Clippy lint · Style · checked with Clippy 0.1.99

clippy::let_and_return: returning the result of a `let` binding from a block

What it meansChecks for let-bindings, which are subsequently returned. It is in the style group and warns by default.

What you see

warning: returning the result of a `let` binding from a block
 --> main.rs:5:5
  |
4 |     let x = String::new();
  |     ---------------------- unnecessary `let` binding
5 |     x
  |     ^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#let_and_return
  = note: `#[warn(clippy::let_and_return)]` on by default
help: return the expression directly
  |
4 ~     
5 ~     String::new()
  |

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 just extraneous code. Remove it to make your code more rusty.

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 foo() -> String {
    let x = String::new();
    x
}

Write this instead

fn foo() -> String {
    String::new()
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
let_and_return = "allow"

One run

cargo clippy -- -A clippy::let-and-return

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.

Stylegroup

warndefault level

yesfixed by cargo clippy --fix

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks for let-bindings, which are subsequently returned.

Why is this bad?

It is just extraneous code. Remove it to make your code more rusty.

Known problems

In the case of some temporaries, e.g. locks, eliding the variable binding could lead to deadlocks. See this issue. This could become relevant if the code is later changed to use the code that would have been bound without first assigning it to a let-binding.

Example

fn foo() -> String {
    let x = String::new();
    x
}

instead, use

fn foo() -> String {
    String::new()
}

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

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