unsizedKeyword

Explanation

unsized has been reserved since the 2015 edition, part of the Rust Reference's original reserved-keyword list. Its role is subtler than the other entries in this section: it isn't pure speculation about some unbuilt feature so much as the unused flip side of a mechanism that already shipped.

Every generic type parameter carries an implicit Sized bound by default (T really means T: Sized unless said otherwise), because most generic code needs to know its parameter's size at compile time. The existing, working way to opt a parameter out of that implicit bound is ?Sized — T: ?Sized relaxes the default, allowing T to be a dynamically-sized type like str or dyn Trait. A bare unsized keyword would presumably spell the same relaxation a different way (unsized T instead of T: ?Sized), but since ?Sized already provides that escape hatch and has since before 1.0, there's no functional gap for a separate unsized keyword to fill. This is likely why it has remained reserved-but-unclaimed rather than becoming real syntax.

Using unsized as an ordinary identifier is a compile error today. The raw-identifier form r#unsized is legal, the same escape hatch every reserved keyword offers.

Usage examples

The `unsized` reservation error and raw-identifier escape hatch

let unsized = 5;     // error: expected identifier, found reserved keyword `unsized`
let r#unsized = 5;   // ok: the raw-identifier form escapes the reservation

Writing generic code

Accepting a dynamically-sized type behind a reference doesn't need the reserved unsized keyword — the existing ?Sized bound already relaxes the implicit Sized requirement on a type parameter.

use std::fmt::Display;

fn describe<T: Display + ?Sized>(value: &T) -> String {
    // <- `?Sized` relaxes the implicit `Sized` bound; `unsized` plays no role here
    format!("value: {value}")
}

let via_str: &str = "reading";
println!("{}", describe(via_str)); // str is unsized; `?Sized` is what makes this compile

Without ?Sized, T: Display alone would implicitly require T: Sized, ruling out str and other dynamically-sized types entirely — the Rust Reference on trait and lifetime bounds documents ?Sized as the one relaxation the language allows on an implicit default bound, which is exactly the gap a hypothetical unsized keyword would otherwise need to fill.

Explanation

Embedded support: Full

Full support. unsized itself is still just a reserved keyword with no defined meaning, identical on every target — that part of the classic Explanation doesn't change under #![no_std]. The one live piece of this page, ?Sized, also carries over unchanged: relaxing a generic parameter's implicit Sized bound, and working with dynamically-sized types like str or dyn Trait behind a reference, works the same way in #![no_std] firmware as anywhere else — the same considerations around dyn Trait (vtable dispatch, no allocator needed for the reference itself) covered on dyn apply here too.

Usage examples

The `unsized` reservation, unaffected by target

let unsized = 5;     // error: expected identifier, found reserved keyword `unsized`, on every target
let r#unsized = 5;   // ok: the raw-identifier form escapes the reservation, on every target