function pointers and callbacks

TODO.md

@@ -27,7 +27,7 @@
    - pointer-only `c_struct` support with target c layout
      - `Some :: c_struct { ... }`: defined c-layout struct
      - `Some :: c_struct`: opaque c-layout struct
-     - defer passing c structs by value until target ABI classification exists
+     - passing c structs by value was deferred until milestone 4.1
 
 2. restricted c header imports (implemented)
    - treat an imported header as a synthetic, file-local package namespace
@@ -55,8 +55,20 @@
    - reject general `u8`/`c_char` interchange, slice-to-pointer coercion, and conversion to mutable c character pointers
 
 4. advanced c interop
-   - by-value records and unions
-   - function pointers and callbacks
+   - by-value records and unions (implemented)
+     - complete plain imported structs/unions and manual `c_struct` values
+     - fixed C arrays inside imported records
+     - keyed struct literals and exactly-one-field union literals
+     - field reads/writes, storage, and fixed-signature calls/returns
+     - aarch64-macos small aggregate, homogeneous float aggregate, and indirect ABI lowering
+     - keep incomplete, bitfield, packed, flexible-array, qualified-field, and otherwise non-plain records pointer-only
+     - keep C variadic record arguments unsupported
+   - function pointers and callbacks (implemented)
+     - imported C function pointer typedefs lower to nullable pointer types
+     - manual `?*c_func(...) T` callback type spelling
+     - concrete `c_func` declarations/definitions can be passed as callback values
+     - postfix calls through non-null function pointers, including `callback?(...)`
+     - fixed and C-variadic callback ABI emission through LLVM indirect calls
    - external variables
    - macros and static inline functions
    - exporting brolang functions to c