fix(md): parse space+tab indented sublists as nested list items by xsourabhsharma · Pull Request #10703 · biomejs/biome

xsourabhsharma · 2026-06-19T12:30:53Z

AI assistance: This PR was developed with guidance from Claude.

Summary

Space+tab-indented sublists were parsed as lazy paragraph continuation instead of nested list items. The fix makes tab expansion column-relative per CommonMark 2.2, and detects sibling list markers at the enclosing item's marker indent.

  * Text

  	- foo
- 	- lreum
- 	- bar
+ 	- lreum
+ 	- bar

Before, only the first - foo became a list item; lreum and bar were absorbed as paragraph text. They are now three sibling <li> items.

Test Plan

cargo test -p biome_markdown_parser (incl. new regression test sublist_loose_space_tab_indent.md)
cargo test -p biome_markdown_formatter
cargo test -p biome_markdown_factory

All pass. Three formatter snapshots (tab.md, nested-tab.md, indent.md) were previously capturing formatter bugs caused by this parser bug; they now match Prettier.

Note: the pure 4-space indent variant ( - foo / - lreum) has a separate pre-existing issue (siblings nesting one level too deep) that is out of scope here.

Tabs were counted as a flat 4 columns and sibling list markers were only matched at the item content indent, so space+tab-indented sublist items were absorbed as lazy paragraph continuation (biomejs#10558). Make tab expansion column-relative per CommonMark 2.2 (TAB_STOP_SPACES minus column mod TAB_STOP_SPACES) and add at_sibling_list_marker to detect sibling bullet and ordered markers at the enclosing item marker indent. The issue input now parses as a single nested ul with three li children. Parser suite (169 spec tests incl. a new regression test), formatter suite (817 tests) and factory tests all pass. The three formatter snapshots that changed were documenting formatter bugs caused by this parser bug and now match Prettier.

changeset-bot · 2026-06-19T12:30:59Z

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Latest commit: ab06831

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github-actions · 2026-06-19T12:31:02Z

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coderabbitai · 2026-06-19T12:34:51Z

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Run ID: 11ac46cf-5e9a-4548-bd7b-afaa3317b175

📥 Commits

Reviewing files that changed from the base of the PR and between 8aa8a54 and ab06831.

📒 Files selected for processing (3)

crates/biome_markdown_parser/src/syntax/list.rs
crates/biome_markdown_parser/src/syntax/mod.rs
crates/biome_markdown_parser/src/token_source.rs

🚧 Files skipped from review as they are similar to previous changes (3)

crates/biome_markdown_parser/src/syntax/mod.rs
crates/biome_markdown_parser/src/token_source.rs
crates/biome_markdown_parser/src/syntax/list.rs

Walkthrough

The Markdown parser gains CommonMark-compliant tab-stop expansion: line_start_leading_indent now advances the running column to the next multiple of TAB_STOP_SPACES per tab rather than using a fixed 4-space increment. compute_marker_indent is extended to raw-scan leading whitespace from the physical line start when a tab is present. A new at_sibling_list_marker predicate checks whether the current position is a sibling list marker at the enclosing item's exact marker indent. at_block_interrupt is wired to call this predicate, preventing sibling markers from being consumed as lazy paragraph continuation. Three test fixtures covering space+tab, two-space+tab, and three-space+tab sublist indentation are added.

Possibly related PRs

biomejs/biome#9717: Modifies compute_marker_indent and list continuation/interrupt detection, directly overlapping with the same functions changed here.
biomejs/biome#9776: Fixes lazy continuation ownership when line indent equals a nested list marker indent, touching the same list continuation logic in list.rs.
biomejs/biome#10376: Adjusts sibling/parent interruption logic in at_block_interrupt to break paragraphs at enclosing marker indents, directly paralleling the change in this PR.

Suggested reviewers

ematipico
dyc3

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the main change: fixing space+tab-indented sublists to parse as nested list items.
Description check	✅ Passed	The description is directly about the parser bug, the CommonMark tab fix, and the new regression test.
Linked Issues check	✅ Passed	The changes address `#10558` by treating the example as nested bullet list items instead of lazy paragraph text.
Out of Scope Changes check	✅ Passed	The extra test fixture and formatter snapshot updates are supporting the fix, with no unrelated changes apparent.

