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redesign stage 0 std ### Summary **Blog post: https://blog.rust-lang.org/inside-rust/2025/05/29/redesigning-the-initial-bootstrap-sequence/** This PR changes how bootstrap builds the stage 1 compiler by switching to precompiled stage 0 standard library instead of building the in-tree one. The goal was to update bootstrap to use the beta standard library at stage 0 rather than compiling it from source (see the motivation at https://github.com/rust-lang/compiler-team/issues/619). Previously, to build a stage 1 compiler bootstrap followed this path: ``` download stage0 compiler -> build in-tree std -> compile stage1 compiler with in-tree std ``` With this PR, the new path is: ``` download stage0 compiler -> compile stage1 compiler with precompiled stage0 std ``` This also means that `cfg(bootstrap)`/`cfg(not(bootstrap))` is no longer needed for library development. ### Building "library" Since stage0 `std` is no longer in-tree `x build/test/check library --stage 0` is now no-op. The minimum supported stage to build `std` is now 1. For the same reason, default stage values in the library profile is no longer 0. Because building the in-tree library now requires a stage1 compiler, I highly recommend library developers to enable `download-rustc` to speed up compilation time. <hr> **Blog post: https://blog.rust-lang.org/inside-rust/2025/05/29/redesigning-the-initial-bootstrap-sequence/** If you encounter a bug or unexpected results please open a topic in the [#t-infra/bootstrap](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap) Zulip channel or create a [bootstrap issue](https://github.com/rust-lang/rust/issues/new?template=bootstrap.md). (Review thread: https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Review.20thread.3A.20stage.200.20redesign.20PR/with/508271433) ~~Blocked on https://github.com/rust-lang/rust/pull/122709~~ try-job: dist-x86_64-linux try-job: `x86_64-msvc*` try-job: `x86_64-apple-*` try-job: `aarch64-apple` try-job: x86_64-gnu try-job: `x86_64-gnu-llvm*`