Clippy lint · Style · checked with Clippy 0.1.99

clippy::unnecessary_lazy_evaluations: unnecessary closure used to substitute value for `Option::None`

What it meansAs the counterpart to or_fun_call, this lint looks for unnecessary lazily evaluated closures on Option and Result. This lint suggests changing the following functions, when eager evaluation results in simpler code: unwrap_or_else to unwrap_or and_then to and or_else to or get_or_insert_with to get_or_insert ok_or_else to ok_or then to then_some (for msrv >= 1.62.0) It is in the style group and warns by default.

What you see

warning: unnecessary closure used to substitute value for `Option::None`
 --> main.rs:5:1
  |
5 | opt.unwrap_or_else(|| 42);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unnecessary_lazy_evaluations
  = note: `#[warn(clippy::unnecessary_lazy_evaluations)]` on by default
help: use `unwrap_or` instead
  |
5 - opt.unwrap_or_else(|| 42);
5 + opt.unwrap_or(42);
  |

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 eager evaluation is shorter and simpler in some cases.

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

let opt: Option<u32> = None;

opt.unwrap_or_else(|| 42);

Write this instead

let opt: Option<u32> = None;

opt.unwrap_or(42);

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
unnecessary_lazy_evaluations = "allow"

One run

cargo clippy -- -A clippy::unnecessary-lazy-evaluations

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

1.48.0added in Rust

Clippy's own documentation

What it does

As the counterpart to or_fun_call, this lint looks for unnecessary lazily evaluated closures on Option and Result.

This lint suggests changing the following functions, when eager evaluation results in simpler code:

unwrap_or_else to unwrap_or
and_then to and
or_else to or
get_or_insert_with to get_or_insert
ok_or_else to ok_or
then to then_some (for msrv >= 1.62.0)

Why is this bad?

Using eager evaluation is shorter and simpler in some cases.

Known problems

It is possible, but not recommended for Deref and Index to have side effects. Eagerly evaluating them can change the semantics of the program.

Example

let opt: Option<u32> = None;

opt.unwrap_or_else(|| 42);

Use instead:

let opt: Option<u32> = None;

opt.unwrap_or(42);

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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