std::io::copy should take Read and Write traits by value

[est31]

This was suggested by @alexcrichton way back in 2015:

mod std::io {
    fn copy<R: Read, W: Write>(r: R, w: W) -> io::Result<u64> { ... }
}

but it wasn't done as it regressed 3 (root) crates in a crater run. Still, ideally if the API breakage policy is ever revisited, it would be better to take Read and Write by-value, as a type impl'ing Read transmits this property to &mut references of it.

@rustbot label T-libs-api rust-2-breakage-wishlist

[rustbot]

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[est31]

FTR this is the error I'm getting if I modify the copy function on a local toolchain based off current master and try to build tar 0.3.1:

error[E0382]: use of moved value: `self`
   --> src/entry.rs:134:48
    |
134 |             if try!(io::copy(self, &mut f)) != self.size {
    |                              ----              ^^^^^^^^^ value used here after move
    |                              |
    |                              value moved here
    |
    = note: move occurs because `self` has type `&mut EntryFields<'_>`, which does not implement the `Copy` trait

error[E0382]: borrow of moved value: `self`
   --> src/entry.rs:145:28
    |
97  |     fn _unpack(&mut self, dst: &Path) -> io::Result<()> {
    |                --------- move occurs because `self` has type `&mut EntryFields<'_>`, which does not implement the `Copy` trait
...
133 |         try!(fs::File::create(dst).and_then(|mut f| {
    |                                             ------- value moved into closure here
134 |             if try!(io::copy(self, &mut f)) != self.size {
    |                              ---- variable moved due to use in closure
...
145 |         if let Ok(mtime) = self.header.mtime() {
    |                            ^^^^^^^^^^^^^^^^^^^ value borrowed here after move

So the borrow checker improvements in 2018-ish haven't fixed the regression here.

tar 0.3.4 is also affected, but latest 0.4.33 is not.

I'm closing this as with current policies this can't be changed.

@rustbot label T-libs-api

[est31]

For the libs team meeting, see #111063 with crater runs for the suggested change as well as my analysis of the results.