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Modify derive_label to support no_std environments #15465

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Merged
merged 2 commits into from
Sep 27, 2024
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Modify derive_label to support no_std environments #15465

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c9bbc5c
Modify `derive_label` to support `no_std` environments
bushrat011899 Sep 27, 2024
80d64a9
Copy useful explanation into a comment
alice-i-cecile Sep 27, 2024
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29 changes: 17 additions & 12 deletions crates/bevy_macro_utils/src/label.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -81,21 +81,26 @@ pub fn derive_label(
.unwrap(),
);
quote! {
impl #impl_generics #trait_path for #ident #ty_generics #where_clause {
fn dyn_clone(&self) -> ::std::boxed::Box<dyn #trait_path> {
::std::boxed::Box::new(::core::clone::Clone::clone(self))
}
// To ensure alloc is available, but also prevent its name from clashing, we place the implementation inside an anonymous constant
const _: () = {
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Why this is needed? 🤔

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If you check the description of the PR the goal is to allow invoking this macro in no_std crates, which is required for Bevy to become no_std.

We would normally use the fully qualified name:

::std::boxed::Box

However, in no_std we can't use the std namespace, so instead we need to use the original definition of Box in the alloc crate:

::alloc::boxed::Box

But that also doesn't work, because access to the alloc namespace requires explicitly linking it in the end-user's crate:

extern crate alloc;

So we need to include that extern statement. However, we can't place it in the global namespace, because it would then clash with other macro invocations.

To ensure alloc is available, but also prevent its name from clashing, we place the implementation inside an anonymous constant:

const _: () = {
    // private namespace so no clashes
};

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Just not understand why code put in const _: () = { ... }. Thanks for detailed explanation!

extern crate alloc;

fn as_dyn_eq(&self) -> &dyn #dyn_eq_path {
self
}
impl #impl_generics #trait_path for #ident #ty_generics #where_clause {
fn dyn_clone(&self) -> alloc::boxed::Box<dyn #trait_path> {
alloc::boxed::Box::new(::core::clone::Clone::clone(self))
}

fn dyn_hash(&self, mut state: &mut dyn ::core::hash::Hasher) {
let ty_id = ::core::any::TypeId::of::<Self>();
::core::hash::Hash::hash(&ty_id, &mut state);
::core::hash::Hash::hash(self, &mut state);
fn as_dyn_eq(&self) -> &dyn #dyn_eq_path {
self
}

fn dyn_hash(&self, mut state: &mut dyn ::core::hash::Hasher) {
let ty_id = ::core::any::TypeId::of::<Self>();
::core::hash::Hash::hash(&ty_id, &mut state);
::core::hash::Hash::hash(self, &mut state);
}
}
}
};
}
.into()
}