Compiler error when using async and WASM

[leroycep]

I'm trying to build a asset loading system using async for WASM, but I'm getting an internal compiler error:

$ ~/Downloads/zig/0.8.0-dev.1120/zig build wasm
broken LLVM module found: Stored value type does not match pointer operand type!
  store %"[]u8"* %27, { %"[]u8", i16 }** %28, align 4
 { %"[]u8", i16 }*Stored value type does not match pointer operand type!
  store %"[]u8"* %36, { %"[]u8", i16 }** %37, align 4
 { %"[]u8", i16 }*
This is a bug in the Zig compiler.thread 1345860 panic:
Unable to dump stack trace: debug info stripped

The code is here: https://github.com/leroycep/zig-wasm-assets/tree/zig-compiler-crash

On the master branch there is a version that works. It works by storing the main task in a global variable, and then relying on the JS runtime to resume once the fetch tasks are complete. https://github.com/leroycep/zig-wasm-assets/blob/master/src/main.zig

[kivikakk]

I worked out what the root cause was. It's these lines here:

fn load_text_files() void {
    var hello_fetch = async env.fetch("hello.txt");
    var world_fetch = async env.fetch("world.txt");

    text_files_opt = TextFiles{
        .hello = await hello_fetch,
        .world = await world_fetch,
    };
}

Note env.fetch's signature:

pub fn fetch(file_name: []const u8) ![]const u8 {

await hello_fetch returns a ![]const u8, but here we try to assign it to struct fields of type []const u8.

This fixes the compile panic:

     text_files_opt = TextFiles{
-        .hello = await hello_fetch,
-        .world = await world_fetch,
+        .hello = try await hello_fetch,
+        .world = try await world_fetch,
     };

The asyncness is causing our type checker to not catch this before it gets to LLVM, I guess.

[leroycep]

Oh, I feel silly now. Thanks for finding what makes the compiler panic!