Fix debuginfo compression in bootstrap by Kobzol · Pull Request #158169 · rust-lang/rust

Kobzol · 2026-06-20T09:29:48Z

Found through https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/weird.20code.20in.20fill_target_compiler/with/604588655.

This PR solves a few issues with debuginfo compression in bootstrap.

The main issue was that debuginfo compression through the -gz flag wasn't enabled, by mistake.

When cc-rs checks if a compiler flag is supported, it tries to read out_dir to generate the source file in. However, when this option is not set, then the check silently fails and the flag is considered to be unsupported. Since we didn't set out_dir, all such added flags were simply ignored.

Normally, it reads OUT_DIR, which is fine if cc-rs is used within a build script. But if it is used elsewhere, then this is a pretty gnarly footgun in cc-rs, because the failure is completely silent. CC @madsmtm or @NobodyXu in case you have thoughts on that :)

After that was fixed, I had to change the used compression flag from -gz to --compress-debug-sections, to support both BFD and LLD linkers.

And to solve it properly, and allow dist CI jobs to configure debuginfo compression for all targets that they build, I added a new bootstrap option to configure debuginfo compression.

I wanted the flag to be configurable both globally and per target. At the same time, the flag applies both to C/C++ and Rust. We currently don't have such config area in bootstrap.toml, and [build] didn't seem like the right place, so I shoved it into [rust] (while documenting that it also applies to C/C++).

r? @bjorn3

bjorn3 · 2026-06-20T09:43:11Z

Thanks!

@bors r+

rust-bors · 2026-06-20T09:43:14Z

📌 Commit 1bc29dc has been approved by bjorn3

It is now in the queue for this repository.

@madsmtm

Fix cc rs flag if supported Found in https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/weird.20code.20in.20fill_target_compiler/with/604588655. When `cc-rs` checks if a compiler flag is supported, it tries to read `out_dir` to generate the source file in. However, when this option is not set, then the check silently fails and the flag is considered to be unsupported. Since we didn't set `out_dir`, all such added flags were simply ignored. Normally, it reads `OUT_DIR`, which is fine if `cc-rs` is used within a build script. But if it is used elsewhere, then this is a pretty gnarly footgun in `cc-rs`, because the failure is [completely silent](https://github.com/rust-lang/cc-rs/blob/main/src/lib.rs#L1483). CC @madsmtm or @NobodyXu in case you have thoughts on that :) r? @bjorn3

rust-bors · 2026-06-20T10:49:00Z

💔 Test for b946733 failed: CI. Failed job:

auto - x86_64-gnu-distcheck (web logs, enhanced plaintext logs)

Kobzol · 2026-06-20T10:55:16Z

@bors try jobs=x86_64-gnu-distcheck

Fix cc rs flag if supported try-job: x86_64-gnu-distcheck

NobodyXu · 2026-06-20T12:55:05Z

Thanks, I've opened an issue #1765, I think this is something we should fix.

We could use a tempfile instead for it

ChrisDenton · 2026-06-20T13:08:30Z

Thanks, I've opened an issue #1765

I think you meant rust-lang/cc-rs#1765 😉

rust-bors · 2026-06-20T13:18:25Z

☀️ Try build successful (CI)
Build commit: d269546 (d269546b8320f5f3255b5c7e6268def1a4808374)
Base parent: 4008bbd (4008bbdf34b583ce44a467043aae3703afc0a76f)

Kobzol · 2026-06-20T13:28:07Z

@bors r=bjorn3

Let's try again.

rust-bors · 2026-06-20T13:28:10Z

📌 Commit d59253e has been approved by bjorn3

It is now in the queue for this repository.

Kobzol · 2026-06-21T09:31:15Z

Oh, it is hardcoded 🤦 This keeps getting better.

So maybe we can detect if the linker is lld, and in that case use -compress-debug-section=zlib instead?

bjorn3 · 2026-06-21T09:37:43Z

Yeah, should work.
Edit: Fat fingered the close button.

rust-bors · 2026-06-21T09:37:46Z

This pull request was unapproved due to being closed.

rustbot · 2026-06-21T11:42:24Z

This PR modifies bootstrap.example.toml.

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

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

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

…ssion with LLD.

