Fence scan

Compile finds fenced code and code spans with one scan, which the steps that leave code as written share. stripAnchors and heading demotion read each line with it, and so does the heading reader behind the refs registry and section slugs. The link passes, codeEvidence, broken-link marking, link lint and table re-alignment also run it on the text of each blockquote, up to ten blockquotes deep. Table re-alignment reads only the fences, since a pipe splits a cell in a code span too. Fence scan limits says where those readers can still disagree about where code is.

The fence scan follows list items. A bullet or numbered list marker followed by one to four columns of whitespace opens an item whose text starts after that whitespace. A marker with no text after it opens an empty item whose text would start one column past the marker, and a blank line right after an empty item ends it. When a marker starts at or past the column where an open item’s text starts, it nests in that item. The item ends at the first non-blank line that starts left of its text, unless that line continues a paragraph. Headings, fences, blockquotes, thematic breaks and list items interrupt a paragraph, and any other line continues it. In its own item, a list item interrupts a paragraph only when it has text and, if numbered, starts at 1.

The container of a fence is the column where the innermost open item’s text starts, or column 0 outside any item. Three or more backticks or tildes open a fence where they start a line’s text less than four columns past the container, or right after a list marker. Tabs advance to the next multiple of four columns, and a backtick fence’s info string can’t hold a backtick.

A run of the same character, at least as long and with nothing after it, closes the fence when it starts less than four columns past the container. A non-blank line that starts left of the container ends the fence along with its list item. A fence with no closer runs to the end of the text unless its list item ends first.

Code spans are paired within each line. A run of backticks opens a span that the next run of the same length closes.

Fence scan limits

The scan doesn’t read blockquotes or HTML. Run on its own, it misses a fence behind >, which the readers that also scan blockquote text find. It doesn’t let a line continue a quoted paragraph, so an unindented line right after a quote in a list item ends the item. A fence line inside an HTML comment or HTML block opens or closes a fence like any other. Indented code blocks aren’t fences either, and the scan reads their lines as prose.

The scan doesn’t open an item for a list marker followed by five or more columns of whitespace. CommonMark opens one there whose first line is indented code, so a fence in that item can run past the line where CommonMark ends it.

Because code spans are paired within each line, a span that wraps onto the line, or a backslash-escaped backtick before it, can shift the pairing.

A reader that starts a new scan on each shard and a reader that scans the whole guide at once can disagree about a shard that follows one ending inside an open fence. Heading demotion starts a new scan on each text it demotes, such as one shard, while the refs registry reads its whole text with one scan. The per-shard link passes and codeEvidence start a new scan on each shard, while broken-link marking and compiled link lint read the whole guide with one scan, as the intra-guide run on the stitched body does.