lint for ref-to-ref transmute, which may contain padding bytes

What it does

detects ref-to-ref transmute, and their type arguments are either:

• unbounded generic type to [u8; core::mem::size_of::<T>()]
• concrete type which has padding to [u8; core::mem::size_of::<T>()]
• enum without integral #[repr] to [u8; core::mem::size_of::<T>()]

Lint Name

transmute_may_include_padding_bytes

Category

suspicious

Advantage

No response

Drawbacks

No response

Example

Fire on https://github.com/rust-lang/rust/issues/76560#issuecomment-1426942424:

impl<T> AsBitPattern for T {
    fn as_bit_pattern<'a>(&'a self) -> &'a [u8] {
        // SAFETY:
        // The following property is always hold for Sized types:
        // - size_of::<T> == size_of::<T> * size_of::<u8>
        // ref-to-ref cast will not truncate nor cause out-of-bound-read.
        unsafe {
            core::mem::transmute::<&'a T, &'a [u8; core::mem::size_of::<T>()]>(self)
        }
    }
}

No source files or tests are named. Start by locating Clippy's lint and test entry points for suspicious lints and the handling of ref-to-ref transmute type arguments. Done means detecting the three listed cases under the lint name transmute_may_include_padding_bytes and covering the provided example.