Kobzol · 2026-06-21T11:49:09Z

I decide to leave this in a better state than it was, and added an option to bootstrap for debuginfo compression.

@bors try @rust-timer queue

Fix debuginfo compression in bootstrap

rust-bors · 2026-06-21T14:12:03Z

☀️ Try build successful (CI)
Build commit: 9372e13 (9372e132b33ebc6bda95a4477c349388793efbd5)
Base parent: a774017 (a7740170e5e2f733b32a8206d5bb439c8e8c2fce)

rust-timer · 2026-06-21T14:52:08Z

Finished benchmarking commit (9372e13): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (primary -2.1%, secondary -2.8%)

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

	mean	range	count
Regressions ❌ (primary)	3.3%	[3.3%, 3.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.9%	[-5.9%, -1.8%]	3
Improvements ✅ (secondary)	-2.8%	[-2.8%, -2.8%]	1
All ❌✅ (primary)	-2.1%	[-5.9%, 3.3%]	4

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%	[2.7%, 2.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 482.353s -> 481.6s (-0.16%)
Artifact size: 390.76 MiB -> 386.51 MiB (-1.09%)

Kobzol · 2026-06-21T15:01:01Z

Nice artifact size wins! Looks like we weren't compressing libstd nor Cargo (I confirmed it using readelf -S --wide locally).

jieyouxu

Thanks, bootstrap changes look good to me, r=me with or without nit.

View changes since this review

jieyouxu · 2026-06-23T01:10:54Z

+                }
+            }
+            StringOrBool::String(value) => CompressDebuginfo::from_str(&value)
+                .map_err(|_| D::Error::invalid_value(Unexpected::Str(&value), &"zlib or off"))?,


Nit:

Suggested change

.map_err(|_| D::Error::invalid_value(Unexpected::Str(&value), &"zlib or off"))?,

.map_err(|_| D::Error::invalid_value(Unexpected::Str(&value), &"`zlib` or `off`"))?,

bjorn3 · 2026-06-23T08:46:10Z

  fi

+  # Compress any remaining debuginfo to make the resulting artifacts slightly smaller
+  RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.compress-debuginfo=zlib"


Maybe this should be in the dist profile?

View changes since the review

Kobzol added 2 commits June 20, 2026 11:22

Fix flag_if_supported in cc-rs by configuring an out_dir

7651c15

Slightly refactor cc code

1bc29dc

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Add missing zlib library to x86_64-gnu-distcheck

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rust-bors Bot pushed a commit that referenced this pull request Jun 20, 2026

Auto merge of #158169 - Kobzol:fix-cc-rs-flag-if-supported, r=<try>

d269546

Fix cc rs flag if supported try-job: x86_64-gnu-distcheck

NobodyXu mentioned this pull request Jun 20, 2026

Fix flag_if_supported when OUT_DIR not set rust-lang/cc-rs#1765

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bjorn3 closed this Jun 21, 2026

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Kobzol added 2 commits June 21, 2026 13:43

Add compress-debuginfo option to bootstrap and fix debuginfo compre…

b15bec6

…ssion with LLD.

Enable debuginfo compression by default in dist builds

ead43fb

Kobzol force-pushed the fix-cc-rs-flag-if-supported branch from 6dcec99 to ead43fb Compare June 21, 2026 11:47

Kobzol changed the title ~~Fix cc rs flag if supported~~ Fix debuginfo compression in bootstrap Jun 21, 2026

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rust-bors Bot pushed a commit that referenced this pull request Jun 21, 2026

Auto merge of #158169 - Kobzol:fix-cc-rs-flag-if-supported, r=<try>

9372e13

Fix debuginfo compression in bootstrap

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Kobzol requested a review from jieyouxu June 21, 2026 15:02

Fix Clippy

057adbd

jieyouxu self-assigned this Jun 22, 2026

jieyouxu approved these changes Jun 23, 2026

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	.map_err(\|_\| D::Error::invalid_value(Unexpected::Str(&value), &"zlib or off"))?,
	.map_err(\|_\| D::Error::invalid_value(Unexpected::Str(&value), &"`zlib` or `off`"))?,

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Overall result: no relevant changes - no action needed

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