Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::ptr_offset_with_cast: use of `offset` with a `usize` cast to an `isize`

What it meansChecks for usage of the offset pointer method with a usize cast to an isize. It is in the complexity group and warns by default.

What you see

warning: use of `offset` with a `usize` cast to an `isize`
 --> main.rs:8:5
  |
8 |     ptr.offset(offset as isize);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#ptr_offset_with_cast
  = note: `#[warn(clippy::ptr_offset_with_cast)]` on by default
help: use `add` instead
  |
8 -     ptr.offset(offset as isize);
8 +     ptr.add(offset);
  |

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?

If we’re always increasing the pointer address, we can avoid the numeric cast by using the add method instead.

Code that does something simple in a complicated way. Warns by default.

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

let vec = vec![b'a', b'b', b'c'];
let ptr = vec.as_ptr();
let offset = 1_usize;

unsafe {
    ptr.offset(offset as isize);
}

Write this instead

let vec = vec![b'a', b'b', b'c'];
let ptr = vec.as_ptr();
let offset = 1_usize;

unsafe {
    ptr.add(offset);
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
ptr_offset_with_cast = "allow"

One run

cargo clippy -- -A clippy::ptr-offset-with-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.

Complexitygroup

warndefault level

yesfixed by cargo clippy --fix

1.30.0added in Rust

Clippy's own documentation

What it does

Checks for usage of the offset pointer method with a usize cast to an isize.

Why is this bad?

If we’re always increasing the pointer address, we can avoid the numeric cast by using the add method instead.

Example

let vec = vec![b'a', b'b', b'c'];
let ptr = vec.as_ptr();
let offset = 1_usize;

unsafe {
    ptr.offset(offset as isize);
}

Could be written:

let vec = vec![b'a', b'b', b'c'];
let ptr = vec.as_ptr();
let offset = 1_usize;

unsafe {
    ptr.add(offset);
}

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

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