Static rustc_lint_defs::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS

pub static NON_EXHAUSTIVE_OMITTED_PATTERNS: &'static Lint

The non_exhaustive_omitted_patterns lint detects when a wildcard (_ or ..) in a pattern for a #[non_exhaustive] struct or enum is reachable.

Example

// crate A
#[non_exhaustive]
pub enum Bar {
    A,
    B, // added variant in non breaking change
}

// in crate B
#![feature(non_exhaustive_omitted_patterns_lint)]

match Bar::A {
    Bar::A => {},
    #[warn(non_exhaustive_omitted_patterns)]
    _ => {},
}

This will produce:

warning: reachable patterns not covered of non exhaustive enum
   --> $DIR/reachable-patterns.rs:70:9
   |
LL |         _ => {}
   |         ^ pattern `B` not covered
   |
 note: the lint level is defined here
  --> $DIR/reachable-patterns.rs:69:16
   |
LL |         #[warn(non_exhaustive_omitted_patterns)]
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = help: ensure that all possible cases are being handled by adding the suggested match arms
   = note: the matched value is of type `Bar` and the `non_exhaustive_omitted_patterns` attribute was found

Explanation

Structs and enums tagged with #[non_exhaustive] force the user to add a (potentially redundant) wildcard when pattern-matching, to allow for future addition of fields or variants. The non_exhaustive_omitted_patterns lint detects when such a wildcard happens to actually catch some fields/variants. In other words, when the match without the wildcard would not be exhaustive. This lets the user be informed if new fields/variants were added.