Conditional compilation by suffix

[SOF3]

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The current dirmod::os!() macro requires a structure like this:

- mod.rs - includes foo, bar, qux
- foo.rs
- bar.rs
- qux/
  - qux/mod.rs - dirmod::os!()
  - qux/windows.rs
  - qux/unix.rs

It might be preferrable to have a structure like this:

- mod.rs - includes foo, bar, qux
- foo.rs
- bar.rs
- qux_windows.rs
- qux_unix.rs

This can be activated by providing a flag like with family suffix; in the dirmod::all!() call in mod.rs.

If qux.rs is also present, there are two possible solutions:

• trigger a compile error
• declare a module like this:

$vis mod qux {
  #[path = "qux.rs"] mod _common;
  pub use _common::*;

  #[cfg(target_family = "windows")] #[path = "qux_windows.rs"] mod _windows;
  pub use _windows::*;

  #[cfg(target_family = "unix")] #[path = "qux_unix.rs"] mod _unix;
  pub use _unix::*;
}

The former is preferred as it leads to less confusion. In particular, the latter solution might be confusing in that the exports from qux_windows.rs and qux_unix.rs actually cannot be used in qux.rs.