Use extern for serialized structs

src/base/CommonEnv.zig

@@ -84,8 +84,9 @@ pub fn serialize(
     return @constCast(offset_self);
 }
 
-/// Serialized representation of ModuleEnv
-pub const Serialized = struct {
+/// Serialized representation of CommonEnv
+/// Uses extern struct to guarantee consistent field layout across optimization levels.
+pub const Serialized = extern struct {
     idents: Ident.Store.Serialized,
     strings: StringLiteral.Store.Serialized,
     exposed_items: ExposedItems.Serialized,

src/base/DataSpan.zig

@@ -1,22 +1,23 @@
 //! Just a small struct to take a span of data in an array
 
-const DataSpan = @This();
+/// DataSpan is used for serialization, so it must be extern struct for consistent layout.
+pub const DataSpan = extern struct {
+    start: u32,
+    len: u32,
 
-start: u32,
-len: u32,
+    /// Creates an empty DataSpan with zero start and zero length.
+    pub fn empty() DataSpan {
+        return DataSpan{ .start = 0, .len = 0 };
+    }
 
-/// Creates an empty DataSpan with zero start and zero length.
-pub fn empty() DataSpan {
-    return DataSpan{ .start = 0, .len = 0 };
-}
+    /// Creates a DataSpan with the specified start position and length.
+    pub fn init(start: u32, len: u32) DataSpan {
+        return DataSpan{ .start = start, .len = len };
+    }
 
-/// Creates a DataSpan with the specified start position and length.
-pub fn init(start: u32, len: u32) DataSpan {
-    return DataSpan{ .start = start, .len = len };
-}
-
-/// Converts this DataSpan into a type that contains a span field.
-/// This is useful for creating wrapper types around DataSpan.
-pub fn as(self: DataSpan, comptime T: type) T {
-    return @as(T, .{ .span = self });
-}
+    /// Converts this DataSpan into a type that contains a span field.
+    /// This is useful for creating wrapper types around DataSpan.
+    pub fn as(self: DataSpan, comptime T: type) T {
+        return @as(T, .{ .span = self });
+    }
+};

src/base/Ident.zig

@@ -105,7 +105,8 @@ pub const Store = struct {
     next_unique_name: u32 = 0,
 
     /// Serialized representation of an Ident.Store
-    pub const Serialized = struct {
+    /// Uses extern struct to guarantee consistent field layout across optimization levels.
+    pub const Serialized = extern struct {
         interner: SmallStringInterner.Serialized,
         attributes: collections.SafeList(Attributes).Serialized,
         next_unique_name: u32,

src/base/PackedDataSpan.zig

@@ -11,7 +11,7 @@
 
 const std = @import("std");
 const testing = std.testing;
-const DataSpan = @import("DataSpan.zig");
+const DataSpan = @import("DataSpan.zig").DataSpan;
 
 /// Configurable packed DataSpan with customizable bit allocation
 pub fn PackedDataSpan(comptime start_bits: u6, comptime length_bits: u6) type {

src/base/Scratch.zig

@@ -2,7 +2,7 @@
 //! when working with recursive operations
 
 const std = @import("std");
-const DataSpan = @import("DataSpan.zig");
+const DataSpan = @import("DataSpan.zig").DataSpan;
 
 /// A stack for easily adding and removing index types when doing recursive operations
 pub fn Scratch(comptime T: type) type {

src/base/SmallStringInterner.zig

@@ -204,7 +204,8 @@ pub fn relocate(self: *SmallStringInterner, offset: isize) void {
 }
 
 /// Serialized representation of a SmallStringInterner
-pub const Serialized = struct {
+/// Uses extern struct to guarantee consistent field layout across optimization levels.
+pub const Serialized = extern struct {
     bytes: collections.SafeList(u8).Serialized,
     hash_table: collections.SafeList(Idx).Serialized,
     entry_count: u32,

src/base/StringLiteral.zig

@@ -97,7 +97,8 @@ pub const Store = struct {
     }
 
     /// Serialized representation of a Store
-    pub const Serialized = struct {
+    /// Uses extern struct to guarantee consistent field layout across optimization levels.
+    pub const Serialized = extern struct {
         buffer: collections.SafeList(u8).Serialized,
 
         /// Serialize a Store into this Serialized struct, appending data to the writer

src/base/mod.zig

@@ -13,7 +13,7 @@ pub const SmallStringInterner = @import("SmallStringInterner.zig");
 pub const safe_memory = @import("safe_memory.zig");
 
 pub const target = @import("target.zig");
-pub const DataSpan = @import("DataSpan.zig");
+pub const DataSpan = @import("DataSpan.zig").DataSpan;
 pub const PackedDataSpan = @import("PackedDataSpan.zig").PackedDataSpan;
 pub const FunctionArgs = @import("PackedDataSpan.zig").FunctionArgs;
 pub const SmallCollections = @import("PackedDataSpan.zig").SmallCollections;