typeofKeyword

Explanation

typeof has been reserved since the 2015 edition, part of the Rust Reference's original reserved-keyword list. It's held for a hypothetical typeof(expr) operator that would give you the compile-time type of an expression, usable itself as a type — similar to C++11's decltype, or to what typeof already means in C and GNU C extensions.

Rust hasn't needed this in practice because type inference already covers most of what people reach for typeof to do in other languages. Writing let x: _ = expr (or simply let x = expr) lets the compiler infer x's type from expr without ever having to name it; generic functions infer their type parameters from arguments the same way. Where C++ needs decltype(expr) to write a type that depends on another expression's type, Rust's inference and generics typically make naming that type unnecessary in the first place. This is likely why typeof has stayed unclaimed since 2015 with no concrete proposal attached to it — the gap it would fill is already mostly closed by other means.

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

Usage examples

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

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

Explanation

Embedded support: Full

No real difference here: typeof's reservation is a lexer-level fact, checked before any target or runtime is even in the picture, so it's exactly as unclaimed and exactly as inert under #![no_std] as in hosted Rust. There's no embedded-specific angle to add — no HAL, register, or interrupt handler changes what this keyword does, because it doesn't do anything yet in either context.

Usage examples

The reservation error is identical under `#![no_std]`

#![no_std]

// let typeof = read_register(); // error: expected identifier, found reserved keyword `typeof`
let r#typeof = 0u32; // ok: the raw-identifier form escapes the reservation, same as hosted Rust