How to call functions hygenically from macros in Rust?

I have a macro in some module that calls a function:

mod a {
  mod b {
    fn foo(_: u32) {}

    macro_rules! bar {
      ( $x:expr ) => (foo($x))
    }
  }
}

This does not work from outside module a::b unless I import foo. Explicitly naming the module works in this case:

macro_rules! bar {
  ( $x:expr ) => (crate::a::b::foo($x))
}

However, this wouldn't work from outside the crate for an exported macro. I tried the following but it didn't work either:

#[macro_export]
macro_rules! bar {
  ( $x:expr ) => (::mycratename::a::b::foo($x))
}

Where mycratename is the name of the library in Cargo.toml.

[lib]
name = "mycratename"
crate-type = ["lib", "cdylib", "staticlib"]

At least it doesn't work when I try to use the macro from within the crate, I didn't try using it from outside. Is there a way to make it work from both within and outside the crate?

This is exactly what the reserved $crate meta variable is for.

It expands to crate within the defining crate and to ::crate_name outside of it.

See Also

Using $crate in Rust's procedural macros?