diff --git a/gen/src/builtin.rs b/gen/src/builtin.rs
index e28a62c..a1393b5 100644
--- a/gen/src/builtin.rs
+++ b/gen/src/builtin.rs
@@ -1,4 +1,5 @@
-use crate::gen::out::Content;
+use crate::gen::ifndef;
+use crate::gen::out::{Content, OutFile};
 
 #[derive(Default, PartialEq)]
 pub struct Builtins {
@@ -25,3 +26,146 @@
         Builtins::default()
     }
 }
+
+pub(super) fn write(out: &mut OutFile) {
+    if out.builtin == Default::default() {
+        return;
+    }
+
+    let include = &mut out.include;
+    let builtin = &mut out.builtin;
+    let out = &mut builtin.content;
+
+    out.begin_block("namespace rust");
+    out.begin_block("inline namespace cxxbridge05");
+    writeln!(out, "// #include \"rust/cxx.h\"");
+
+    ifndef::write(out, builtin.panic, "CXXBRIDGE05_PANIC");
+
+    if builtin.rust_string {
+        out.next_section();
+        writeln!(out, "struct unsafe_bitcopy_t;");
+    }
+
+    if builtin.rust_error {
+        out.begin_block("namespace");
+        writeln!(out, "template <typename T>");
+        writeln!(out, "class impl;");
+        out.end_block("namespace");
+    }
+
+    ifndef::write(out, builtin.rust_string, "CXXBRIDGE05_RUST_STRING");
+    ifndef::write(out, builtin.rust_str, "CXXBRIDGE05_RUST_STR");
+    ifndef::write(out, builtin.rust_slice, "CXXBRIDGE05_RUST_SLICE");
+    ifndef::write(out, builtin.rust_box, "CXXBRIDGE05_RUST_BOX");
+    ifndef::write(out, builtin.unsafe_bitcopy, "CXXBRIDGE05_RUST_BITCOPY");
+    ifndef::write(out, builtin.rust_vec, "CXXBRIDGE05_RUST_VEC");
+    ifndef::write(out, builtin.rust_fn, "CXXBRIDGE05_RUST_FN");
+    ifndef::write(out, builtin.rust_error, "CXXBRIDGE05_RUST_ERROR");
+    ifndef::write(out, builtin.rust_isize, "CXXBRIDGE05_RUST_ISIZE");
+
+    if builtin.manually_drop {
+        out.next_section();
+        include.utility = true;
+        writeln!(out, "template <typename T>");
+        writeln!(out, "union ManuallyDrop {{");
+        writeln!(out, "  T value;");
+        writeln!(
+            out,
+            "  ManuallyDrop(T &&value) : value(::std::move(value)) {{}}",
+        );
+        writeln!(out, "  ~ManuallyDrop() {{}}");
+        writeln!(out, "}};");
+    }
+
+    if builtin.maybe_uninit {
+        out.next_section();
+        writeln!(out, "template <typename T>");
+        writeln!(out, "union MaybeUninit {{");
+        writeln!(out, "  T value;");
+        writeln!(out, "  MaybeUninit() {{}}");
+        writeln!(out, "  ~MaybeUninit() {{}}");
+        writeln!(out, "}};");
+    }
+
+    out.begin_block("namespace");
+
+    if builtin.trycatch
+        || builtin.rust_error
+        || builtin.rust_str_new_unchecked
+        || builtin.rust_str_repr
+    {
+        out.begin_block("namespace repr");
+        writeln!(out, "struct PtrLen final {{");
+        writeln!(out, "  const void *ptr;");
+        writeln!(out, "  size_t len;");
+        writeln!(out, "}};");
+        out.end_block("namespace repr");
+    }
+
+    if builtin.rust_str_new_unchecked || builtin.rust_str_repr {
+        out.next_section();
+        writeln!(out, "template <>");
+        writeln!(out, "class impl<Str> final {{");
+        writeln!(out, "public:");
+        if builtin.rust_str_new_unchecked {
+            writeln!(
+                out,
+                "  static Str new_unchecked(repr::PtrLen repr) noexcept {{",
+            );
+            writeln!(out, "    Str str;");
+            writeln!(out, "    str.ptr = static_cast<const char *>(repr.ptr);");
+            writeln!(out, "    str.len = repr.len;");
+            writeln!(out, "    return str;");
+            writeln!(out, "  }}");
+        }
+        if builtin.rust_str_repr {
+            writeln!(out, "  static repr::PtrLen repr(Str str) noexcept {{");
+            writeln!(out, "    return repr::PtrLen{{str.ptr, str.len}};");
