improve format impl for literals

[conradludgate]

The basic idea of this change can be seen here https://godbolt.org/z/MT37cWoe1.

Updates the format impl to have a fast path for string literals and the default path for regular format args.

This change will allow format!("string literal") to be used interchangably with "string literal".to_owned().

This would be relevant in the case of f!"string literal" being legal (rust-lang/rfcs#3267) in which case it would be the easiest way to create owned strings from literals, while also being just as efficient as any other impl

[rust-highfive]

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[rust-highfive]

r? @joshtriplett

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[joshtriplett]

@bors r+ rollup=never

[bors]

📌 Commit 738d100438d9b099fed604cfa3b1eae47a191a43 has been approved by joshtriplett

[conradludgate]

How does one re-request a review 😅

[Dylan-DPC]

@bors r=joshtriplett

[bors]

📌 Commit 5dd0fe3 has been approved by joshtriplett

[bors]

⌛ Testing commit 5dd0fe3 with merge 4a8d2e3...

[bors]

☀️ Test successful - checks-actions
Approved by: joshtriplett
Pushing 4a8d2e3 to master...

[rust-timer]

Finished benchmarking commit (4a8d2e3): comparison url.

Instruction count

• Primary benchmarks: 😿 relevant regressions found
• Secondary benchmarks: 🎉 relevant improvement found

	mean1	max	count2
Regressions 😿 (primary)	0.4%	0.5%	4
Regressions 😿 (secondary)	N/A	N/A	0
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	-1.5%	-1.5%	1
All 😿🎉 (primary)	0.4%	0.5%	4

Max RSS (memory usage)

Results

• Primary benchmarks: 😿 relevant regressions found
• Secondary benchmarks: 🎉 relevant improvements found

	mean1	max	count2
Regressions 😿 (primary)	2.3%	5.7%	4
Regressions 😿 (secondary)	N/A	N/A	0
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	-4.6%	-4.8%	2
All 😿🎉 (primary)	2.3%	5.7%	4

Cycles

Results

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: mixed results

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	2.2%	2.2%	1
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	-3.0%	-3.0%	1
All 😿🎉 (primary)	N/A	N/A	0

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression

Footnotes

1. the arithmetic mean of the percent change ↩ ↩² ↩³

2. number of relevant changes ↩ ↩² ↩³

[pnkfelix]

Visiting for weekly rustc-perf performance triage.

• This is adding a fast-path so that format!("literal") will compile into the same code as "literal".to_owned().
• The primary regression is solely contained to bitmaps.
• Its possible that the regression to bitmaps is due to format!("literal") being totally unused in that code; all instances of format! there take an additional argument.
  So its possible that the extra code to check about whether to use the fast-path is slowing things down there.
• But I personally don't believe that explanation here: Unless I'm misunderstanding the code, there is some amount of macro-expansion into multiple instances of format!, but
  most of the expanded code is going to be dominated by all the impl blocks, not the relatively few format! instances. (Unless I massively misunderstand how the macros and/or codegen and/or inlining end up linking up here.)
• So: I don't believe the best hypothesis I have for what is happening here.
• But I also do not think the regression here is large enough to warrant further investigation.
• Marking as triaged.

@rustbot label: perf-regression-triaged