Implement HeapSizeOf for PathBuf and friends

[Michael-F-Bryan]

In a project I found myself needing to calculate the memory used by a PathBuf and ended up just ignoring the field. It'd be better if PathBuf implemented HeapSizeOf though.

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I did a little spelunking through the std source code and it looks like PathBuf is just a newtype'd OsString, which is in turn just a newtype'd std::sys::Buf (os-specific string buffer). I stopped trying to write a PR soon after that because I'd started doing mem::transmutes between newtypes and digging into std::sys's os-specific internals, which felt a bit too hacky.

I think you could get pretty far with something like this:

use std::path::PathBuf;
use std::ffi::OsString;
use std::mem;

impl HeapSizeOf for PathBuf {
  fn heap_size_of_children(&self) -> usize {
    // PathBuf is just a newtype'd OsString
    let os_str: &OsString = unsafe { mem::transmute(self) };
    os_str.heap_size_of_children()
  }
}

impl HeapSizeOf for OsString {
  fn heap_size_of_children(&self) -> usize {
    // the easy case, should work 99% of time.
    if let Some(s) = self.to_str() {
      return unsafe { heap_size_of(s.as_ptr()) };
    }

    // fall back to a sane default or alternate implementation
    ...
  }
}

[Mark-Simulacrum]

See also https://github.com/rust-lang/rust/issues/58234.