+            writeln!(out, "  }}");
+        }
+        writeln!(out, "}};");
+    }
+
+    if builtin.rust_error {
+        out.next_section();
+        writeln!(out, "template <>");
+        writeln!(out, "class impl<Error> final {{");
+        writeln!(out, "public:");
+        writeln!(out, "  static Error error(repr::PtrLen repr) noexcept {{");
+        writeln!(out, "    Error error;");
+        writeln!(out, "    error.msg = static_cast<const char *>(repr.ptr);");
+        writeln!(out, "    error.len = repr.len;");
+        writeln!(out, "    return error;");
+        writeln!(out, "  }}");
+        writeln!(out, "}};");
+    }
+
+    out.end_block("namespace");
+    out.end_block("namespace cxxbridge05");
+
+    if builtin.trycatch {
+        out.begin_block("namespace behavior");
+        include.exception = true;
+        include.type_traits = true;
+        include.utility = true;
+        writeln!(out, "class missing {{}};");
+        writeln!(out, "missing trycatch(...);");
+        writeln!(out);
+        writeln!(out, "template <typename Try, typename Fail>");
+        writeln!(out, "static typename ::std::enable_if<");
+        writeln!(
+            out,
+            "    ::std::is_same<decltype(trycatch(::std::declval<Try>(), ::std::declval<Fail>())),",
+        );
+        writeln!(out, "                 missing>::value>::type");
+        writeln!(out, "trycatch(Try &&func, Fail &&fail) noexcept try {{");
+        writeln!(out, "  func();");
+        writeln!(out, "}} catch (const ::std::exception &e) {{");
+        writeln!(out, "  fail(e.what());");
+        writeln!(out, "}}");
+        out.end_block("namespace behavior");
+    }
+
+    out.end_block("namespace rust");
+}
diff --git a/gen/src/write.rs b/gen/src/write.rs
index fc093e7..5dcf91c 100644
--- a/gen/src/write.rs
+++ b/gen/src/write.rs
@@ -1,6 +1,6 @@
 use crate::gen::namespace_organizer::NamespaceEntries;
 use crate::gen::out::OutFile;
-use crate::gen::{ifndef, include, Opt};
+use crate::gen::{builtin, include, Opt};
 use crate::syntax::atom::Atom::{self, *};
 use crate::syntax::symbol::Symbol;
 use crate::syntax::{
@@ -31,7 +31,7 @@
         write_generic_instantiations(out);
     }
 
-    write_builtins(out);
+    builtin::write(out);
     include::write(out);
 
     out_file
@@ -240,149 +240,6 @@
     }
 }
 
-fn write_builtins(out: &mut OutFile) {
-    if out.builtin == Default::default() {
-        return;
-    }
-
-    let include = &mut out.include;
-    let builtin = &mut out.builtin;
-    let out = &mut builtin.content;
-
-    out.begin_block("namespace rust");
-    out.begin_block("inline namespace cxxbridge05");
-    writeln!(out, "// #include \"rust/cxx.h\"");
-
-    ifndef::write(out, builtin.panic, "CXXBRIDGE05_PANIC");
-
-    if builtin.rust_string {
-        out.next_section();
-        writeln!(out, "struct unsafe_bitcopy_t;");
-    }
-
-    if builtin.rust_error {
-        out.begin_block("namespace");
-        writeln!(out, "template <typename T>");
-        writeln!(out, "class impl;");
-        out.end_block("namespace");
-    }
-
-    ifndef::write(out, builtin.rust_string, "CXXBRIDGE05_RUST_STRING");
-    ifndef::write(out, builtin.rust_str, "CXXBRIDGE05_RUST_STR");
-    ifndef::write(out, builtin.rust_slice, "CXXBRIDGE05_RUST_SLICE");
-    ifndef::write(out, builtin.rust_box, "CXXBRIDGE05_RUST_BOX");
-    ifndef::write(out, builtin.unsafe_bitcopy, "CXXBRIDGE05_RUST_BITCOPY");
-    ifndef::write(out, builtin.rust_vec, "CXXBRIDGE05_RUST_VEC");
-    ifndef::write(out, builtin.rust_fn, "CXXBRIDGE05_RUST_FN");
-    ifndef::write(out, builtin.rust_error, "CXXBRIDGE05_RUST_ERROR");
-    ifndef::write(out, builtin.rust_isize, "CXXBRIDGE05_RUST_ISIZE");
-
-    if builtin.manually_drop {
-        out.next_section();
-        include.utility = true;
-        writeln!(out, "template <typename T>");
