What is "curious inner-module" hack?

MMK · July 10, 2023, 7:30pm

While reading this, I came across this concept , I couldn't find a single normal description, can anyone tell me something?

cuviper · July 11, 2023, 12:39am

I'm not sure, but from this description:

We introduce a special macro metavar $crate which expands to ::foo when a macro was imported through crate ident foo, and to nothing when it was defined in the crate where it is being expanded. $crate::bar::baz will be an absolute path either way.

I'm guessing the hack was something like having a root module with the same name as your crate, so ::foo written in the macro would refer to that module when used within the crate, while that same ::foo referred to the crate itself when used from outside.

Also, 2018 edition changed the meaning of such paths:
https://doc.rust-lang.org/edition-guide/rust-2018/path-changes.html#the-crate-keyword-refers-to-the-current-crate

The prefix :: previously referred to either the crate root or an external crate; it now unambiguously refers to an external crate. For instance, ::foo::bar always refers to the name bar inside the external crate foo.

MMK · July 11, 2023, 10:28am

Thank you!!!, I didn't even think about it, that's the only explanation

example from old version of std :

github.com

rust-lang/rust/blob/0d3bd7720c50e3ada4bac77331d43926493be4fe/src/libstd/lib.rs#L273-L277


      
          // A curious inner-module that's not exported that contains the binding
          // 'std' so that macro-expanded references to std::error and such
          // can be resolved within libstd.
          #[doc(hidden)]
          mod std {

the usage of underline function of println! macro (also reexported from mod std):

github.com

rust-lang/rust/blob/0d3bd7720c50e3ada4bac77331d43926493be4fe/src/libstd/macros.rs#L291


      
          /// information, see `std::fmt` and `std::io::stdio`.
          ///
          /// # Example
          ///
          /// ```
          /// println!("hello there!");
          /// println!("format {} arguments", "some");
          /// ```
          #[macro_export]
          macro_rules! println(
              ($($arg:tt)*) => (format_args!(::std::io::stdio::println_args, $($arg)*))
          )
          
          /// Declare a task-local key with a specific type.
          ///
          /// # Example
          ///
          /// ```
          /// local_data_key!(my_integer: int)
          ///
          /// my_integer.replace(Some(2));