lang 2.1: Pack as a type-system value

Add a `pack` variant to IR `TypeInfo` — an ordered, interned sequence of
per-position element types (`PackInfo { elements: []const TypeId }`) — with
constructor (`packType`), structural equality + hashing, and a `pack(T0, …)`
printer. A pack is comptime-only: it lowers to flat positional args before
codegen and has no runtime layout, so `sizeOf` and `toLLVMType` bail loudly
rather than inventing a size. 5 unit tests (N=0/1/3, dedup, order/arity
distinctness, distinct-from-tuple, printer).

Also: give TypeTable an arena for the slices its constructors dupe (freed at
deinit), and add the missing `usize`/`isize` arms to `sizeOf` (a latent
non-exhaustive switch) so types.test.zig compiles and runs leak-free.

src/ir/types.zig

@@ -67,6 +67,7 @@ pub const TypeInfo = union(enum) {
     closure: ClosureInfo,
     optional: OptionalInfo,
     tuple: TupleInfo,
+    pack: PackInfo,
     any,
     protocol: ProtocolInfo,
     noreturn,
@@ -162,6 +163,14 @@ pub const TypeInfo = union(enum) {
         names: ?[]const StringId,
     };
 
+    /// A heterogeneous variadic pack as a first-class type-system value: an
+    /// ordered sequence of per-position element types. Comptime-only — a pack
+    /// lowers to flat positional args before codegen and has NO runtime layout
+    /// (sizeOf panics). `elements.len == 0` is a valid empty pack.
+    pub const PackInfo = struct {
+        elements: []const TypeId,
+    };
+
     pub const ProtocolInfo = struct {
         name: StringId,
         methods: []const Method,
@@ -245,6 +254,12 @@ pub const TypeTable = struct {
     /// Maps TypeInfo → TypeId for dedup of structural types
     intern_map: std.HashMap(TypeKey, TypeId, TypeKeyContext, 80),
     alloc: Allocator,
+    /// Owns the element/param slices duped by the type constructors
+    /// (`functionType*`, `closureType*`, `packType`). Freed wholesale in
+    /// `deinit` — these slices live as long as the table, so an arena avoids
+    /// per-slice bookkeeping and the owned-vs-borrowed ambiguity that blocks
+    /// freeing them individually.
+    slice_arena: std.heap.ArenaAllocator,
     /// Target pointer size in bytes (4 for wasm32, 8 for 64-bit targets).
     pointer_size: u8 = 8,
     /// Borrowed pointer to Lowering's `type_alias_map`. When set,
@@ -262,6 +277,7 @@ pub const TypeTable = struct {
             .strings = StringPool.init(alloc),
             .intern_map = std.HashMap(TypeKey, TypeId, TypeKeyContext, 80).init(alloc),
             .alloc = alloc,
+            .slice_arena = std.heap.ArenaAllocator.init(alloc),
         };
 
         // Pre-populate builtin slots 0–15 (must match TypeId enum order)
@@ -295,6 +311,7 @@ pub const TypeTable = struct {
         self.infos.deinit(self.alloc);
         self.strings.deinit(self.alloc);
         self.intern_map.deinit();
+        self.slice_arena.deinit();
     }
 
     /// Look up the TypeInfo for a given TypeId.
@@ -366,22 +383,22 @@ pub const TypeTable = struct {
     }
 
     pub fn functionTypeCC(self: *TypeTable, params: []const TypeId, ret: TypeId, cc: TypeInfo.CallConv) TypeId {
-        const owned_params = self.alloc.dupe(TypeId, params) catch unreachable;
+        const owned_params = self.slice_arena.allocator().dupe(TypeId, params) catch unreachable;
         return self.intern(.{ .function = .{ .params = owned_params, .ret = ret, .call_conv = cc } });
     }
 
     pub fn functionTypePack(self: *TypeTable, params: []const TypeId, ret: TypeId, cc: TypeInfo.CallConv, pack_start: u32) TypeId {
-        const owned_params = self.alloc.dupe(TypeId, params) catch unreachable;
+        const owned_params = self.slice_arena.allocator().dupe(TypeId, params) catch unreachable;
         return self.intern(.{ .function = .{ .params = owned_params, .ret = ret, .call_conv = cc, .pack_start = pack_start } });
     }
 
