Allow storing `format_args!()` in variable #140748

m-ou-se · 2025-05-07T16:01:52Z

Tracking issue for super let: #139076

Tracking issue for format_args: #99012

This change allows:

let name = "world";
let f = format_args!("hello {name}!"); // New: Store format_args!() for later!

println!("{f}");

This will need an FCP.

This implementation makes use of super let, which is unstable and might not exist in the future in its current form. However, it is entirely reasonable to assume future Rust will always have a way of expressing temporary lifetimes like this, since the (stable) pin!() macro needs this too. (This was also the motivation for merging #139114.)

(This is a second version of #139135)

m-ou-se · 2025-05-07T16:14:39Z

@bors try @rust-timer queue

bors · 2025-05-07T16:15:56Z

⌛ Trying commit d261de6 with merge beb8d4e...

Allow storing format_args!() in variable Fixes rust-lang#92698 Tracking issue for super let: rust-lang#139076 This change allows: ```rust let name = "world"; let f = format_args!("hello {name}!"); println!("{f}"); ``` This will need an FCP. This implementation makes use of `super let`, which is unstable and might not exist in the future in its current form. However, it is entirely reasonable to assume future Rust will always have _a_ way of expressing temporary lifetimes like this, since the (stable) `pin!()` macro needs this. (This was also the motivation for merging rust-lang#139114.) > [!NOTE] > This PR causes many subtle changes in diagnostics output. Most of those are good. Some of those are bad. I've collected all the bad ones in the last commit. Those still need fixing. Marking this PR as draft. (This is a second version of rust-lang#139135)

bors · 2025-05-07T18:25:44Z

☀️ Try build successful - checks-actions
Build commit: beb8d4e (beb8d4ea463c3ae10548aabc62ac2cff505be075)

rust-timer · 2025-05-07T20:03:49Z

Finished benchmarking commit (beb8d4e): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.2%, 1.1%]	24
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.4%	[-4.5%, -0.2%]	2
Improvements ✅ (secondary)	-1.8%	[-1.9%, -1.7%]	6
All ❌✅ (primary)	0.2%	[-4.5%, 1.1%]	26

Max RSS (memory usage)

Results (primary 0.4%, secondary -0.2%)

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)	1.7%	[0.4%, 12.3%]	27
Regressions ❌ (secondary)	3.8%	[3.8%, 3.8%]	1
Improvements ✅ (primary)	-2.2%	[-6.1%, -0.5%]	13
Improvements ✅ (secondary)	-2.3%	[-3.2%, -1.3%]	2
All ❌✅ (primary)	0.4%	[-6.1%, 12.3%]	40

Cycles

Results (primary 0.1%, secondary 2.4%)

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)	0.7%	[0.4%, 2.0%]	14
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-2.6%	[-5.4%, -0.5%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-5.4%, 2.0%]	17

Binary size

Results (primary 0.1%, secondary 0.1%)

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)	0.1%	[0.0%, 0.8%]	87
Regressions ❌ (secondary)	0.1%	[0.0%, 0.2%]	15
Improvements ✅ (primary)	-0.1%	[-0.4%, -0.0%]	6
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.4%, 0.8%]	93

Bootstrap: 770.356s -> 770.437s (0.01%)
Artifact size: 365.20 MiB -> 365.11 MiB (-0.02%)

fmease · 2025-06-16T11:58:28Z

tests/ui/borrowck/span-semicolon-issue-139049.rs

Following the discussion in https://github.com/rust-lang/rust/pull/142543/files/6ff3713e0fc60146a36774b447204677ab7b8d5a#r2149545623, instead of fully removing this test, could you replace it with the one I suggested in #142543 (comment) or modify tests/ui/nll/issue-54556-used-vs-unused-tails.rs to add it there.

That would be very appreciated but not 'required' since it's more important to get your PR merged in the first place :) Urgau or I can simply re-add a test as a follow-up, no problem.

bors · 2025-06-17T21:53:14Z

☔ The latest upstream changes (presumably #142613) made this pull request unmergeable. Please resolve the merge conflicts.

This uses `super let` to allow let f = format_args!("Hello {}", world); println!("{f}"); to work.

