[WIP] TryFromBytes

TODO:
- Should we require `FromBytes: FromZeroes + TryFromBytes` or
  `FromZeroes: TryFromBytes`? We obviously only need one of these, but
  which one?

Makes progress on #5

src/derive_util.rs

@@ -62,3 +62,34 @@ macro_rules! union_has_padding {
         false $(|| core::mem::size_of::<$t>() != core::mem::size_of::<$ts>())*
     };
 }
+
+#[doc(hidden)]
+pub use project::project as __project;
+
+#[doc(hidden)] // `#[macro_export]` bypasses this module's `#[doc(hidden)]`.
+#[macro_export]
+macro_rules! impl_try_from_bytes {
+    ($ty:ty { $($f:tt: $f_ty:ty),* } $(=> $validation_method:ident)?) => {
+        #[allow(unused_qualifications)]
+        unsafe impl zerocopy::TryFromBytes for $ty {
+            fn is_bit_valid(bytes: &zerocopy::MaybeValid<Self>) -> bool {
+                true $(&& {
+                    let f: &zerocopy::MaybeValid<$f_ty>
+                        = zerocopy::derive_util::__project!(&bytes.$f);
+                    zerocopy::TryFromBytes::is_bit_valid(f)
+                })*
+                $(&& {
+                    let bytes_ptr: *const zerocopy::MaybeValid<Self> = bytes;
+                    let slf = unsafe { &*bytes_ptr.cast::<Self>() };
+                    // TODO: What about interior mutability? One approach would
+                    // be to have the validation method operate on a
+                    // `#[repr(transparent)]` `Freeze` container that implements
+                    // `Projectable`. If we eventually get a `Freeze` or
+                    // `NoCell` trait, that container could implement `Deref`
+                    // for types which don't contain any cells.
+                    slf.$validation_method()
+                })?
+            }
+        }
+    }
+}