chore(ci): disable branch coverage for now

[m4tx]

We've pinned the Rust nightly to a specific version, because it caused some problems when generating the coverage. Seems like the issue hasn't been fixed yet, while a newer nightly version introduced some formatter changes, so it's difficult to stay on the pinned version any longer.

This disables the nightly-only branch coverage calculation and upgrades the Rust nightly version in the CI.

Relevant issues:

• Crash Report (SIGSEGV) llvm/llvm-project#119558
• Branch coverage taiki-e/cargo-llvm-cov#8

[Copilot]

Pull request overview

This PR updates the Rust CI workflow to stop computing nightly-only branch coverage (due to upstream LLVM/cargo-llvm-cov issues) and unpins/upgrades the Rust nightly used elsewhere in CI to a newer snapshot.

Changes:

• Update the pinned Rust nightly toolchain version to nightly-2026-02-28.
• Switch the coverage job to use the stable toolchain instead of nightly.
• Remove --branch from cargo llvm-cov commands to disable branch coverage generation.

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[github-actions]

Bencher Report

Branch	stable-coverage
Testbed	github-ubuntu-latest

Click to view all benchmark results

Benchmark	Latency	Benchmark Result microseconds (µs) (Result Δ%)	Upper Boundary microseconds (µs) (Limit %)
empty_router/empty_router	📈 view plot 🚷 view threshold	4,630.60 µs (-21.68%) Baseline: 5,912.69 µs	7,018.51 µs (65.98%)
json_api/json_api	📈 view plot 🚷 view threshold	883.04 µs (-13.43%) Baseline: 1,020.02 µs	1,162.74 µs (75.94%)
nested_routers/nested_routers	📈 view plot 🚷 view threshold	801.26 µs (-14.81%) Baseline: 940.50 µs	1,067.05 µs (75.09%)
single_root_route/single_root_route	📈 view plot 🚷 view threshold	781.51 µs (-13.28%) Baseline: 901.17 µs	1,025.37 µs (76.22%)
single_root_route_burst/single_root_route_burst	📈 view plot 🚷 view threshold	15,185.00 µs (-13.19%) Baseline: 17,492.42 µs	20,659.90 µs (73.50%)

🐰 View full continuous benchmarking report in Bencher

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
rust	90.00% <ø> (+0.34%)	⬆️

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see 30 files with indirect coverage changes

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[seqre]

LGTM once formatting is fixed

[m4tx]

LGTM once formatting is fixed

@seqre the CI is green, though?

[seqre]

@seqre the CI is green, though?

I'm confused, how is cot/src/lib.rs fine here, but the same change is failed in other PR???

[m4tx]

@seqre the CI is green, though?

I'm confused, how is cot/src/lib.rs fine here, but the same change is failed in other PR???

As per this PR's description:

while a newer nightly version introduced some formatter changes, so it's difficult to stay on the pinned version any longer.