AD-0006 — Markdown via dprint-plugin-markdown, and retiring dprint

Context

FR-1.1/1.1a/1.6 require canonical Markdown with hard-wrapped prose, guardrails (never break inline code, split links, or wrap tables/fenced code; preserve hard breaks), and verbatim fenced code. This is the last per-format pass. The repo already formatted its Markdown with dprint (the markdown wasm plugin, lineWidth 80, textWrap always), gated by a required CI job — so prim taking over Markdown overlaps and, per the issue, should replace it.

Decision: dprint-plugin-markdown as a library

dprint-plugin-markdown = "0.22" — the same engine the repo already used, now a Rust dependency of prim-fmt, in a markdown module. prim_fmt::format dispatches FileKind::Markdown to it.

• format_text(text, &Configuration, code_block_cb) -> anyhow::Result<Option<String>>.
• Config from Style: line_width = max_line_length.unwrap_or(80), text_wrap = TextWrap::Always (FR-1.1 hard wrap). EOL/final newline stay with hygiene.
• FR-1.6 via the callback: format_code_block_text returns Ok(None), so dprint never reformats embedded code — fenced blocks pass through verbatim.
• dprint’s defaults give FR-1.1 canonical output (ATX headings, dash list markers, padded tables, normalized blank lines) and its wrapper honors the FR-1.1a guardrails (inline code atomic, links not split, tables/code not wrapped, hard breaks preserved).
• Markdown is effectively infallible (CommonMark accepts any input), so the FormatError::Parse arm is defensive and unreachable in practice.

Because prim uses the same engine and config as the repo’s dprint setup, prim’s output matches the existing Markdown byte-for-byte — the migration produced zero reformatting churn.

Decision: disable dprint-core debug assertions in the dev profile

dprint-core’s printer carries a debug_assert that panics on valid Markdown containing an inline code span with an embedded newline (e.g. a long backticked span that a previous wrap split across two source lines). Release builds — and the dprint wasm plugins — compile the assertion out, which is why dprint itself never crashed. prim’s dev/test builds hit it.

A targeted profile override in the workspace Cargo.toml disables debug assertions for the dprint-core package only:

[profile.dev.package.dprint-core]
debug-assertions = false

prim’s own assertions are unaffected; only this dependency’s over-aggressive debug check is silenced, so prim is robust on such input in every build. A regression test (markdown::tests::inline_code_spanning_a_newline_does_not_panic) pins the behaviour.

Decision: retire dprint

dprint existed in this repo solely to format Markdown. With prim owning it:

• dprint.json is deleted.
• justfile fmt/lint call prim instead of dprint fmt/dprint check.
• The CI Dprint job is replaced by a prim self-check job (cargo run -p prim-cli -- --check .); the gate’s needs is updated.
• markdownlint stays as an independent lint (it checks content rules prim does not). prim and markdownlint agree on the repo’s Markdown.

prim now formats all of its own connective tissue — its stated purpose.

Consequences

dprint-plugin-markdown (and the shared dprint-core/jsonc-parser stack from AD-0003) are prim-fmt dependencies. With Markdown done, all per-format passes (FR-1.1–1.6) are implemented; Milestone 3 is complete. The .md-as-hygiene- vehicle behavioural tests were retargeted to .txt orphans, completing the migration of those tests off owned-but-now-structured file types.

Satisfies: FR-1.1 (Markdown canonical + prose wrap), FR-1.1a (wrap guardrails), FR-1.6 (fenced code verbatim), FR-3.4/6.2 (no reorder / data unchanged).
Related: AD-0003 (JSON via dprint-plugin-json; the fallible format API and the shared dprint-core stack), docs/design/system.md, crates/prim-fmt/src/markdown.rs.