Warn on &mut to a temporary value

What it does

Warn when &mut is taken on a temporary value, especially when type implements Copy. Code below doesn't modify buf, because what is being passed to t is a &mut to a temporary array created by a <[u8]>::try_into():

&mut reference implies that a receiver will modify value, modifying temporary value, which user has no access to is pointless IMHO. More likely than not, it is not user intention.

Lint Name

mut-ref-rvalue

Category

suspicious

Advantage

No response

Drawbacks

No response

Example

#[inline(never)]
pub fn t(b: &mut [u8;4]) {
    b[0] = 99;
}

#[inline(never)]
pub fn tt() -> [u8;16] {
    let buf = [0u8; 16];
    t(&mut buf[..4].try_into().unwrap());
    buf
}

Could be written as:

#[inline(never)]
pub fn t(b: &mut [u8;4]) {
    b[0] = 99;
}

#[inline(never)]
pub fn tt() -> [u8;16] {
    let mut buf = [0u8; 16];
    t((&mut buf[..4]).try_into().unwrap());
    buf
}

The issue names no implementation files or tests. Start with the proposed examples and inspect existing Clippy suspicious lints to define the temporary-value and Copy cases; done means a mut-ref-rvalue lint warns on the shown temporary reference while accepting the mutable-local rewrite.