Unstable features
Introduction
The Rust language is stable, i.e. it promises backwards compatibility within the same edition, with a few exceptions, such as reserving the right to patch safety holes. The kernel currently uses Edition 2021, which is the latest.
On top of that, the kernel uses some Rust unstable features. These features can only be accessed by opting into them. They are typically used as a way to introduce new features into the language, library and toolchain to allow end users to experiment with them and provide feedback before committing to them.
"Unstable" in this context means the feature may change in future versions, i.e. backwards compatibility is not promised for those features. It does not imply that the features are broken. For instance, unstable features may be production-ready and ready for stabilization or they may be experimental, incomplete or internal to the compiler.
When unstable features are deemed mature enough, they may get promoted into stable Rust. In other cases, they may get dropped altogether. Some features are internal to the compiler or perma-unstable.
There are ongoing discussions around stability within the Rust project, such as potentially defining extra phases. These finer-grained levels could be useful for the kernel.
Usage in the kernel
The unstable features used in the kernel are tracked at issue #2.
Removing the need for these is a priority in order to be able to eventually declare a minimum Rust version for the kernel.
Therefore, the set of unstable features used in the kernel needs to be carefully considered. Typically, for each of them, we need to consider:
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Whether there is no other way around the issue they help with, or whether the alternative is considered to have bigger downsides than using the unstable feature.
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Whether they would be required to build the kernel.
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Whether stabilization is likely, whether they are internal to the compiler and whether they are used in the standard library.
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Whether other features that are on the critical path will likely take longer to get stabilized anyway.
Moreover, most of the features are only allowed within the kernel crate, i.e. for abstractions. Elsewhere (e.g. drivers), only a minimal set is allowed (see the rust_allowed_features variable in scripts/Makefile.build).
If you would like to use a new Rust unstable feature in the kernel, then please contact us.
alloc
alloc is part of the Rust standard library and its implementation uses many unstable features. Normally, this library (as well as core and others) is provided by the compiler, and thus those unstable features do not break users' code.
Currently, the kernel contains a fork of alloc (matched to the supported Rust version by the kernel) with some additions on top. This complicates compiling the kernel with a different compiler version due to those unstable features, but this fork is meant to be temporary: the original plan for alloc discussed with upstream Rust (and others) is documented in-tree in the rust/alloc/README.md file.
Discussions with upstream Rust are ongoing to see what the eventual solution for alloc will be.