Resource Types & fn drop(self)

Some values represent things in the outside world — open files, sockets, GPU buffers, lock guards — and need explicit cleanup. Miri's answer is the fn drop(self) method. Defining one on a class or struct turns it into a resource type:

use system.io

// A type with `fn drop(self)` is a *resource type*.
// Miri runs `drop` exactly once when the value's lifetime ends.
class FileHandle
    private path String

    fn init(p String)
        self.path = p
        println(f"opened {self.path}")

    fn drop(self)
        println(f"closed {self.path}")

fn main()
    let f = FileHandle(p: "config.toml")
    println("doing work...")
    // f goes out of scope here — `drop` fires automatically.
    // No manual .close() needed.

• Miri guarantees drop runs exactly once when the value's lifetime ends.
• Resource types are single-owner — they cannot be shared via reference counting and they do not participate in Copy-on-Write.
• Resource types are intentionally not Cloneable — most external resources cannot be meaningfully duplicated.
• Aliasing a resource (var alias = original) consumes the original, just like passing it to a function.

This is how Miri delivers RAII without manual .close() calls and without exception-handling ceremony.