refactor: refactor utf8 path conversions

[hardfist]

What changed

Rspack support UTF8 only Path so it's not necessary for rspack-resolver to support non utf8-path

• Replaced lossy path-to-string conversions in resolver, tsconfig, filesystem, NAPI, and tests with strict UTF-8 conversions.
• Added a small shared path_to_str helper for core path assertions.
• Kept path extension and alias string construction on borrowed UTF-8 path strings where possible.

Why

rspack-resolver only needs to support UTF-8 paths, so lossy conversions can silently hide invalid path data and add unnecessary conversion work.

Performance notes

CodSpeed reports this PR improves three resolver benchmarks:

Benchmark	Base	Head	Efficiency
resolver[multi-thread]	65.1 ms	63 ms	+3.29%
resolver[resolve from symlinks multi thread]	123.7 ms	115.1 ms	+7.5%
resolver[resolve from symlinks]	195.6 ms	186 ms	+5.15%

Local callgrind comparison for resolver/resolve from symlinks (8975e38 -> 85e0cec) shows the same direction:

Metric	Base	Head	Delta
Ir	3,073,208,626	3,000,439,756	-72,768,870 / -2.37%
Dr	798,591,590	787,908,689	-1.34%
Dw	594,749,952	592,901,782	-0.31%
accesses	4,466,550,168	4,381,250,227	-1.91%

The main reason is removing the core::str::lossy::Utf8Chunks::next hot path from repeated to_string_lossy() calls. In the base profile it accounts for about 52.5M Ir; after this PR it is replaced by cheaper strict UTF-8 validation (core::str::converts::from_utf8, about 7.7M Ir). The affected hot paths are primarily load_extensions and load_alias_or_file, where resolver candidate paths are converted repeatedly.

Measurement command shape:

RUSTFLAGS='-C debuginfo=1 -C strip=none -g' cargo bench --bench resolver --no-run
valgrind --tool=callgrind --cache-sim=yes \
  --callgrind-out-file=optimization-artifacts/valgrind/callgrind/pr219-symbols/{base,head}/callgrind.out \
  target/release/deps/resolver-* 'resolve from symlinks'

Validation

• cargo fmt --all --check
• cargo check --workspace
• cargo check --workspace --features enable_instrument
• cargo clippy --all-features -- -D warnings
• cargo test -p rspack_resolver --lib -- --skip pnp
• GitHub Actions PR checks are passing, including Clippy, tests, benchmark, coverage, build, doc, wasm, format.

Full cargo test --workspace was also run locally. It still fails on existing environment-sensitive fixtures: PnP tests need fixtures/pnp/.pnp.cjs, and integration resolve tests need local pnpm node_modules content.

[codspeed-hq]

Merging this PR will improve performance by 5.3%

⚡ 3 improved benchmarks
✅ 9 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	resolver[multi-thread]	65.1 ms	63 ms	+3.29%
⚡	Simulation	resolver[resolve from symlinks multi thread]	123.7 ms	115.1 ms	+7.5%
⚡	Simulation	resolver[resolve from symlinks]	195.6 ms	186 ms	+5.15%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing codex/utf8-path-only (85e0cec) with main (8975e38)}

[Copilot]

Pull request overview

This PR refactors path-to-string conversion to use strict UTF-8 handling instead of lossy conversion across resolver internals, tsconfig handling, filesystem code, NAPI output, and tests.

Changes:

• Adds a shared path_to_str helper for strict UTF-8 path assertions.
• Replaces to_string_lossy() conversions with to_str().expect(...) or path_to_str(...).
• Keeps extension and alias path construction on borrowed UTF-8 strings where possible.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/path.rs	Adds the shared strict UTF-8 path conversion helper.
src/lib.rs	Applies strict path conversion in resolver tracing, alias, extension, and error construction paths.
src/tsconfig.rs	Uses strict conversion for tsconfig tracing and ${configDir} substitution.
src/file_system.rs	Uses strict UTF-8 conversion when rebuilding wasm symlink paths.
napi/src/lib.rs	Converts resolved paths to JS strings with strict UTF-8 handling.
src/tests/alias.rs	Updates alias tests to use strict path conversion.
src/tests/browser_field.rs	Updates browser field fallback path conversion.
src/tests/memory_fs.rs	Adds a local helper and uses strict paths for the in-memory filesystem.
src/tests/missing.rs	Updates alias path setup to strict conversion.
src/tests/pnp.rs	Updates PnP path assertions and formatting to strict conversion.
src/tests/resolve.rs	Updates fixture path conversion to strict UTF-8.
src/tests/roots.rs	Updates absolute path requests to strict UTF-8.
src/tests/windows.rs	Updates Windows path assertions to strict UTF-8.

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