Build rustc and codegen backends with -Zembed-metadata=no

[bjorn3]

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While this won't change much for rustc and rustc-dev, it should make codegen backends a bit smaller by omitting their crate metadata. In addition it should reduce disk usage while compiling.

Follow up to #145343

[rustbot]

r? @jieyouxu

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[bjorn3]

cc @Kobzol

@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: d9668cf (d9668cf85dd8b3cac5d57f7b5b8b1508112db899, parent: b2a322beb29110e22a1782e2ce5ed2a0719b81ed)

[bjorn3]

And the baseline to compare the disk space against for try builds:

@bors try

[Kobzol]

r? @Kobzol

[rust-bors]

☀️ Try build successful (CI)
Build commit: db6fee5 (db6fee556e8bf32d307f378d2acbb52ba25b5a8b, parent: db1484bdee1cb065b322a5bd512b39c49dadc9e5)

[bjorn3]

For the try build it saves 2GB or 4% of the total space the build takes (46GB -> 44GB). Hoped it would be a bit more, but still not bad I think. And maybe it gives a higher relative saving when not collecting PGO profiles?

@rustbot ready

[Kobzol]

Let's do a full try build to see the codegen backend savings (we can compare them with the parent commit, should be enough), and also run perf., just in case.

@bors try jobs=dist-x86_64-linux @rust-timer queue

[bjorn3]

For cg_clif the .rmeta section of the codegen backend dylib when built from the cg_clif repo is almost 1MB out of the 15MB the full dylib uses. The savings from this PR should be near identical, though the full size of the dylib may be different due to different codegen options.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 2cbe7f7 (2cbe7f7eda161d598e47752d03fc34a1539dad7c, parent: 9b81629631b382fd49ee3a20ac47797b1467e52d)

[rust-timer]

Finished benchmarking commit (2cbe7f7): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (secondary -1.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.6%	[2.5%, 2.7%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.1%	[-6.1%, -2.2%]	3
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 2.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.7%	[2.1%, 3.1%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 476.337s -> 475.884s (-0.10%)
Artifact size: 383.08 MiB -> 383.14 MiB (0.02%)

[Kobzol]

Before:

2026-01-13T17:34:05.4564183Z -rw-r--r-- 1 root root  2.5M Jan 13 16:45 rustc-codegen-cranelift-nightly-x86_64-unknown-linux-gnu.tar.xz
2026-01-13T17:34:05.4565056Z -rw-r--r-- 1 root root  1.1M Jan 13 17:19 rustc-codegen-gcc-nightly-x86_64-unknown-linux-gnu.tar.xz

After:

2026-01-13T22:55:02.7071176Z -rw-r--r-- 1 root root  2.3M Jan 13 22:07 rustc-codegen-cranelift-nightly-x86_64-unknown-linux-gnu.tar.xz
2026-01-13T22:55:02.7072077Z -rw-r--r-- 1 root root  827K Jan 13 22:40 rustc-codegen-gcc-nightly-x86_64-unknown-linux-gnu.tar.xz

And no other components seem to have changed their size. Yeah, pretty nice, a solid ~10%!

[Kobzol]

Could you please rebase this onto #161102, and apply it to everything in Builder::cargo?

[Kobzol]

You can r=me if CI passes.

[Kobzol]

Hmm, there are some steps that run with in-tree Cargo even without self.local_rebuild. I guess that it will be better to wait for the beta bump, which should happen soon, and then use -Zembed-metadata=no unconditionally.

[bjorn3]

@rustbot blocked

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[bjorn3]

@rustbot ready

[Kobzol]

@bors try jobs=x86_64-gnu,x86_64-msvc*,aarch64-apple*

[rust-bors]

☀️ Try build successful (CI)
Build commit: d2c6d50 (d2c6d50e3778d6d894ccb1d84a50782edfb1039e)
Base parent: 16a623a (16a623ad672a92409b5c04beb303583c6cf72a7e)

[Kobzol]

Ok, looks good! Let's try.

@bors r+ rollup=iffy

[rust-bors]

📌 Commit bbc9e66 has been approved by Kobzol

It is now in the queue for this repository.