build-std: context #3873
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build-std: context #3873
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This was referenced Oct 31, 2025
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This was referenced Oct 31, 2025
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I'm going to propose to go ahead and merge this RFC. Although it doesn't really have a distinct set of changes being defined, it does lay out the scope and intention of the direction we are planning to go. I'd like to direct reviewers attention to the Motivation section as a key part of this RFC. It essentially outlines the key objectives and scope of this project. The following RFCs are explaining exactly how that will be achieved. The idea here is for us to agree that these are worthy goals and that we understand what's in and out of scope (in order to make this project tractable). Getting this merged also helps with the subsequent RFCs which are built on top of this. @rfcbot fcp merge compiler,cargo,crates-io,libs-api |
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Team member @ehuss has proposed to merge this. The next step is review by the rest of the tagged team members:
No concerns currently listed. Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up! See this document for info about what commands tagged team members can give me. |
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As part of rust-lang#3883, we have switched the template to stop using level-1 headings which isn't the way mdbook is intended to be used.
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🔔 This is now entering its final comment period, as per the review above. 🔔 |
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…json, r=Kivooeo Destabilise `target-spec-json` Per rust-lang/compiler-team#944: > Per rust-lang#71009, the ability to load target spec JSONs was stabilised accidentally. Within the team, we've always considered the format to be unstable and have changed it freely. This has been feasible as custom targets can only be used with core, like any other target, and so custom targets de-facto require nightly to be used (i.e. to build core manually or use Cargo's -Zbuild-std). > > Current build-std RFCs (rust-lang/rfcs#3873, rust-lang/rfcs#3874) propose a mechanism for building core on stable (at the request of Rust for Linux), which combined with a stable target-spec-json format, permit the current format to be used much more widely on stable toolchains. This would prevent us from improving the format - making it less tied to LLVM, switching to TOML, enabling keys in the spec to be stabilised individually, etc. > > De-stabilising the format gives us the opportunity to improve the format before it is too challenging to do so. Internal company toolchains and projects like Rust for Linux already use target-spec-json, but must use nightly at some point while doing so, so while it could be inconvenient for those users to destabilise this, it is hoped that an minimal alternative that we could choose to stabilise can be proposed relatively quickly.
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…json, r=Kivooeo Destabilise `target-spec-json` Per rust-lang/compiler-team#944: > Per rust-lang#71009, the ability to load target spec JSONs was stabilised accidentally. Within the team, we've always considered the format to be unstable and have changed it freely. This has been feasible as custom targets can only be used with core, like any other target, and so custom targets de-facto require nightly to be used (i.e. to build core manually or use Cargo's -Zbuild-std). > > Current build-std RFCs (rust-lang/rfcs#3873, rust-lang/rfcs#3874) propose a mechanism for building core on stable (at the request of Rust for Linux), which combined with a stable target-spec-json format, permit the current format to be used much more widely on stable toolchains. This would prevent us from improving the format - making it less tied to LLVM, switching to TOML, enabling keys in the spec to be stabilised individually, etc. > > De-stabilising the format gives us the opportunity to improve the format before it is too challenging to do so. Internal company toolchains and projects like Rust for Linux already use target-spec-json, but must use nightly at some point while doing so, so while it could be inconvenient for those users to destabilise this, it is hoped that an minimal alternative that we could choose to stabilise can be proposed relatively quickly.
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While Rust's pre-built standard library has proven itself sufficient for the majority of use cases, there are a handful of use cases that are not well supported:
Proposals to solve these problems come broadly under the umbrella of "build-std" and date back over 10 years ago, though no complete solution has yet reached consensus.
This RFC does not propose any changes directly, only document the background, history and motivations for build-std. It is part of a series of build-std RFCs and later RFCs will reference this one. This RFC is part of the build-std project goal.
build-std="always"(build-std: always #3874)build-std="compatible"(RFC not opened yet)build-std="match-profile"(RFC not opened yet)There is also a Zulip channel where you can ask questions about any of the build-std RFCs. This series of RFCs was drafted over many months with the help of stakeholders from many Rust project teams, we thank them for their help!
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