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rust: macros: support #[cfg] properly in #[vtable] macro.

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Currently, we generate `HAS_` constants as long as the definition exists in
the source, regardless if it is cfg-ed out or not.

Currently, uses of `#[cfg]` present in both trait and impl, so it is not a
problem; however if only the impl side uses `#[cfg]` then `HAS_` constants
will incorrectly be true while it shouldnt't.

With `syn` support, we can now implement `#[cfg]` handling properly by
propagating the `#[cfg]` attributes to the constants.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://patch.msgid.link/20260113170529.2240744-1-gary@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This commit is contained in:
Gary Guo 2026-01-13 17:05:21 +00:00 • committed by Miguel Ojeda
parent be97f3c820
commit 779f6e3e1c
2 changed files with 10 additions and 9 deletions
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6
rust/macros/helpers.rs
View file

@ -7,6 +7,7 @@ use syn::{
Parse,
ParseStream, //
},
Attribute,
Error,
LitStr,
Result, //
@ -53,3 +54,8 @@ pub(crate) fn file() -> String {
proc_macro::Span::call_site().file()
}
}
/// Obtain all `#[cfg]` attributes.
pub(crate) fn gather_cfg_attrs(attr: &[Attribute]) -> impl Iterator<Item = &Attribute> + '_ {
attr.iter().filter(|a| a.path().is_ident("cfg"))
}

13
rust/macros/vtable.rs
View file

@ -23,7 +23,6 @@ use syn::{
fn handle_trait(mut item: ItemTrait) -> Result<ItemTrait> {
let mut gen_items = Vec::new();
let mut gen_consts = HashSet::new();
gen_items.push(parse_quote! {
/// A marker to prevent implementors from forgetting to use [`#[vtable]`](vtable)
@ -38,22 +37,17 @@ fn handle_trait(mut item: ItemTrait) -> Result<ItemTrait> {
&format!("HAS_{}", name.to_string().to_uppercase()),
name.span(),
);
// Skip if it's declared already -- this can happen if `#[cfg]` is used to selectively
// define functions.
// FIXME: `#[cfg]` should be copied and propagated to the generated consts.
if gen_consts.contains(&gen_const_name) {
continue;
}
// We don't know on the implementation-site whether a method is required or provided
// so we have to generate a const for all methods.
let cfg_attrs = crate::helpers::gather_cfg_attrs(&fn_item.attrs);
let comment =
format!("Indicates if the `{name}` method is overridden by the implementor.");
gen_items.push(parse_quote! {
#(#cfg_attrs)*
#[doc = #comment]
const #gen_const_name: bool = false;
});
gen_consts.insert(gen_const_name);
}
}
@ -87,10 +81,11 @@ fn handle_impl(mut item: ItemImpl) -> Result<ItemImpl> {
if defined_consts.contains(&gen_const_name) {
continue;
}
let cfg_attrs = crate::helpers::gather_cfg_attrs(&fn_item.attrs);
gen_items.push(parse_quote! {
#(#cfg_attrs)*
const #gen_const_name: bool = true;
});
defined_consts.insert(gen_const_name);
}
}