Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::fn_to_numeric_cast_any: casting function pointer `fn1` to `usize`

What it meansChecks for casts of a function pointer to any integer type. It is in the restriction group and off by default.

What you see

warning: casting function pointer `fn1` to `usize`
 --> main.rs:7:9
  |
7 | let _ = fn1 as usize;
  |         ^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#fn_to_numeric_cast_any
  = note: requested on the command line with `-W clippy::fn-to-numeric-cast-any`
help: did you mean to invoke the function?
  |
7 | let _ = fn1() as usize;
  |            ++

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?

Casting a function pointer to an integer can have surprising results and can occur accidentally if parentheses are omitted from a function call. If you aren’t doing anything low-level with function pointers then you can opt out of casting functions to integers in order to avoid mistakes. Alternatively, you can use this lint to audit all uses of function pointer casts in your code.

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

// fn1 is cast as `usize`
fn fn1() -> u16 {
    1
};
let _ = fn1 as usize;

Write this instead

// maybe you intended to call the function?
fn fn2() -> u16 {
    1
};
let _ = fn2() as usize;

// or

// maybe you intended to cast it to a function type?
fn fn3() -> u16 {
    1
}
let _ = fn3 as fn() -> u16;

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]
fn_to_numeric_cast_any = "warn"

In code

#![warn(clippy::fn_to_numeric_cast_any)]

One run

cargo clippy -- -W clippy::fn-to-numeric-cast-any

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

1.58.0added in Rust

Clippy's own documentation

What it does

Checks for casts of a function pointer to any integer type.

Why restrict this?

Example

// fn1 is cast as `usize`
fn fn1() -> u16 {
    1
};
let _ = fn1 as usize;

Use instead:

// maybe you intended to call the function?
fn fn2() -> u16 {
    1
};
let _ = fn2() as usize;

// or

// maybe you intended to cast it to a function type?
fn fn3() -> u16 {
    1
}
let _ = fn3 as fn() -> u16;

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

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