design flaw: direct parameter passing to coroutines

[andrewrk]

Directly passing a parameter is not guaranteed to be a copy; neither is it guaranteed to be a reference. This is a footgun when combined with coroutines, where accessing a reference across a suspend point could be undefined behavior:

const std = @import("std");
const assert = std.debug.assert;

const Foo = struct {
    x: i32,
    y: i32,
};

async fn callee(foo: Foo) void {
    suspend;
    assert(foo.y == 5678);
}

test "pass directly" {
    const h = bar();
    blowUpStack(10);
    resume h;
}

fn bar() promise {
    var x: i32 = 1234;
    var y: i32 = 5678;
    var foo = Foo{ .x = x, .y = y };
    return async<std.debug.global_allocator> callee(foo) catch unreachable;
}

fn blowUpStack(x: u32) void {
    if (x == 0) return;
    blowUpStack(x - 1);
}

Test 1/1 pass directly...assertion failure
/home/andy/dev/zig/build/lib/zig/std/debug/index.zig:118:13: 0x2055a9 in ??? (test)
            @panic("assertion failure");
            ^
/home/andy/dev/zig/build/test.zig:11:11: 0x2247c9 in ??? (test)
    assert(foo.y == 5678);
          ^
/home/andy/dev/zig/build/test.zig:17:5: 0x205657 in ??? (test)
    resume h;
    ^
/home/andy/dev/zig/build/lib/zig/std/special/test_runner.zig:13:25: 0x22368a in ??? (test)
        if (test_fn.func()) |_| {
                        ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:96:22: 0x22343b in ??? (test)
            root.main() catch |err| {
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:70:20: 0x2233b5 in ??? (test)
    return callMain();
                   ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:64:39: 0x223218 in ??? (test)
    std.os.posix.exit(callMainWithArgs(argc, argv, envp));
                                      ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:37:5: 0x2230d0 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Tests failed. Use the following command to reproduce the failure:
/home/andy/dev/zig/build/zig-cache/test

[andrewrk]

Duplicate of #1155 which is solved with the merge of #3033