Public items in a library crate compose the published API surface. Consumers discover what to call by reading the docs; an undocumented pub fn is a discoverability gap that turns into a support cost the first time someone has to read the source to figure out what it does.

The check fires on pub declarations of the four item kinds that make up the API surface: fn, struct, enum, trait. Restricted visibilities (pub(crate), pub(super), pub(in path)) are exempt — they're internal API, not the published contract.

What counts as documented:

• a preceding /// ... line doc comment,
• a preceding /** ... */ block doc comment,
• a preceding #[doc = "..."] attribute,
• or #[doc(hidden)] (deliberately undocumented — same opt-out as clippy's missing_docs).

Example

// FIRES: no doc comment.
pub fn parse_id(s: &str) -> i64 {
    s.parse::<i64>().unwrap_or(0)
}

// DOES NOT FIRE: outer doc comment.
/// Parse `s` as an `i64`, returning 0 on failure.
pub fn parse_id_documented(s: &str) -> i64 {
    s.parse::<i64>().unwrap_or(0)
}

// DOES NOT FIRE: restricted visibility (internal API).
pub(crate) fn helper() {}

// DOES NOT FIRE: explicitly hidden.
#[doc(hidden)]
pub fn internal_thing() {}

What to do

• Add a /// doc comment naming what the item does, its arguments, and any non-obvious failure modes. A one-liner is fine for self-evident helpers; longer docs are valuable for anything stateful.
• If the item shouldn't be part of the public surface, narrow the visibility to pub(crate) or remove the pub entirely.
• If the item is part of the public surface but deliberately undocumented (private extension hook, deprecated alias being removed), apply #[doc(hidden)] so the suppression is visible in source.

Suppression

For items that are temporarily undocumented (work-in-progress trait methods, scaffolding), use an inline directive linked to a tracking issue:

// cofferdam-ignore: Rust.MissingPubDoc: docs land with <issue>
pub fn next_step() -> i32 { 0 }

A blanket file-wide suppression on this check tends to silently grow as new public items are added; prefer per-item directives.