refactor(main): remove one layer of indirection

This commit removes the BotAdapter.erased_callback function, which
served as a generic version of BotAdapter.callback that was legible
for zircon.MessageClosure.

However, it is clearer to document how that mechanism works. Therefore
we remove the BotAdapter.erased_callback and instead makes
BotAdapter.callback match the zircon.MessageClosure.callbackFn
signature.

src/main.zig

@@ -44,11 +44,6 @@ pub fn main() !void {
     };
 }
 
-pub const Adapter = struct {
-    ptr: *anyopaque,
-    callbackFn: *const fn (*anyopaque, zircon.Message) ?zircon.Message,
-};
-
 /// BotAdapter is the closure that we register in zircon as the
 /// message callback.
 ///
@@ -82,10 +77,23 @@ pub const BotAdapter = struct {
     /// See
     /// - https://modern.ircdocs.horse/, for what kinds of messages exists in the IRC protocol documentation,
     /// - https://github.com/Jassob/zircon/blob/main/src/message.zig, for zircon documentation.
-    pub fn callback(self: *BotAdapter, message: zircon.Message) ?zircon.Message {
+    ///
+    /// NOTE: This function does not have a self-parameter, this is
+    ///       because this function is called as a "generic" function
+    ///       that is parameterized in the pointer argument (otherwise
+    ///       zircon library would not be able to be reused).
+    ///
+    ///       That's why the first thing we do is perform some casting
+    ///       magic to convert our pointer back to a BotAdapter
+    ///       pointer. This is safe as long as we know that we
+    ///       registered a BotAdapter closure and there are no other
+    ///       callbacks registered.
+    pub fn callback(ptr: *anyopaque, message: zircon.Message) ?zircon.Message {
+        const self: *@This() = @ptrCast(@alignCast(ptr));
+
         switch (message) {
             .PRIVMSG => |msg| {
-                const nick = if (msg.prefix != null and msg.prefix.?.nick != null) msg.prefix.?.nick.? else "unknown";
+                const nick = nickFromPrefix(msg.prefix);
 
                 // create message
                 const bot_message = BotMessage.init_owned(
@@ -125,19 +133,23 @@ pub const BotAdapter = struct {
         return .{ .PRIVMSG = .{ .prefix = null, .targets = "#eru-admin", .text = err_msg } };
     }
 
-    pub fn erased_callback(self: *anyopaque, message: zircon.Message) ?zircon.Message {
-        const a: *@This() = @ptrCast(@alignCast(self));
-        return a.callback(message);
-    }
-
     pub fn closure(self: *BotAdapter) zircon.MessageClosure {
         return .{
             .ptr = self,
-            .callbackFn = BotAdapter.erased_callback,
+            .callbackFn = BotAdapter.callback,
         };
     }
 };
 
+/// nickFromPrefix returns a nick if it exists in the prefix or
+/// "unknown" otherwise.
+fn nickFromPrefix(prefix: ?zircon.Prefix) []const u8 {
+    if (prefix == null or prefix.?.nick == null) {
+        return "unknown";
+    }
+    return prefix.?.nick.?;
+}
+
 /// toIRC converts a bot response and converts it to a IRC message.
 fn toIRC(response: BotResponse) zircon.Message {
     switch (response) {
@@ -150,30 +162,27 @@ fn toIRC(response: BotResponse) zircon.Message {
     }
 }
 
-test "substitute" {
+fn priv_msg(text: []const u8) zircon.Message {
-    var bot_adapter = try BotAdapter.init(std.testing.allocator);
+    return .{ .PRIVMSG = .{
-    defer bot_adapter.deinit();
+        .prefix = .{ .nick = "jassob", .host = "localhost", .user = "jassob" },
-    const prefix = zircon.Prefix{ .nick = "jassob", .user = "jassob", .host = "localhost" };
-    const msg = zircon.Message{
-        .PRIVMSG = .{
-            .prefix = prefix,
         .targets = "#test",
-            .text = "hello world",
+        .text = text,
-        },
+    } };
-    };
+}
-    if (bot_adapter.callback(msg)) |_| {
+
+test "substitute" {
+    var adapter = try BotAdapter.init(std.testing.allocator);
+    defer adapter.deinit();
+    if (BotAdapter.callback(&adapter, priv_msg("hello world"))) |_| {
         @panic("unexpected response");
     }
-    const cmd_msg = zircon.Message{
+    const response = BotAdapter.callback(&adapter, priv_msg("s/world/zig/"));
-        .PRIVMSG = .{
+
-            .prefix = prefix,
-            .targets = "#test",
-            .text = "s/world/zig/",
-        },
-    };
-    const response = bot_adapter.callback(cmd_msg);
     try std.testing.expect(response != null);
-    try std.testing.expectEqualStrings("jassob: \"hello zig\"", response.?.PRIVMSG.text);
+    try std.testing.expectEqualStrings(
+        "jassob: \"hello zig\"",
+        response.?.PRIVMSG.text,
+    );
 }
 
 test "get empty backlog message" {
@@ -182,5 +191,9 @@ test "get empty backlog message" {
     const msg = zircon.Message{
         .PRIVMSG = .{ .prefix = null, .targets = "#eru-admin", .text = "!backlog 0" },
     };
-    try std.testing.expectEqualDeep("no matching message", bot_adapter.callback(msg).?.PRIVMSG.text);
+
+    try std.testing.expectEqualDeep(
+        "no matching message",
+        BotAdapter.callback(&bot_adapter, msg).?.PRIVMSG.text,
+    );
 }