How to call blocks written in C from Rust

example output.txt

Rust: key = 0x10299ff88, block = 0x16d462c20
   C: answer = 42
   C: len = 18
Rust: key = 0x0, block = 0x0
   C: len = 0

lib.rs

use std::ffi::{c_char, c_int, c_void};

#[repr(C)]
struct Block {
    isa: *const c_void,
    flags: c_int,
    reserved: c_int,
    invoke: unsafe extern "C" fn(*const Self, *const c_char) -> c_int,
}

#[no_mangle]
extern "C" fn interop_invoke(key: *const c_char, block: *const Block) -> c_int {
    println!("Rust: key = {key:?}, block = {block:?}");

    if block.is_null() {
        0
    } else {
        unsafe { ((*block).invoke)(block, key) }
    }
}

main.c

#include <stdio.h>

extern int interop_invoke(const char*, int (^)(const char*));

int main() {
  int answer = 42;
  int len = interop_invoke("answer", ^(const char* key) {
    return printf("   C: %s = %d\n", key, answer);
  });
  printf("   C: len = %d\n", len);

  len = interop_invoke(NULL, NULL);
  printf("   C: len = %d\n", len);

  return 0;
}

Makefile

.PHONY: all clean

all: main

main: main.c libblock.dylib

libblock.dylib: lib.rs
	rustc --crate-type cdylib $^ -o $@

clean:
	rm -f main libblock.dylib

• Language Specification for Blocks
• Block Implementation Specification