We no longer error in this case!

This message is no longer generated. This is probably a good thing. The relevant span is entirely in user code, and "format_args_nl" is an implementation detail with a name that isn't even public.

Diagnostics should know that the `&` for arguments in format_args!() come from the macro expansion rather than from the original source.

compiler/rustc_ast_lowering/src/format.rs

tests/ui/borrowck/issue-114374-invalid-help-fmt-args.stderr

jdonszelmann · 2025-06-19T13:28:41Z

bors · 2025-06-19T13:28:44Z

📌 Commit b4f2cac has been approved by jdonszelmann

It is now in the queue for this repository.

bors · 2025-06-19T19:13:36Z

⌛ Testing commit b4f2cac with merge 255aa22...

bors · 2025-06-19T22:18:07Z

☀️ Test successful - checks-actions
Approved by: jdonszelmann
Pushing 255aa22 to master...

github-actions · 2025-06-19T22:21:21Z

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 8de4c72 (parent) -> 255aa22 (this PR)

Test differences

Show 8 test diffs

Stage 1

[ui] tests/ui/borrowck/issue-114374-invalid-help-fmt-args.rs: pass -> [missing] (J1)
fmt::test_lifetime: [missing] -> pass (J2)

Stage 2

[ui] tests/ui/borrowck/issue-114374-invalid-help-fmt-args.rs: pass -> [missing] (J0)

Additionally, 5 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 255aa220821c05c3eac7605fce4ea1c9ab2cbdb4 --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

dist-aarch64-msvc: 14983.1s -> 8413.0s (-43.9%)
dist-aarch64-apple: 6723.7s -> 4689.2s (-30.3%)
x86_64-apple-2: 4537.4s -> 3543.9s (-21.9%)
dist-x86_64-apple: 11510.2s -> 9048.3s (-21.4%)
x86_64-apple-1: 7805.5s -> 6181.5s (-20.8%)
dist-apple-various: 8283.5s -> 6874.9s (-17.0%)
x86_64-gnu-llvm-19-2: 5280.0s -> 5852.9s (10.9%)
x86_64-gnu-aux: 5956.6s -> 6537.8s (9.8%)
mingw-check-tidy: 70.4s -> 77.1s (9.6%)
dist-various-2: 3377.5s -> 3110.7s (-7.9%)

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 · 2025-06-20T00:48:00Z

Finished benchmarking commit (255aa22): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.2%, 1.0%]	24
Regressions ❌ (secondary)	1.6%	[0.3%, 4.7%]	12
Improvements ✅ (primary)	-3.7%	[-7.1%, -0.3%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-7.1%, 1.0%]	26

Max RSS (memory usage)

Results (primary 1.0%, secondary 2.3%)

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

	mean	range	count
Regressions ❌ (primary)	3.0%	[2.1%, 4.3%]	4
Regressions ❌ (secondary)	3.8%	[2.6%, 6.5%]	4
Improvements ✅ (primary)	-3.2%	[-3.2%, -3.1%]	2
Improvements ✅ (secondary)	-3.7%	[-3.7%, -3.7%]	1
All ❌✅ (primary)	1.0%	[-3.2%, 4.3%]	6

Cycles

Results (primary -6.8%, secondary 5.1%)

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)	5.1%	[4.0%, 7.2%]	4
Improvements ✅ (primary)	-6.8%	[-6.8%, -6.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-6.8%	[-6.8%, -6.8%]	1

Binary size

Results (primary 0.1%, secondary 1.9%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 0.7%]	60
Regressions ❌ (secondary)	1.9%	[0.0%, 17.0%]	35
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	5
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.2%, 0.7%]	65

Bootstrap: 691.904s -> 692.261s (0.05%)
Artifact size: 372.02 MiB -> 371.96 MiB (-0.02%)

Kobzol · 2025-06-20T04:55:27Z

Weird, somehow this makes binary size and linking time of workspaces with a lot of crates much worse.

m-ou-se added T-lang Relevant to the language team T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. A-fmt Area: `core::fmt` labels May 7, 2025

m-ou-se removed T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) A-tidy Area: The tidy tool labels May 7, 2025

This was referenced May 7, 2025

Allow storing format_args!() in variable or const #139135

Closed

Allow storing format_args! in a let binding #92698

Closed

m-ou-se force-pushed the super-format-args3 branch from 4971602 to d261de6 Compare May 7, 2025 16:08

rustbot added A-tidy Area: The tidy tool T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) labels May 7, 2025

m-ou-se removed T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) A-tidy Area: The tidy tool labels May 7, 2025

This comment has been minimized.

