Idea

How would you like an effective way of controlling all of the logging within the “lib” part of your crate? Of course, still allowing each mod/file instance to be individually set to its own level.

Let’s say you have something like this project ( my_project) structure:

• src/
  • lib.rs
  • core/
    • control.rs
    • mod.rs

You might have:

• src/lib.rs

  mod my_core;
  
  pub(crate) use flogging::*;
  pub use my_core::*;
  
  //
  // Cargo.toml
  //
  // [dependencies]
  // ctor = "0.5.0"
  use ctor::*;
  
  pub(crate) const DEBUG_LEVEL:Level = Level::ALL;
  // pub(crate) const DEBUG_LEVEL:Level = Level::OFF;
  
  ///
  /// Reset the log file each time `my_project` is loaded.
  ///
  /// This is an alternative to using `remove_file()` in
  /// the individual mod/file setup commands.\
  /// Only useful if all child mods are using the same log file.
  ///
  #[ctor]
  fn reset_log(){
      Logger::remove_file("test_logs/usage.log");
  }
  
  #[cfg(test)]
  mod tests{
      use super::*;
  
      #[test]
      fn control(){
          my_core::control::do_it();
      }
  
      #[test]
      fn my_core(){
          my_core::do_it();
      }
  
  }

• src/core/control.rs
  Notice the use of DEBUG_LEVEL:

  use crate::*;
  
  const_logger!({
      Logger::builder(module_path!())
          .add_console_handler()
          .add_file_handler("test_logs/usage.log")
          .set_level(DEBUG_LEVEL)
          //         ^^^^^^^^^^^
          .build()
  });
  
  #[logger]
  pub fn do_it() {
      entering!();
      info!("Hello from `Control`.");
      exiting!();
  }

• src/core/mod.rs
  Notice the use of DEBUG_LEVEL, and add_pconsole_handler():

  pub mod control;
  
  use crate::*;
  
  const_logger!({
      Logger::builder(module_path!())
          .add_pconsole_handler()
          //   ^^^^^^^^
          .add_file_handler("test_logs/usage.log")
          // .set_level(Level::INFO)
          .set_level(DEBUG_LEVEL)
          //         ^^^^^^^^^^^
          .build()
  });
  
  #[logger]
  pub fn do_it() {
      entering!();
      info!("This text came from the file `src/core/mod.rs`, just to let you know.");
      exiting!();
  }

The possible output from running the src/lib.rs tests is:

• console

  ---- tests::control stdout ----
  my_project::my_core::control->do_it [FINER  ] Entry
  my_project::my_core::control->do_it [INFO   ] Hello from `Control`.
  my_project::my_core::control->do_it [FINER  ] Return
  
  ---- tests::my_core stdout ----
  my_project::my_core->do_it [FINER  ] Entry
  This text came from the file `src/core/mod.rs`, just to let you know.
  my_project::my_core->do_it [FINER  ] Return
  

• test_logs/usage.log

  2025-08-27T10:05:48.581421788+08:00 my_project::my_core::control->do_it [FINER  ] Entry
  2025-08-27T10:05:48.581421917+08:00 my_project::my_core->do_it [FINER  ] Entry
  2025-08-27T10:05:48.581505388+08:00 my_project::my_core::control->do_it [INFO   ] Hello from `Control`.
  2025-08-27T10:05:48.581510841+08:00 my_project::my_core->do_it [INFO   ] This text came from the file `src/core/mod.rs`, just to let you know.
  2025-08-27T10:05:48.581528496+08:00 my_project::my_core::control->do_it [FINER  ] Return
  2025-08-27T10:05:48.581533425+08:00 my_project::my_core->do_it [FINER  ] Return
  

Now let’s prepare for a production build.

First we change the global DEBUG_LEVEL to OFF, then we modify src/core/mod.rs to be set to Level::INFO.

• src/lib.rs

  ...
      pub(crate) const DEBUG_LEVEL:Level = Level::OFF;
  ...

• src/core/mod.rs

  ...
         .set_level(Level::INFO)
  ...

Now to see what comes out:

• console

  ---- tests::my_core stdout ----
  This text came from the file `src/core/mod.rs`, just to let you know.
  

• test_logs/usage.log

  2025-08-27T10:07:36.124653478+08:00 my_project::my_core->do_it [INFO   ] This text came from the file `src/core/mod.rs`, just to let you know.
  

Next, let’s get rid of the log file (part of src/core/mod.rs):

...

const_logger!({
    Logger::builder(module_path!())
        .add_pconsole_handler()
        //   ^^^^^^^^
        // .add_file_handler("test_logs/usage.log")
        // .set_level(DEBUG_LEVEL)
        .set_level(Level::INFO)
        //         ^^^^^^^^^^^
        .build()
});

...

And the final production output is:

• console

  ---- tests::my_core stdout ----
  This text came from the file `src/core/mod.rs`, just to let you know.
  

• test_logs/usage.log