redesign stage 0 std

[onur-ozkan]

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 rust-lang/compiler-team#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.

---

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 Zulip channel or create a bootstrap issue.

(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 #122709 and #137215

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*

[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[onur-ozkan]

@rustbot ready

r? bootstrap
cc @rust-lang/libs

[onur-ozkan]

@rustbot author (currently stage 2 std is copied from stage 1 and this behaviour should change with the beta std change)

[onur-ozkan]

Rebased.

[onur-ozkan]

@bors try

[bors]

⌛ Trying commit a577bcc with merge a925d4d...

[bors]

☀️ Try build successful - checks-actions
Build commit: a925d4d (a925d4d1b51c93d4e0087978a9fb129f8f7127f3)

[Kobzol]

I can confirm that the above try build commit works with rustc-perf, at least locally.

[Kobzol]

@bors r=albertlarsan68,jieyouxu,mark-simulacrum,kobzol,jyn514,Noratrieb,WaffleLapkin,RalfJung,bjorn3

@bors p=1

This PR was reviewed by a lot of people :D Let's try to approve with a small priority bump to get the ball rolling.

[bors]

📌 Commit a577bcc has been approved by albertlarsan68,jieyouxu,mark-simulacrum,kobzol,jyn514,Noratrieb,WaffleLapkin,RalfJung,bjorn3

It is now in the queue for this repository.

[bors]

⌛ Testing commit a577bcc with merge 52882f6...

[bors]

☀️ Test successful - checks-actions
Approved by: albertlarsan68,jieyouxu,mark-simulacrum,kobzol,jyn514,Noratrieb,WaffleLapkin,RalfJung,bjorn3
Pushing 52882f6 to master...

[github-actions]

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 91fad92 (parent) -> 52882f6 (this PR)

Test differences

Show 3 test diffs

Stage 0

• core::builder::tests::ci_rustc_if_unchanged_invalidate_on_library_changes_in_ci: pass -> [missing] (J0)

Additionally, 2 doctest diffs were found. These are ignored, as they are noisy.

Job group index

• J0: aarch64-apple, aarch64-gnu, i686-gnu-2, i686-gnu-nopt-2, i686-msvc-2, x86_64-apple-1, x86_64-gnu, x86_64-gnu-llvm-19-1, x86_64-gnu-llvm-19-3, x86_64-gnu-llvm-20-1, x86_64-gnu-llvm-20-3, x86_64-gnu-nopt, x86_64-gnu-stable, x86_64-mingw-2, x86_64-msvc-2

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 52882f6522ae9f34f1d574b2efabc4b18e691ae0 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

1. x86_64-apple-2: 7074.7s -> 4265.1s (-39.7%)
2. dist-x86_64-apple: 11819.4s -> 9527.8s (-19.4%)
3. aarch64-gnu-debug: 4413.6s -> 3872.1s (-12.3%)
4. x86_64-gnu-llvm-19-1: 3945.8s -> 3707.9s (-6.0%)
5. aarch64-apple: 5149.6s -> 5447.6s (5.8%)
6. x86_64-gnu-llvm-19-3: 7233.4s -> 6817.9s (-5.7%)
7. x86_64-gnu-llvm-20-3: 7297.2s -> 6895.9s (-5.5%)
8. dist-apple-various: 6596.2s -> 6918.6s (4.9%)
9. i686-gnu-nopt-2: 6796.9s -> 6487.0s (-4.6%)
10. dist-x86_64-msvc-alt: 7741.1s -> 7396.8s (-4.4%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

[rust-timer]

Finished benchmarking commit (52882f6): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (secondary -0.8%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.6%	[0.6%, 2.8%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.4%	[-9.0%, -0.4%]	12
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -0.4%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.7%	[0.5%, 0.8%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.9%	[-1.3%, -0.6%]	8
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 774.333s -> 743.462s (-3.99%)
Artifact size: 372.28 MiB -> 372.27 MiB (-0.01%)