A better example would be computation expressions in F#, the async {} and query {} builders are super easy to work with and don't rely on HKT's to work (since there's no such thing in .Net). I think such a design would work pretty well in rust, and doesn't require adding a bunch of single-purpose keywords - something I wish the C#/.Net team did considering F# had async first.

async blocks require keyword in rust because it conflicts with a struct constructor.

F# works around this by having expression builders be normal types, `async` is just an alias for Control.AsyncBuilder (actually, it's an instance of it if I want to be technical), it's not even a reserved word in the language specification. This is why I feel such an implementation would be a great fit for Rust, the parser already has to figure out different contexts curly braces could be used (struct initializers, lexical scoping constructs, closures, the list goes on) - you can avoid adding an extra reserved keywords and get support for more than just async expressions.

Async is strictly more limited than the more generic LINQ comprehension syntax.

Actually in F# computation expressions are way more powerful, as they can define their own "keywords" within given semantics. This is for example, how `query{}` computation expression works - and unlike linq it can support things like left joins natively just because entire sql-like syntax is defined by library, not hardcoded into language. Same for async, yield generators etc.