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.
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.