Add specialization for deque1.prepend(deque2.drain(range)) (VecDeque::prepend and extend_front)

[antonilol]

Tracking issue: #146975

This specialization makes sure deque1.prepend(deque2.drain(..)) gets similar speed to deque1.append(&mut deque2). deque1.prepend(deque2.drain(..)) is the equivalent of deque1.append(&mut deque2) but appending to the front (see the second example of prepend, prepend is from the same tracking issue).

[rustbot]

r? @jhpratt

rustbot has assigned @jhpratt.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[jhpratt]

@bors r+ rollup=never

[bors]

📌 Commit 7e425b8 has been approved by jhpratt

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7e425b8 with merge f280e76...

[bors]

☀️ Test successful - checks-actions
Approved by: jhpratt
Pushing f280e76 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 e25ee6b (parent) -> f280e76 (this PR)

Test differences

Show 4 test diffs

Stage 1

• vec_deque::test_extend_front_specialization_deque_drain: [missing] -> pass (J0)

Stage 2

• vec_deque::test_extend_front_specialization_deque_drain: [missing] -> pass (J1)

Additionally, 2 doctest diffs were found. These are ignored, as they are noisy.

Job group index

• J0: pr-check-2, x86_64-gnu-llvm-20-3, x86_64-gnu-llvm-21-3
• J1: aarch64-apple, aarch64-gnu, aarch64-gnu-llvm-20-1, aarch64-msvc-1, arm-android, armhf-gnu, dist-i586-gnu-i586-i686-musl, i686-gnu-1, i686-gnu-nopt-1, i686-msvc-1, test-various, x86_64-gnu, x86_64-gnu-aux, x86_64-gnu-llvm-20-2, x86_64-gnu-llvm-21-2, x86_64-gnu-nopt, x86_64-gnu-stable, x86_64-mingw-1, x86_64-msvc-1

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard f280e764d51976d62da12033ddcbf72d0076a969 --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. pr-check-1: 2006.6s -> 1506.0s (-24.9%)
2. aarch64-apple: 11262.2s -> 8644.7s (-23.2%)
3. x86_64-gnu-debug: 7951.6s -> 6297.6s (-20.8%)
4. x86_64-rust-for-linux: 3088.5s -> 2665.5s (-13.7%)
5. aarch64-gnu-debug: 4711.8s -> 4113.3s (-12.7%)
6. dist-powerpc-linux: 5115.8s -> 5650.6s (+10.5%)
7. aarch64-gnu: 7232.1s -> 7977.3s (+10.3%)
8. i686-gnu-1: 8242.4s -> 7423.0s (-9.9%)
9. x86_64-gnu-miri: 4950.6s -> 4458.6s (-9.9%)
10. dist-ohos-x86_64: 4214.6s -> 4620.1s (+9.6%)

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 (f280e76): 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)

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

Cycles

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

Binary size

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

Bootstrap: 473.697s -> 474.288s (0.12%)
Artifact size: 390.82 MiB -> 390.82 MiB (-0.00%)