Sign in to view

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label May 7, 2025

This comment has been minimized.

Sign in to view

rustbot added the perf-regression Performance regression. label May 7, 2025

rustbot added the T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) label Jun 16, 2025

fmease reviewed Jun 16, 2025

View reviewed changes

fmease mentioned this pull request Jun 16, 2025

Add DesugaringKind::FormatLiteral #142594

Open

m-ou-se added 9 commits June 18, 2025 10:20

Allow storing format_args!() in let.

8ff2c1a

This uses `super let` to allow let f = format_args!("Hello {}", world); println!("{f}"); to work.

Add test for new format_args!() temporary lifetimes.

5b763fb

Remove outdated test.

ddd9ee9

We no longer error in this case!

Update mir-opt tests.

4ca0f01

Remove 'this error originates in format_args_nl' from test output.

f608da2

This message is no longer generated. This is probably a good thing. The relevant span is entirely in user code, and "format_args_nl" is an implementation detail with a name that isn't even public.

Update unpretty tests.

58a5ab5

Fix span of AddrOf in format_args!() expansion.

42fc5ae

Diagnostics should know that the `&` for arguments in format_args!() come from the macro expansion rather than from the original source.

Bless/update tests.

a693c52

Update/bless clippy tests.

2c175e1

m-ou-se force-pushed the super-format-args3 branch from ae3bfda to 2c175e1 Compare June 18, 2025 08:32

jdonszelmann reviewed Jun 19, 2025

View reviewed changes

compiler/rustc_ast_lowering/src/format.rs Show resolved Hide resolved

compiler/rustc_ast_lowering/src/format.rs Outdated Show resolved Hide resolved

tests/ui/borrowck/issue-114374-invalid-help-fmt-args.stderr Show resolved Hide resolved

m-ou-se added 2 commits June 19, 2025 14:08

Use expr_ref.

2da3a66

Remove old format_args diagnostic.

b4f2cac

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 19, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Jun 19, 2025

bors merged commit 255aa22 into rust-lang:master Jun 19, 2025
11 checks passed

rustbot added this to the 1.89.0 milestone Jun 19, 2025

m-ou-se deleted the super-format-args3 branch June 20, 2025 06:04

Allow storing format_args!() in variable #140748

Allow storing format_args!() in variable #140748

Uh oh!

Conversation

m-ou-se commented May 7, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

m-ou-se commented May 7, 2025

Uh oh!

This comment has been minimized.

bors commented May 7, 2025

Uh oh!

This comment has been minimized.

bors commented May 7, 2025

Uh oh!

This comment has been minimized.

rust-timer commented May 7, 2025

Overall result: ❌✅ regressions and improvements - please read the text below

Uh oh!

fmease Jun 16, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

bors commented Jun 17, 2025

Uh oh!

Uh oh!

Uh oh!

Uh oh!

jdonszelmann commented Jun 19, 2025

Uh oh!

bors commented Jun 19, 2025

Uh oh!

bors commented Jun 19, 2025

Uh oh!

bors commented Jun 19, 2025

Uh oh!

Uh oh!

github-actions bot commented Jun 19, 2025

Test differences

Stage 1

Stage 2

Job duration changes

Uh oh!

rust-timer commented Jun 20, 2025

Overall result: ❌✅ regressions and improvements - please read the text below

Uh oh!

Kobzol commented Jun 20, 2025

Uh oh!

Uh oh!

Allow storing `format_args!()` in variable #140748

Allow storing `format_args!()` in variable #140748

m-ou-se commented May 7, 2025 •

edited

Loading

fmease Jun 16, 2025 •

edited

Loading