diff --git a/src/astgen.zig b/src/astgen.zig
index d940c787f1..4e3fdccb6d 100644
--- a/src/astgen.zig
+++ b/src/astgen.zig
@@ -1152,12 +1152,12 @@ fn varDecl(
             var init_scope: Scope.GenZir = .{
                 .parent = scope,
                 .force_comptime = scope.isComptime(),
-                .zir_code = scope.getGenZir().zir_code,
+                .zir_code = gz.zir_code,
             };
             defer init_scope.instructions.deinit(mod.gpa);
 
-            var resolve_inferred_alloc: ?zir.Inst.Ref = null;
-            var opt_type_inst: ?zir.Inst.Ref = null;
+            var resolve_inferred_alloc: zir.Inst.Ref = 0;
+            var opt_type_inst: zir.Inst.Ref = 0;
             if (var_decl.ast.type_node != 0) {
                 const type_inst = try typeExpr(mod, &init_scope.base, var_decl.ast.type_node);
                 opt_type_inst = type_inst;
@@ -1169,6 +1169,9 @@ fn varDecl(
             }
             const init_result_loc: ResultLoc = .{ .block_ptr = &init_scope };
             const init_inst = try expr(mod, &init_scope.base, init_result_loc, var_decl.ast.init_node);
+            const zir_tags = gz.zir_code.instructions.items(.tag);
+            const zir_datas = gz.zir_code.instructions.items(.data);
+
             const parent_zir = &gz.instructions;
             if (init_scope.rvalue_rl_count == 1) {
                 // Result location pointer not used. We don't need an alloc for this
@@ -1179,16 +1182,15 @@ fn varDecl(
                 try parent_zir.ensureCapacity(mod.gpa, expected_len);
                 for (init_scope.instructions.items) |src_inst| {
                     if (src_inst == init_scope.rl_ptr) continue;
-                    const src_inst_dtag = init_scope.zir_code.instructions.get(src_inst);
-                    if (src_inst_dtag.tag == .store_to_block_ptr) {
-                        if (src_inst_dtag.data.bin.lhs == init_scope.rl_ptr) continue;
+                    if (zir_tags[src_inst] == .store_to_block_ptr) {
+                        if (zir_datas[src_inst].bin.lhs == init_scope.rl_ptr) continue;
                     }
                     parent_zir.appendAssumeCapacity(src_inst);
                 }
                 assert(parent_zir.items.len == expected_len);
-                const casted_init = if (opt_type_inst) |type_inst|
+                const casted_init = if (opt_type_inst != 0)
                     try gz.addPlNode(.as_node, var_decl.ast.type_node, zir.Inst.As{
-                        .dest_type = type_inst,
+                        .dest_type = opt_type_inst,
                         .operand = init_inst,
                     })
                 else
@@ -1211,17 +1213,16 @@ fn varDecl(
             const expected_len = parent_zir.items.len + init_scope.instructions.items.len;
             try parent_zir.ensureCapacity(mod.gpa, expected_len);
             for (init_scope.instructions.items) |src_inst| {
-                const src_inst_dtag = init_scope.zir_code.instructions.get(src_inst);
-                if (src_inst_dtag.tag == .store_to_block_ptr) {
-                    if (src_inst_dtag.data.bin.lhs == init_scope.rl_ptr) {
-                        init_scope.zir_code.instructions.items(.tag)[src_inst] = .store_to_inferred_ptr;
+                if (zir_tags[src_inst] == .store_to_block_ptr) {
+                    if (zir_datas[src_inst].bin.lhs == init_scope.rl_ptr) {
+                        zir_tags[src_inst] = .store_to_inferred_ptr;
                     }
                 }
                 parent_zir.appendAssumeCapacity(src_inst);
             }
             assert(parent_zir.items.len == expected_len);
-            if (resolve_inferred_alloc) |inst| {
-                _ = try gz.addUnTok(.resolve_inferred_alloc, inst, tree.firstToken(node));
+            if (resolve_inferred_alloc != 0) {
+                _ = try gz.addUnNode(.resolve_inferred_alloc, resolve_inferred_alloc, node);
             }
             const sub_scope = try block_arena.create(Scope.LocalPtr);
             sub_scope.* = .{
@@ -1234,7 +1235,7 @@ fn varDecl(
             return &sub_scope.base;
         },
         .keyword_var => {
-            var resolve_inferred_alloc: ?zir.Inst.Ref = null;
+            var resolve_inferred_alloc: zir.Inst.Ref = 0;
             const var_data: struct {
                 result_loc: ResultLoc,
                 alloc: zir.Inst.Ref,
@@ -1249,8 +1250,8 @@ fn varDecl(
                 break :a .{ .alloc = alloc, .result_loc = .{ .inferred_ptr = alloc } };
             };
             const init_inst = try expr(mod, scope, var_data.result_loc, var_decl.ast.init_node);
-            if (resolve_inferred_alloc) |inst| {
-                _ = try gz.addUnNode(.resolve_inferred_alloc, inst, node);
+            if (resolve_inferred_alloc != 0) {
+                _ = try gz.addUnNode(.resolve_inferred_alloc, resolve_inferred_alloc, node);
             }
             const sub_scope = try block_arena.create(Scope.LocalPtr);
             sub_scope.* = .{
diff --git a/src/zir.zig b/src/zir.zig
index ed3ad8fd2b..2c43c453f7 100644
--- a/src/zir.zig
+++ b/src/zir.zig
@@ -746,6 +746,9 @@ pub const Inst = struct {
         store_to_block_ptr,
         /// Same as `store` but the type of the value being stored will be used to infer
         /// the pointer type.
+        /// Uses the `bin` union field - astgen.zig depends on the ability to change
+        /// the tag of an instruction from `store_to_block_ptr` to `store_to_inferred_ptr`
+        /// without changing the data.
         store_to_inferred_ptr,
         /// String Literal. Makes an anonymous Decl and then takes a pointer to it.
         /// Uses the `str` union field.