Make #[repr(C)] optional

You can either use `std::os::raw::c_int` or `libc::c_int`.

Unfortunately, the feature name is awkward to work around
dtolnay/trybuild#171.

Cargo.toml

@@ -11,6 +11,10 @@ keywords = ["enum", "open", "integer", "newtype"]
 include = ["/src", "/LICENSE", "Cargo.toml"]
 
 [features]
+# This is not named libc because it would require "dep:libc"
+# which doesn't work with trybuild: https://github.com/dtolnay/trybuild/issues/171.
+libc_ = ["libc", "open-enum-derive/repr_c"]
+std = ["open-enum-derive/repr_c"]
 
 # Needed to test #[deny(missing_docs)], which doesn't trigger in unit or integration tests.
 [[bin]]
@@ -19,8 +23,7 @@ path = "src/test-lints.rs"
 
 [dependencies]
 open-enum-derive = { path = "derive", version = "0.2.0" }
-# TODO: consider making this optional - I tried and trybuild started failing
-libc = { version = "0.2" }
+libc = { version = "0.2", optional = true }
 
 [dev-dependencies]
 trybuild = "1"

derive/Cargo.toml

@@ -13,9 +13,10 @@ include = ["/src", "../LICENSE", "Cargo.toml"]
 proc-macro = true
 
 [features]
+# Enable #[repr(C)] open enums
+repr_c = []
 
 [dependencies]
 syn = { version = "1", features = ["full"] }
 quote = "1"
 proc-macro2 = "1.0.43"
-libc = { version = "0.2" }

derive/src/lib.rs

@@ -113,6 +113,7 @@ enum Repr {
     I64,
     Usize,
     Isize,
+    #[cfg(feature = "repr_c")]
     C,
 }
 
@@ -157,6 +158,7 @@ impl Repr {
             Repr::I64 => "i64",
             Repr::Usize => "usize",
             Repr::Isize => "isize",
+            #[cfg(feature = "repr_c")]
             Repr::C => "C",
         }
     }
@@ -167,6 +169,7 @@ impl ToTokens for Repr {
         tokens.extend([match self {
             // Technically speaking, #[repr(C)] on an enum isn't always `c_int`,
             // but those who care can fix it if they need.
+            #[cfg(feature = "repr_c")]
             Repr::C => quote!(::open_enum::__private::c_int),
             x => x.name().parse::<TokenStream>().unwrap(),
         }])
@@ -187,7 +190,15 @@ impl Parse for Repr {
             "u64" => Repr::U64,
             "usize" => Repr::Usize,
             "isize" => Repr::Isize,
+            #[cfg(feature = "repr_c")]
             "C" => Repr::C,
+            #[cfg(not(feature = "repr_c"))]
+            "C" => {
+                return Err(Error::new(
+                    ident.span(),
+                    "#[repr(C)] requires either the `std` or `libc_` feature",
+                ))
+            }
             _ => {
                 return Err(Error::new(
                     ident.span(),

src/lib.rs

@@ -199,5 +199,12 @@ pub use open_enum_derive::open_enum;
 /// Utility items only to be used by macros. Do not expect API stability.
 #[doc(hidden)]
 pub mod __private {
+    #[cfg(all(feature = "libc", not(feature = "std")))]
     pub use libc::c_int;
+
+    #[cfg(feature = "std")]
+    extern crate std;
+
+    #[cfg(feature = "std")]
+    pub use std::os::raw::c_int;
 }

tests/basic.rs

@@ -59,7 +59,6 @@ enum Color {
     Azure,
 }
 
-
 #[test]
 fn values() {
     assert_eq!(Fruit::Apple.0, 0);
@@ -114,19 +113,24 @@ fn other_derive() {
     HasHash(20).hash(&mut hasher);
 }
 
+#[cfg(any(feature = "libc", feature = "std"))]
 #[test]
 fn repr_c() {
     use core::mem::size_of;
 
     #[repr(C)]
     #[open_enum]
     enum AnimalSound {
-        Moo, Honk, Bahhh,
+        Moo,
+        Honk,
+        Bahhh,
     }
-    use libc::c_int;
 
     assert_eq!(AnimalSound::Moo.0, 0);
-    assert_eq!(size_of::<AnimalSound>(), size_of::<c_int>());
+    assert_eq!(
+        size_of::<AnimalSound>(),
+        size_of::<open_enum::__private::c_int>()
+    );
 }
 
 #[test]