Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::as_underscore: using `as _` conversion

What it meansChecks for the usage of as _ conversion using inferred type. It is in the restriction group and off by default.

What you see

warning: using `as _` conversion
 --> main.rs:5:5
  |
5 | foo(n as _);
  |     ^^^^^-
  |          |
  |          help: consider giving the type explicitly: `usize`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#as_underscore
  = note: requested on the command line with `-W clippy::as-underscore`

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 conversion might include lossy conversion or a dangerous cast that might go undetected due to the type being inferred. The lint is allowed by default as using _ is less wordy than always specifying the type.

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 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(n: usize) {}
let n: u16 = 256;
foo(n as _);

Write this instead

fn foo(n: usize) {}
let n: u16 = 256;
foo(n as usize);

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]
as_underscore = "warn"

In code

#![warn(clippy::as_underscore)]

One run

cargo clippy -- -W clippy::as-underscore

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

yesfixed by cargo clippy --fix

1.63.0added in Rust

Clippy's own documentation

What it does

Checks for the usage of as _ conversion using inferred type.

Why restrict this?

The conversion might include lossy conversion or a dangerous cast that might go undetected due to the type being inferred.

The lint is allowed by default as using _ is less wordy than always specifying the type.

Example

fn foo(n: usize) {}
let n: u16 = 256;
foo(n as _);

Use instead:

fn foo(n: usize) {}
let n: u16 = 256;
foo(n as usize);

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

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