Debugging in function chain calling

Sometimes I will write some code like:

let client = reqwest::Client::new();
let res = client
    .post("example...")
    .json(&payload)
    .send()
    .await
    .unwrap()
    .json::<Response>()
    .await
    .unwrap();

And there might be some error during the query, then the final json parser will get an unexpected JSON response that I need to figure out the actual content. But it is inconvenient to have to rewrite the code and break the chain call to debug.

let response = client.post(...).response();
println!("{response:?}")

What if we can…

// ...
    .send()
    .debug()
    .json()
    .debug()

Solution

We can inherit and extend the std::fmt::Debug trait:

pub trait DebugExt {
    fn debug(&self) -> &Self
    where
        Self: Debug,
    {
        println!("{self:?}");
        self
    }
}

impl<D: Debug> DebugExt for D {}

Then any struct that has the Debug trait implied can automatically have DebugExt trait implied, so we can put .debug() into the chain and inspect the value without breaking the existing code.

Playground link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=d283e02ea1b1041e04b21fc478f10271

Credit

Thanks to @SpriteOvO for teaching me about the trait inheritance part.