feat(core, macros): add r_append! macro and RenderResult::append_other

Implement `RenderResult::append_other()` to merge buffered content and
immediate output behavior from one result into another. Add the
`r_append!` macro with both explicit buffer (`dst, src`) and implicit
buffer (`src`) usage forms, re-exported from `mingling::prelude`.

2 files changed, 87 insertions, 0 deletions

diff --git a/mingling_macros/src/func/r_print.rs b/mingling_macros/src/func/r_print.rs
index 0f77b2b..86ea52f 100644
--- a/mingling_macros/src/func/r_print.rs
+++ b/mingling_macros/src/func/r_print.rs
@@ -68,3 +68,60 @@ pub(crate) fn r_eprintln(input: TokenStream) -> TokenStream {
 pub(crate) fn r_eprint(input: TokenStream) -> TokenStream {
     expand_print(input, "eprint")
 }
+
+/// Parsed input for `r_append!`.
+///
+/// Two forms:
+/// - `(dst, src)`       — explicit buffer and source
+/// - `(src)`            — implicit `__render_result_buffer`
+struct AppendInput {
+    dst: Option<Ident>,
+    src: proc_macro2::TokenStream,
+}
+
+impl Parse for AppendInput {
+    fn parse(input: ParseStream) -> syn::Result<Self> {
+        if input.peek(Ident) && input.peek2(Token![,]) {
+            let dst: Ident = input.parse()?;
+            let _comma: Token![,] = input.parse()?;
+            let src: TokenStream2 = input.parse()?;
+            Ok(AppendInput {
+                dst: Some(dst),
+                src,
+            })
+        } else {
+            let src: TokenStream2 = input.parse()?;
+            Ok(AppendInput { dst: None, src })
+        }
+    }
+}
+
+/// `r_append!` macro: appends the contents of another `RenderResult` to this one.
+///
+/// Two forms:
+/// - `r_append!(dst, src)` — appends `src` into `dst`
+/// - `r_append!(src)`      — appends `src` into `__render_result_buffer`
+pub(crate) fn r_append(input: TokenStream) -> TokenStream {
+    let parsed: AppendInput = match syn::parse(input) {
+        Ok(p) => p,
+        Err(e) => return e.to_compile_error().into(),
+    };
+
+    let dst_ident = parsed.dst.clone();
+    let src_tokens = parsed.src;
+
+    let expanded = match dst_ident {
+        Some(dst) => {
+            quote! {
+                #dst.append_other(#src_tokens);
+            }
+        }
+        None => {
+            quote! {
+                __render_result_buffer.append_other(#src_tokens);
+            }
+        }
+    };
+
+    expanded.into()
+}
diff --git a/mingling_macros/src/lib.rs b/mingling_macros/src/lib.rs
index 5e7a40f..5d8d7e0 100644
--- a/mingling_macros/src/lib.rs
+++ b/mingling_macros/src/lib.rs
@@ -1478,6 +1478,36 @@ pub fn r_eprint(input: TokenStream) -> TokenStream {
     func::r_print::r_eprint(input)
 }
 
+/// Appends the contents of one `RenderResult` to another.
+///
+/// # Implicit buffer (inside `#[buffer]` functions)
+///
+/// ```rust,ignore
+/// use mingling::macros::{buffer, r_append};
+///
+/// #[buffer]
+/// fn render() {
+///     let other = make_other_result();
+///     r_append!(other);
+/// }
+/// ```
+///
+/// # Explicit buffer
+///
+/// ```rust,ignore
+/// use mingling::macros::r_append;
+/// use mingling::RenderResult;
+///
+/// let mut dst = RenderResult::new();
+/// let src = RenderResult::from("hello");
+/// r_append!(dst, src);
+/// assert!(!dst.is_empty());
+/// ```
+#[proc_macro]
+pub fn r_append(input: TokenStream) -> TokenStream {
+    func::r_print::r_append(input)
+}
+
 /// Derive macro for automatically implementing the `Grouped` trait on a struct.
 ///
 /// The `#[derive(Grouped)]` macro: