Auto merge of #119899 - onur-ozkan:redesign-stage0-std, r=albertlarsan68,jieyouxu,mark-simulacrum,kobzol,jyn514,Noratrieb,WaffleLapkin,RalfJung,bjorn3

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*`

9 files changed, 48 insertions, 19 deletions

diff --git a/src/ci/docker/host-x86_64/mingw-check/Dockerfile b/src/ci/docker/host-x86_64/mingw-check-1/Dockerfile
index df73c7382b5..a877de1f7b2 100644
--- a/src/ci/docker/host-x86_64/mingw-check/Dockerfile
+++ b/src/ci/docker/host-x86_64/mingw-check-1/Dockerfile
@@ -34,31 +34,23 @@ RUN npm install es-check@6.1.1 eslint@8.6.0 typescript@5.7.3 -g
 COPY scripts/sccache.sh /scripts/
 RUN sh /scripts/sccache.sh
 
-COPY host-x86_64/mingw-check/reuse-requirements.txt /tmp/
+COPY host-x86_64/mingw-check-1/reuse-requirements.txt /tmp/
 RUN pip3 install --no-deps --no-cache-dir --require-hashes -r /tmp/reuse-requirements.txt
 
-COPY host-x86_64/mingw-check/check-default-config-profiles.sh /scripts/
-COPY host-x86_64/mingw-check/validate-toolstate.sh /scripts/
-COPY host-x86_64/mingw-check/validate-error-codes.sh /scripts/
+COPY host-x86_64/mingw-check-1/check-default-config-profiles.sh /scripts/
+COPY host-x86_64/mingw-check-1/validate-toolstate.sh /scripts/
+COPY host-x86_64/mingw-check-1/validate-error-codes.sh /scripts/
 
 # Check library crates on all tier 1 targets.
 # We disable optimized compiler built-ins because that requires a C toolchain for the target.
 # We also skip the x86_64-unknown-linux-gnu target as it is well-tested by other jobs.
 ENV SCRIPT \
-           python3 ../x.py check --stage 0 --set build.optimized-compiler-builtins=false core alloc std --target=aarch64-unknown-linux-gnu,i686-pc-windows-msvc,i686-unknown-linux-gnu,x86_64-apple-darwin,x86_64-pc-windows-gnu,x86_64-pc-windows-msvc && \
            /scripts/check-default-config-profiles.sh && \
-           python3 ../x.py check compiletest --set build.compiletest-use-stage0-libtest=true && \
-           python3 ../x.py check --target=x86_64-pc-windows-gnu --host=x86_64-pc-windows-gnu && \
-           python3 ../x.py clippy ci && \
            python3 ../x.py build --stage 0 src/tools/build-manifest && \
            python3 ../x.py test --stage 0 src/tools/compiletest && \
-           python3 ../x.py test --stage 0 core alloc std test proc_macro && \
-           # Build both public and internal documentation.
-           RUSTDOCFLAGS=\"--document-private-items --document-hidden-items\" python3 ../x.py doc --stage 0 library && \
-           mkdir -p /checkout/obj/staging/doc && \
-           cp -r build/x86_64-unknown-linux-gnu/doc /checkout/obj/staging && \
-           RUSTDOCFLAGS=\"--document-private-items --document-hidden-items\" python3 ../x.py doc --stage 0 compiler && \
-           RUSTDOCFLAGS=\"--document-private-items --document-hidden-items\" python3 ../x.py doc --stage 0 library/test && \
+           python3 ../x.py check compiletest --set build.compiletest-use-stage0-libtest=true && \
+           python3 ../x.py check --stage 1 --target=i686-pc-windows-gnu --host=i686-pc-windows-gnu && \
+           python3 ../x.py check --stage 1 --set build.optimized-compiler-builtins=false core alloc std --target=aarch64-unknown-linux-gnu,i686-pc-windows-msvc,i686-unknown-linux-gnu,x86_64-apple-darwin,x86_64-pc-windows-gnu,x86_64-pc-windows-msvc && \
            /scripts/validate-toolstate.sh && \
            /scripts/validate-error-codes.sh && \
            reuse --include-submodules lint && \
diff --git a/src/ci/docker/host-x86_64/mingw-check/check-default-config-profiles.sh b/src/ci/docker/host-x86_64/mingw-check-1/check-default-config-profiles.sh
index 0c85d4b449d..0c85d4b449d 100755
--- a/src/ci/docker/host-x86_64/mingw-check/check-default-config-profiles.sh
+++ b/src/ci/docker/host-x86_64/mingw-check-1/check-default-config-profiles.sh
diff --git a/src/ci/docker/host-x86_64/mingw-check/reuse-requirements.in b/src/ci/docker/host-x86_64/mingw-check-1/reuse-requirements.in
index d7c2d3fde5b..d7c2d3fde5b 100644
--- a/src/ci/docker/host-x86_64/mingw-check/reuse-requirements.in
+++ b/src/ci/docker/host-x86_64/mingw-check-1/reuse-requirements.in
diff --git a/src/ci/docker/host-x86_64/mingw-check/reuse-requirements.txt b/src/ci/docker/host-x86_64/mingw-check-1/reuse-requirements.txt
index 8784e18864b..8784e18864b 100644
--- a/src/ci/docker/host-x86_64/mingw-check/reuse-requirements.txt
+++ b/src/ci/docker/host-x86_64/mingw-check-1/reuse-requirements.txt
diff --git a/src/ci/docker/host-x86_64/mingw-check/validate-error-codes.sh b/src/ci/docker/host-x86_64/mingw-check-1/validate-error-codes.sh
index e9aa948eb87..e9aa948eb87 100755
--- a/src/ci/docker/host-x86_64/mingw-check/validate-error-codes.sh
+++ b/src/ci/docker/host-x86_64/mingw-check-1/validate-error-codes.sh
diff --git a/src/ci/docker/host-x86_64/mingw-check/validate-toolstate.sh b/src/ci/docker/host-x86_64/mingw-check-1/validate-toolstate.sh
index a5691da8cda..a5691da8cda 100755
--- a/src/ci/docker/host-x86_64/mingw-check/validate-toolstate.sh
+++ b/src/ci/docker/host-x86_64/mingw-check-1/validate-toolstate.sh
diff --git a/src/ci/docker/host-x86_64/mingw-check-2/Dockerfile b/src/ci/docker/host-x86_64/mingw-check-2/Dockerfile
new file mode 100644
index 00000000000..0c75f116aa0
--- /dev/null
+++ b/src/ci/docker/host-x86_64/mingw-check-2/Dockerfile
@@ -0,0 +1,37 @@
+FROM ubuntu:22.04
+
+ARG DEBIAN_FRONTEND=noninteractive
+RUN apt-get update && apt-get install -y --no-install-recommends \
+  g++ \
+  make \
+  ninja-build \
+  file \
+  curl \
+  ca-certificates \
+  python3 \
+  python3-pip \
+  python3-pkg-resources \
+  git \
+  cmake \
+  sudo \
+  gdb \
+  xz-utils \
+  libssl-dev \
+  pkg-config \
+  mingw-w64 \
+  && rm -rf /var/lib/apt/lists/*
+
+ENV RUST_CONFIGURE_ARGS="--set rust.validate-mir-opts=3"
+
+COPY scripts/sccache.sh /scripts/
+RUN sh /scripts/sccache.sh
+
+ENV SCRIPT \
+        python3 ../x.py clippy ci && \
+        python3 ../x.py test --stage 1 core alloc std test proc_macro && \
+        # Build both public and internal documentation.
+        RUSTDOCFLAGS=\"--document-private-items --document-hidden-items\" python3 ../x.py doc --stage 0 compiler && \
+        RUSTDOCFLAGS=\"--document-private-items --document-hidden-items\" python3 ../x.py doc --stage 1 library && \
+        mkdir -p /checkout/obj/staging/doc && \
+        cp -r build/x86_64-unknown-linux-gnu/doc /checkout/obj/staging && \
+        RUSTDOCFLAGS=\"--document-private-items --document-hidden-items\" python3 ../x.py doc --stage 1 library/test
diff --git a/src/ci/docker/host-x86_64/mingw-check-tidy/Dockerfile b/src/ci/docker/host-x86_64/mingw-check-tidy/Dockerfile
index d17f7ed7171..006a697af21 100644
--- a/src/ci/docker/host-x86_64/mingw-check-tidy/Dockerfile
+++ b/src/ci/docker/host-x86_64/mingw-check-tidy/Dockerfile
@@ -34,12 +34,12 @@ COPY host-x86_64/mingw-check-tidy/eslint.version /tmp/
 COPY scripts/sccache.sh /scripts/
 RUN sh /scripts/sccache.sh
 
