Rust does allow you to split out generic specifications into a separate "declaration block" so things don't get too busy. Like you could write the original Rust code as:

    pub fn read<P>(path: P) -> io::Result<Vec<u8>>
    where
        P: AsRef<Path>,
    {
        // ...
    }

Personally I don't find your example with the type signature completely separate to be easier to read. Having to look in more than one place doesn't really make sense to me.

Funny, though, Rust's 'where' syntax sorta vaguely superficially reminds me of K&R pre-ANSI C:

    unsigned char *read(path)
        const char *path;
    {
        /* ... */
    }