entertaining error message on bad macro syntax

[dhardy]

UPDATE: Mentoring instructions below.

---

Try compiling this code:

use std::io;

fn main(){
    let x: io::IoResult<()> = Ok(());
    match x {
        Err(ref e) if e.kind == io::EndOfFile {
            return
        }
        _ => {}
    }
}

I (using Rust 0.11.0) get the very misleading message:

$ rustc match_syntax_err.rs
match_syntax_err.rs:7:13: 7:19 error: found `return` in ident position
match_syntax_err.rs:7             return
                                  ^~~~~~
match_syntax_err.rs:8:9: 8:10 error: expected `:` but found `}`
match_syntax_err.rs:8         }

when actually all that's wrong is a missing =>. Not really a good brain teaser on a Friday evening ;)

[steveklabnik]

This error message is the same today. How awkward.

[bombless]

I guess we can summary a pattern here (missing => between pattern and block) to the parsing rule that allow us to add something similar as check_no_chained_comparison.

I suspect another valid approach would be, don't mention the ident thing, just focus on the fact that a value is needed here (a struct literal, in fact), so we can just say "invalid struct literal, unexpected return here" or even simpler, "return not allowed inside pattern matching". At least this will warn the user that the return is not really inside a block.

[gchp]

On 1.10.0-nightly this gives:

test.rs:5:30: 5:31 error: expected one of `.`, `=>`, `?`, or an operator, found `{`
test.rs:5         Err(ref e) if 1 == 2 {
                                       ^
error: aborting due to previous error

This seems accurate enough to me, the compiler is pointing out where it is expecting to find the =>, and mentioning it in the error message. Do we need something more specific than this?

[dhardy]

The error I see is still pretty misleading

[estebank]

Current output:

error: expected identifier, found keyword `return`
 --> src/main.rs:7:13
  |
7 |             return
  |             ^^^^^^

---

parse_pat can parse a struct and recover from an error. It should attach a secondary label pointing at the start of the struct constructor being parsed (I believe it should be lo.to(self.prev_span)) with a label along the lines of "while parsing the fields for this struct". This would end up looking close to this:

error: expected identifier, found keyword `return`
 --> src/main.rs:7:13
  |
6 |         Err(ref e) if e.kind == io::EndOfFile {
  |                                 ------------- while parsing the fields for this struct
7 |             return
  |             ^^^^^^ expected identifier, found keyword