What is the mechanism for converting a function to a trait in Rust?

Question

An example from actix-web is as follows:

use actix_web::{web, App, Responder, HttpServer}; async fn index() -> impl Responder { "Hello world!" } #[actix_rt::main] async fn main() -> std::io::Result<()> { HttpServer::new(|| { App::new().service( web::scope("/app").route("/index.html", web::get().to(index)), ) }) .bind("127.0.0.1:8088")? .run() .await }

My question is around how the statement to(index) works in Rust.

Looking at the source code for to we see:

pub fn to<F, T, R, U>(mut self, handler: F) -> Self where F: Factory<T, R, U>, // --- snip

Where Factory is defined as:

pub trait Factory<T, R, O>: Clone + 'static where R: Future<Output = O>, O: Responder, { fn call(&self, param: T) -> R; }

What is the mechanism by which the function async fn index() -> impl Responder gets converted to Factory<T, R, O>?

This is, indeed, a good question. I'm expectating.

Netwave
– Netwave

2020-09-10 11:17:28 +00:00
Commented Sep 10, 2020 at 11:17 — Netwave
– Netwave, Commented Sep 10, 2020 at 11:17

Masklinn · Accepted Answer · 2020-09-10 11:22:20Z

There is an implementation of the trait just after your snippet:

impl<F, R, O> Factory<(), R, O> for F where F: Fn() -> R + Clone + 'static, R: Future<Output = O>, O: Responder, { fn call(&self, _: ()) -> R { (self)() } }

This can be read as: if a type F implements Fn() -> Future<Output = impl Responder> + ... then it also implements Factory<(), _, _>.

And an async fn is syntactic sugar for a function which returns a Future of some sort (and can use .await internally), so async fn index() -> impl Responder implements Fn() -> impl Future<Output = impl Responder> so it also implements Factory<(), _, _>.

Also lingo correction: there is no conversion in the code posted, instead there is a constraint: F needs to implement the specified trait, but its identity doesn't change.

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