Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::match_as_ref: manual implementation of `Option::as_ref`

What it meansChecks for match which is used to add a reference to an Option value. It is in the complexity group and warns by default.

What you see

warning: manual implementation of `Option::as_ref`
 --> main.rs:5:22
  |
5 |   let r: Option<&()> = match x {
  |  ______________________^
6 | |     None => None,
7 | |     Some(ref v) => Some(v),
8 | | };
  | |_^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#match_as_ref
  = note: `#[warn(clippy::match_as_ref)]` on by default
help: use `Option::as_ref()` directly
  |
5 - let r: Option<&()> = match x {
6 -     None => None,
7 -     Some(ref v) => Some(v),
8 - };
5 + let r: Option<&()> = x.as_ref();
  |

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?

Using as_ref() or as_mut() instead is shorter.

Code that does something simple in a complicated way. 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

let x: Option<()> = None;

let r: Option<&()> = match x {
    None => None,
    Some(ref v) => Some(v),
};

Write this instead

let x: Option<()> = None;

let r: Option<&()> = x.as_ref();

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
match_as_ref = "allow"

One run

cargo clippy -- -A clippy::match-as-ref

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.

Complexitygroup

warndefault level

yesfixed by cargo clippy --fix

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks for match which is used to add a reference to an Option value.

Why is this bad?

Using as_ref() or as_mut() instead is shorter.

Example

let x: Option<()> = None;

let r: Option<&()> = match x {
    None => None,
    Some(ref v) => Some(v),
};

Use instead:

let x: Option<()> = None;

let r: Option<&()> = x.as_ref();

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

Up next

Also worth your time

Did this answer your question?