False positive in trivially_copy_pass_by_ref where reference is used

[ogham]

I've encountered a false positive with the trivially_copy_pass_by_ref lint. While Clippy is able to tell that a function takes a &u8 instead of a u8, it doesn't seem to be checking whether that reference is passed to another function that requires a reference rather than a value. My example:

// Clippy says:
// error: this argument (1 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
pub fn contains(&self, target: &u8) -> bool {
    exists_within(some_bytes, target)
}

pub fn exists_within<'a, T>(slice: &'a [T], reference: &'a T) -> bool {
    unimplemented!()
}

Its recommended fix (to change it to a u8) won't work here, because the function the reference is passed to requires it to be a reference.

I made a quick Rust playground with the exact code that ran into this issue.

Thanks for Clippy!

---

$ cargo clippy -V
clippy 0.0.212 (204bb9b5 2020-03-17)

[flip1995]

rust-clippy/clippy_lints/src/trivially_copy_pass_by_ref.rs

Line 118 in 7ca335a

Applicability::Unspecified,

This should be MaybeIncorrect

To fix this issue and suggest for every occurrence of the parameter inside a function to add a & if necessary, is really hard. Also I'm not really sure if this would be worth it, since it would convolute the suggestion really hard.