genKeyword

Explanation

gen is the newest reservation in this group: it became a reserved keyword only in the 2024 edition, unlike the rest of this section's keywords, most of which have been reserved since 2015. That recency matches how recent the design work behind it is.

gen is earmarked for a possible future generator/coroutine syntax — gen fn or gen { } blocks that produce a sequence of values one at a time via yield, the same way async fn produces a Future that resolves via .await. The analogy is close and deliberate: async fn already desugars into a state machine that implements Future; a gen fn would desugar into a state machine that implements Iterator instead. Where async suspends waiting for a value that isn't ready yet, gen would suspend after producing a value, resuming from that exact point on the next call for the next one.

This has real, relatively recent design momentum: there is an experimental nightly implementation gated behind #![feature(gen_blocks)] that supports gen { } block syntax today on nightly Rust, and it's this active work that justifies gen being reserved specifically in 2024 rather than having sat unclaimed since 2015 like abstract or do. None of this is stable yet, and reservation doesn't guarantee eventual stabilization — but of this section's keywords, gen and become are the two with genuine forward motion behind them.

Using gen as an ordinary identifier is a compile error in the 2024 edition (and later); it was never reserved in 2015/2018/2021, so existing code using gen as a variable or function name from those editions is unaffected until it migrates. The raw-identifier form r#gen is legal in any edition, the same escape hatch every reserved keyword offers.

Usage examples

let gen = 5;     // error (2024 edition): expected identifier, found reserved keyword `gen`
let r#gen = 5;   // ok: the raw-identifier form escapes the reservation

// Today's real, stable parallel: an async fn suspends at `.await`
// points and produces a `Future`; a future `gen fn` would suspend at
// `yield` points and produce an `Iterator` instead — same shape,
// different trait.
async fn fetch_count(id: u32) -> u32 {
    // suspends here, waiting for a value that isn't ready yet
    lookup(id).await
}

async fn lookup(id: u32) -> u32 {
    id * 2
}