Can Copy types own memory (that this should track)?

[Aeledfyr]

A blanket impl of DeepSize for all copy types would cover a significant amount of the library comparability code in this library. Can Copy types own memory, in the sense that this library tracks?

The only type that I can think of that that conflicts with this are references; they are currently defined to not measure the referenced structure, but other structures may want special logic for that.

One way to solve it would be with specialization and a default impl of DeepSize for types with Copy, but that's still a long way off. The other would be to just make a normal impl (which would be a breaking change) and hope that no one runs into cases where a specialized implementation is needed.

[matklad]

Yeah, I think

impl<T: Copy> DeepSizeOf for T {
    fn deep_size_of_children(&self, context: &mut Context) -> usize {
        0
    }
}

would be really great. The problem here is that this, sadly, doesn't work due to those pesky coherence rules:

error[E0119]: conflicting implementations of trait `DeepSizeOf` for type `std::path::PathBuf`
  --> src/lib.rs:51:1
   |
51 | impl<T: Copy> DeepSizeOf for T {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `std::path::PathBuf`
   |
  ::: src/default_impls.rs:90:5
   |
90 |     impl DeepSizeOf for PathBuf {
   |     --------------------------- first implementation here
   |
   = note: upstream crates may add a new impl of trait `std::marker::Copy` for type `std::path::PathBuf` in future versions

BUT! It seems we can use auroref specialization in the deref macro to a similar effect!

https://github.com/dtolnay/case-studies/blob/master/autoref-specialization/README.md

See poc here: matklad@a8b43bd