cknitt · Oct 10, 2026 · Oct 10, 2026 · Oct 10, 2026 · Oct 10, 2026
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -53,6 +53,7 @@
 - Speed up inlining decisions for large functions. https://github.com/rescript-lang/rescript/pull/8771
 - Speed up the rewriting of JS globals shadowed by local bindings in nested functions. https://github.com/rescript-lang/rescript/pull/8772
 - Speed up dead code elimination of toplevel bindings in large modules. https://github.com/rescript-lang/rescript/pull/8773
+- Make .cmt and .cmti files smaller by no longer embedding a copy of the .cmi. https://github.com/rescript-lang/rescript/pull/8774
 - Represent explicit expression braces as `Pexp_braces` in parsetree v1 and format `else` branches consistently with `if` branches. https://github.com/rescript-lang/rescript/pull/8678
 - Omit redundant braces around multi-statement switch case bodies when formatting. https://github.com/rescript-lang/rescript/pull/8677
 - Avoid running `rescript-schema-ppx` and `sury-ppx` on source files without an `@schema` annotation. https://github.com/rescript-lang/rescript/pull/8662

diff --git a/compiler/core/js_implementation.ml b/compiler/core/js_implementation.ml
@@ -60,11 +60,10 @@ let after_parsing_sig ppf outputprefix ast =
       Delayed_checks.force_delayed_checks ();
       Warnings.check_fatal ();
       let deprecated = Builtin_attributes.deprecated_of_sig ast in
-      let sg =
-        Env.save_signature ~deprecated sg modulename (outputprefix ^ ".cmi")
-      in
+      ignore
+        (Env.save_signature ~deprecated sg modulename (outputprefix ^ ".cmi"));
       Typemod.save_signature modulename tsg outputprefix !Location.input_name
-        initial_env sg;
+        initial_env;
       process_with_gentype (outputprefix ^ ".cmti"))
 
 let interface ~parser ppf ?outputprefix fname =

diff --git a/compiler/ml/cmt_format.ml b/compiler/ml/cmt_format.ml
@@ -1,6 +1,6 @@
 include Cmt_format_common
 
-let save_cmt filename modname binary_annots sourcefile initial_env cmi =
+let save_cmt filename modname binary_annots sourcefile initial_env =
   Cmt_format_persistence.save_cmt filename modname binary_annots sourcefile
-    initial_env cmi;
+    initial_env;
   clear ()
diff --git a/compiler/ml/cmt_format.mli b/compiler/ml/cmt_format.mli
@@ -15,19 +15,16 @@
 
 (** cmt and cmti files format. *)
 
-(** The layout of a cmt file is as follows:
-      <cmt> := \{<cmi>\} <cmt magic> \{cmt infos\} \{<source info>\}
+(** The layout of a cmt or cmti file is as follows:
+      <cmt> := \{<cmi>\} <cmt magic> \{cmt infos\}
     where <cmi> is the cmi file format:
       <cmi> := <cmi magic> <cmi info>.
-    More precisely, the optional <cmi> part must be present if and only if
-    the file is:
-    - a cmti, or
-    - a cmt, for a ml file which has no corresponding mli (hence no
-    corresponding cmti).
+    The compiler no longer writes the optional <cmi> part: the signature is
+    read from the .cmi file. Files written by older compilers have it in a
+    cmti, and in the cmt of a module without an interface.
 
-    Thus, we provide a common reading function for cmi and cmt(i)
-    files which returns an option for each of the three parts: cmi
-    info, cmt info, source info. *)
+    [read] accepts cmi, cmt and cmti files, with or without the <cmi> part,
+    and returns an option for the cmi info and the cmt info. *)
 
 open Typedtree
 
@@ -83,10 +80,8 @@ val save_cmt :
   (* source file *)
   Env.t ->
   (* initial env *)
-  Cmi_format.cmi_infos option ->
-  (* if a .cmi was generated *)
   unit
-(** [save_cmt filename modname binary_annots sourcefile initial_env cmi]
+(** [save_cmt filename modname binary_annots sourcefile initial_env]
     writes a cmt(i) file.  *)
 
 (* Miscellaneous functions *)

diff --git a/compiler/ml/cmt_format_common.ml b/compiler/ml/cmt_format_common.ml
@@ -111,9 +111,8 @@ let input_cmt ic = (input_value ic : cmt_infos)
     it exists, and the cmt_infos, if it exists. Thus, it can be used
     with .cmi, .cmt and .cmti files.
 
