From aeb950b86fcb7921d527e3c221c4bade40a5e1b6 Mon Sep 17 00:00:00 2001
From: agra <alex@swipelab.co>
Date: Wed, 27 May 2026 21:48:45 +0300
Subject: [PATCH] =?UTF-8?q?ffi=20M5.A.next.5.1.B:=20build=5Fblock=5Fconver?=
 =?UTF-8?q?t=20added=20to=20stdlib=20=E2=80=94=20make-green?=
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`build_block_convert(args: []Type, $ret: Type) -> string` emits
the convert-body source for the generic `Into(Block) for
Closure(..$args) -> $R` impl (step 5.2):

  1. A nested `__invoke :: (block_self: *Block, arg0: T0, ...) ->
     R callconv(.c) { ... }` trampoline matching the per-shape
     Apple Block ABI.
  2. A `return Block.{ ... };` literal whose `invoke` slot points
     at the nested trampoline via `xx @__invoke`.

Void-returning shapes emit `typed_fn(block_self.sx_env, args...)`;
non-void emits `return typed_fn(...)`. Per-position arg names
follow `arg0`, `arg1`, ... in declaration order; the typed-fn
cast reconstructs the closure's call signature so the trampoline
hands control back to `sx_fn` with the right argument layout.

`examples/176-build-block-convert.sx` flips green (216/216).
---
 library/modules/std/objc_block.sx | 67 +++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)

diff --git a/library/modules/std/objc_block.sx b/library/modules/std/objc_block.sx
index 09d73cb..0d0f3cc 100644
--- a/library/modules/std/objc_block.sx
+++ b/library/modules/std/objc_block.sx
@@ -107,3 +107,70 @@ impl Into(Block) for Closure(bool) -> void {
         };
     }
 }
+
+// Comptime builder for the generic `Into(Block) for Closure(..$args)
+// -> $R` impl body. Receives the closure's per-position pack types
+// (`args`, a comptime `[]Type`) and the closure's return type
+// (`$ret`), emits source that:
+//
+//   1. Declares a nested `__invoke` `callconv(.c)` trampoline whose
+//      signature matches the per-shape Apple Block ABI: first arg is
+//      `block_self: *Block`, then the pack types verbatim. The body
+//      reconstructs a typed fn-pointer from `block_self.sx_fn`,
+//      prepends `block_self.sx_env`, and tail-calls the sx closure.
+//   2. Returns a stack Block literal whose `invoke` slot points at
+//      the nested trampoline via `xx @__invoke`.
+//
+// The host impl wraps the emitted source via `#insert
+// build_block_convert($args, $R);`. Per-call-shape monomorphisation
+// of the impl body re-runs this builder with the concrete types
+// bound, so each closure shape gets its own dedicated trampoline.
+//
+// Void return type emits `typed_fn(...)`; non-void emits
+// `return typed_fn(...)`.
+build_block_convert :: (args: []Type, $ret: Type) -> string {
+    ret_name := type_name(ret);
+    code := "__invoke :: (block_self: *Block";
+    i : s64 = 0;
+    while i < args.len {
+        code = concat(code, ", arg");
+        code = concat(code, int_to_string(i));
+        code = concat(code, ": ");
+        code = concat(code, type_name(args[i]));
+        i = i + 1;
+    }
+    code = concat(code, ") -> ");
+    code = concat(code, ret_name);
+    code = concat(code, " callconv(.c) { typed_fn : (*void");
+    i = 0;
+    while i < args.len {
+        code = concat(code, ", ");
+        code = concat(code, type_name(args[i]));
+        i = i + 1;
+    }
+    code = concat(code, ") -> ");
+    code = concat(code, ret_name);
+    code = concat(code, " = xx block_self.sx_fn; ");
+    if ret_name == "void" {
+        code = concat(code, "typed_fn(block_self.sx_env");
+    } else {
+        code = concat(code, "return typed_fn(block_self.sx_env");
+    }
+    i = 0;
+    while i < args.len {
+        code = concat(code, ", arg");
+        code = concat(code, int_to_string(i));
+        i = i + 1;
+    }
+    code = concat(code, "); } ");
+    code = concat(code, "return .{ ");
+    code = concat(code, "isa = @_NSConcreteStackBlock, ");
+    code = concat(code, "flags = 0, ");
+    code = concat(code, "reserved = 0, ");
+    code = concat(code, "invoke = xx @__invoke, ");
+    code = concat(code, "descriptor = xx @__sx_block_descriptor, ");
+    code = concat(code, "sx_env = self.env, ");
+    code = concat(code, "sx_fn = self.fn_ptr, ");
+    code = concat(code, "};");
+    return code;
+}