[Wesley Wigham (weswigham)]

This is a followup to #64997 since it sets up the SIMD build stuff - the formatting scanner is something simd should lend some benefits to, but most of the improvements here are actually just reduced allocations and repeated work and not simd-specific. This is like a 30% perf improvement for formatting checker.ts, and about 2/3rds of that isn't simd-related.

[Copilot]

🔵 Needs a closer look

The unsafe, default-on SIMD paths rely on experimental Go 1.27 APIs and vector-mask layout, while AMD64/ARM64 correctness and allocation validation across SIMD/scalar builds and vector widths remains unverified.

No open issues 🎉

What changed in this PR

This PR builds on #64997 to improve formatting performance in the native TypeScript compiler and language server. It reduces allocations and repeated work, adds SIMD-assisted text scanning, and reports roughly 30% faster formatting of checker.ts.

Changes:

• Optimize trivia scanning, string escaping, line maps, and position maps with scalar and SIMD paths.
• Cache formatter line lookups and avoid redundant trivia copies.
• Add regression tests, fuzz coverage, and benchmarks; enable SIMD by default while retaining non-SIMD CI coverage.

File	Description
tsc/​internal/​stringutil/​textscan.go	Add shared ASCII prefix-scanning helpers.
tsc/​internal/​stringutil/​textscan_test.go	Test and benchmark prefix scans.
tsc/​internal/​stringutil/​textscan_simd.go	Implement SIMD prefix scanning.
tsc/​internal/​stringutil/​textscan_simd_test.go	Verify SIMD mask offsets and byte order.
tsc/​internal/​stringutil/​textscan_scalar.go	Provide non-SIMD scanning implementations.
tsc/​internal/​scanner/​scanner.go	Accelerate trivia scanning and line-end lookup.
tsc/​internal/​scanner/​scanner_test.go	Cover bulk trivia and stopping options.
tsc/​internal/​scanner/​lines_test.go	Verify line-end positions across newline encodings.
tsc/​internal/​printer/​utilities.go	Integrate optimized escaping and hexadecimal output.
tsc/​internal/​printer/​textscan_test.go	Add escaping boundary tests, fuzzing, and benchmarks.
tsc/​internal/​printer/​escaping.go	Add allocation-conscious string escaping.
tsc/​internal/​printer/​escaping_test.go	Check allocations and UTF-16 escape output.
tsc/​internal/​printer/​escaping_reference_test.go	Preserve an independent reference escaping implementation.
tsc/​internal/​parser/​parser_test.go	Separate parsing and position-map benchmarks.
tsc/​internal/​fourslash/​tests/​formatBulkTrivia_test.go	Verify editor formatting with long trivia and Unicode.
tsc/​internal/​format/​span.go	Use cached formatter line lookups.
tsc/​internal/​format/​scanner.go	Share trivia buffers instead of cloning them.
tsc/​internal/​format/​scanner_test.go	Verify saved trivia survives scanner operations.
tsc/​internal/​format/​rulecontext.go	Use cached line lookups for semicolon decisions.
tsc/​internal/​format/​README.md	Document benchmarking and profiling commands.
tsc/​internal/​format/​format_test.go	Test bulk-trivia formatting and idempotence.
tsc/​internal/​format/​context.go	Cache line starts and the last line lookup.
tsc/​internal/​format/​context_test.go	Check cached lookups in both traversal directions.
tsc/​internal/​format/​benchmark_test.go	Add document and editor-operation benchmarks.
tsc/​internal/​core/​textscan_test.go	Test and benchmark line starts and UTF-16 lengths.
tsc/​internal/​core/​linestarts_simd.go	Add SIMD line-start computation.
tsc/​internal/​core/​linestarts_simd_test.go	Verify line-break mask packing.
tsc/​internal/​core/​linestarts_scalar.go	Retain scalar line-start computation.
tsc/​internal/​core/​core.go	Optimize line-start and UTF-16 length calculations.
tsc/​internal/​ast/​positionmap.go	Skip ASCII runs when building position maps.
tsc/​internal/​ast/​positionmap_scan_test.go	Test position-map boundaries and benchmark scanning.
tools/​scripts/​gen/​generatedFile.test.mts	Verify SIMD defaults preserve explicit experiment settings.
Herebyfile.mjs	Default tasks to SIMD unless explicitly configured.
.github/​workflows/​ci.yml	Enable SIMD and add a non-SIMD test job.

_{Review effort: Balanced}