     pub fn closureType(self: *TypeTable, params: []const TypeId, ret: TypeId) TypeId {
-        const owned_params = self.alloc.dupe(TypeId, params) catch unreachable;
+        const owned_params = self.slice_arena.allocator().dupe(TypeId, params) catch unreachable;
         return self.intern(.{ .closure = .{ .params = owned_params, .ret = ret } });
     }
 
     pub fn closureTypePack(self: *TypeTable, params: []const TypeId, ret: TypeId, pack_start: u32) TypeId {
-        const owned_params = self.alloc.dupe(TypeId, params) catch unreachable;
+        const owned_params = self.slice_arena.allocator().dupe(TypeId, params) catch unreachable;
         return self.intern(.{ .closure = .{ .params = owned_params, .ret = ret, .pack_start = pack_start } });
     }
 
@@ -389,6 +406,13 @@ pub const TypeTable = struct {
         return self.intern(.{ .vector = .{ .element = element, .length = length } });
     }
 
+    /// Construct (and intern) a heterogeneous pack type from an ordered
+    /// element-type list. `elements.len == 0` yields the empty pack.
+    pub fn packType(self: *TypeTable, elements: []const TypeId) TypeId {
+        const owned = self.slice_arena.allocator().dupe(TypeId, elements) catch unreachable;
+        return self.intern(.{ .pack = .{ .elements = owned } });
+    }
+
     /// Size in bytes for a type (pointer-sized = 8 on 64-bit).
     pub fn sizeOf(self: *const TypeTable, id: TypeId) u32 {
         const info = self.get(id);
@@ -449,6 +473,10 @@ pub const TypeTable = struct {
                 return if (total == 0) 8 else total;
             },
             .protocol => 16, // {ctx, vtable}
+            .usize, .isize => 8, // pointer-sized (this path is not target-aware; see typeSizeBytes)
+            // Comptime-only: a pack must be expanded to flat positional args
+            // before codegen. Reaching runtime layout means a pack leaked.
+            .pack => @panic("pack type has no runtime layout (comptime-only)"),
         };
     }
 
@@ -740,6 +768,17 @@ pub const TypeTable = struct {
                 buf.append(alloc, ')') catch break :blk "(?)";
                 break :blk buf.toOwnedSlice(alloc) catch "(?)";
             },
+            .pack => |pk| blk: {
+                var buf = std.ArrayList(u8).empty;
+                defer buf.deinit(alloc);
+                buf.appendSlice(alloc, "pack(") catch break :blk "pack(?)";
+                for (pk.elements, 0..) |e, i| {
+                    if (i > 0) buf.appendSlice(alloc, ", ") catch break :blk "pack(?)";
+                    buf.appendSlice(alloc, self.formatTypeName(alloc, e)) catch break :blk "pack(?)";
+                }
+                buf.append(alloc, ')') catch break :blk "pack(?)";
+                break :blk buf.toOwnedSlice(alloc) catch "pack(?)";
+            },
             .signed => |w| std.fmt.allocPrint(alloc, "s{d}", .{w}) catch "s?",
             .unsigned => |w| std.fmt.allocPrint(alloc, "u{d}", .{w}) catch "u?",
             else => self.typeName(id),
@@ -812,6 +851,9 @@ fn hashTypeInfo(h: *std.hash.Wyhash, info: TypeInfo) void {
             for (t.fields) |f| h.update(std.mem.asBytes(&f));
             if (t.names) |ns| for (ns) |n| h.update(std.mem.asBytes(&n));
         },
+        .pack => |p| {
+            for (p.elements) |e| h.update(std.mem.asBytes(&e));
+        },
     }
 }
 
@@ -871,5 +913,13 @@ fn typeInfoEql(a: TypeInfo, b: TypeInfo) bool {
             }
             return true;
         },
+        .pack => |p| {
+            const q = b.pack;
+            if (p.elements.len != q.elements.len) return false;
+            for (p.elements, q.elements) |pe, qe| {
+                if (pe != qe) return false;
+            }
+            return true;
+        },
     };
 }