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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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…, 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
adamgemmell
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Feb 26, 2026
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@rfcbot fcp resolve cargo-config-shorthand |
epage
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Mar 3, 2026
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| the well-tested and supported pre-built std. Later proposals will extend the | ||
| `build-std` option with customised standard library builds that use the user's | ||
| profile. |
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This seems like this should be covered under Future Possibilities. Specifically, I'd like to feel comfortable that there are options for how to reasonably transition to this
Contributor
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@rfcbot fcp concern user-profile See https://github.com/rust-lang/rfcs/pull/3874/changes#r2879155524 |
epage
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Mar 3, 2026
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| ### Why does "always" rebuild in release profile? | ||
| [rationale-release-profile]: #why-does-always-rebuild-in-release-profile | ||
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| The release profile most closely matches the existing pre-built standard | ||
| library, which has proven itself suitable for a majority of use cases. | ||
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| By minimising the differences between a newly-built std and a pre-built std, | ||
| there is less chance of the user experiencing bugs or unexpected behaviour from | ||
| the well-tested and supported pre-built std. Later proposals will extend the | ||
| `build-std` option with customised standard library builds that use the user's | ||
| profile. |
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Isn't a part of this the idea that building std should be a drop-in for using the regular std so we should build them the same way by default?
epage
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| ### Should the standard library inherit RUSTFLAGS? | ||
| [unresolved-inherit-rustflags]: #should-the-standard-library-inherit-rustflags | ||
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| Existing designs for *[Opaque dependencies]* intended that `RUSTFLAGS` would not | ||
| apply to the opaque dependency. However, if a target modifier were set using | ||
| `RUSTFLAGS` and build-std ignored the variable, then rustc would fail to build | ||
| the user's project due to incompatible target modifiers. This would necessitate | ||
| that every stable target modifier be exposed via Cargo to be usable in practice. |
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Profiles use the workspace's version with a future possibility to use the user's profiles, see #3874 (comment)
RUSTFLAGS should be independent of the local build also but we are not doing it for target modifiers.
This feels inconsistent. If an opaque dependency's profile is independent of the local build unless there is an opt-in, why aren't we doing the same for RUSTFLAGS?
Contributor
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@rfcbot fcp concern rustflags-profile-inconsistency See https://github.com/rust-lang/rfcs/pull/3874/changes#r2879178332 |
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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!
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