fix!: avoid "comptime var" error by returning array instead of slice

this changes the semantics of `comptimeEncode`; users will need to,
e.g. use

```zig
const encoding = comptimeEncode(T);
passToObjc(&encoding)
```

to pass the encoding by pointer rather than by value.

build.zig

@@ -5,7 +5,11 @@ pub fn build(b: *std.Build) void {
     const target = b.standardTargetOptions(.{});
     const add_paths = b.option(bool, "add-paths", "add macos SDK paths from dependency") orelse false;
 
-    const objc = b.addModule("objc", .{ .root_source_file = .{ .path = "src/main.zig" } });
+    const objc = b.addModule("objc", .{
+        .root_source_file = .{ .path = "src/main.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
     if (add_paths) @import("macos_sdk").addPathsModule(objc);
     objc.linkSystemLibrary("objc", .{});
     objc.linkFramework("Foundation", .{});

build.zig.zon

@@ -10,8 +10,8 @@
     },
     .dependencies = .{
         .macos_sdk = .{
-            .url = "https://github.com/mitchellh/zig-build-macos-sdk/archive/4186e9fd445d12041651abe59ea5f396499b0844.tar.gz",
-            .hash = "1220bc2612b57b0cfaaecbcac38e3144e5a9362ff668d71eb8334e895047bdbb7148",
+            .url = "https://github.com/mitchellh/zig-build-macos-sdk/archive/ee70f27c08680307fa35ada92e6b2c36e0ff84c6.tar.gz",
+            .hash = "1220b415f529f1c04ed876c2b481e9f8119d353d4e3d4d7c8607ee302d2142e13eca",
         },
     },
 }

src/block.zig

@@ -41,7 +41,7 @@ pub fn Block(
             .size = @sizeOf(Context),
             .copy_helper = &descCopyHelper,
             .dispose_helper = &descDisposeHelper,
-            .signature = objc.comptimeEncode(InvokeFn).ptr,
+            .signature = &objc.comptimeEncode(InvokeFn),
         };
 
         /// This is the function type that is called back.

src/class.zig

@@ -76,7 +76,7 @@ pub const Class = struct {
             self.value,
             objc.sel(name).value,
             @ptrCast(&imp),
-            encoding.ptr,
+            &encoding,
         ));
     }
 

src/encoding.zig

@@ -4,22 +4,30 @@ const c = @import("c.zig");
 const assert = std.debug.assert;
 const testing = std.testing;
 
-/// Encode a type into a comptime string.
-pub fn comptimeEncode(comptime T: type) [:0]const u8 {
+/// how much space do we need to encode this type?
+fn comptimeN(comptime T: type) usize {
     comptime {
         const encoding = objc.Encoding.init(T);
 
         // Figure out how much space we need
         var counting = std.io.countingWriter(std.io.null_writer);
         try std.fmt.format(counting.writer(), "{}", .{encoding});
+        return counting.bytes_written;
+    }
+}
+
+/// Encode a type into a comptime string.
+pub fn comptimeEncode(comptime T: type) [comptimeN(T):0]u8 {
+    comptime {
+        const encoding = objc.Encoding.init(T);
 
         // Build our final signature
-        var buf: [counting.bytes_written + 1]u8 = undefined;
-        var fbs = std.io.fixedBufferStream(buf[0..counting.bytes_written]);
+        var buf: [comptimeN(T) + 1]u8 = undefined;
+        var fbs = std.io.fixedBufferStream(buf[0 .. buf.len - 1]);
         try std.fmt.format(fbs.writer(), "{}", .{encoding});
-        buf[counting.bytes_written] = 0;
+        buf[buf.len - 1] = 0;
 
-        return buf[0..counting.bytes_written :0];
+        return buf[0 .. buf.len - 1 :0].*;
     }
 }