derives panic when a discriminant is given by a macro

[AdnoC]

If you use set the discriminant of a C-like enum with a macro the derive panics. This happens with both ToPrimitive and FromPrimitive.

Minimal example:

#[macro_use]
extern crate num_derive;
extern crate num_traits;

macro_rules! get_an_isize {
    () => (0_isize)
}

#[derive(FromPrimitive)]
pub enum CLikeEnum {
    VarA = get_an_isize!(), // panics
    VarB = 2,
}

Errors with

error: proc-macro derive panicked
 --> src\lib.rs:9:10
  |
9 | #[derive(FromPrimitive)]
  |          ^^^^^^^^^^^^^
  |
  = help: message: called `Result::unwrap()` on an `Err` value: ParseError(Some("failed to parse derive input: failed to parse"))

Cargo.toml:

[dependencies]
num-traits = "0.2"
num-derive = "0.2"

Rust version:
rustc 1.27.0 (3eda71b00 2018-06-19)

[AdnoC]

Actually, seems like it is syn's fault, as syn::parse is returning an Err.

[cuviper]

You may need to enable the "full-syntax" feature here, which enables "syn/full".

[AdnoC]

Yeah, just figured that out. Going to make a ticket for syn since it should be able to parse valid custom derive input.

[bcantrill]

I hit this as well -- thank you @AdnoC and @cuviper for what ended up being a quick fix to an otherwise vexing problem! Was there ever an issue open on syn? If not, I think I'd like to get an issue open over there (and/or commented upon); the failure mode here is pretty brutal...

[cuviper]

@bcantrill I don't know if there was ever a syn issue, but the error now seems worse:

error: proc-macro derive panicked
 --> src/lib.rs:9:10
  |
9 | #[derive(FromPrimitive)]
  |          ^^^^^^^^^^^^^
  |
  = help: message: called `Result::unwrap()` on an `Err` value: Error("expected `,`")

I guess it's sort of more specific than "failed to parse", but there's not enough context to really be useful.

Maybe instead of syn::parse(input).unwrap(), we should try the syn::parse_macro_input! macro. If that prints span info, it will be a lot more helpful. Or maybe we can manually do the same while combining a message about the "full-syntax" feature.

[cuviper]

Re-opening since I think we can indeed do better on num-derive's part.

I have it looking like this now:

error: expected `,`
  --> src/test.rs:12:24
   |
10 |     VarA = get_an_isize!(), // panics
   |                        ^

error: this might need the "full-syntax" feature of `num-derive`
  --> src/test.rs:12:24
   |
10 |     VarA = get_an_isize!(), // panics
   |                        ^

error: aborting due to 2 previous errors

But I'd like to connect the second message back to the derive(FromPrimitive) span if possible.

[cuviper]

Aha, Span::call_site() is what I want. I'll get a PR up shortly, once I figure out how to test this...

[bcantrill]

Thanks for opening this back up! The failure mode was indeed really confusing -- but your new error message is great, and would have sent me to the right spot more or less immediately; thanks again!