On-Stack Dynamic Dispatch

Description

We can dynamically dispatch over multiple values, however, to do so, we need to declare multiple variables to bind differently-typed objects. To extend the lifetime as necessary, we can use deferred conditional initialization, as seen below:

Example

use std::io;
use std::fs;

# fn main() -> Result<(), Box<dyn std::error::Error>> {
# let arg = "-";

// These must live longer than `readable`, and thus are declared first:
let (mut stdin_read, mut file_read);

// We need to ascribe the type to get dynamic dispatch.
let readable: &mut dyn io::Read = if arg == "-" {
    stdin_read = io::stdin();
    &mut stdin_read
} else {
    file_read = fs::File::open(arg)?;
    &mut file_read
};

// Read from `readable` here.

# Ok(())
# }

Motivation

Rust monomorphises code by default. This means a copy of the code will be generated for each type it is used with and optimized independently. While this allows for very fast code on the hot path, it also bloats the code in places where performance is not of the essence, thus costing compile time and cache usage.

Luckily, Rust allows us to use dynamic dispatch, but we have to explicitly ask for it.

Advantages

We do not need to allocate anything on the heap. Neither do we need to initialize something we won't use later, nor do we need to monomorphize the whole code that follows to work with both File or Stdin.

Disadvantages

The code needs more moving parts than the Box-based version:

// We still need to ascribe the type for dynamic dispatch.
let readable: Box<dyn io::Read> = if arg == "-" {
    Box::new(io::stdin())
} else {
    Box::new(fs::File::open(arg)?)
};
// Read from `readable` here.

Discussion

Rust newcomers will usually learn that Rust requires all variables to be initialized before use, so it's easy to overlook the fact that unused variables may well be uninitialized. Rust works quite hard to ensure that this works out fine and only the initialized values are dropped at the end of their scope.

The example meets all the constraints Rust places on us:

All variables are initialized before using (in this case borrowing) them
Each variable only holds values of a single type. In our example, stdin is of type Stdin, file is of type File and readable is of type &mut dyn Read
Each borrowed value outlives all the references borrowed from it

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