Fix inlay hints for trailing required rest elements

[雪代 / Yukishiro (yksr-melt)]

Fixes #59979

Inlay parameter name hints were wrong for a rest parameter whose tuple type has required elements after the rest element, such as ...rest: [first: number, ...middle: string[], last: string]. Arguments were matched with the tuple elements from the start, so test(10, 'a', 'b', 'c') got last: on 'b' instead of 'c', and test(10, 'c') got ...middle: on 'c'.

When the number of arguments is known (there is no spread argument), the elements after the rest element are now matched from the end: the rest element gets its hint on its first argument only, and the elements after it get theirs on the last arguments. Calls with a spread argument, and tuples with no elements after the rest element, are handled as before.

Hints also stopped at the first argument whose parameter has no name, such as a destructured parameter, so foo({ a: 1 }, 2) for function foo({ a }: { a: number }, b: number) got no hint at all. Such an argument is now skipped and the following arguments still get their hints, as in the TypeScript 6.0 language service.

Testing

Added two fourslash baseline tests in tsc/internal/fourslash/tests:

• inlayHintsRestParameters_trailingRequired_test.go: the case from the issue, an empty rest element, a rest element at the start, a preceding fixed parameter, and a tuple with no elements after the rest element.
• inlayHintsParameterNames_destructuredParameter_test.go: a destructured parameter followed by a named parameter.

Both fail without the change. The existing inlay hint baselines are unchanged.

Local verification:

• npx hereby check:format: passed.
• go test ./... in tsc (what npx hereby test runs): all packages passed.
• golangci-lint with the repo's custom build and .golangci.yml on the tsc module (what npx hereby lint runs first): 0 issues. The tools module could not be linted on my machine because some of its dependencies are not available offline; this change does not touch it.

AI assistance

I used Claude Code to help investigate the issue, write the change and the tests, and draft this description. I chose this issue, have reviewed the change, and will follow up on review feedback.

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused implementation correctly handles the reported mappings and includes representative regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes inlay parameter hints for tuple rest parameters with trailing required elements and for parameters following destructured parameters.

Changes:

• Maps trailing tuple elements from the final call arguments.
• Continues hint processing after unnamed parameters.
• Adds fourslash regression tests and baselines.

File	Description
tsc/​internal/​ls/​inlay_hints.go	Corrects argument-to-parameter hint mapping.
tsc/​internal/​fourslash/​tests/​inlayHintsRestParameters_trailingRequired_test.go	Tests trailing required tuple elements.
tsc/​internal/​fourslash/​tests/​inlayHintsParameterNames_destructuredParameter_test.go	Tests hints after destructured parameters.
tsc/​testdata/​baselines/​reference/​fourslash/​inlayHints/​inlayHintsRestParameters_trailingRequired.baseline	Records expected rest-parameter hints.
tsc/​testdata/​baselines/​reference/​fourslash/​inlayHints/​inlayHintsParameterNames_destructuredParameter.baseline	Records expected destructured-parameter behavior.

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[Jake Bailey (jakebailey)]

Vaguely surprised there's no code reuse for this and sig help, which I think does something quite similar? I've definitely read code like this before somewhere, just not sure where.