Any way to make Rust understand an `fn(T) -> impl Future` always returns the same type?

Question

I've a function which takes a generic parameter, gets what it needs out of that, then returns a future. The future does not actually store the generic data, it is completely monomorphic.

For convenience I wanted to use an async fn to create the future, which I understand requires returning an impl Future as async fn return opaque types: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c3b061d12a126dc30099ac3bd018c273

use std::io::{Read, stdin};
use std::fs::File;
use std::future::Future;
use std::path::Path;

fn caller(p: Option<&Path>) -> impl Future<Output=()> {
    if let Some(p) = p {
        f(File::open(p).unwrap())
    } else {
        f(stdin())
    }
}

fn f<R: Read>(_: R) -> impl Future<Output=()> {
    fut()
}

async fn fut() {}

However rustc freaks out in the conditional as the caller side is absolutely convinced the future must somehow be different between the two branches:

error[E0308]: `if` and `else` have incompatible types
  --> src/lib.rs:10:9
   |
7  | /     if let Some(p) = p {
8  | |         f(File::open(p).unwrap())
   | |         ------------------------- expected because of this
9  | |     } else {
10 | |         f(stdin())
   | |         ^^^^^^^^^^ expected struct `File`, found struct `Stdin`
11 | |     }
   | |_____- `if` and `else` have incompatible types
...
14 |   fn f<R: Read>(_: R) -> impl Future<Output=()> {
   |                          ---------------------- the found opaque type
   |
   = note:     expected type `impl Future<Output = ()>` (struct `File`)
           found opaque type `impl Future<Output = ()>` (struct `Stdin`)

Is there a way to fix this aside from boxing the future or hand-rolling fut in order to end up with a single concrete type?

Answer 1

I don't think you can avoid boxing, but at least you can avoid boxing the future itself:

use std::io::{Read, stdin};
use std::fs::File;
use std::future::Future;
use std::path::Path;

fn caller(p: Option<&Path>) -> impl Future<Output=()> {
    let read = if let Some(p) = p {
        Box::new(File::open(p).unwrap()) as Box<dyn Read>
    } else {
        Box::new(stdin())
    };
    f(read)
}

fn f<R: Read>(_: R) -> impl Future<Output=()> {
    fut()
}

async fn fut() {}

As my understanding, the problem is not the future, but that is actually building different types for the parameters, and somehow floods into the return type. It would makes sense it wouldn't though.

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