Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::confusing_method_to_numeric_cast: casting function pointer `u16::max` to `usize`

What it meansChecks for casts of a primitive method pointer like max/min to any integer type. It is in the suspicious group and warns by default.

What you see

warning: casting function pointer `u16::max` to `usize`
 --> main.rs:3:9
  |
3 | let _ = u16::max as usize;
  |         ^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#confusing_method_to_numeric_cast
  = note: `#[warn(clippy::confusing_method_to_numeric_cast)]` on by default
help: did you mean to use the associated constant?
  |
3 - let _ = u16::max as usize;
3 + let _ = u16::MAX 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.

Code that is most likely wrong or useless. Warns by default.

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 _ = u16::max as usize;

Write this instead

let _ = u16::MAX as usize;

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::confusing_method_to_numeric_cast)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
confusing_method_to_numeric_cast = "allow"

One run

cargo clippy -- -A clippy::confusing-method-to-numeric-cast

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.

Suspiciousgroup

warndefault level

nofixed by cargo clippy --fix

1.89.0added in Rust

Clippy's own documentation

What it does

Checks for casts of a primitive method pointer like max/min to any integer type.

Why restrict this?

Example

let _ = u16::max as usize;

Use instead:

let _ = u16::MAX 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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