Add const Default impls for HashSet and HashMap with custom Hasher

[estebank]

Follow up to #134628. Tracking issue #143894.

r? @fmease

cc @fee1-dead @oli-obk

This doesn't allow for const H: HashSet<()> = Default::default();, but would allow for it to work with a custom hasher, which would enable the Fx* family to work.

[rustbot]

fmease is not on the review rotation at the moment.
They may take a while to respond.

[oli-obk]

@bors r+ rollup

[rust-bors]

📌 Commit 5f58acb has been approved by oli-obk

It is now in the queue for this repository.

[rust-bors]

☀️ Test successful - CI
Approved by: oli-obk
Pushing cd7ad97 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 fe98ddc (parent) -> cd7ad97 (this PR)

Test differences

Show 226 test diffs

226 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 cd7ad97987010603f0007288547927e648e9a369 --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-aarch64-apple: 6992.9s -> 8163.6s (+16.7%)
2. dist-x86_64-apple: 7357.9s -> 8247.1s (+12.1%)
3. dist-apple-various: 3943.8s -> 3507.2s (-11.1%)
4. x86_64-gnu-llvm-21-3: 5986.2s -> 6640.1s (+10.9%)
5. x86_64-gnu-tools: 3276.4s -> 3631.3s (+10.8%)
6. x86_64-gnu-llvm-20-2: 5055.2s -> 5598.8s (+10.8%)
7. aarch64-msvc-2: 6089.5s -> 6716.2s (+10.3%)
8. dist-aarch64-msvc: 5441.3s -> 5992.0s (+10.1%)
9. x86_64-msvc-2: 8303.7s -> 9141.8s (+10.1%)
10. dist-x86_64-msvc-alt: 8978.1s -> 9846.4s (+9.7%)

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 (cd7ad97): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (primary 1.0%)

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

	mean	range	count
Regressions ❌ (primary)	4.7%	[4.7%, 4.7%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.8%	[-2.8%, -2.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.0%	[-2.8%, 4.7%]	2

Cycles

Results (primary -4.8%, secondary 4.1%)

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)	4.1%	[3.6%, 4.6%]	2
Improvements ✅ (primary)	-4.8%	[-4.8%, -4.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-4.8%	[-4.8%, -4.8%]	1

Binary size

Results (primary -0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.1%, 0.1%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.6%, -0.1%]	5
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-0.6%, 0.1%]	7

Bootstrap: 473.229s -> 472.349s (-0.19%)
Artifact size: 383.52 MiB -> 383.53 MiB (0.00%)