Don't lint transmute_undefined_repr on unions.

Author: Jarcho

clippy_lints/src/transmute/transmute_undefined_repr.rs

@@ -312,6 +312,8 @@ fn reduce_ty<'tcx>(cx: &LateContext<'tcx>, mut ty: Ty<'tcx>) -> ReducedTy<'tcx>
             ty::Adt(def, _) if def.is_enum() && (def.variants().is_empty() || is_c_void(cx, ty)) => {
                 ReducedTy::TypeErasure
             },
+            // TODO: Check if the conversion to or from at least one of a union's fields is valid.
+            ty::Adt(def, _) if def.is_union() => ReducedTy::TypeErasure,
             ty::Foreign(_) => ReducedTy::TypeErasure,
             ty::Ref(_, ty, _) => ReducedTy::Ref(ty),
             ty::RawPtr(ty) => ReducedTy::Ref(ty.ty),

tests/ui/transmute_undefined_repr.rs

@@ -2,7 +2,7 @@
 #![allow(clippy::unit_arg, clippy::transmute_ptr_to_ref)]
 
 use core::ffi::c_void;
-use core::mem::{size_of, transmute};
+use core::mem::{size_of, transmute, MaybeUninit};
 
 fn value<T>() -> T {
     unimplemented!()
@@ -103,5 +103,8 @@ fn main() {
         trait Trait {}
         let _: (isize, isize) = transmute(value::<&dyn Trait>()); // Ok
         let _: &dyn Trait = transmute(value::<(isize, isize)>()); // Ok
+
+        let _: MaybeUninit<Ty2<u32, u32>> = transmute(value::<Ty2<u32, u32>>()); // Ok
+        let _: Ty2<u32, u32> = transmute(value::<MaybeUninit<Ty2<u32, u32>>>()); // Ok
     }
 }