Improve error message of generated static assertions

Author: meithecatte

enumflags_derive/src/lib.rs

@@ -147,12 +147,13 @@ fn verify_flag_values<'a>(
                     Span::call_site()); // call_site because def_site is unstable
                 // adapted from static-assertions-rs by nvzqz (MIT/Apache-2.0)
                 deferred_checks.push(quote_spanned!(variant.span() =>
-                    #[allow(unknown_lints, eq_op)]
-                    const #assertion_name: [(); 0 - !(
-                        (#_type_name::#variant_name as u64).wrapping_sub(1) &
-                        (#_type_name::#variant_name as u64) == 0 &&
-                        (#_type_name::#variant_name as u64) != 0
-                    ) as usize] = [];
+                    const #assertion_name: fn() = || {
+                        ::enumflags2::_internal::assert_exactly_one_bit_set::<[(); (
+                            (#_type_name::#variant_name as u64).wrapping_sub(1) &
+                            (#_type_name::#variant_name as u64) == 0 &&
+                            (#_type_name::#variant_name as u64) != 0
+                        ) as usize]>();
+                    };
                 ));
             }
             Err(why) => return Err(why.into()),

src/lib.rs

@@ -214,6 +214,26 @@ pub mod _internal {
     pub mod core {
         pub use core::{convert, option, ops};
     }
+
+    pub struct AssertionSucceeded;
+    pub struct AssertionFailed;
+    pub trait ExactlyOneBitSet {}
+    impl ExactlyOneBitSet for AssertionSucceeded {}
+
+    pub trait AssertionHelper {
+        type Status;
+    }
+
+    impl AssertionHelper for [(); 1] {
+        type Status = AssertionSucceeded;
+    }
+
+    impl AssertionHelper for [(); 0] {
+        type Status = AssertionFailed;
+    }
+
+    pub fn assert_exactly_one_bit_set<T: AssertionHelper>()
+        where T::Status: ExactlyOneBitSet {}
 }
 
 // Internal debug formatting implementations