Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::let_underscore_untyped: non-binding `let` without a type annotation

What it meansChecks for let _ = <expr> without a type annotation, and suggests to either provide one, or remove the let keyword altogether. It is in the restriction group and off by default.

What you see

warning: non-binding `let` without a type annotation
 --> main.rs:6:1
  |
6 | let _ = foo();
  | ^^^^^^^^^^^^^^
  |
help: consider adding a type annotation
 --> main.rs:6:6
  |
6 | let _ = foo();
  |      ^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#let_underscore_untyped
  = note: requested on the command line with `-W clippy::let-underscore-untyped`

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?

The let _ = <expr> expression ignores the value of <expr>, but will continue to do so even if the type were to change, thus potentially introducing subtle bugs. By supplying a type annotation, one will be forced to re-visit the decision to ignore the value in such cases.

Lints that forbid language features for specific codebases (no unwrap, no indexing...). Off by default; pick them one by one, never the whole group.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

fn foo() -> Result<u32, ()> {
    Ok(123)
}
let _ = foo();

Write this instead

fn foo() -> Result<u32, ()> {
    Ok(123)
}
// Either provide a type annotation:
let _: Result<u32, ()> = foo();
// …or drop the let keyword:
_ = foo();

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
let_underscore_untyped = "warn"

In code

#![warn(clippy::let_underscore_untyped)]

One run

cargo clippy -- -W clippy::let-underscore-untyped

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.

Restrictiongroup

allowdefault level

nofixed by cargo clippy --fix

1.69.0added in Rust

Clippy's own documentation

What it does

Checks for let _ = <expr> without a type annotation, and suggests to either provide one, or remove the let keyword altogether.

Why restrict this?

The let _ = <expr> expression ignores the value of <expr>, but will continue to do so even if the type were to change, thus potentially introducing subtle bugs. By supplying a type annotation, one will be forced to re-visit the decision to ignore the value in such cases.

Known problems

The _ = <expr> is not properly supported by some tools (e.g. IntelliJ) and may seem odd to many developers. This lint also partially overlaps with the other let_underscore_* lints.

Example

fn foo() -> Result<u32, ()> {
    Ok(123)
}
let _ = foo();

Use instead:

fn foo() -> Result<u32, ()> {
    Ok(123)
}
// Either provide a type annotation:
let _: Result<u32, ()> = foo();
// …or drop the let keyword:
_ = foo();

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

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