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codspeed-hq · 2026-06-19T12:37:40Z

Merging this PR will not alter performance

✅ 28 untouched benchmarks
⏩ 228 skipped benchmarks¹

_{Comparing xsourabhsharma:fix/md-space-tab-sublist (ab06831) with main (f62fb8b)²}

228 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩
No successful run was found on main (f719af6) during the generation of this report, so f62fb8b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

compute_marker_indent now scans through leading tabs (column-relative) to the marker when the line's leading whitespace contains a tab, so 2space+tab and 3space+tab indents that expand to column 4 are measured as 4 rather than as the space count only. Pure-space lines keep the original behavior. Adds regression tests for both variants. All fuzz genLooseTabSublist cases (space+tab, 2space+tab, 3space+tab, pure tab) now parse as three sibling list items.

Replace the pure-space char-count loop with chars().count() so the loop variable is no longer unused. Behavior is unchanged.

ematipico

Looks good, but the docs need some. Please look after them

ematipico · 2026-06-24T08:42:32Z

+            // A sibling list item sits at the enclosing item's *marker* indent,
+            // which can be below its content (`required`) indent when the marker
+            // is followed by more than one space (or a tab). Detect siblings at
+            // the marker indent too, otherwise such lines are absorbed as lazy
+            // paragraph continuation (biomejs/biome#10558).


Do you understand this comment? What does the mention of "required" mean? Also, why is there a mention of an issue? We're fixing a bug.

ematipico · 2026-06-24T08:44:50Z

+    /// Tabs expand column-relative to the next multiple of `TAB_STOP_SPACES` (4),
+    /// per CommonMark §2.2. Counting a tab as a flat 4 would over-count when the
+    /// tab does not start at a column that is already a multiple of 4 (e.g. a
+    /// leading ` \t` is 4 columns, not 5), which mis-classifies space+tab-indented
+    /// sub-list markers as lazy paragraph continuation (biomejs/biome#10558).


Did you read the comment? This isn't documentation, it's Claude Code writing a diary. Please reword it so it's sounds like proper documentation. There's also no need to track the issue. We're fixing a bug.

xsourabhsharma · 2026-06-24T11:23:26Z

give me 20min i think need to do myself.

xsourabhsharma mentioned this pull request Jun 19, 2026

🐛 [md] Incorrect parsing of sub lists #10558

Closed

1 task

github-actions Bot added A-Parser Area: parser A-Formatter Area: formatter L-Markdown Language: Markdown labels Jun 19, 2026

xsourabhsharma marked this pull request as draft June 19, 2026 12:53

chore: add changeset for md space+tab sublist fix

40123c6

xsourabhsharma marked this pull request as ready for review June 20, 2026 02:40

xsourabhsharma marked this pull request as draft June 20, 2026 03:08

xsourabhsharma added 2 commits June 20, 2026 09:19

fix: resolve clippy unused variable in compute_marker_indent

8aa8a54

Replace the pure-space char-count loop with chars().count() so the loop variable is no longer unused. Behavior is unchanged.

xsourabhsharma marked this pull request as ready for review June 20, 2026 06:53

ematipico reviewed Jun 24, 2026

View reviewed changes

Delete .changeset/md-space-tab-sublist.md

9a1b546

docs(md): reword comment blocks per review feedback

ab06831

ematipico approved these changes Jun 24, 2026

View reviewed changes

ematipico merged commit 13ee358 into biomejs:main Jun 24, 2026
31 checks passed

xsourabhsharma deleted the fix/md-space-tab-sublist branch June 24, 2026 12:38

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fix(md): parse space+tab indented sublists as nested list items#10703

fix(md): parse space+tab indented sublists as nested list items#10703
ematipico merged 6 commits into
biomejs:mainfrom
xsourabhsharma:fix/md-space-tab-sublist

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Summary

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Merging this PR will not alter performance

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