-COPY host-x86_64/mingw-check/reuse-requirements.txt /tmp/
+COPY host-x86_64/mingw-check-1/reuse-requirements.txt /tmp/
 RUN pip3 install --no-deps --no-cache-dir --require-hashes -r /tmp/reuse-requirements.txt \
     && pip3 install virtualenv
 
-COPY host-x86_64/mingw-check/validate-toolstate.sh /scripts/
-COPY host-x86_64/mingw-check/validate-error-codes.sh /scripts/
+COPY host-x86_64/mingw-check-1/validate-toolstate.sh /scripts/
+COPY host-x86_64/mingw-check-1/validate-error-codes.sh /scripts/
 
 # NOTE: intentionally uses python2 for x.py so we can test it still works.
 # validate-toolstate only runs in our CI, so it's ok for it to only support python3.
diff --git a/src/ci/docker/host-x86_64/x86_64-gnu-nopt/Dockerfile b/src/ci/docker/host-x86_64/x86_64-gnu-nopt/Dockerfile
index d8113e06723..1b57ae7c8da 100644
--- a/src/ci/docker/host-x86_64/x86_64-gnu-nopt/Dockerfile
+++ b/src/ci/docker/host-x86_64/x86_64-gnu-nopt/Dockerfile
@@ -29,5 +29,5 @@ RUN echo "optimize = false" >> /config/nopt-std-config.toml
 ENV RUST_CONFIGURE_ARGS --build=x86_64-unknown-linux-gnu \
   --disable-optimize-tests \
   --set rust.test-compare-mode
-ENV SCRIPT python3 ../x.py test --stage 0 --config /config/nopt-std-config.toml library/std \
+ENV SCRIPT python3 ../x.py test --stage 1 --config /config/nopt-std-config.toml library/std \
   && python3 ../x.py --stage 2 test