1 files changed, 378 insertions, 0 deletions
diff --git a/mingling_macros/src/attr/chain.rs b/mingling_macros/src/attr/chain.rs
new file mode 100644
index 0000000..dbd87dd
--- /dev/null
+++ b/mingling_macros/src/attr/chain.rs
@@ -0,0 +1,378 @@
+#![allow(clippy::too_many_arguments)]
+
+use crate::res_injection::{
+    ResourceInjection, extract_args_info, generate_immut_resource_bindings,
+    wrap_body_with_mut_resources, wrap_body_with_mut_resources_async,
+};
+use proc_macro::TokenStream;
+use quote::{ToTokens, quote};
+use syn::spanned::Spanned;
+use syn::{Ident, ItemFn, Pat, ReturnType, Signature, Type, TypePath, parse_macro_input};
+
+/// Checks whether the return type is `()`
+fn is_unit_return_type(sig: &Signature) -> bool {
+    match &sig.output {
+        ReturnType::Type(_, ty) => match &**ty {
+            Type::Tuple(tuple) => tuple.elems.is_empty(),
+            _ => false,
+        },
+        ReturnType::Default => true,
+    }
+}
+
+/// Validates that the return type is acceptable.
+/// Accepts `()`, `Next`, `ChainProcess<...>`, or any type that can
+/// be converted to `ChainProcess` via `.into()` (i.e. any pack type).
+fn validate_return_type(sig: &Signature) -> Result<(), proc_macro2::TokenStream> {
+    // `()` or omitted is always valid
+    if is_unit_return_type(sig) {
+        return Ok(());
+    }
+
+    Ok(())
+}
+
+/// Builds the `proc` function implementation inside the generated `Chain` impl.
+///
+/// Instead of inlining the user's body, the trait method calls the original
+/// function by name, with resources injected from the application context.
+fn generate_proc_fn(
+    fn_name: &Ident,
+    has_resources: bool,
+    resources: &[ResourceInjection],
+    program_type: &proc_macro2::TokenStream,
+    previous_type: &TypePath,
+    is_async_fn: bool,
+    is_unit_return: bool,
+    origin_return_type: &proc_macro2::TokenStream,
+) -> proc_macro2::TokenStream {
+    let immut_resource_stmts = generate_immut_resource_bindings(resources.iter(), program_type);
+    let mut_resources: Vec<_> = resources.iter().filter(|r| r.is_mut).collect();
+
+    // Use a fixed parameter name `prev` for the trait method, regardless of
+    // the user's original parameter name (which may be `_` and cannot be
+    // referenced in expression position).
+    let fixed_prev: Pat = syn::parse_quote!(prev);
+
+    // Build the call to the original function with resource arguments injected.
+    // The variable names come from the resource injection bindings
+    // (immut bindings are `let #name = …`, mut closures receive `|#name: &mut T|`),
+    // which match the original function's parameter names.
+    let resource_args: Vec<_> = resources
+        .iter()
+        .map(|res| {
+            let var_name = &res.var_name;
+            quote! { #var_name }
+        })
+        .collect();
+
+    let fn_call = if has_resources {
+        let call = quote! { #fn_name(#fixed_prev, #(#resource_args),*) };
+        if is_async_fn {
+            quote! { #call.await }
+        } else {
+            call
+        }
+    } else {
+        let call = quote! { #fn_name(#fixed_prev) };
+        if is_async_fn {
+            quote! { #call.await }
+        } else {
+            call
+        }
+    };
+
+    // Convert the function call to a syn::Stmt so existing wrapping functions can use it
+    let fn_call_expr: syn::Expr = syn::parse_quote! { #fn_call };
+    let fn_call_stmt = syn::Stmt::Expr(fn_call_expr, None);
+
+    let wrapped_body = if is_async_fn && !mut_resources.is_empty() {
+        wrap_body_with_mut_resources_async(&[fn_call_stmt], &mut_resources, program_type)
+    } else {
+        wrap_body_with_mut_resources(
+            &[fn_call_stmt],
+            &mut_resources,
+            program_type,
+            is_unit_return,
+        )
+    };
+
+    let proc_body = if is_unit_return {
+        let body_with_ending = if has_resources {
+            quote! {
+                #(#immut_resource_stmts)*
+                #wrapped_body;
+                <crate::ResultEmpty as ::mingling::Routable::<crate::ThisProgram>>
+                ::to_chain(crate::ResultEmpty)
+            }
+        } else {
+            quote! {
+                #wrapped_body;
+                <crate::ResultEmpty as ::mingling::Routable::<crate::ThisProgram>>
+                ::to_chain(crate::ResultEmpty)
+            }
+        };
+        quote! { #body_with_ending }
+    } else {
+        let body = if has_resources {
+            quote! {
+                #(#immut_resource_stmts)*
+                #wrapped_body
+            }
+        } else {
+            quote! { #wrapped_body }
+        };
+        quote! {
+            let __chain_result = { #body };
+            <#origin_return_type as ::std::convert::Into<
+                ::mingling::ChainProcess<#program_type>
+            >>::into(__chain_result)
+        }
+    };
+
+    #[cfg(feature = "async")]
+    {
+        quote! {
+            async fn proc(#fixed_prev: #previous_type) -> ::mingling::ChainProcess<#program_type> {
+                #proc_body
+            }
+        }
+    }
+
+    #[cfg(not(feature = "async"))]
+    {
+        quote! {
+            fn proc(#fixed_prev: #previous_type) -> ::mingling::ChainProcess<#program_type> {
+                #proc_body
+            }
+        }
+    }
+}
+
+/// Assembles the final expanded output: hidden struct, `register_chain!` invocation,
+/// `Chain` impl with the `proc` method, and the preserved original function.
