Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::default_numeric_fallback: default numeric fallback might occur

What it meansChecks for usage of unconstrained numeric literals which may cause default numeric fallback in type inference. Default numeric fallback means that if numeric types have not yet been bound to concrete types at the end of type inference, then integer type is bound to i32, and similarly floating type is bound to f64. See RFC0212 for more information about the fallback. It is in the restriction group and off by default.

What you see

warning: default numeric fallback might occur
 --> main.rs:3:9
  |
3 | let i = 10;
  |         ^^ help: consider adding suffix: `10_i32`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#default_numeric_fallback
  = note: requested on the command line with `-W clippy::default-numeric-fallback`

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?

To ensure that every numeric type is chosen explicitly rather than implicitly.

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 (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

let i = 10;
let f = 1.23;

Write this instead

let i = 10_i32;
let f = 1.23_f64;

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]
default_numeric_fallback = "warn"

In code

#![warn(clippy::default_numeric_fallback)]

One run

cargo clippy -- -W clippy::default-numeric-fallback

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

1.52.0added in Rust

Clippy's own documentation

What it does

Checks for usage of unconstrained numeric literals which may cause default numeric fallback in type inference.

Default numeric fallback means that if numeric types have not yet been bound to concrete types at the end of type inference, then integer type is bound to i32, and similarly floating type is bound to f64.

See RFC0212 for more information about the fallback.

Why restrict this?

To ensure that every numeric type is chosen explicitly rather than implicitly.

Known problems

This lint is implemented using a custom algorithm independent of rustc’s inference, which results in many false positives and false negatives.

Example

let i = 10;
let f = 1.23;

Use instead:

let i = 10_i32;
let f = 1.23_f64;

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

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