rust: pin-init: add #[default_error(<type>)] attribute to initializer macros

The `#[default_error(<type>)]` attribute can be used to supply a default
type as the error used for the `[pin_]init!` macros. This way one can
easily define custom `try_[pin_]init!` variants that default to your
project specific error type. Just write the following declarative macro:

    macro_rules! try_init {
        ($($args:tt)*) => {
            ::pin_init::init!(
                #[default_error(YourCustomErrorType)]
                $($args)*
            )
        }
    }

Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <lossin@kernel.org>

rust/pin-init/internal/src/init.rs

@@ -8,12 +8,13 @@ use syn::{
     parse_quote,
     punctuated::Punctuated,
     spanned::Spanned,
-    token, Block, Expr, ExprCall, ExprPath, Ident, Path, Token, Type,
+    token, Attribute, Block, Expr, ExprCall, ExprPath, Ident, Path, Token, Type,
 };
 
 use crate::diagnostics::{DiagCtxt, ErrorGuaranteed};
 
 pub(crate) struct Initializer {
+    attrs: Vec<InitializerAttribute>,
     this: Option<This>,
     path: Path,
     brace_token: token::Brace,
@@ -54,8 +55,17 @@ impl InitializerField {
     }
 }
 
+enum InitializerAttribute {
+    DefaultError(DefaultErrorAttribute),
+}
+
+struct DefaultErrorAttribute {
+    ty: Box<Type>,
+}
+
 pub(crate) fn expand(
     Initializer {
+        attrs,
         this,
         path,
         brace_token,
@@ -69,14 +79,23 @@ pub(crate) fn expand(
 ) -> Result<TokenStream, ErrorGuaranteed> {
     let error = error.map_or_else(
         || {
-            if let Some(default_error) = default_error {
+            if let Some(default_error) = attrs.iter().fold(None, |acc, attr| {
+                #[expect(irrefutable_let_patterns)]
+                if let InitializerAttribute::DefaultError(DefaultErrorAttribute { ty }) = attr {
+                    Some(ty.clone())
+                } else {
+                    acc
+                }
+            }) {
+                default_error
+            } else if let Some(default_error) = default_error {
                 syn::parse_str(default_error).unwrap()
             } else {
                 dcx.error(brace_token.span.close(), "expected `? <type>` after `}`");
                 parse_quote!(::core::convert::Infallible)
             }
         },
-        |(_, err)| err,
+        |(_, err)| Box::new(err),
     );
     let slot = format_ident!("slot");
     let (has_data_trait, data_trait, get_data, init_from_closure) = if pinned {
@@ -358,6 +377,7 @@ fn make_field_check(
 
 impl Parse for Initializer {
     fn parse(input: syn::parse::ParseStream<'_>) -> syn::Result<Self> {
+        let attrs = input.call(Attribute::parse_outer)?;
         let this = input.peek(Token![&]).then(|| input.parse()).transpose()?;
         let path = input.parse()?;
         let content;
@@ -389,7 +409,19 @@ impl Parse for Initializer {
             .peek(Token![?])
             .then(|| Ok::<_, syn::Error>((input.parse()?, input.parse()?)))
             .transpose()?;
+        let attrs = attrs
+            .into_iter()
+            .map(|a| {
+                if a.path().is_ident("default_error") {
+                    a.parse_args::<DefaultErrorAttribute>()
+                        .map(InitializerAttribute::DefaultError)
+                } else {
+                    Err(syn::Error::new_spanned(a, "unknown initializer attribute"))
+                }
+            })
+            .collect::<Result<Vec<_>, _>>()?;
         Ok(Self {
+            attrs,
             this,
             path,
             brace_token,
@@ -400,6 +432,12 @@ impl Parse for Initializer {
     }
 }
 
+impl Parse for DefaultErrorAttribute {
+    fn parse(input: syn::parse::ParseStream<'_>) -> syn::Result<Self> {
+        Ok(Self { ty: input.parse()? })
+    }
+}
+
 impl Parse for This {
     fn parse(input: syn::parse::ParseStream<'_>) -> syn::Result<Self> {
         Ok(Self {