+fn generate_struct_and_impl(
+    fn_attrs: &[syn::Attribute],
+    vis: &syn::Visibility,
+    struct_name: &Ident,
+    previous_type: &TypePath,
+    previous_type_str: &proc_macro2::TokenStream,
+    program_type: &proc_macro2::TokenStream,
+    proc_fn: &proc_macro2::TokenStream,
+    original_fn: &proc_macro2::TokenStream,
+) -> proc_macro2::TokenStream {
+    quote! {
+        #(#fn_attrs)*
+        #[doc(hidden)]
+        #[allow(non_camel_case_types)]
+        #vis struct #struct_name;
+
+        ::mingling::macros::register_chain!(#previous_type_str, #struct_name);
+
+        impl ::mingling::Chain<#program_type> for #struct_name {
+            type Previous = #previous_type;
+
+            #proc_fn
+        }
+
+        // Keep the original function unchanged
+        #original_fn
+    }
+}
+
+/// Ensures the function is not async when the `async` feature is disabled.
+#[cfg(not(feature = "async"))]
+fn reject_async(sig: &Signature) -> Result<(), proc_macro2::TokenStream> {
+    if sig.asyncness.is_some() {
+        return Err(syn::Error::new(
+            sig.span(),
+            "Chain function cannot be async when async feature is disabled",
+        )
+        .to_compile_error());
+    }
+    Ok(())
+}
+
+pub(crate) fn chain_attr(attr: TokenStream, item: TokenStream) -> TokenStream {
+    // Reject non-empty attribute arguments; #[chain] must be bare
+    if !attr.is_empty() {
+        return syn::Error::new(
+            attr.into_iter().next().unwrap().span().into(),
+            "#[chain] does not accept arguments",
+        )
+        .to_compile_error()
+        .into();
+    }
+
+    // Parse the function item
+    let input_fn = parse_macro_input!(item as ItemFn);
+
+    // Handle async feature gate
+    #[cfg(feature = "async")]
+    let is_async_fn = input_fn.sig.asyncness.is_some();
+
+    #[cfg(not(feature = "async"))]
+    {
+        if let Err(err) = reject_async(&input_fn.sig) {
+            return err.into();
+        }
+    }
+
+    // Check if return type is unit
+    let is_unit_return = is_unit_return_type(&input_fn.sig);
+
+    // Validate return type
+    if let Err(err) = validate_return_type(&input_fn.sig) {
+        return err.into();
+    }
+
+    // Extract the previous type, parameter name, and resource injection params
+    let (_, previous_type, resources) = match extract_args_info(&input_fn.sig) {
+        Ok(info) => info,
+        Err(e) => return e.to_compile_error().into(),
+    };
+
+    // Prepare building blocks
+    let mut fn_attrs = input_fn.attrs.clone();
+    fn_attrs.retain(|attr| !attr.path().is_ident("chain"));
+    let vis = &input_fn.vis;
+    let fn_name = &input_fn.sig.ident;
+    let has_resources = !resources.is_empty();
+
+    // Generate struct name
+    let internal_name = format!(
+        "__internal_chain_{}",
+        just_fmt::snake_case!(fn_name.to_string())
+    );
+    let struct_name = Ident::new(&internal_name, fn_name.span());
+
+    // Always use the default crate-defined program path
+    let program_type = crate::default_program_path();
+
+    // Extract the user's return type for the explicit Into turbofish
+    let origin_return_type = match &input_fn.sig.output {
+        ReturnType::Type(_, ty) => quote! { #ty },
+        ReturnType::Default => quote! { () },
+    };
+
+    // Generate the `proc` function for the Chain impl
+    let proc_fn = generate_proc_fn(
+        fn_name,
+        has_resources,
+        &resources,
+        &program_type,
+        &previous_type,
+        #[cfg(feature = "async")]
+        is_async_fn,
+        #[cfg(not(feature = "async"))]
+        false,
+        is_unit_return,
+        &origin_return_type,
+    );
+
+    // Preserve the original function untouched
+    // Note: do NOT add `#vis` here — `input_fn` (ItemFn) already contains its own visibility.
