The serializer escapes a backtick that cannot form a code span where it sits

[Azganoth]

Summary

Saving escapes a backtick that cannot open a code span. A code span needs a closing run of the same length inside the same block, and a backtick with no such run is literal text as written. The serializer escapes it anyway, so the file gains a backslash the author did not write and the document is unchanged.

corpus/commonmark/blocks.md carries the case under the heading Block structure takes precedence over inline parsing, where two list items each hold one backtick and the span cannot cross the boundary between them. That is one instance of the general rule rather than a case of its own.

Steps to reproduce

1. Open a document containing a followed by a single backtick and b.
2. Save it without editing.
3. Read the file.

Expected behavior

The line is unchanged. Nothing closes the backtick, so nothing needs holding literal.

Actual behavior

Measured against 8bc36754 by driving the editor mount used by the plugin tests.

Document	Saved
a ` b	a \` b
`unclosed	\`unclosed
- `a code span does not / - cross a list-item boundary`	both backticks escaped
a `\n\nb ` (one in each of two paragraphs)	both backticks escaped
> a ` b	> a \` b
# a ` b	# a \` b

A backtick that does close is unaffected, because the pair parses as a code span and never reaches this path: a ` b ` c is read as an inlineCode node and written a `b` c.

Related context

• Related issues or pull requests: Preserve the authored form of a Markdown file on save #251 is the parent. The serializer escapes tildes that cannot form a strikethrough where they sit #256 is the closest precedent, relaxing tildes that cannot form a strikethrough where they sit; The serializer escapes a construct it can only see one text node of #292 and The serializer escapes a bracket run whose link tail it never parses #293 relax constructs the serializer can see only one text node of. The serializer escapes a marker run too short to open a thematic break #373 is the sibling class for a block marker rather than an inline one.
• Documentation, decisions, or prior investigation: docs/decisions.md under Preserve the form a file was written in; corpus/commonmark/blocks.md under Block structure takes precedence over inline parsing.

Done when

• A backtick with no closing run of its own length in the same block is written without an escape.
• A backtick that could still be closed keeps its escape, and a real code span is unaffected.
• Regression coverage asserts the saved bytes and the document the saved file reopens as, since closure already holds and cannot catch this.

Notes, logs, screenshots

Diagnosis

No relaxation pass reconsiders a backtick. serializeMarkdownText runs relaxAttentionEscapes, relaxBlockMarkerEscapes, relaxBracketEscapes, relaxAngleEscapes, relaxAutolinkLiteralEscapes, and relaxCharacterReferenceEscapes. A backtick falls into none of them: ATTENTION_CHARACTERS is *_~, and relaxBlockMarkerEscapes branches only on #, the ordered-list markers, and |. The escape state.safe emits is therefore never revisited.

The machinery to decide it already exists. skipCodeSpan walks a block for exactly this question and returns the opening offset unchanged when no closing run of equal length follows, which is the condition under which the escape is unnecessary. It is currently used only by findBracketLinks.

Out of scope

• The ** and -- block-marker escapes in the same file, which are The serializer escapes a marker run too short to open a thematic break #373 and its hyphen sibling.
• Tilde runs, which The serializer escapes tildes that cannot form a strikethrough where they sit #256 already covers.