tock-registers: fix Copy and Clone implementation on Field

A Field contains a PhantomData over a generic R: RegisterLongName, and
a PhantomData<R> is copyable regardless of whether R: Copy. However,
using #[derive(Copy, Clone)] will generate

    #[automatically_derived]
    #[allow(unused_qualifications)]
    impl<T: ::core::marker::Copy + IntLike,
         R: ::core::marker::Copy + RegisterLongName>
            ::core::marker::Copy for Field<T, R> {}

and hence Field will only implement Copy when R: Copy.

Manually implementing Clone and Copy works around this issue.

See rust-lang/rust#26925 for more
information on this.

Signed-off-by: Leon Schuermann <[email protected]>

Author: lschuermann

libraries/tock-register-interface/src/registers.rs

@@ -518,7 +518,6 @@ impl<T: IntLike, R: RegisterLongName> InMemoryRegister<T, R> {
 /// Specific section of a register.
 ///
 /// For the Field, the mask is unshifted, ie. the LSB should always be set.
-#[derive(Copy, Clone)]
 pub struct Field<T: IntLike, R: RegisterLongName> {
     pub mask: T,
     pub shift: usize,
@@ -552,6 +551,33 @@ impl<T: IntLike, R: RegisterLongName> Field<T, R> {
     }
 }
 
+// #[derive(Copy, Clone)] won't work here because it will use
+// incorrect bounds, as a result of using a PhantomData over the
+// generic R. The PhantomData<R> implements Copy regardless of whether
+// R does, but the #[derive(Copy, Clone)] generates
+//
+//    #[automatically_derived]
+//    #[allow(unused_qualifications)]
+//    impl<T: ::core::marker::Copy + IntLike,
+//         R: ::core::marker::Copy + RegisterLongName>
+//            ::core::marker::Copy for Field<T, R> {}
+//
+// , so Field will only implement Copy if R: Copy.
+//
+// Manually implementing Clone and Copy works around this issue.
+//
+// Relevant Rust issue: https://github.com/rust-lang/rust/issues/26925
+impl<T: IntLike, R: RegisterLongName> Clone for Field<T, R> {
+    fn clone(&self) -> Self {
+        Field {
+            mask: self.mask,
+            shift: self.shift,
+            associated_register: self.associated_register,
+        }
+    }
+}
+impl<T: IntLike, R: RegisterLongName> Copy for Field<T, R> {}
+
 macro_rules! Field_impl_for {
     ($type:ty) => {
         impl<R: RegisterLongName> Field<$type, R> {