+    let original_fn = quote! {
+        #(#fn_attrs)*
+        #input_fn
+    };
+
+    // Assemble the final output
+    let previous_type_str = quote! { #previous_type };
+    let expanded = generate_struct_and_impl(
+        &fn_attrs,
+        vis,
+        &struct_name,
+        &previous_type,
+        &previous_type_str,
+        &program_type,
+        &proc_fn,
+        &original_fn,
+    );
+
+    expanded.into()
+}
+
+/// Builds a match arm for chain mapping
+pub(crate) fn build_chain_arm(
+    struct_name: &Ident,
+    previous_type: &TypePath,
+) -> proc_macro2::TokenStream {
+    let enum_variant = &previous_type.path.segments.last().unwrap().ident;
+    quote! {
+        #struct_name => #enum_variant,
+    }
+}
+
+/// Builds a match arm for chain existence check
+pub(crate) fn build_chain_exist_arm(previous_type: &TypePath) -> proc_macro2::TokenStream {
+    let enum_variant = &previous_type.path.segments.last().unwrap().ident;
+    quote! {
+        Self::#enum_variant => true,
+    }
+}
+
+pub(crate) fn register_chain(input: TokenStream) -> TokenStream {
+    // Parse the input as a comma-separated list of arguments
+    let input_parsed = syn::parse_macro_input!(input with syn::punctuated::Punctuated<syn::Expr, syn::Token![,]>::parse_terminated);
+
+    // Check that there are exactly two elements
+    if input_parsed.len() != 2 {
+        return syn::Error::new(
+            input_parsed.span(),
+            "Expected exactly two comma-separated arguments: `PreviousType, StructName`",
+        )
+        .to_compile_error()
+        .into();
+    }
+
+    // Extract the two elements
+    let previous_type_expr = &input_parsed[0];
+    let struct_name_expr = &input_parsed[1];
+
+    // Convert expressions to TypePath and Ident
+    let previous_type = match syn::parse2::<TypePath>(previous_type_expr.to_token_stream()) {
+        Ok(ty) => ty,
+        Err(e) => return e.to_compile_error().into(),
+    };
+
+    let struct_name = match syn::parse2::<syn::Ident>(struct_name_expr.to_token_stream()) {
+        Ok(ident) => ident,
+        Err(e) => return e.to_compile_error().into(),
+    };
+
+    // Record the chain mapping: previous_type => struct_name
+    let chain_entry = build_chain_arm(&struct_name, &previous_type);
+
+    // Record the chain existence check
+    let chain_exist_entry = build_chain_exist_arm(&previous_type);
+
+    let mut chains = crate::get_global_set(&crate::CHAINS).lock().unwrap();
+    let mut chain_exist = crate::get_global_set(&crate::CHAINS_EXIST).lock().unwrap();
+
+    let chain_entry_str = chain_entry.to_string();
+    let chain_exist_entry_str = chain_exist_entry.to_string();
+
+    // Check for duplicate variant before inserting
+    let variant_name = previous_type
+        .path
+        .segments
+        .last()
+        .unwrap()
+        .ident
+        .to_string();
+    if let Err(err) = crate::check_duplicate_variant(
+        &chains,
+        &chain_entry_str,
+        &variant_name,
+        "chain",
+        previous_type.span(),
+    ) {
+        return err.into();
+    }
+
+    chains.insert(chain_entry_str);
+    chain_exist.insert(chain_exist_entry_str);
+
+    quote! {}.into()
+}