my-bad-idea.rs

/// Proposed

/// A file which may or may not be async
struct ?async File {
    file_descriptor: std::os::RawFd,  // shared field in all contexts
    async waker: Waker,               // field only available in async contexts
    !async meta: Metadata,            // field only available in non-async contexts
}

impl ?async File {
    /// Attempts to open a file in read-only mode.
    ?async fn open(path: Path) -> io::Result<Self> {
        if is_async() {   // compiler built-in function
            // create an async `File` here; can use `.await`
        } else {
            // create a non-async `File` here
        }
    }
}

/// My Bad Idea?
struct async(Either) File {
    file_descriptor: std::os::RawFd, // shared field in all contexts
    // or maybe async(Enabled)
    async(Required) waker: Waker,    // field only available in async contexts
    // or maybe async(Disabled)
    async(Never) Metadata,           // field only available in non-async contexts
}

impl async(Either) File {
    /// Attempts to open a file in read-only mode.
    async(Either) fn open(path: Path) -> io::Result<Self> {
        match async_context() {
            async::Enabled =>  { /* create an async `File` here; can use `.await` */ },
            async::Disabled =>  { /* create a non-async `File` here */ },
        }
    }
}