[pascaldekloe]

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The fallback to Dragon was placed in the Grisu module. As a result tests don't test what they appear to test, and some benchmarks from Grisu measured Dragon instead.

The corrected values in the tests haven been verified against the Go implementation.

[rustbot]

Some changes occurred in float parsing

cc @tgross35

[rustbot]

r? @scottmcm

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[tgross35]

Thank you for preparing this!

I only had time to go through the first commit here, which mostly looks pretty good to me with a few small requests. If you'd like to fix those then put only the first commit in a separate PR, I'd be happy to approve that bit and shrink the remaining todo here.

As a noncritical note, I'm having a bit of trouble understanding the commit summaries. The messages usually have details, but I'm not entirely sure what something like "write out of flt2dec/random.rs" or "test flt2dec front of format_short & format_fixed" means. We don't really have a guideline but sticking to the usual style with changes described in imperative mood would help a bit (e.g. something like flt2dec: test: Don't use `write!` in random tests or flt2dec: test: Test `format_{short,fixed}` via the main entrypoint).

I'll review the rest a little later.

View changes since this review

[pascaldekloe]

Pushed the commit explainer plus distinct summary lines in Rust docs @tgross35. :-) I'll wait for the rest of the review. Some urgency would be nice because I have time to do this now. 🙏

The git messages ain't readable sections on their own indeed. You need the code in conjunction to comprehend the listings. I'll work on that.

[tgross35]

Next batch, mostly for the second commit

View changes since this review

[tgross35]

Let's make sure there's no perf effect.

@bors try @rust-timer queue

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[rust-bors]

☀️ Try build successful (CI)
Build commit: cb7a7f2 (cb7a7f2a841fd88e9e55ffb325dfabdc4476a5b2, parent: 4933094f25ba1edf2de0a203665976832fc85bc9)

[rust-timer]

Finished benchmarking commit (cb7a7f2): 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 (secondary 3.9%)

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)	3.9%	[1.8%, 6.0%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

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

Bootstrap: 488.817s -> 487.773s (-0.21%)
Artifact size: 393.48 MiB -> 393.50 MiB (0.01%)

[pascaldekloe]

All done with a few options open @tgross35.

@rustbot ready

[tgross35]

Enclosed in rollup,
@bors yield

[rust-bors]

Auto build was cancelled. Cancelled workflows:

• https://github.com/rust-lang/rust/actions/runs/39189607083

The next pull request likely to be tested is #164554.

[tgross35]

@bors r-

#164554 (comment)

[rust-bors]

This pull request was unapproved.

This PR was contained in a rollup (#164554), which was unapproved.

Auto build was cancelled due to unapproval. Cancelled workflows:

• https://github.com/rust-lang/rust/actions/runs/39196524511

View changes since this unapproval

[tgross35]

@bors try jobs=test-x86_64-mingw-1

[tgross35]

@bors r+

[rust-bors]

📌 Commit 59d1c07 has been tentatively approved by tgross35

It will be put into the queue for this repository once PR CI succeeds.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 8f0ec02 (8f0ec02643410e51fc7ac533d03ca29e79b7a97d)
Base parent: b6eeb93 (b6eeb93c992027a03e1fd120b6652546396b142c)

[rust-log-analyzer]

The job test-x86_64-gnu-aux failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

failures:

---- num::flt2dec::f32_fixed_sanity_test stdout ----

thread 'num::flt2dec::f32_fixed_sanity_test' (2708) panicked at library/coretests/tests/num/flt2dec/mod.rs:289:5:
assertion `left == right` failed: dragon short format of 12676506_f32 * -102_f32.exp2()
  left: ([51], -23)
 right: ([50], -23)
---- num::flt2dec::f32_fixed_sanity_test stdout end ----

failures:
    num::flt2dec::f32_fixed_sanity_test

test result: FAILED. 2813 passed; 1 failed; 23 ignored; 0 measured; 0 filtered out; finished in 1632.61s

error: test failed, to rerun pass `-p coretests --test coretests`
Bootstrap failed while executing `miri --stage 2 library/core library/alloc --tests`
Currently active steps:
test::Crate { build_compiler: Compiler { stage: 2, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, mode: Std, crates: ["core", "alloc"] } at src/bootstrap/src/core/build_steps/test.rs:3622
Build completed unsuccessfully in 0:52:48
make: *** [Makefile:56: check-aux] Error 1
  local time: Mon Oct 19 09:03:20 UTC 2026
  network time: Mon, 19 Oct 2026 09:03:20 GMT
##[error]Process completed with exit code 2.
##[group]Run echo "disk usage:"
echo "disk usage:"

[rust-bors]

💔 Test for 2a5f389 failed: CI. Failed job:

• auto - test-x86_64-gnu-aux (web logs, enhanced plaintext logs)