Clippy lints / Style
Clippy lint · Style · checked with Clippy 0.1.99
clippy::fn_to_numeric_cast_with_truncation: casting function pointer `fn1` to `i32`, which truncates the value
What it meansChecks for casts of a function pointer to a numeric type not wide enough to store an address. It is in the style group and warns by default.
What you see
warning: casting function pointer `fn1` to `i32`, which truncates the value --> main.rs:6:9 | 6 | let _ = fn1 as i32; | ^^^^^^^^^^ help: try: `fn1 as usize` | = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#fn_to_numeric_cast_with_truncation = note: `#[warn(clippy::fn_to_numeric_cast_with_truncation)]` on by default
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?
Such a cast discards some bits of the function’s address. If this is intended, it would be more clearly expressed by casting to usize first, then casting the usize to the intended type (with a comment) to perform the truncation.
Code that works but is not idiomatic Rust. 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
fn fn1() -> i16 {
1
};
let _ = fn1 as i32;
Write this instead
// Cast to usize first, then comment with the reason for the truncation
fn fn1() -> i16 {
1
};
let fn_ptr = fn1 as usize;
let fn_ptr_truncated = fn_ptr as i32;
Turn it off, or on
Allow it where it does not fit your code, as narrowly as you can:
On one item
#[allow(clippy::fn_to_numeric_cast_with_truncation)]
fn my_function() { /* ... */ }Whole crate (Cargo.toml)
[lints.clippy]
fn_to_numeric_cast_with_truncation = "allow"One run
cargo clippy -- -A clippy::fn-to-numeric-cast-with-truncationThe [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.
Clippy's own documentation
What it does
Checks for casts of a function pointer to a numeric type not wide enough to store an address.
Why is this bad?
Such a cast discards some bits of the function’s address. If this is intended, it would be more
clearly expressed by casting to usize first, then casting the usize to the intended type (with
a comment) to perform the truncation.
Example
fn fn1() -> i16 {
1
};
let _ = fn1 as i32;
Use instead:
// Cast to usize first, then comment with the reason for the truncation
fn fn1() -> i16 {
1
};
let fn_ptr = fn1 as usize;
let fn_ptr_truncated = fn_ptr as i32;
From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.
Related lints
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- new_without_defaultChecks for public types with a pub fn new() -> Self method and no implementation of Default
- len_zeroChecks for getting the length of something via .len() just to compare to zero, and suggests using .is_empty() where applicable
- field_reassign_with_defaultChecks for immediate reassignment of fields initialized with Default::default()
- filter_map_bool_thenChecks for usage of bool::then in Iterator::filter_map
- fn_to_numeric_castcasting function pointer `fun` to `i64`
- for_kv_mapChecks for iterating a map (HashMap or BTreeMap) and ignoring either the keys or values
- from_over_intoan implementation of `From` is preferred since it gives you `Into<_>` for free where the reverse isn't true
- from_str_radix_10Checks for function invocations of the form primitive::from_str_radix(s, 10)
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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