Allow wrapped function to return error union

When an error is returned by a wrapped host function,
it causes a trap to be passed back to wasm. The trap's
message will mention the error enum's name.

README.md

@@ -13,9 +13,9 @@ const wat_bytes =
     \\  (func (export "main") (call $hello)))
 ;
 
-fn print_hello() void {
+fn print_hello() !void {
     const stdout = @import("std").io.getStdOut().writer();
-    stdout.print("Hello, Slime!\n", .{}) catch unreachable;
+    try stdout.print("Hello, Slime!\n", .{});
 }
 
 pub fn main() !void {

src/func.zig

@@ -1,6 +1,7 @@
 const std = @import("std");
 const assert = std.debug.assert;
 const panic = std.debug.panic;
+const c_alloc = std.heap.c_allocator;
 
 const Store = @import("./store.zig").Store;
 const Extern = @import("./extern.zig").Extern;
@@ -98,7 +99,7 @@ pub const Func = extern struct {
                 .callback = &(struct {
                     const meta = @typeInfo(@TypeOf(func)).Fn;
                     const ArgsType = std.meta.ArgsTuple(@TypeOf(func));
-                    const ResultType = meta.return_type orelse void;
+                    const ReturnType = meta.return_type orelse void;
 
                     fn callback(
                         env: ?*anyopaque,
@@ -118,7 +119,7 @@ pub const Func = extern struct {
                         }
 
                         assert(num_args == meta.params.len - offset);
-                        assert(num_results == getNumResultsFromResultType(ResultType));
+                        assert(num_results == getNumResultsFromReturnType(ReturnType));
 
                         // Turn the "env" parameter back into a function pointer.
                         const func_ptr: *const @TypeOf(func) = @ptrCast(env);
@@ -129,13 +130,24 @@ pub const Func = extern struct {
                                 panic("Cannot convert {} to {s}", .{ arg, @typeName(p.type.?) });
 
                         // Call the function with the arguments, get the results.
-                        const func_results = @call(.auto, func_ptr, func_args);
+                        const func_results_maybe_error = @call(.auto, func_ptr, func_args);
+
+                        // If function returns an ErrorUnion, convert it to
+                        // its payload, or return a trap if an error occurred.
+                        const func_results = if (@typeInfo(@TypeOf(func_results_maybe_error)) == .ErrorUnion)
+                            func_results_maybe_error catch |err| {
+                                const fmt = "Error '{s}' occurred in host code";
+                                const msg = std.fmt.allocPrintZ(c_alloc, fmt, .{@errorName(err)});
+                                return Trap.init(msg catch "Error occurred in host code");
+                            }
+                        else
+                            func_results_maybe_error;
 
                         // Pass the result(s) back to WASM.
-                        if (comptime std.meta.trait.isTuple(ResultType)) {
+                        if (comptime std.meta.trait.isTuple(@TypeOf(func_results))) {
                             inline for (func_results, 0..) |res, i|
                                 results[i] = Val.fromValue(@TypeOf(res), res);
-                        } else if (ResultType != void)
+                        } else if (@TypeOf(func_results) != void)
                             results[0] = Val.fromValue(@TypeOf(func_results), func_results);
 
                         return null;
@@ -165,7 +177,7 @@ pub const Func = extern struct {
             val.* = Val.fromValue(@TypeOf(arg), arg);
 
         // Set up function results.
-        const num_results = comptime getNumResultsFromResultType(TResult);
+        const num_results = comptime getNumResultsFromReturnType(TResult);
         var results_array: [num_results]Val = undefined;
 
         var trap: ?*Trap = null;
@@ -198,10 +210,16 @@ pub const Func = extern struct {
         };
     }
 
-    fn getNumResultsFromResultType(comptime T: type) usize {
-        return comptime if (std.meta.trait.isTuple(T))
-            @typeInfo(T).Struct.fields.len
-        else if (T != void) 1 else 0;
+    fn getNumResultsFromReturnType(comptime T: type) usize {
+        const meta = @typeInfo(T);
+        return comptime if (meta == .ErrorUnion)
+            getNumResultsFromReturnType(meta.ErrorUnion.payload)
+        else if (std.meta.trait.isTuple(T))
+            meta.Struct.fields.len
+        else if (T != void)
+            1
+        else
+            0;
     }
 
     pub const Callback = fn (
@@ -257,7 +275,11 @@ pub const FuncType = opaque {
         inline for (meta.params[offset..], 0..) |p, i|
             params.data[i] = ValType.fromType(p.type.?);
 
-        const ResultType = meta.return_type orelse void;
+        const ResultType = switch (@typeInfo(meta.return_type orelse void)) {
+            .ErrorUnion => |u| u.payload,
+            else => |t| t,
+        };
+
         var results = if (comptime std.meta.trait.isTuple(ResultType)) blk: {
             var res = ValTypeVec.initUninitialized(ResultType.len);
             inline for (ResultType, res.toSlice()) |T, *r|