Short-circuit calculate_fallback_to_f32 when no float vars

[inq]

Sharing a small fix.

When no unresolved variable is a float_vid, no f32 fallback can apply,
so we can skip from_float_for_f32_root_vids (which walks the proof tree
of every pending obligation under the new solver — O(N × M) on crates
with many large obligations).

On ReShell: stage1 wall-clock 2m35s → 1m46s (-49s, ~31%).

Context + profile breakdown will be linked at
rust-lang/trait-system-refactor-initiative#254 after this PR opens.

r? @lcnr

[lcnr]

👍
@bors r+ rollup=never

[rust-bors]

📌 Commit 55ff2e1 has been approved by lcnr

It is now in the queue for this repository.

[lcnr]

@bors r-

[rust-bors]

This pull request was unapproved.

View changes since this unapproval

[lcnr]

@bors r+

[rust-bors]

📌 Commit 55ff2e1 has been approved by lcnr

It is now in the queue for this repository.

[rust-bors]

☀️ Test successful - CI
Approved by: lcnr
Duration: 3h 12m
Pushing cb40c25 to main...

[github-actions]

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 1d72d7e (parent) -> cb40c25 (this PR)

Test differences

Show 6 test diffs

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

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard cb40c25f6aebb637163d26bf76a680ed6e5d1eda --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

1. dist-apple-various: 1h 52m -> 1h 18m (-29.8%)
2. dist-various-2: 35m 25s -> 44m 17s (+25.0%)
3. dist-armv7-linux: 1h 14m -> 1h 32m (+24.3%)
4. x86_64-mingw-2: 2h 46m -> 2h 9m (-21.8%)
5. x86_64-rust-for-linux: 55m 31s -> 45m 40s (-17.7%)
6. x86_64-gnu-tools: 1h 5m -> 54m 15s (-17.1%)
7. i686-gnu-1: 2h 22m -> 1h 58m (-16.7%)
8. test-various: 2h 13m -> 1h 51m (-16.0%)
9. dist-x86_64-musl: 2h 12m -> 1h 52m (-15.5%)
10. x86_64-gnu-stable: 2h 25m -> 2h 4m (-14.1%)

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]

Finished benchmarking commit (cb40c25): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

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
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-21.0%	[-87.6%, -0.0%]	18
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 0.3%, secondary -0.6%)

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

	mean	range	count
Regressions ❌ (primary)	2.2%	[2.2%, 2.2%]	1
Regressions ❌ (secondary)	4.8%	[2.5%, 6.8%]	4
Improvements ✅ (primary)	-0.7%	[-0.9%, -0.4%]	2
Improvements ✅ (secondary)	-3.3%	[-9.3%, -0.4%]	8
All ❌✅ (primary)	0.3%	[-0.9%, 2.2%]	3

Cycles

Results (primary 3.0%, secondary -18.9%)

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

	mean	range	count
Regressions ❌ (primary)	3.0%	[2.6%, 3.3%]	2
Regressions ❌ (secondary)	5.2%	[2.8%, 7.2%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-34.2%	[-84.6%, -0.4%]	11
All ❌✅ (primary)	3.0%	[2.6%, 3.3%]	2

Binary size

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

Bootstrap: 494.198s -> 497.794s (0.73%)
Artifact size: 394.42 MiB -> 396.40 MiB (0.50%)

[panstromek]

perf triage:

include-blob regressions are noise.

@rustbot label: +perf-regression-triaged