refactor(router-core): use one path interpolation implementation

[Sheraff]

What changed

• remove the server-gated plain-template implementation from interpolatePath
• remove the internal server interpolation option and router call-site plumbing
• use the canonical segment parser in every environment
• expand interpolation parity coverage across params, missing params, splats, duplicate slashes, and custom decoders
• add a focused interpolation benchmark

Why

Path interpolation had two implementations of the route grammar: a server-gated plain-template fast path and the canonical parser. The internal options.server value was also runtime plumbing rather than a compile-time server boundary, so it was not an appropriate dead-code-elimination mechanism. A single path keeps grammar behavior and maintenance centralized.

This PR intentionally does not include the separate parseSegment branch reorder. It is correct and mergeable on its own; the parser optimization is complementary rather than required.

Impact

• plain templates measured 6.329 ms versus the captured 6.253 ms baseline for 10,000 interpolations (about +1.2%, within the scale of run-to-run noise)
• braced templates measured 11.322 ms versus 11.278 ms (about +0.4%)
• react-router.minimal decreases from 85,919 to 85,907 bytes gzip (-12 bytes; raw -47 bytes)

Validation

• @tanstack/router-core:test:unit: 105 files / 1,495 tests passed, plus 3 expected failures
• @tanstack/router-core:test:types: TypeScript 5.6 through 7.0 passed
• @tanstack/router-core:test:eslint: zero errors (existing warnings remain)
• focused tests/path.bench.ts run
• react-router.minimal bundle-size scenario on main and this branch
• confirmed no interpolation server option or router call-site prop remains
• git diff --check

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View your CI Pipeline Execution ↗ for commit 7a25c7c

Command	Status	Duration	Result
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☁️ Nx Cloud last updated this comment at 2026-08-08 12:37:09 UTC

[github-actions]

🚀 Changeset Version Preview

No changeset entries found. Merging this PR will not cause a version bump for any packages.

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commit: 7a25c7c

[github-actions]

Bundle Size Benchmarks

• Commit: 426830291b4e
• Measured at: 2026-08-08T12:27:39.136Z
• Baseline source: history:c568caf3f376
• Dashboard: bundle-size history

The following scenarios have bundle-size changes compared with the baseline:

Scenario	Current (gzip)	Delta vs baseline	Initial gzip	Raw	Brotli	Trend
react-router.minimal	83.89 KiB	-12 B (-0.01%)	83.76 KiB	262.57 KiB	73.00 KiB	█████████▁▁▁
react-router.full	87.39 KiB	-8 B (-0.01%)	87.25 KiB	274.29 KiB	76.09 KiB	█████████▁▁▁
solid-router.minimal	33.20 KiB	-15 B (-0.04%)	33.08 KiB	96.41 KiB	29.92 KiB	█████████▁▁▁
solid-router.full	38.03 KiB	-18 B (-0.05%)	37.91 KiB	111.04 KiB	34.19 KiB	█████████▁▁▁
vue-router.minimal	49.48 KiB	-13 B (-0.03%)	49.36 KiB	138.16 KiB	44.61 KiB	█████████▁▁▁
vue-router.full	55.06 KiB	-14 B (-0.02%)	54.93 KiB	156.33 KiB	49.54 KiB	█████████▁▁▁
react-start.minimal	96.65 KiB	-10 B (-0.01%)	96.52 KiB	304.74 KiB	83.69 KiB	█████████▁▁▁
react-start.deferred-hydration	97.38 KiB	-10 B (-0.01%)	96.54 KiB	306.10 KiB	84.30 KiB	█████████▁▁▁
react-start.full	99.82 KiB	-14 B (-0.01%)	99.68 KiB	314.45 KiB	86.54 KiB	█████████▁▁▁
react-start.rsbuild.minimal	99.98 KiB	-18 B (-0.02%)	99.80 KiB	315.28 KiB	86.19 KiB	▁▁▁▇▇▇▇████▆
react-start.rsbuild.minimal-iife	100.38 KiB	-21 B (-0.02%)	100.22 KiB	316.22 KiB	86.54 KiB	▁▁▁▇▇▆▆████▆
react-start.rsbuild.full	103.29 KiB	-18 B (-0.02%)	103.12 KiB	325.38 KiB	88.96 KiB	▁▁▁▆▆▅▅████▆
solid-start.minimal	45.93 KiB	-16 B (-0.03%)	45.80 KiB	137.41 KiB	40.88 KiB	█████████▁▁▁
solid-start.deferred-hydration	49.00 KiB	-9 B (-0.02%)	45.86 KiB	144.87 KiB	43.65 KiB	█████████▁▁▁
solid-start.full	51.00 KiB	-16 B (-0.03%)	50.88 KiB	152.80 KiB	45.21 KiB	█████████▁▁▁
vue-start.minimal	65.47 KiB	-17 B (-0.03%)	65.35 KiB	188.92 KiB	58.25 KiB	█████████▁▁▁
vue-start.full	69.28 KiB	-8 B (-0.01%)	69.15 KiB	201.19 KiB	61.57 KiB	█████████▁▁▁

Current gzip tracks all emitted client JS chunks. Initial gzip tracks only the entry/import graph. Trend sparkline is historical current gzip ending with this PR measurement; lower is better.

[codspeed-hq]

Merging this PR will degrade performance by 7.49%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 4 improved benchmarks
❌ 8 (👁 6) regressed benchmarks
✅ 168 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	ssr dehydrate rich types (solid)	87.3 ms	95.7 ms	-8.77%
❌	Simulation	ssr dehydrate plain control (solid)	119.6 ms	129.2 ms	-7.49%
⚡	Memory	mem server error-paths redirect (solid)	588.2 KB	281.8 KB	×2.1
⚡	Memory	mem client preload-churn (vue)	776.2 KB	741.4 KB	+4.7%
⚡	Simulation	ssr control-flow unmatched 404 (react)	67.2 ms	64.3 ms	+4.55%
⚡	Memory	mem client navigation-churn (solid)	570.2 KB	553 KB	+3.11%
👁	Memory	mem server server-fn-churn (vue)	263.1 KB	273.4 KB	-3.78%
👁	Memory	mem server error-paths not-found (solid)	416.3 KB	733.4 KB	-43.23%
👁	Memory	mem server streaming-peak chunked (solid)	30.3 MB	32.2 MB	-5.91%
👁	Memory	mem server error-paths not-found (react)	256.3 KB	291.3 KB	-12.01%
👁	Memory	mem server error-paths redirect (react)	204.1 KB	227.1 KB	-10.13%
👁	Memory	mem server error-paths unmatched (react)	263.6 KB	542.4 KB	-51.4%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing agent/interpolate-path-remove-server-fast-path (7a25c7c) with main (c568caf)}