drop_ref should not trigger when drop is used in a match branch

[ia0]

Summary

This bug is similar to #9482 but for drop_ref instead of drop_copy.

I would similarly argue that if drop() is called on the result of a function (or other expression with side-effects) as a match branch (i.e. the code contains something like => drop(expr_with_side_effects),) then the drop_ref lint should not trigger because drop() is not used to drop the value but to ignore the value.

The fix may be similar to #9491.

Lint Name

drop_ref

Reproducer

I tried this code:

fn foo(xs: &mut [u8; 3]) -> &mut u8 {
    println!("doing foo");
    &mut xs[2] // result is not always useful, the side-effect matters
}
fn bar() {
    println!("doing bar");
}

fn main() {
    let mut xs = *b"abc";
    match 42 {
        0 => drop(foo(&mut xs)),  // drop is needed because we only care about side-effects
        1 => bar(),
        _ => (),  // doing nothing (no side-effects needed here)
    }
}

I saw this happen:

error: calls to `std::mem::drop` with a reference instead of an owned value. Dropping a reference does nothing
  --> src/main.rs:12:14
   |
12 |         0 => drop(foo(&mut xs)),  // drop is needed because we only care about side-effects
   |              ^^^^^^^^^^^^^^^^^^
   |
note: argument has type `&mut u8`
  --> src/main.rs:12:19
   |
12 |         0 => drop(foo(&mut xs)),  // drop is needed because we only care about side-effects
   |                   ^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#drop_ref

I expected to see this happen:

No error

Version

rustc 1.67.0-nightly (c97b539e4 2022-11-30)
binary: rustc
commit-hash: c97b539e408ea353f4fde2f9251d598291fec421
commit-date: 2022-11-30
host: x86_64-unknown-linux-gnu
release: 1.67.0-nightly
LLVM version: 15.0.4

Additional Labels

No response

[ia0]

Thanks @ericwu2003 for the fix!