Fix: Handle Negative VLQ Offsets in Source Maps for VSCode Jump to Definition

[sethconvex]

Problem

The getMappedLocation function was creating inverted text ranges when source maps contained negative VLQ (Variable Length Quantity) offsets. Negative offsets are legal and valid in the source map specification - they occur when declarations in the generated .d.ts file are out of order relative to the source file (non-monotonic mappings).

When VSCode received these inverted ranges in LSP location responses, it mis-interpreted them, causing jump to definition to fail or navigate to incorrect locations. This was particularly problematic for projects like Convex, which generate api.d.ts files with potentially out-of-order references that result in negative VLQ offsets in their source maps.

Root cause:

• Source maps use relative VLQ encoding where positions are stored as deltas
• When declaration order differs from source order, these deltas can be negative (going backwards)
• Example: Position 100 in .d.ts maps to position 800 in source, but position 200 in .d.ts maps to position 200 in source
• When mapping a range from position 100-200 in .d.ts, the end position (200) would be before the start position (800) in the source, creating an invalid inverted range
• VSCode cannot handle inverted ranges, breaking jump to definition functionality

Solution

Added validation logic in getMappedLocation to ensure the end position is always valid:

• Clamps the end position to the start position when:
  • The end position is before the start position (inverted range)
  • The end position is in a different file than the start position
• This results in a zero-length range when clamping occurs, which is a valid and safe fallback

Changes

• internal/ls/source_map.go: Added validation and clamping logic to prevent inverted ranges
• internal/ls/source_map_test.go: Added comprehensive test suite (219 lines) covering:
  • Inverted ranges being clamped correctly
  • Valid ranges not being modified
  • Different file scenarios being handled
  • Zero-length ranges being valid

Testing

The fix includes extensive test coverage with multiple test cases:

• TestGetMappedLocation_NonMonotonicMappings: Tests the core fix for non-monotonic mappings
• TestGetMappedLocation_ZeroLengthRange: Verifies zero-length ranges are handled correctly
• TestValidateAndClampSpanLogic: Directly tests the validation logic

All tests pass and verify that:

1. Inverted ranges are clamped to prevent negative-length ranges
2. Valid ranges remain unchanged
3. Cross-file scenarios are handled safely

Impact

• Fixes: Restores jump to definition functionality in VSCode when source maps contain negative VLQ offsets
• Real-world impact: Resolves issues for Convex projects and other codebases that generate .d.ts files with out-of-order declarations
• Backward compatible: No breaking changes; only adds validation for edge cases
• Safe fallback: Zero-length ranges are valid and prevent VSCode from mis-interpreting the location
• Standards compliant: Properly handles negative VLQ offsets as specified in the source map specification

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