remove redundant clones via mir opt?

[matthiaskrgr]

I wonder if it is possible to have an optimization pass that removes redundant clones if we know that .clone() does not do anything funky and that the original object would have been untouched and dropped in the same block where it was cloned.

There's a clippy lint which tries to warn about useless clones https://rust-lang.github.io/rust-clippy/master/index.html#redundant_clone

The pass could optimize something like

#[derive(Clone)]
pub struct S {
    inner: String,
}

pub fn a(a: S) -> S {
    a.clone()
}

to

#[derive(Clone)]
pub struct S {
    inner: String,
}

pub fn a(a: S) -> S {
    a // just return the object
}

[Aaron1011]

I tried something similar to this (rewriting Clone impls in terms of Copy when all of the Clone impls are derived), but the perf results weren't very good: #72409

[matthiaskrgr]

It seems that a.into() is already optimized, but to_owned() and
to_string() are not.