CRATER: Analyze effect of `format_args!` deduplication by traviscross · Pull Request #154205 · rust-lang/rust

traviscross · 2026-03-22T08:55:25Z

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This is an instrumented rustc for doing a crater run to analyze:

cc @dianne @nikomatsakis

Co-authored-by: Shoyu Vanilla (Flint) <modulo641@gmail.com>

traviscross · 2026-03-22T08:55:47Z

@bors try

…cation, r=<try> CRATER: Analyze effect `format_args!` deduplication

rust-bors · 2026-03-22T11:07:02Z

☀️ Try build successful (CI)
Build commit: 34547a6 (34547a62f24f000c096164e9e68c12381940788d, parent: 562dee4820c458d823175268e41601d4c060588a)

When crater tests a crate, `cargo build` compiles all of its dependencies too, and each dependency's `format_args!` invocations were producing `[FMTARGS]` lines in the same log. This inflates the totals -- a popular crate like `serde` gets counted once per test subject that depends on it -- and makes per-crate attribution meaningless. Cargo already distinguishes root crates from dependencies: it passes `--cap-lints allow` to rustc for dependencies, which is how dependency warnings are suppressed. Let's use the same signal. We check `self.tcx.sess.opts.lint_cap` and skip the instrumentation output when it's `Some(Allow)`. We also add a `"crate"` field to the JSON output with the crate name (via `tcx.crate_name(LOCAL_CRATE)`) for sanity checking. With this, each crater log contains `[FMTARGS]` lines only from the crate being tested, which gives correct ecosystem-wide counts without post-hoc deduplication.

traviscross · 2026-03-23T08:38:13Z

@bors try

…cation, r=<try> CRATER: Analyze effect of `format_args!` deduplication

rust-bors · 2026-03-23T10:50:10Z

☀️ Try build successful (CI)
Build commit: 1463461 (1463461488056994d1b84e8b752e7c11c7fae7aa, parent: bbe853615821442ef11d6cd42a30a73432b38d89)

traviscross · 2026-03-23T11:28:12Z

@craterbot run mode=check-only crates=top-10000 p=1

craterbot · 2026-03-23T11:28:17Z

👌 Experiment pr-154205 created and queued.
🤖 Automatically detected try build 1463461
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-03-24T07:02:57Z

🚧 Experiment pr-154205 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-03-24T14:27:28Z

🎉 Experiment pr-154205 is completed!
📊 0 regressed and 0 fixed (10000 total)
📊 47 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-154205/retry-regressed-list.txt ↩

The structural deduplication we used in the first crater run can't distinguish different format strings that happen to have identical property profiles. Without a source location identifier, two `println!("{x} is valid")` and `println!("{y} is ready")` in the same crate produce identical JSON and collapse into one "unique" invocation. Let's add a `"loc"` field containing `file:line:col` derived from `fmt.span.lo()`. With this, the analysis script can deduplicate on `(crate, loc)` -- correctly collapsing multi-target compilations of the same source-level `format_args!` while keeping structurally similar but distinct invocations separate.

traviscross · 2026-03-25T03:50:06Z

@bors try

…cation, r=<try> CRATER: Analyze effect of `format_args!` deduplication

rust-bors · 2026-03-25T06:00:01Z

☀️ Try build successful (CI)
Build commit: 38c2997 (38c2997544af84fe301d600d6896397738163002, parent: 8a703520e80d87d4423c01f9d4fbc9e5f6533a02)

traviscross · 2026-03-25T06:00:58Z

@craterbot run mode=check-only crates=top-10000 p=1

craterbot · 2026-03-25T06:01:04Z

👌 Experiment pr-154205-1 created and queued.
🤖 Automatically detected try build 38c2997
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-03-26T06:33:55Z

🚧 Experiment pr-154205-1 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-03-26T14:02:04Z

🎉 Experiment pr-154205-1 is completed!
📊 1 regressed and 0 fixed (10000 total)
📊 40 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-154205-1/retry-regressed-list.txt ↩

traviscross · 2026-03-27T04:16:03Z

Thanks craterbot.

traviscross · 2026-03-27T09:10:23Z

Apparently top-{n} is broken. See rust-lang/crater#813.

@craterbot check

craterbot · 2026-03-27T09:10:44Z

👌 Experiment pr-154205-2 created and queued.
🤖 Automatically detected try build 38c2997
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

dianne and others added 3 commits March 22, 2026 06:48

add tests for duplicated format args

fbbcac8

Co-authored-by: Shoyu Vanilla (Flint) <modulo641@gmail.com>

format_args!: only de-duplicate captured places

388b002

Add crater instrumentation for format_args dedup analysis

706867b

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Mar 22, 2026

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rust-bors bot pushed a commit that referenced this pull request Mar 22, 2026

Auto merge of #154205 - traviscross:TC/crater-for-format_args-dedupli…

34547a6

…cation, r=<try> CRATER: Analyze effect `format_args!` deduplication

traviscross changed the title ~~CRATER: Analyze effect format_args! deduplication~~ CRATER: Analyze effect of format_args! deduplication Mar 22, 2026

traviscross mentioned this pull request Mar 22, 2026

Do not deduplicate captured args while expanding format_args! #149926

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rust-bors bot pushed a commit that referenced this pull request Mar 23, 2026

Auto merge of #154205 - traviscross:TC/crater-for-format_args-dedupli…

1463461

…cation, r=<try> CRATER: Analyze effect of `format_args!` deduplication

craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Mar 23, 2026

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craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Mar 24, 2026

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rust-bors bot pushed a commit that referenced this pull request Mar 25, 2026

Auto merge of #154205 - traviscross:TC/crater-for-format_args-dedupli…

38c2997

…cation, r=<try> CRATER: Analyze effect of `format_args!` deduplication

craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 25, 2026

craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Mar 26, 2026

traviscross closed this Mar 27, 2026

rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Mar 27, 2026

traviscross reopened this Mar 27, 2026

rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Mar 27, 2026

craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Mar 27, 2026

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traviscross commented Mar 22, 2026 •

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