Build rustc and codegen backends with -Zno-embed-metadata #151061

bjorn3 · 2026-01-13T13:49:23Z

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

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.

rustbot · 2026-01-13T13:49:28Z

r? @jieyouxu

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bjorn3 · 2026-01-13T13:49:36Z

cc @Kobzol

@bors try

Build rustc and codegen backends with -Zno-embed-metadata

bjorn3 · 2026-01-13T13:50:31Z

src/bootstrap/src/core/builder/cargo.rs

-        if matches!(mode, Mode::Std) {
+        if matches!(mode, Mode::Std | Mode::Rustc | Mode::Codegen) {
            cargo.arg("-Zno-embed-metadata");
        }


Maybe we could move this into run_cargo as that is the only function which supports -Zno-embed-metadata currently?

Yeah, it would be nice, we could then do some asserts, e.g. that you are not using OnlyRlib (although it's tricky - OnlyDylib is in theory also not valid for -Zno-embed-metadata, but it works if the thing you build doesn't need to be linked to through normal Rust means).

OnlyDylib is only used for codegen backends, which don't need the crate metadata. I've moved the -Zno-embed-metadata to run_cargo.

rust-bors · 2026-01-13T16:05:54Z

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

bjorn3 · 2026-01-13T16:22:21Z

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

@bors try

Build rustc and codegen backends with -Zno-embed-metadata

Kobzol · 2026-01-13T16:37:34Z

r? @Kobzol

rust-bors · 2026-01-13T18:37:30Z

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

bjorn3 · 2026-01-13T19:34:27Z

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 · 2026-01-13T19:41:07Z

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

Build rustc and codegen backends with -Zno-embed-metadata try-job: dist-x86_64-linux

bjorn3 · 2026-01-13T19:47:04Z

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 · 2026-01-13T22:55:41Z

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

rust-timer · 2026-01-13T23:36:26Z

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 · 2026-01-14T07:07:10Z

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%!

src/bootstrap/src/core/build_steps/compile.rs

Kobzol · 2026-01-14T07:09:15Z

src/bootstrap/src/core/builder/cargo.rs

-        if matches!(mode, Mode::Std) {
+        if matches!(mode, Mode::Std | Mode::Rustc | Mode::Codegen) {
            cargo.arg("-Zno-embed-metadata");
        }


Yeah, it would be nice, we could then do some asserts, e.g. that you are not using OnlyRlib (although it's tricky - OnlyDylib is in theory also not valid for -Zno-embed-metadata, but it works if the thing you build doesn't need to be linked to through normal Rust means).

rust-log-analyzer · 2026-01-14T10:55:00Z

The job pr-check-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   |
69 | extern crate proc_macro2;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^

error: only metadata stub found for `rlib` dependency `quote`
       please provide path to the corresponding .rmeta file with full metadata
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_derive-1.0.228/src/lib.rs:70:1
   |
70 | extern crate quote;
   | ^^^^^^^^^^^^^^^^^^^

error: only metadata stub found for `rlib` dependency `syn`
       please provide path to the corresponding .rmeta file with full metadata
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_derive-1.0.228/src/lib.rs:71:1
   |
71 | extern crate syn;
   | ^^^^^^^^^^^^^^^^^

error[E0432]: unresolved imports `quote::quote_spanned`, `quote::ToTokens`, `quote::quote_spanned`, `quote::quote_spanned`, `quote::quote_spanned`, `quote::ToTokens`, `quote::quote_spanned`, `quote::quote_spanned`, `quote::format_ident`, `quote::quote_spanned`
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_derive-1.0.228/src/de.rs:8:20
   |
 8 | use quote::{quote, quote_spanned, ToTokens};
   |                    ^^^^^^^^^^^^^  ^^^^^^^^
   |
  ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_derive-1.0.228/src/de/enum_adjacently.rs:15:20
   |
15 | use quote::{quote, quote_spanned};
   |                    ^^^^^^^^^^^^^
   |
  ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_derive-1.0.228/src/de/enum_externally.rs:19:20
   |
19 | use quote::{quote, quote_spanned};
   |                    ^^^^^^^^^^^^^
   |
  ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_derive-1.0.228/src/de/enum_untagged.rs:18:20
   |
18 | use quote::{quote, quote_spanned};
   |                    ^^^^^^^^^^^^^
   |
  ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_derive-1.0.228/src/de/identifier.rs:10:20
   |
10 | use quote::{quote, ToTokens};
   |                    ^^^^^^^^
   |
  ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_derive-1.0.228/src/de/struct_.rs:13:20
   |
13 | use quote::{quote, quote_spanned};
   |                    ^^^^^^^^^^^^^
   |
  ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_derive-1.0.228/src/de/tuple.rs:9:20
   |
 9 | use quote::{quote, quote_spanned};
   |                    ^^^^^^^^^^^^^
   |
  ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_derive-1.0.228/src/pretend.rs:4:13
   |
 4 | use quote::{format_ident, quote};
   |             ^^^^^^^^^^^^
   |
  ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_derive-1.0.228/src/ser.rs:8:20
   |
 8 | use quote::{quote, quote_spanned};
   |                    ^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0432`.
[RUSTC-TIMING] serde_derive test:false 0.125
error: could not compile `serde_derive` (lib) due to 4 previous errors

bjorn3 · 2026-01-14T11:09:50Z

I can't reproduce the CI failure locally. Could it be related to rust-lang/cargo#16503?

Kobzol · 2026-01-14T16:52:29Z

Possibly, although my first guess is that we are not copying rustc .rmeta files somewhere in bootstrap.

Build rustc and codegen backends with -Zno-embed-metadata

b6605c3

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.

rustbot assigned jieyouxu Jan 13, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) labels Jan 13, 2026

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Auto merge of #151061 - bjorn3:rustc_no_embed_metadata, r=<try>

d9668cf

Build rustc and codegen backends with -Zno-embed-metadata

bjorn3 commented Jan 13, 2026

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rust-bors bot pushed a commit that referenced this pull request Jan 13, 2026

Auto merge of #151061 - bjorn3:rustc_no_embed_metadata, r=<try>

db6fee5

Build rustc and codegen backends with -Zno-embed-metadata

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rustbot assigned Kobzol and unassigned jieyouxu Jan 13, 2026

bjorn3 force-pushed the rustc_no_embed_metadata branch from 6e25c8e to b6605c3 Compare January 13, 2026 19:29

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 13, 2026

rust-bors bot pushed a commit that referenced this pull request Jan 13, 2026

Auto merge of #151061 - bjorn3:rustc_no_embed_metadata, r=<try>

2cbe7f7

Build rustc and codegen backends with -Zno-embed-metadata try-job: dist-x86_64-linux

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 13, 2026

Kobzol reviewed Jan 14, 2026

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Pass -Zno-embed-metadata in run_cargo

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Build rustc and codegen backends with -Zno-embed-metadata #151061

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Build rustc and codegen backends with -Zno-embed-metadata #151061

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rust-timer commented Jan 13, 2026

Overall result: no relevant changes - no action needed

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