sx/library/modules/std/objc_block.sx

// Obj-C blocks bridged to sx closures.
//
// Apple's block ABI (clang's "Block Implementation Specification"): a block
// pointer is a struct whose first five fields are { isa, flags, reserved,
// invoke, descriptor } followed by per-block captured state. When an API
// like `[UIView animateWithDuration:animations:]` receives a block, the
// runtime reads `invoke` and calls it with the block pointer as the first
// argument. UIKit / Foundation callers always `_Block_copy` synchronously
// before returning, so a stack-allocated block is safe to pass directly.
//
// We layer the sx closure onto Apple's layout by appending two pointer
// fields to the standard 32-byte header: `sx_env` (the closure's captured
// environment pointer) and `sx_fn` (the closure trampoline). The per-
// signature `__block_invoke_*` C-ABI fn knows the offsets and calls
// through to `sx_fn(sx_env, args...)`.
//
// ── Lifetime contract ───────────────────────────────────────────────────
// `xx <closure> : *Block` returns a pointer into the surrounding sx
// function's stack frame. Same rule as `&local_var`: pass it directly to
// a callee that consumes it immediately or `_Block_copy`s internally
// (UIKit/Foundation always do). Don't store the pointer to use after the
// caller returns. If you need that, ship a `Block_copy`-backed sibling
// API and use it instead.

// Standard 32-byte block header plus two trailing slots for the sx closure
// it wraps. Total = 48 bytes.
Block :: struct {
    isa: *void;
    flags: s32;
    reserved: s32;
    invoke: *void;
    descriptor: *void;
    sx_env: *void;
    sx_fn: *void;
}

// Per-block-shape metadata. The runtime reads `size` when copying the
// block to the heap, so it must equal the actual instance size.
BlockDescriptor :: struct {
    reserved: u64;
    size: u64;
}

// libSystem isa pointer for stack-allocated blocks. Resolved at link time
// (auto-linked on every Apple target via libSystem).
_NSConcreteStackBlock : *void #foreign;

// Shared descriptor for the 48-byte sx-block layout. All Into impls below
// point their `descriptor` field at this.
__sx_block_descriptor : BlockDescriptor = .{
    reserved = 0,
    size = 48,
};

// Single generic impl covers every closure shape. The compiler
// monomorphises this body per call shape; inside each mono,
// `$args` is the bound pack of arg types and `$R` is the bound
// return type. The `#insert` evaluates `build_block_convert` at
// comptime and substitutes the resulting source — a nested
// `callconv(.c)` trampoline + the Block literal that points its
// `invoke` slot at it. One impl in stdlib replaces every per-
// signature hand-rolled `__block_invoke_*` + `Into(Block)` pair.
impl Into(Block) for Closure(..$args) -> $R {
    convert :: (self: Closure(..$args) -> $R) -> Block {
        #insert build_block_convert($args, $R);
    }
}

// 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;
}