Clippy lints / Style
Clippy lint · Style · checked with Clippy 0.1.99
clippy::fn_to_numeric_cast: casting function pointer `fun` to `i64`
What it meansChecks for casts of function pointers to something other than usize. It is in the style group and warns by default.
What you see
warning: casting function pointer `fun` to `i64` --> main.rs:4:9 | 4 | let _ = fun as i64; | ^^^^^^^^^^ help: try: `fun as usize` | = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#fn_to_numeric_cast = note: `#[warn(clippy::fn_to_numeric_cast)]` 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?
Casting a function pointer to anything other than usize/isize is not portable across architectures. If the target type is too small the address would be truncated, and target types larger than usize are unnecessary. Casting to isize also doesn’t make sense, since addresses are never signed.
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 fun() -> i32 { 1 }
let _ = fun as i64;
Write this instead
let _ = fun 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::fn_to_numeric_cast)]
fn my_function() { /* ... */ }Whole crate (Cargo.toml)
[lints.clippy]
fn_to_numeric_cast = "allow"One run
cargo clippy -- -A clippy::fn-to-numeric-castThe [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 function pointers to something other than usize.
Why is this bad?
Casting a function pointer to anything other than usize/isize is
not portable across architectures. If the target type is too small the
address would be truncated, and target types larger than usize are
unnecessary.
Casting to isize also doesn’t make sense, since addresses are never
signed.
Example
fn fun() -> i32 { 1 }
let _ = fun as i64;
Use instead:
let _ = fun as usize;
From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.
Related lints
- needless_returnunneeded `return` statement
- 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
- excessive_precisionfloat has excessive precision
- 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_cast_with_truncationcasting function pointer `fn1` to `i32`, which truncates the value
- 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
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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