src/doc/unstable-book/src/language-features/trace-macros.md

   1 # `trace_macros`
   2
   3 The tracking issue for this feature is [#29598].
   4
   5 [#29598]: https://github.com/rust-lang/rust/issues/29598
   6
   7 ------------------------
   8
   9 With `trace_macros` you can trace the expansion of macros in your code.
  10
  11 ## Examples
  12
  13 ```rust
  14 #![feature(trace_macros)]
  15
  16 fn main() {
  17     trace_macros!(true);
  18     println!("Hello, Rust!");
  19     trace_macros!(false);
  20 }
  21 ```
  22
  23 The `cargo build` output:
  24
  25 ```txt
  26 note: trace_macro
  27  --> src/main.rs:5:5
  28   |
  29 5 |     println!("Hello, Rust!");
  30   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
  31   |
  32   = note: expanding `println! { "Hello, Rust!" }`
  33   = note: to `print ! ( concat ! ( "Hello, Rust!" , "\n" ) )`
  34   = note: expanding `print! { concat ! ( "Hello, Rust!" , "\n" ) }`
  35   = note: to `$crate :: io :: _print ( format_args ! ( concat ! ( "Hello, Rust!" , "\n" ) )
  36           )`
  37
  38     Finished dev [unoptimized + debuginfo] target(s) in 0.60 secs
  39 ```