ra_syntax: errors on a type cast + comparison

[ordian]

fn is_empty<T: ?Sized>(ptr: *const T) -> bool {
    return ptr as *const usize as usize <= 256;
    //                                   ^^^   
    // ra_syntax expects a type, COMMA, R_ANGLE or SEMI 
}

[DJMcNab]

The relevant code in libsyntax starts here - @matklad, I'm not sure how to make this work in our infra?

[matklad]

@DJMcNab missing link to linsyntax source?

[DJMcNab]

Oops. I'm on mobile so can't link now, but it's a line about creating a Snapshot and reloading it on syntax errors in the parsed trait bound

[matklad]

I think that logic exists solely for the purpose of better error message. it should be possible to parse this without lookahead. See rust-lang/rust#42578 and the state of the code before that PR for how this should be done.

We'll add backtracking later, of course, but for the start it's better to keep things simple.

[matklad]

Specifically, the following is still error in rustc:

return ptr as *const usize as usize < 256;

[kiljacken]

This is working now, so issue can be closed.