Doesn't work with repr(C)

[elmarco]

Hi

An enum with a repr(C) will fail to compile:

diff --git a/tests/test.rs b/tests/test.rs
index e7e9eff..34ea048 100644
--- a/tests/test.rs
+++ b/tests/test.rs
@@ -4,7 +4,7 @@ mod small_prime {
     use super::*;
 
     #[derive(Serialize_repr, Deserialize_repr, PartialEq, Debug)]
-    #[repr(u8)]
+    #[repr(C)]
     enum SmallPrime {
         Two = 2,
         Three = 3,

   Compiling serde_repr v0.1.6 (/home/elmarco/src/serde-repr)
error[E0412]: cannot find type `C` in this scope
 --> tests/test.rs:7:12
  |
6 |     #[derive(Serialize_repr, Deserialize_repr, PartialEq, Debug)]
  |              -------------- similarly named type parameter `S` defined here
7 |     #[repr(C)]
  |            ^ help: a type parameter with a similar name exists: `S`

error[E0412]: cannot find type `C` in this scope
 --> tests/test.rs:7:12
  |
6 |     #[derive(Serialize_repr, Deserialize_repr, PartialEq, Debug)]
  |                              ---------------- similarly named type parameter `D` defined here
7 |     #[repr(C)]
  |            ^ help: a type parameter with a similar name exists: `D`

Is there a workaround or possible solution ? (is it possible to get the underlying chosen int?)

[dtolnay]

This is not supported by this crate. The repr is required to be a specific integer type.

But cannot find type `C` in this scope is not a good error message. I'll make a PR to change that.

To support repr(C), you could look for, or create, an attribute macro that inspects a set of enum discriminants and expands to the same integer repr that would be chosen by a C compiler.

#[infer_C_repr]
#[derive(Serialize_repr)]
pub enum Enum {
    ...
}

// expands to:
#[repr(i16)] // for example
#[derive(Serialize_repr)]
pub enum Enum {
    ...
}