Why this builder pattern doesn't work?

[frederikhors]

Hi guys, I'm learning Rust and I'm trying to setup tracing lib and I have this code:

use tracing_subscriber;

pub fn init(is_pretty: bool) {
  let subscriber = tracing_subscriber::fmt();
  
  if is_pretty {
    &subscriber.pretty();
  }

  subscriber.init();
}

But the compiler errors with:

use of moved value: `subscriber`
value used here after move rustc E0382

tracing.rs(8, 21): `subscriber` moved due to this method call

tracing.rs(4, 9): move occurs because `subscriber` has type `tracing_subscriber::fmt::SubscriberBuilder<tracing_subscriber::fmt::format::DefaultFields, tracing_subscriber::fmt::format::Format, tracing_subscriber::EnvFilter>`, which does not implement the `Copy` trait

mod.rs(718, 9): this function takes ownership of the receiver `self`, which moves `subscriber`

I tried this too:

use tracing_subscriber;

pub fn init(is_pretty: bool) {
  let mut subscriber = tracing_subscriber::fmt();
  
  if is_pretty {
    subscriber = subscriber.pretty();
  }

  subscriber.init();
}

But the compiler errors with:

mismatched types

expected struct `tracing_subscriber::fmt::SubscriberBuilder<tracing_subscriber::fmt::format::DefaultFields, tracing_subscriber::fmt::format::Format<tracing_subscriber::fmt::format::Full>, _>`

found struct `tracing_subscriber::fmt::SubscriberBuilder<tracing_subscriber::fmt::format::Pretty, tracing_subscriber::fmt::format::Format<tracing_subscriber::fmt::format::Pretty>, _>`rustcE0308

tracing.rs(12, 26): expected due to this value

I asked the same question on SO but apparently no one is able to correctly answer.

Can you hep me?

[davidbarsky]

I suggest looking at the documentation here: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/layer/index.html#runtime-configuration-with-layers. This might help a bit.