-        writeln!(out, "union ManuallyDrop {{");
-        writeln!(out, "  T value;");
-        writeln!(
-            out,
-            "  ManuallyDrop(T &&value) : value(::std::move(value)) {{}}",
-        );
-        writeln!(out, "  ~ManuallyDrop() {{}}");
-        writeln!(out, "}};");
-    }
-
-    if builtin.maybe_uninit {
-        out.next_section();
-        writeln!(out, "template <typename T>");
-        writeln!(out, "union MaybeUninit {{");
-        writeln!(out, "  T value;");
-        writeln!(out, "  MaybeUninit() {{}}");
-        writeln!(out, "  ~MaybeUninit() {{}}");
-        writeln!(out, "}};");
-    }
-
-    out.begin_block("namespace");
-
-    if builtin.trycatch
-        || builtin.rust_error
-        || builtin.rust_str_new_unchecked
-        || builtin.rust_str_repr
-    {
-        out.begin_block("namespace repr");
-        writeln!(out, "struct PtrLen final {{");
-        writeln!(out, "  const void *ptr;");
-        writeln!(out, "  size_t len;");
-        writeln!(out, "}};");
-        out.end_block("namespace repr");
-    }
-
-    if builtin.rust_str_new_unchecked || builtin.rust_str_repr {
-        out.next_section();
-        writeln!(out, "template <>");
-        writeln!(out, "class impl<Str> final {{");
-        writeln!(out, "public:");
-        if builtin.rust_str_new_unchecked {
-            writeln!(
-                out,
-                "  static Str new_unchecked(repr::PtrLen repr) noexcept {{",
-            );
-            writeln!(out, "    Str str;");
-            writeln!(out, "    str.ptr = static_cast<const char *>(repr.ptr);");
-            writeln!(out, "    str.len = repr.len;");
-            writeln!(out, "    return str;");
-            writeln!(out, "  }}");
-        }
-        if builtin.rust_str_repr {
-            writeln!(out, "  static repr::PtrLen repr(Str str) noexcept {{");
-            writeln!(out, "    return repr::PtrLen{{str.ptr, str.len}};");
-            writeln!(out, "  }}");
-        }
-        writeln!(out, "}};");
-    }
-
-    if builtin.rust_error {
-        out.next_section();
-        writeln!(out, "template <>");
-        writeln!(out, "class impl<Error> final {{");
-        writeln!(out, "public:");
-        writeln!(out, "  static Error error(repr::PtrLen repr) noexcept {{");
-        writeln!(out, "    Error error;");
-        writeln!(out, "    error.msg = static_cast<const char *>(repr.ptr);");
-        writeln!(out, "    error.len = repr.len;");
-        writeln!(out, "    return error;");
-        writeln!(out, "  }}");
-        writeln!(out, "}};");
-    }
-
-    out.end_block("namespace");
-    out.end_block("namespace cxxbridge05");
-
-    if builtin.trycatch {
-        out.begin_block("namespace behavior");
-        include.exception = true;
-        include.type_traits = true;
-        include.utility = true;
-        writeln!(out, "class missing {{}};");
-        writeln!(out, "missing trycatch(...);");
-        writeln!(out);
-        writeln!(out, "template <typename Try, typename Fail>");
-        writeln!(out, "static typename ::std::enable_if<");
-        writeln!(
-            out,
-            "    ::std::is_same<decltype(trycatch(::std::declval<Try>(), ::std::declval<Fail>())),",
-        );
-        writeln!(out, "                 missing>::value>::type");
-        writeln!(out, "trycatch(Try &&func, Fail &&fail) noexcept try {{");
-        writeln!(out, "  func();");
-        writeln!(out, "}} catch (const ::std::exception &e) {{");
-        writeln!(out, "  fail(e.what());");
-        writeln!(out, "}}");
-        out.end_block("namespace behavior");
-    }
-
-    out.end_block("namespace rust");
-}
-
 fn write_struct(out: &mut OutFile, strct: &Struct) {
     let guard = format!("CXXBRIDGE05_STRUCT_{}", strct.ident.cxx.to_symbol());
     writeln!(out, "#ifndef {}", guard);