Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::lossy_float_literal: literal cannot be represented as the underlying type without loss of precision

What it meansChecks for whole number float literals that cannot be represented as the underlying type without loss. It is in the restriction group and off by default.

What you see

warning: literal cannot be represented as the underlying type without loss of precision
 --> main.rs:3:14
  |
3 | let _: f32 = 16_777_217.0; // 16_777_216.0
  |              ^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#lossy_float_literal
  = note: requested on the command line with `-W clippy::lossy-float-literal`
help: consider changing the type or replacing it with
  |
3 - let _: f32 = 16_777_217.0; // 16_777_216.0
3 + let _: f32 = 16_777_216.0; // 16_777_216.0
  |

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?

If the value was intended to be exact, it will not be. This may be especially surprising when the lost precision is to the left of the decimal point.

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

let _: f32 = 16_777_217.0; // 16_777_216.0

Write this instead

let _: f32 = 16_777_216.0;
let _: f64 = 16_777_217.0;

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]
lossy_float_literal = "warn"

In code

#![warn(clippy::lossy_float_literal)]

One run

cargo clippy -- -W clippy::lossy-float-literal

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

1.43.0added in Rust

Clippy's own documentation

What it does

Checks for whole number float literals that cannot be represented as the underlying type without loss.

Why restrict this?

If the value was intended to be exact, it will not be. This may be especially surprising when the lost precision is to the left of the decimal point.

Example

let _: f32 = 16_777_217.0; // 16_777_216.0

Use instead:

let _: f32 = 16_777_216.0;
let _: f64 = 16_777_217.0;

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

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