Rust: Clarify that `expr`/`stmt` in macros can may only be followed by one of: => , ;

[dashed]

For the code:

macro_rules! foo {
    ($something:expr invalid) => {{
        // ...
    }};
}

macro_rules! bar {
    ($something:stmt invalid) => {{
        // ...
    }};
}

fn main() {
    foo!(true invalid);
    bar!(true invalid);
}

https://is.gd/TFIoq1

The compiler would give the following errors for this

error: `$something:expr` is followed by `invalid`, which is not allowed for `expr` fragments
 --> <anon>:2:22
2 |>     ($something:expr invalid) => {{
  |>                      ^^^^^^^

error: `$something:stmt` is followed by `invalid`, which is not allowed for `stmt` fragments
 --> <anon>:8:22
8 |>     ($something:stmt invalid) => {{
  |>    

---

I think it would be better if the error clarified that expr/stmt in macros can may only be followed by one of: => , ;

[durka]

I'm working on cleaning up the follow set checker to make this easier.

[pnkfelix]

cc @LeoTestard

[Mark-Simulacrum]

@durka Did that cleanup ever happen? The errors are still the same today.

[durka]

Um... I have no idea what I was planning to do.

[durka]

What should be done (IMO) is to refactor is_in_follow() so that instead of a match, it uses a map from &str to some kind of FollowSet type which is a list of tokens/idents and also has a Display impl, so then the error can do:

format!("{} cannot be followed by {} -- the follow set is {}",
    frag_spec, actual_token, follow_sets[frag_spec])

[Phlosioneer]

Issue triage: Issue still present, and still very annoying. This should probably be labeled Easy and Compliler-Errors.