Clippy lint · Style · checked with Clippy 0.1.99

clippy::collapsible_match: this `match` can be collapsed into the outer `match`

What it meansFinds nested match or if let expressions where the patterns may be “collapsed” together without adding any branches. Note that this lint is not intended to find all cases where nested match patterns can be merged, but only cases where merging would most likely make the code more readable. It is in the style group and warns by default.

What you see

warning: this `match` can be collapsed into the outer `match`
 --> main.rs:5:20
  |
5 |           Some(n) => match n {
  |  ____________________^
6 | |             Ok(n) => n,
7 | |             _ => return,
8 | |         }
  | |_________^
  |
help: the outer pattern can be modified to include the inner pattern
 --> main.rs:5:14
  |
5 |         Some(n) => match n {
  |              ^ replace this binding
6 |             Ok(n) => n,
  |             ^^^^^ with this pattern
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#collapsible_match
  = note: `#[warn(clippy::collapsible_match)]` on by default

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

It is unnecessarily verbose and complex.

Code that works but is not idiomatic Rust. Warns by default.

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

fn func(opt: Option<Result<u64, String>>) {
    let n = match opt {
        Some(n) => match n {
            Ok(n) => n,
            _ => return,
        }
        None => return,
    };
}

Write this instead

fn func(opt: Option<Result<u64, String>>) {
    let n = match opt {
        Some(Ok(n)) => n,
        _ => return,
    };
}

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::collapsible_match)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
collapsible_match = "allow"

One run

cargo clippy -- -A clippy::collapsible-match

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Settings in clippy.toml

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

Stylegroup

warndefault level

yesfixed by cargo clippy --fix

1.50.0added in Rust

Clippy's own documentation

What it does

Finds nested match or if let expressions where the patterns may be “collapsed” together without adding any branches.

Note that this lint is not intended to find all cases where nested match patterns can be merged, but only cases where merging would most likely make the code more readable.

Why is this bad?

It is unnecessarily verbose and complex.

Example

fn func(opt: Option<Result<u64, String>>) {
    let n = match opt {
        Some(n) => match n {
            Ok(n) => n,
            _ => return,
        }
        None => return,
    };
}

Use instead:

fn func(opt: Option<Result<u64, String>>) {
    let n = match opt {
        Some(Ok(n)) => n,
        _ => return,
    };
}

Configuration

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

(default: current version)

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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