Don't use [!Default; 0]: Default impl (#150)

There is an effort to remove Rust's unconditional `[T; 0]: Default` impl
for `T: !Default`, see rust-lang/rust#145457.
foundations depends on this impl in its public API, so fixing this is a
breaking change.

However, after consideration we decided that we will break semver for
this change. We don't expect any users of foundations to depend directly
on `[T; 0]: Settings`. We also put a best-effort workaround in place for the
`#[derive(Settings)]` use case inside foundations.

Co-authored-by: Leo Blöcher <lblocher@cloudflare.com>

Author: WaffleLapkin

foundations-macros/src/settings.rs

@@ -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.";
@@ -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,)*][..]);
@@ -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();

foundations/src/settings/basic_impls.rs

@@ -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);
@@ -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> {