Attribute proc_macro 

Defines a function-like procedural macro.

Applied to a pub function at the root of a proc-macro crate, proc_macro makes that function usable as a macro invoked as foo!(...) in other crates. The function receives the tokens written inside the invocation as a TokenStream and returns the TokenStream that replaces the invocation:

use proc_macro::TokenStream;

#[proc_macro]
pub fn foo(input: TokenStream) -> TokenStream {
   "fn answer() -> u32 { 67 }".parse().unwrap()
}

The macro can only be invoked from other crates, not from the crate where it is defined:

use my_macro_crate::foo;

// Expands to `fn answer() -> u32 { 67 }`.
foo!();

fn main() {
   println!("{}", answer()); // Prints 67
}

The attribute is only usable with crates of the proc-macro crate type, which is set in the crate’s Cargo.toml with proc-macro = true in the [lib] section. Using it anywhere else is a compilation error:

error: the `#[proc_macro]` attribute is only usable with crates of the `proc-macro` crate type
--> src/lib.rs:4:1
 |
4| #[proc_macro]
 | ^^^^^^^^^^^^

For more information, see the Reference on function-like procedural macros and the proc_macro crate documentation.