-    .cmti files always contain a cmi_infos at the beginning. .cmt files
-    only contain a cmi_infos at the beginning if there is no associated
-    .cmti file.
+    The compiler no longer writes a cmi_infos in front of the cmt_infos in
+    .cmt and .cmti files, but files from older compilers may contain one.
 *)
 let read filename =
   (*  Printf.fprintf stderr "Cmt_format.read %s\n%!" filename; *)

diff --git a/compiler/ml/platform/native/cmt_format_persistence.ml b/compiler/ml/platform/native/cmt_format_persistence.ml
@@ -19,16 +19,12 @@ let output_cmt output_channel cmt =
   output_string output_channel Config.cmt_magic_number;
   output_value output_channel (cmt : cmt_infos)
 
-let save_cmt filename modname binary_annots sourcefile initial_env cmi =
+(* Unlike OCaml, no copy of the .cmi is written in front of the cmt infos:
+   tools read the .cmi itself. *)
+let save_cmt filename modname binary_annots sourcefile initial_env =
   if !Clflags.binary_annotations then
     Misc.output_to_bin_file_directly filename
-      (fun temp_file_name output_channel ->
-        let interface_digest =
-          match cmi with
-          | None -> None
-          | Some cmi ->
-            Some (Cmi_format.output_cmi temp_file_name output_channel cmi)
-        in
+      (fun _temp_file_name output_channel ->
         let cmt =
           {
             cmt_modname = modname;
@@ -44,7 +40,7 @@ let save_cmt filename modname binary_annots sourcefile initial_env cmi =
               (if need_to_clear_env then keep_only_summary initial_env
                else initial_env);
             cmt_imports = List.sort compare (Env.imports ());
-            cmt_interface_digest = interface_digest;
+            cmt_interface_digest = None;
             cmt_use_summaries = need_to_clear_env;
             cmt_extra_info = {deprecated_used = deprecated_uses ()};
           }

diff --git a/compiler/ml/platform/playground/cmt_format_persistence.ml b/compiler/ml/platform/playground/cmt_format_persistence.ml
@@ -1,2 +1 @@
-let save_cmt _filename _modname _binary_annots _sourcefile _initial_env _cmi =
-  ()
+let save_cmt _filename _modname _binary_annots _sourcefile _initial_env = ()
diff --git a/compiler/ml/typemod.ml b/compiler/ml/typemod.ml
@@ -1738,7 +1738,7 @@ let type_implementation_more ?check_exists sourcefile outputprefix modulename
          so that value declarations which are not used internally but exported
          are not reported as being unused. *)
       Cmt_format.save_cmt (outputprefix ^ ".cmt") modulename
-        (Cmt_format.Implementation str) (Some sourcefile) initial_env None;
+        (Cmt_format.Implementation str) (Some sourcefile) initial_env;
       (str, coercion, finalenv, dclsig)
       (* identifier is useless might read from serialized cmi files*))
     else
@@ -1753,26 +1753,24 @@ let type_implementation_more ?check_exists sourcefile outputprefix modulename
          the value being exported. We can still capture unused
          declarations like "let x = true;; let x = 1;;", because in this
          case, the inferred signature contains only the last declaration. *)
-      (if not !Clflags.dont_write_files then
-         let deprecated = Builtin_attributes.deprecated_of_str ast in
-         let cmi =
-           Env.save_signature ?check_exists ~deprecated simple_sg modulename
-             (outputprefix ^ ".cmi")
-         in
-         Cmt_format.save_cmt (outputprefix ^ ".cmt") modulename
-           (Cmt_format.Implementation str) (Some sourcefile) initial_env
-           (Some cmi));
+      if not !Clflags.dont_write_files then (
+        let deprecated = Builtin_attributes.deprecated_of_str ast in
+        ignore
+          (Env.save_signature ?check_exists ~deprecated simple_sg modulename
+             (outputprefix ^ ".cmi"));
+        Cmt_format.save_cmt (outputprefix ^ ".cmt") modulename
+          (Cmt_format.Implementation str) (Some sourcefile) initial_env);
       (str, coercion, finalenv, simple_sg)
   with e ->
     Cmt_format.save_cmt (outputprefix ^ ".cmt") modulename
       (Cmt_format.Partial_implementation
          (Array.of_list (Cmt_format.get_saved_types ())))
-      (Some sourcefile) initial_env None;
+      (Some sourcefile) initial_env;
     raise e
 
-let save_signature modname tsg outputprefix source_file initial_env cmi =
+let save_signature modname tsg outputprefix source_file initial_env =
   Cmt_format.save_cmt (outputprefix ^ ".cmti") modname
-    (Cmt_format.Interface tsg) (Some source_file) initial_env (Some cmi)
+    (Cmt_format.Interface tsg) (Some source_file) initial_env
 
 (* "Packaging" of several compilation units into one unit
    having them as sub-modules. *)

diff --git a/compiler/ml/typemod.mli b/compiler/ml/typemod.mli
@@ -45,13 +45,7 @@ val type_open_ :
   Path.t * Env.t
 
 val save_signature :
-  string ->
-  Typedtree.signature ->
-  string ->
-  string ->
-  Env.t ->
-  Cmi_format.cmi_infos ->
-  unit
+  string -> Typedtree.signature -> string -> string -> Env.t -> unit
 
 type error =
   | Cannot_apply of module_type