Unable to print in a procedural macro function declaration in Rust

Question

I am using a proc_macro and want to print some verbose for debugging. The println! statement doesn't print anything.

This is the macro call:

decl_module! {

    /// The module declaration.
    pub struct Module<T: Trait> for enum Call where origin: T::Origin {
        // A default function for depositing events
        fn deposit_event() = default;

        /// Allow a user to claim ownership of an unclaimed proof
        fn create_claim(origin, proof: Vec<u8>) -> DispatchResult {
            // Verify that the incoming transaction is signed and store who the
            // caller of this function is.
            let sender = ensure_signed(origin)?;
            println!("send is: {}", sender);

            // Verify that the specified proof has not been claimed yet or error with the message
            ensure!(!Proofs::<T>::exists(&proof), "This proof has already been claimed.");

            // Call the `system` pallet to get the current block number
            let current_block = <system::Module<T>>::block_number();

            // Store the proof with the sender and the current block number
            Proofs::<T>::insert(&proof, (sender.clone(), current_block));

            // Emit an event that the claim was created
            Self::deposit_event(RawEvent::ClaimCreated(sender, proof));
            Ok(())
        }

        /// Allow the owner to revoke their claim
        fn revoke_claim(origin, proof: Vec<u8>) -> DispatchResult {
            // Determine who is calling the function
            let sender = ensure_signed(origin)?;

            // Verify that the specified proof has been claimed
            ensure!(Proofs::<T>::exists(&proof), "This proof has not been stored yet.");

            // Get owner of the claim
            let (owner, _) = Proofs::<T>::get(&proof);

            // Verify that sender of the current call is the claim owner
            ensure!(sender == owner, "You must own this claim to revoke it.");

            // Remove claim from storage
            Proofs::<T>::remove(&proof);

            // Emit an event that the claim was erased
            Self::deposit_event(RawEvent::ClaimRevoked(sender, proof));
            Ok(())
        }
    }
}

It is taken from here . I added the following line:

println!("send is: {}", sender);

I am running a blockchain(Polkadot dApp), in the terminal (or anywhere), I could not see the output message. Note: everything is working fine but I am unable to print.

Answer 1

This macro call generates code that contains a print statement. It does not run that code. You will not see the print output until the code is run and you call create_claim .

If you want to debug your macro call, there are severaltools for macros-by-example, but I don't know if they also work for procedural macros or if there are equivalents.