use extern crate as zerocopy to support intenral derive use (#604)

This commit also alters the derives to refer to items as
`::zerocopy`, instead of `zerocopy`, which unambiguously refers to
the top-level item, and not any local modules that may have the
same name.

src/lib.rs

@@ -228,6 +228,13 @@
     __INTERNAL_USE_ONLY_NIGHLTY_FEATURES_IN_TESTS,
     feature(layout_for_ptr, strict_provenance)
 )]
+
+// This is a hack to allow zerocopy-derive derives to work in this crate. They
+// assume that zerocopy is linked as an extern crate, so they access items from
+// it as `zerocopy::Xxx`. This makes that still work.
+#[cfg(any(feature = "derive", test))]
+extern crate self as zerocopy;
+
 #[macro_use]
 mod macros;
 
@@ -290,14 +297,6 @@ use {
 #[allow(unused_imports)]
 use crate::util::polyfills::NonNullExt as _;
 
-// This is a hack to allow zerocopy-derive derives to work in this crate. They
-// assume that zerocopy is linked as an extern crate, so they access items from
-// it as `zerocopy::Xxx`. This makes that still work.
-#[cfg(any(feature = "derive", test))]
-mod zerocopy {
-    pub(crate) use crate::*;
-}
-
 #[rustversion::nightly]
 #[cfg(all(test, not(__INTERNAL_USE_ONLY_NIGHLTY_FEATURES_IN_TESTS)))]
 const _: () = {

zerocopy-derive/src/lib.rs

@@ -549,7 +549,7 @@ fn impl_block<D: DataExt>(
     let field_types = data.field_types();
 
     let field_type_bounds = require_trait_bound_on_field_types
-        .then(|| field_types.iter().map(|ty| parse_quote!(#ty: zerocopy::#trait_ident)))
+        .then(|| field_types.iter().map(|ty| parse_quote!(#ty: ::zerocopy::#trait_ident)))
         .into_iter()
         .flatten()
         .collect::<Vec<_>>();
@@ -560,8 +560,8 @@ fn impl_block<D: DataExt>(
         let fields = field_types.iter();
         let validator_macro = check.validator_macro_ident();
         parse_quote!(
-            zerocopy::macro_util::HasPadding<#type_ident, {zerocopy::#validator_macro!(#type_ident, #(#fields),*)}>:
-                zerocopy::macro_util::ShouldBe<false>
+            ::zerocopy::macro_util::HasPadding<#type_ident, {::zerocopy::#validator_macro!(#type_ident, #(#fields),*)}>:
+                ::zerocopy::macro_util::ShouldBe<false>
         )
     });
 
@@ -579,7 +579,7 @@ fn impl_block<D: DataExt>(
         // We currently only support deriving for sized types; this code will
         // fail to compile for unsized types.
         Some(quote!(
-            const LAYOUT: zerocopy::DstLayout = zerocopy::DstLayout::for_type::<Self>();
+            const LAYOUT: ::zerocopy::DstLayout = ::zerocopy::DstLayout::for_type::<Self>();
 
             // SAFETY: `.cast` preserves address and provenance.
             //
@@ -620,7 +620,7 @@ fn impl_block<D: DataExt>(
         // TODO(#553): Add a test that generates a warning when
         // `#[allow(deprecated)]` isn't present.
         #[allow(deprecated)]
-        unsafe impl < #(#params),* > zerocopy::#trait_ident for #type_ident < #(#param_idents),* >
+        unsafe impl < #(#params),* > ::zerocopy::#trait_ident for #type_ident < #(#param_idents),* >
         where
             #(#bounds,)*
         {