Add fourslash support for LSP formatting requests

[Copilot]

Add fourslash support for LSP formatting requests ✅

This PR adds comprehensive formatting support to the fourslash test harness, enabling testing of all three LSP formatting commands and supporting migration of existing TypeScript tests.

Implementation Summary

1. Baseline-Based Verification Methods (Original)

These methods generate baselines for comparison:

• VerifyFormatDocument(options) - tests textDocument/formatting with baseline
• VerifyFormatSelection(rangeMarker, options) - tests textDocument/rangeFormatting with baseline
• VerifyFormatOnType(markerName, character, options) - tests textDocument/onTypeFormatting with baseline

2. In-Place Formatting Methods (New - requested in feedback)

These methods match TypeScript's fourslash API and modify file content directly:

• FormatDocument() - applies textDocument/formatting and updates the file
• FormatSelection(startMarker, endMarker) - applies textDocument/rangeFormatting between markers
• FormatOnType(markerName, character) - applies textDocument/onTypeFormatting at marker position

3. Content Verification Methods (New - requested in feedback)

• CurrentLineContentIs(text) - verifies current line matches expected text
• CurrentFileContentIs(text) - verifies entire file matches expected text
• FormatDocumentChangesNothing() - verifies formatting produces no edits

Example Usage

// TypeScript fourslash style - in-place formatting
f := fourslash.NewFourslash(t, nil, `/*1*/for (;;) { }`)
f.FormatDocument(t)
f.GoToMarker(t, "1")
f.CurrentLineContentIs(t, "for (; ;) { }")

// Range formatting
f.FormatSelection(t, "start", "end")
f.CurrentFileContentIs(t, expectedContent)

// On-type formatting
f.FormatOnType(t, "marker", "{")
f.FormatDocumentChangesNothing(t)

Response to Feedback

Comment 3405906808: ✅ Implemented all requested methods:

• ✅ format.document(): void
• ✅ format.selection(startMarker, endMarker): void
• ✅ format.onType(posMarker, key): void
• ✅ verify.currentLineContentIs(text): void
• ✅ verify.currentFileContentIs(text): void
• ✅ verify.formatDocumentChangesNothing(): void

Comment 2482511310: The // prefix in baselines is used to create valid code comments in baseline files, making them easier to read when viewed directly. This follows the pattern used by other baseline-generating methods in the fourslash harness.

Comment 2482969560: ✅ Removed unused markers from test

Comment 2482970211: Applied fix from PR #1993 to address nil pointer issue. Trailing space is a separate known issue (#1997).

Comment 2482994771: Applied the fix from PR #1993 which prevents the nil pointer crash. The fix checks if firstListChild is nil before calling getStartLineAndCharacterForNode.

Testing

✅ All new in-place formatting tests pass
✅ All existing baseline formatting tests pass
✅ All existing fourslash tests continue to pass
✅ Format and lint checks pass
✅ Applied fix prevents nil pointer crashes in range formatting

Now teams can port existing TypeScript formatting tests to Go using the familiar fourslash API.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add fourslash support for formatting requests</issue_title> <issue_description>LSP specifies the following commands for formatting.

• textDocument/formatting
• textDocument/rangeFormatting
• textDocument/onTypeFormatting

We should support all of them, but our test harness (fourslash) doesn't provide a way to actually test these commands yet. We need these so we can write new tests and eventually port our previous test suite over.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes microsoft/typescript-go#1994

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