lint on dbg! of non-reference with return value not used

[joshtriplett]

The dbg! macro takes its argument by value, so that it can return that same value. See #59943 for some background.

We should lint on code that calls dbg! on a non-reference value (thus consuming that value) and doesn't do anything with the return value. The lint could suggest taking a reference.

[Mark-Simulacrum]

Is there a direct reason for doing this? i.e., has some code come up that is hurt by the consumption?

I've personally hit the "cannot use after move" error with dbg!(x) a few times, and I feel like a lint would never have been helpful, to my recollection.

[Centril]

I concur with @Mark-Simulacrum and have the same experience of hitting the error.

I think it's also perfectly fine to use dbg!(val) where val is by-move as opposed to by-copy or by-reference. After all, the point of dbg!(...) is quick and dirty debugging, so even consuming the value by-move accidentally is better than getting an annoying lint in the way. Worse yet, if you have deny(warnings) on, this would cause another edit-compile-debug cycle, which seems counter-productive.

Ostensibly we could improve the diagnostics, but those are also good:

error[E0382]: use of moved value: `s`
 --> src/main.rs:4:10
  |
2 |     let s = "hello".to_owned();
  |         - move occurs because `s` has type `std::string::String`, which does not implement the `Copy` trait
3 |     dbg!(s);
  |     -------- value moved here
4 |     dbg!(s);
  |          ^ value used here after move
  |
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)