build-std: always #3874
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This was referenced Oct 31, 2025
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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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Thanks everyone! I think we're at a stage where we can start a proposal for FCP. @rfcbot fcp merge cargo,compiler,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:
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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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| `build-std` is a short-hand for an object which sets the `when` key: | ||
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| ```toml | ||
| [build] | ||
| build-std = { when = "always" } | ||
| ``` |
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Something that came up with config-include is deciding whether the primary form of a table should be a standard table or inline table and that can help us decide whether short-hands are appropriate or not.
Is there a reason you are using an inline table rather than a standard table?
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Not especially, just for conciseness in the RFC text. My gut feeling is that build-std.when would be most common because it'll eventually be the only option that users need to set.
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If our end point is people will only use build-std.when then using dotted keys makes sense.
Still a bit unsure about the value add of offering a shorthand.
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…vooeo 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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…vooeo 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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…vooeo 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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Rollup merge of #150151 - destabilise-target-spec-json, r=Kivooeo Destabilise `target-spec-json` Per rust-lang/compiler-team#944: > Per #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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@rfcbot fcp concern cargo-config-shorthand See #3874 (comment) |
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Destabilise `target-spec-json` Per rust-lang/compiler-team#944: > Per rust-lang/rust#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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…, r=Kobzol Move bootstrap configuration to library workspace This creates a new "dist" profile in the standard library which contains configuration for the distributed std artifacts previously contained in bootstrap, in order for a future build-std implementation to use. bootstrap.toml settings continue to override these defaults, as would any RUSTFLAGS provided. I've left some cargo features driven by bootstrap for a future patch. Unfortunately, profiles aren't expressive enough to express per-target overrides, so [this risc-v example](https://github.com/rust-lang/rust/blob/c8f22ca269a1f2653ac962fe2bc21105065fd6cd/src/bootstrap/src/core/build_steps/compile.rs#L692) was not able to be moved across. This could go in its own profile which Cargo would have to know to use, and then the panic-abort rustflags overrides would need duplicating again. Doesn't seem like a sustainable solution as a couple similar overrides would explode the number of lines here. We could use a cargo config in the library workspace for this, but this then would have to be respected by Cargo's build-std implementation and I'm not yet sure about the tradeoffs there. This patch also introduces a build probe to deal with the test crate's stability which is obviously not ideal, I'm open to other solutions here or can back that change out for now if anyone prefers. cc @Mark-Simulacrum rust-lang/rfcs#3874
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…, r=Kobzol Move bootstrap configuration to library workspace This creates a new "dist" profile in the standard library which contains configuration for the distributed std artifacts previously contained in bootstrap, in order for a future build-std implementation to use. bootstrap.toml settings continue to override these defaults, as would any RUSTFLAGS provided. I've left some cargo features driven by bootstrap for a future patch. Unfortunately, profiles aren't expressive enough to express per-target overrides, so [this risc-v example](https://github.com/rust-lang/rust/blob/c8f22ca269a1f2653ac962fe2bc21105065fd6cd/src/bootstrap/src/core/build_steps/compile.rs#L692) was not able to be moved across. This could go in its own profile which Cargo would have to know to use, and then the panic-abort rustflags overrides would need duplicating again. Doesn't seem like a sustainable solution as a couple similar overrides would explode the number of lines here. We could use a cargo config in the library workspace for this, but this then would have to be respected by Cargo's build-std implementation and I'm not yet sure about the tradeoffs there. This patch also introduces a build probe to deal with the test crate's stability which is obviously not ideal, I'm open to other solutions here or can back that change out for now if anyone prefers. cc @Mark-Simulacrum rust-lang/rfcs#3874
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Add a new Cargo configuration option,
build-std = "always|never", which will unconditionally rebuild standard library dependencies. The set of standard library dependencies can optionally be customised with a newbuild-std-cratesoption. It also describes how Cargo (or external tools) should build the standard library crates on stable (i.e., which flags to pass and features to enable).This proposal limits the ways the built standard library can be customised (such as by settings in the profile) and intends that the build standard library matches the prebuilt one (if available) as closely as possible.
This RFC is is part of the build-std project goal and a series of build-std RFCs:
build-std="always"(this RFC)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!
FCP
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