size_of_in_element_count false positive with u8 element type

[jturner314]

Lint name: size_of_in_element_count

I tried this code:

fn u8_as_bytes(x: &u8) -> &[u8] {
    unsafe {
        let ptr: *const u8 = x;
        std::slice::from_raw_parts(ptr.cast::<u8>(), std::mem::size_of::<u8>())
    }
}

I expected to see this happen: Clippy should not give a size_of_in_element_count error.

Instead, this happened for rustup run nightly cargo clippy:

error: found a count of bytes instead of a count of elements of `T`
 --> src/main.rs:4:54
  |
4 |         std::slice::from_raw_parts(ptr.cast::<u8>(), std::mem::size_of::<u8>())
  |                                                      ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[deny(clippy::size_of_in_element_count)]` on by default
  = help: use a count of elements instead of a count of bytes, it already gets multiplied by the size of the type
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#size_of_in_element_count

In contrast, clippy (correctly) does not give an error for this code:

fn u32_as_bytes(x: &u32) -> &[u8] {
    unsafe {
        let ptr: *const u32 = x;
        std::slice::from_raw_parts(ptr.cast::<u8>(), std::mem::size_of::<u32>())
    }
}

In other words, I've seen the false positive only for the u8 case.

The false positive also occurs for the following similar code, but again, only for the u8 case:

fn u8_slice_as_bytes(slice: &[u8]) -> &[u8] {
    unsafe {
        std::slice::from_raw_parts(
            slice.as_ptr().cast::<u8>(),
            slice.len() * std::mem::size_of::<u8>(),
        )
    }
}

You might wonder why anyone would write a functions like u8_as_bytes or u8_slice_as_bytes. This came up for me because they're in a macro that implements a trait, and the macro is used for all the numeric primitives, including u8. (Here's the CI failure for my crate due to the clippy lint.)

Meta

• rustup run nightly cargo clippy -V: clippy 0.1.51 (a62a760 2021-01-13)
• rustup run nightly rustc -Vv:

  rustc 1.51.0-nightly (a62a76047 2021-01-13)
  binary: rustc
  commit-hash: a62a76047ea24aad7639f14eb3ce0e620b77bdb7
  commit-date: 2021-01-13
  host: x86_64-unknown-linux-gnu
  release: 1.51.0-nightly
  

[ThibsG]

Seems related to #6511 where there is the same use of std::slice::from_raw_parts function

[ghost]

When the result is &[u8], the number of elements is the same as the number of bytes and we don't need to lint. Right? 🤔

[jturner314]

Yeah, when the result of std::slice::from_raw_parts is &[u8], the size_of_in_element_count lint shouldn't apply. The description of the lint says:

What it does

Detects expressions where size_of::<T> or size_of_val::<T> is used as a count of elements of type T

Why is this bad

These functions expect a count of T and not a number of bytes

When the output is &[u8], it's clear that providing a number of bytes to the function is correct.