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Merged
merged 2 commits into from
Oct 28, 2025
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Don't use [!Default; 0]: Default impl #150

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39f2da7
don't use `[!Default; 0]: Default` impl
WaffleLapkin Oct 26, 2025
a252ad9
Best-effort compatibility for [T; 0] in #[derive(Settings)]
TheJokr Oct 28, 2025
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39 changes: 35 additions & 4 deletions foundations-macros/src/settings.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ use syn::parse::{Parse, ParseStream};
use syn::spanned::Spanned;
use syn::{
parse_macro_input, parse_quote, Attribute, Expr, ExprLit, Field, Fields, Ident, Item, ItemEnum,
ItemStruct, Lit, LitStr, Meta, MetaNameValue, Path,
ItemStruct, Lit, LitStr, Meta, MetaNameValue, Path, Type,
};

const ERR_NOT_STRUCT_OR_ENUM: &str = "Settings should be either structure or enum.";
Expand Down Expand Up @@ -238,12 +238,19 @@ fn impl_settings_trait_for_field(

impl_for_field.append_all(quote_spanned! { span=>
let mut key = parent_key.to_vec();

key.push(#name_str.into());

#crate_path::settings::Settings::add_docs(&self.#name, &key, docs);
});

// foundations#150: `[T; 0]` used to impl Settings for `T: !Default`, but this
// is not possible anymore. We thus can't call `Settings::add_docs` for such
// fields, but it was a noop anyway.
// TODO(lblocher): Remove this compatibility hack with the next major release
if !is_array_zst(&field.ty) {
impl_for_field.append_all(quote_spanned! { span =>
#crate_path::settings::Settings::add_docs(&self.#name, &key, docs);
});
}

if !docs.is_empty() {
impl_for_field.append_all(quote! {
docs.insert(key, &[#(#docs,)*][..]);
Expand Down Expand Up @@ -286,6 +293,30 @@ fn extract_doc_comments(attrs: &[Attribute]) -> Vec<LitStr> {
comments
}

/// Returns whether `ty` is exactly `[T; 0]` (for some T)
fn is_array_zst(ty: &Type) -> bool {
let Type::Array(array) = ty else {
return false;
};

let mut expr = &array.len;
while let Expr::Cast(cast) = expr {
expr = &cast.expr;
}

let Expr::Lit(ExprLit {
lit: Lit::Int(len_lit),
..
}) = expr
else {
return false;
};
len_lit
.base10_parse::<usize>()
.map(|len| len == 0)
.unwrap_or(false)
}

fn impl_serde_aware_default(item: &ItemStruct) -> proc_macro2::TokenStream {
let name = &item.ident;
let (impl_generics, ty_generics, where_clause) = item.generics.split_for_impl();
Expand Down
9 changes: 4 additions & 5 deletions foundations/src/settings/basic_impls.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -30,7 +30,6 @@ macro_rules! impl_noop {
}

impl_noop!(<T> Settings for PhantomData<T> where T: 'static);
impl_noop!(<T> Settings for [T; 0] where T: Debug + Clone + 'static);
impl_noop!(<Idx> Settings for Range<Idx> where Idx: Debug + Serialize + DeserializeOwned + Clone + Default + 'static);
impl_noop!(<T> Settings for Reverse<T> where T: Settings);
impl_noop!(<T> Settings for Wrapping<T> where T: Settings);
Expand Down Expand Up @@ -119,10 +118,10 @@ macro_rules! impl_for_array {
}

impl_for_array! {
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
31 32
0 1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18 19
20 21 22 23 24 25 26 27 28 29
30 31 32
}

impl<T: Settings> Settings for Option<T> {
Expand Down