NON_EXHAUSTIVE_OMITTED_PATTERNS in rustc_lint::builtin - Rust
Description: The `non_exhaustive_omitted_patterns` lint aims to help consumers of a `#[non_exhaustive]` struct or enum who want to match all of its fields/variants explicitly.

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## NON_EXHAUSTIVE_OMITTED_PATTERNS

# Static NON_EXHAUSTIVE_OMITTED_PATTERNS

Source

    pub static NON_EXHAUSTIVE_OMITTED_PATTERNS: &'static Lint

Expand description

The `non_exhaustive_omitted_patterns` lint aims to help consumers of a `#[non_exhaustive]` struct or enum who want to match all of its fields/variants explicitly.

The `#[non_exhaustive]` annotation forces matches to use wildcards, so exhaustiveness checking cannot be used to ensure that all fields/variants are matched explicitly. To remedy this, this allow-by-default lint warns the user when a match mentions some but not all of the fields/variants of a `#[non_exhaustive]` struct or enum.

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

This will produce:

    warning: some variants are not matched explicitly
       --> $DIR/reachable-patterns.rs:70:9
       |
    LL |         match Bar::A {
       |               ^ pattern `Bar::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 variants are matched explicitly by adding the suggested match arms
       = note: the matched value is of type `Bar` and the `non_exhaustive_omitted_patterns` attribute was found

Warning: setting this to `deny` will make upstream non-breaking changes (adding fields or variants to a `#[non_exhaustive]` struct or enum) break your crate. This goes against expected semver behavior.

#### §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.