121 wasm freestanding target builds

// Pins that a Koru program builds for a target with NO operating system and NO
// libc — bare wasm — and that the produced artifact is named where the compiler
// looks for it.
//
// Three things in the emitted program used to assume a hosted platform, none of
// them written by the user:
//
//   - the allocator spine named `std.heap.c_allocator`, which cannot be NAMED
//     on a target with no libc to bind malloc to;
//   - the leak-check epilogue reached for stderr via `debug.print` and for a
//     syscall via `process.exit`, neither of which exists freestanding;
//   - the driver copied `zig-out/bin/output`, while Zig names a wasm executable
//     `output.wasm` — so a SUCCESSFUL cross-compile reported FileNotFound.
//
// The first two are compile errors in code the user never wrote; the third is a
// green build reported as a failure. All three are invisible to every other
// target, because on a hosted platform each assumption happens to hold.
//
// `entry = .disabled` plus `rdynamic` is what makes the artifact a REACTOR — a
// module with exports a host calls, rather than a command that runs once and
// exits. That is the shape a request handler needs.
//
// COMPILE_ONLY because the artifact is a wasm module, which needs a wasm host
// to run and cannot execute on the build machine. Compiling IS the assertion.

const std = @import("std");

~import std/build

~std/build:config {
    "target": "wasm32-freestanding"
}

~std/build:requires {
    exe.entry = .disabled;
    exe.rdynamic = true;
    exe.root_module.link_libc = false;
}

~pub tor assert-target { }

~proc assert-target|zig {
    const builtin = @import("builtin");
    if (builtin.cpu.arch != .wasm32)
        @compileError("build:config target did not reach the backend: compiled for " ++ @tagName(builtin.cpu.arch) ++ ", expected wasm32");
    if (builtin.os.tag != .freestanding)
        @compileError("build:config target did not reach the backend: compiled for " ++ @tagName(builtin.os.tag) ++ ", expected freestanding");
}

// A symbol the host side reaches in. Its survival into the module is what
// `rdynamic` buys; without it the export is stripped and there is nothing to
// call.
export fn koru_add(a: i32, b: i32) i32 {
    return a + b;
}

const fire = true;

~if(fire)
| then |> assert-target()
| else |> _