a conditional attribute upon compilation in the absence of the std feature.

#![cfg_attr(not(feature = "std"), no_std)]
fn my_function () -> bool {
    true
}

And you can drop the std feature at compile time like this

$ cargo build --no-default-features

Or consume the crate as a dependency in your toml like this:

[dependencies]
nostdable-crate = { version = "0.1.1", default-features = false }

optionally use core:: and alloc::

So, let’s say we have the following function:

#![cfg_attr(not(feature = "std"), no_std)]

fn my_function () -> String {
    String::from("foobar")
}

Here we are using std::String which is not available, this is going to fail on a no_std setup as is. libcore’s core::str will only work for static strings as there is no allocation available. In these environments you need an allocator so that we can import the alloc:: crate, if you have an allocator primitive you need to implement a GlobalAlloc and initialize it with #[global_allocator]. This is no needed in UEFI so I didn’t have to do it.
So the question is, how do I set things up so that I can use core and alloc types conditionally? This would be it:

#![cfg_attr(not(feature = "std"), no_std)]

// SETUP ALLOCATOR WITH #[global_allocator]
// see: https://doc.rust-lang.org/std/alloc/trait.GlobalAlloc.html

#[cfg(not(feature = "std"))]
extern crate alloc;

#[cfg(not(feature = "std"))]
use alloc::string::String;

fn my_function () -> String {
    String::from("foobar")
}

If you are using Vec<> the same applies, you’d have to conditionally use it from alloc:: or from std:: accordingly.

Conclusions

I really thought that porting an existing crate to #[no_std] was a lot more work and a lot more constraining. In general, if you depend or could port your code to stuff that is present in both std:: and core:: + alloc:: you should be good to go. If you want to target an environment where an allocator is not possible then porting relatively common Rust code becomes a lot more complicated as you need to find a way to write your code with no allocations whatsoever.

In my original implementation I did some file io:: operations so my entire crate API was filled with -> io::Result<,>. io:: extirpation was 90% of the porting efforts as I didn’t have any external dependencies. If you have a crate that relies on a complicated dependency tree you might have a harder time porting your existing code.

If you want to have a look at my Boot Loader Spec crate for a full example it’s in my gitlab.

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