Fix section offsets in generatedMappings() for indexed source maps - #1976
Merged
robhogan merged 2 commits intoSep 26, 2026
Merged
Conversation
SectionsConsumer#generatedMappings() added a section's column offset to mappings on every line of the section, while originalPositionFor() only applies it to the first line. It also emitted the unmapped segment that precedes a section's first mapping at line 1, column 0 instead of at the section's offset. composeSourceMaps iterates the last map with eachMapping(), so composing with an indexed map as the last map could put mappings on the wrong column.
Composes an indexed map whose second section starts at a nonzero column and spans two lines, and checks that the composed map resolves each position the same way the indexed map does. Without the SectionsConsumer fix, both the unmapped start of the section and its second line resolve differently.
robhogan
approved these changes
Sep 26, 2026
robhogan
left a comment
Collaborator
There was a problem hiding this comment.
Nice find, tidy fix - thanks!
kwy404
added a commit
to kwy404/kwy404
that referenced
this pull request
Sep 26, 2026
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Summary
For indexed (sectioned) source maps,
generatedMappings()onmetro-source-map'sConsumer(whicheachMapping()iterates) applies section offsets differently fromoriginalPositionFor():originalPositionFor()already does (see the existingcolumn offset only applies to first line of sectiontest).composeSourceMapswalks the last map witheachMapping(), so when that map is an indexed map, composed mappings can land on the wrong column. For example, with a second section at{line: 0, column: 4}that spans two lines, the composed map puts that section's second line mapping at column 4 instead of 0. Looking up line 2, column 0 in the composed map then returns no source, while the indexed map itself maps that position.This code has not changed since the consumer was added in ccd508c. The fix applies the column offset only to mappings on the first line of a section, and places the inserted unmapped segment at the section's offset.
Changelog: [Fix] Fix generated positions when iterating the mappings of indexed source maps, e.g. in
composeSourceMapsTest plan
Added a test to
packages/metro-source-map/src/__tests__/Consumer-test.js: an indexed map whose second section starts at column 4, has no mapping at its start, and spans two lines.[1, 0]instead of[1, 4], and the second line mapping at[2, 4]instead of[2, 0].yarn jest packages/metro-source-map: 7 suites, 191 tests passed.jestrun on Windows with Node 24.16.0 (soscripts/jestFilter.jsskips the suites it lists as broken on Windows): 125 suites passed, 1921 tests passed, 24 skipped.flow check: no errors.eslintandprettier --checkon both changed files: clean.