From 1f22b2cbb2d9cdc4b7e436328e7018f473a57388 Mon Sep 17 00:00:00 2001
From: Justus Klausecker <justus@klausecker.de>
Date: Sat, 2 May 2026 11:36:50 +0200
Subject: [PATCH] LowerZon: fix `packed` containers

Since `packed` containers are now internally represented by a `bitpack`,
they need special handling on initialization: they need to be either
bitpacked or bitcasted to their backing integer. `Sema` already did this,
but `LowerZon` didn't yet.
---
 src/Sema/LowerZon.zig               | 44 ++++++++++++++++++++++-------
 test/behavior/zon.zig               | 27 ++++++++++++++++++
 test/behavior/zon/packed_struct.zon |  5 ++++
 test/behavior/zon/packed_union.zon  |  3 ++
 4 files changed, 69 insertions(+), 10 deletions(-)
 create mode 100644 test/behavior/zon/packed_struct.zon
 create mode 100644 test/behavior/zon/packed_union.zon

diff --git a/src/Sema/LowerZon.zig b/src/Sema/LowerZon.zig
index 96b854885f6ab1083497d9608c9ebdc36a7b2d0e..0ba12a8813477ce26016c34443c2ea3825630740 100644
--- a/src/Sema/LowerZon.zig
+++ b/src/Sema/LowerZon.zig
@@ -748,7 +748,8 @@ fn lowerTuple(self: *LowerZon, node: Zoir.Node.Index, res_ty: Type) !InternPool.
 
 fn lowerStruct(self: *LowerZon, node: Zoir.Node.Index, res_ty: Type) !InternPool.Index {
     const pt = self.sema.pt;
-    const comp = pt.zcu.comp;
+    const zcu = pt.zcu;
+    const comp = zcu.comp;
     const gpa = comp.gpa;
     const io = comp.io;
     const ip = &pt.zcu.intern_pool;
@@ -807,7 +808,28 @@ fn lowerStruct(self: *LowerZon, node: Zoir.Node.Index, res_ty: Type) !InternPool
         if (value.* == .none) return self.fail(node, "missing field '{f}'", .{name.fmt(ip)});
     }
 
-    return (try self.sema.pt.aggregateValue(res_ty, field_values)).toIntern();
+    const result: Value = switch (struct_info.layout) {
+        .auto, .@"extern" => try pt.aggregateValue(res_ty, field_values),
+        .@"packed" => result: {
+            const arena = self.sema.arena;
+            const buf = try arena.alloc(u8, @intCast((res_ty.bitSize(zcu) + 7) / 8));
+            var bit_offset: u16 = 0;
+            for (field_values) |field_ip| {
+                const field_val: Value = .fromInterned(field_ip);
+                field_val.writeToPackedMemory(zcu, buf, bit_offset) catch |err| switch (err) {
+                    error.ReinterpretDeclRef => unreachable, // bitpack fields cannot be pointers
+                    error.OutOfMemory => |e| return e,
+                };
+                bit_offset += @intCast(field_val.typeOf(zcu).bitSize(zcu));
+            }
+            assert(bit_offset == res_ty.bitSize(zcu));
+            break :result Value.readFromPackedMemory(res_ty, pt, buf, 0, arena) catch |err| switch (err) {
+                error.IllDefinedMemoryLayout => unreachable, // bitpacks have well-defined layout
+                error.OutOfMemory => |e| return e,
+            };
+        },
+    };
+    return result.toIntern();
 }
 
 fn lowerSlice(self: *LowerZon, node: Zoir.Node.Index, res_ty: Type) !InternPool.Index {
@@ -944,22 +966,24 @@ fn lowerUnion(self: *LowerZon, node: Zoir.Node.Index, res_ty: Type) !InternPool.
     };
     const tag = try self.sema.pt.enumValueFieldIndex(.fromInterned(union_info.enum_tag_type), name_index);
     const field_type: Type = .fromInterned(union_info.field_types.get(ip)[name_index]);
-    const val = if (maybe_field_node) |field_node| b: {
+    const val: Value = if (maybe_field_node) |field_node| b: {
         if (field_type.toIntern() == .void_type) {
             return self.fail(field_node, "expected type 'void'", .{});
         }
-        break :b try self.lowerExprKnownResTy(field_node, field_type);
+        break :b .fromInterned(try self.lowerExprKnownResTy(field_node, field_type));
     } else b: {
         if (field_type.toIntern() != .void_type) {
             return error.WrongType;
         }
-        break :b .void_value;
+        break :b .void;
     };
-    return ip.getUnion(gpa, io, self.sema.pt.tid, .{
-        .ty = res_ty.toIntern(),
-        .tag = tag.toIntern(),
-        .val = val,
-    });
+    const result: Value = switch (union_info.layout) {
+        .auto, .@"extern" => try pt.unionValue(res_ty, tag, val),
+        .@"packed" => try self.sema.bitCastVal(val, res_ty, 0, 0, 0) orelse {
+            unreachable; // `null` is only possible if the input value contains a pointer, which a packed union cannot.
+        },
+    };
+    return result.toIntern();
 }
 
 fn lowerVector(self: *LowerZon, node: Zoir.Node.Index, res_ty: Type) !InternPool.Index {
diff --git a/test/behavior/zon.zig b/test/behavior/zon.zig
index 8a43b4894e3e9a200b855c52fc96b52f3a19e742..6de22701d5bd644b492ba321bdc9d78b3fa3b87b 100644
--- a/test/behavior/zon.zig
+++ b/test/behavior/zon.zig
@@ -571,3 +571,30 @@ test "build.zig.zon" {
 
     try expectEqual(.{ "build.zig", "build.zig.zon", "src" }, build.paths);
 }
+
+test "packed" {
+    {
+        const U = packed union {
+            x: f32,
+            y: u32,
+        };
+
+        const u: U = @import("zon/packed_union.zon");
+
+        try expectEqual(0.5, u.x);
+        try expectEqual(@as(u32, @bitCast(@as(f32, 0.5))), u.y);
+    }
+    {
+        const S = packed struct {
+            x: u3,
+            y: f32,
+            z: i7,
+        };
+
+        const s: S = @import("zon/packed_struct.zon");
+
+        try expectEqual(2, s.x);
+        try expectEqual(0.5, s.y);
+        try expectEqual(-10, s.z);
+    }
+}
diff --git a/test/behavior/zon/packed_struct.zon b/test/behavior/zon/packed_struct.zon
new file mode 100644
index 0000000000000000000000000000000000000000..31996ea9fca4962b9130b8e30465d503d7d847d3
--- /dev/null
+++ b/test/behavior/zon/packed_struct.zon
@@ -0,0 +1,5 @@
+.{
+    .x = 2,
+    .y = 0.5,
+    .z = -10,
+}
diff --git a/test/behavior/zon/packed_union.zon b/test/behavior/zon/packed_union.zon
new file mode 100644
index 0000000000000000000000000000000000000000..9911dfd9b101fd97c49968ef96702e824d462afc
--- /dev/null
+++ b/test/behavior/zon/packed_union.zon
@@ -0,0 +1,3 @@
+.{
+    .x = 0.5,
+}
-- 
2.54.0