Sep 4, 2025 · Sep 4, 2025 · Sep 4, 2025 · Sep 4, 2025 · Sep 5, 2025 · Sep 5, 2025
diff --git a/AGENTS.md b/AGENTS.md
 #CLAUDE.md
 #AGENTS.md

 This file provides guidance toClaude Code (claude.ai/code) whenworkingwith codein this repository.
 Shared guidance for code agents (Claude, Gemini, Codex, etc.)working in this repository. The files `CLAUDE.md` and `GEMINI.md` are symlinks to this document. For Codex‑specific workflow details also see `CODEX.md`.

 ## Project Overview

 - Avoid introducing meaningless symbols
 - Maintain readable JavaScript output
 - Consider compilation speed impact
 - Use appropriate optimization passes in the Lambda and JS IRs
 - Use appropriate optimization passes in the Lambda and JS IRs

 ## Agent Notes and Gotchas

 - **Node version features:** Runtime docstring tests conditionally enable features by Node major version:
  - 20+: array `toReversed`/`toSorted`
  - 22+: new `Set` APIs and `Promise.withResolvers`
  - 24+: `RegExp.escape`
  Tests auto‑skip unsupported features based on `process.version`.
 - **CPU count in sandboxes:** Some CI/sandboxed environments report `os.cpus().length === 0`.
  - Clamp concurrency/batch size to at least 1 when using `os.cpus()`.
  - This pattern is used in `tests/docstring_tests/DocTest.res` and `cli/rescript-legacy/format.js`.
 - **Formatting in tests:** `make test` checks formatting (OCaml, ReScript, JS, Rust). If it fails locally, run `make format` and re‑run tests.
 - **Executables location:** Build copies platform binaries into `packages/@rescript/<platform>/bin/` and convenience folders like `darwinarm64/`.
 - **Direct dune usage:** You can use `dune build`/`dune build -w`, but prefer `make` targets which also copy executables.

 ## References

 - `CODEX.md`: detailed setup, build, testing, and workflows for agented development
 - `README.md`: high‑level repo overview and usage
 - `Makefile`: authoritative list of build/test targets
diff --git a/CHANGELOG.md b/CHANGELOG.md
 - Fix generation of interfaces for module types containing multiple type constraints. https://github.com/rescript-lang/rescript/pull/7825
 - JSX preserve mode: fix "make is not a valid component name". https://github.com/rescript-lang/rescript/pull/7831
 - Rewatch: include parser arguments of experimental features. https://github.com/rescript-lang/rescript/pull/7836
 - Suppress spurious “constructor … is never used” warnings when constructors are introduced via variant coercions; still warn for constructors only present on the target side. https://github.com/rescript-lang/rescript/pull/7839

 #### :memo: Documentation

diff --git a/compiler/ml/typecore.ml b/compiler/ml/typecore.ml
           raise (Error (loc, env, Not_subtype (tr1, tr2, ctx))));
      (arg, ty', cty')
    in
    (* After a successful coercion, if we coerced between concrete variant
       types, mark the intersection of constructors as positively used for the
       target type to avoid spurious "constructor ... is never used" warnings
       when values are introduced via coercions. *)
    (try
       let _p_src, _p_src_conc, src_decl =
         Ctype.extract_concrete_typedecl env arg.exp_type
       in
       let _, p_tgt_conc, tgt_decl = Ctype.extract_concrete_typedecl env ty' in
       match (src_decl.type_kind, tgt_decl.type_kind) with
       | Type_variant src_cons, Type_variant tgt_cons ->
         let module StringSet = Set.Make (String) in
         let src_set =
           List.fold_left
             (fun acc (c : Types.constructor_declaration) ->
               StringSet.add (Ident.name c.cd_id) acc)
             StringSet.empty src_cons
         in
         let has_src name = StringSet.mem name src_set in
         let tgt_ty_name_conc = Path.last p_tgt_conc in
         (* Mark usage for the concrete target decl (implementation view). *)
         List.iter
           (fun (c : Types.constructor_declaration) ->
             let cname = Ident.name c.cd_id in
             if has_src cname then
               Env.mark_constructor_used Env.Positive env tgt_ty_name_conc
                 tgt_decl cname)
           tgt_cons
       | _ -> ()
     with Not_found -> ());
    rue
      {
        exp_desc = arg.exp_desc;
diff --git a/tests/build_tests/super_errors/expected/VariantCoercionWarnOnC.res.expected b/tests/build_tests/super_errors/expected/VariantCoercionWarnOnC.res.expected

  [1;33mWarning number 37[0m
  [36m/.../fixtures/VariantCoercionWarnOnC.res[0m:[2m9:3-11:7[0m

   7 [2m│[0m   let log: a => unit
   8 [2m│[0m } = {
   [1;33m9[0m [2m│[0m   [1;33mtype b =[0m
  [1;33m10[0m [2m│[0m [1;33m    | ...a[0m
  [1;33m11[0m [2m│[0m [1;33m    | C[0m
  12 [2m│[0m
  13 [2m│[0m   let log = (x: a) => Js.log((x :> b))

  unused constructor C.
diff --git a/tests/build_tests/super_errors/fixtures/VariantCoercionWarnOnC.res b/tests/build_tests/super_errors/fixtures/VariantCoercionWarnOnC.res
 // Ensure unused-constructor warning still fires for new members introduced
 // only on the target side of a coercion.

 type a = A | B

 module M: {
  let log: a => unit
 } = {
  type b =
    | ...a
    | C

  let log = (x: a) => Js.log((x :> b))
 }

 let _ = M.log(A)
diff --git a/tests/docstring_tests/DocTest.res b/tests/docstring_tests/DocTest.res
  }
 }

 let batchSize = OS.cpus()->Array.length
 // Some environments may report 0 CPUs; clamp to at least 1 to avoid zero-sized batches
 let batchSize = switch OS.cpus()->Array.length {
 | n if n > 0 => n
 | _ => 1
 }

 let runtimePath = Path.join(["packages", "@rescript", "runtime"])

diff --git a/tests/docstring_tests/DocTest.res.js b/tests/docstring_tests/DocTest.res.js
diff --git a/tests/tests/src/VariantCoercionConstructUsed.mjs b/tests/tests/src/VariantCoercionConstructUsed.mjs
 // Generated by ReScript, PLEASE EDIT WITH CARE


 function log(x) {
  console.log(x);
 }

 let DoNotWarn = {
  log: log
 };

 console.log("A");

 export {
  DoNotWarn,
 }
 /*  Not a pure module */
diff --git a/tests/tests/src/VariantCoercionConstructUsed.res b/tests/tests/src/VariantCoercionConstructUsed.res
 // Repro for incorrect "constructor ... is never used" warning with coercions
 // This should compile cleanly without warnings when coercing from a -> b.

 type a = A | B
 module DoNotWarn: {
  let log: a => unit
 } = {
  type b =
    | ...a
    | C

  let log = (x: a) => Js.log((x :> b))
 }

 let _ = DoNotWarn.log(A)
Original file line number	Diff line number	Diff line change
		@@ -1,6 +1,6 @@
		#CLAUDE.md
		#AGENTS.md

		This file provides guidance toClaude Code (claude.ai/code) whenworkingwith codein this repository.
		Shared guidance for code agents (Claude, Gemini, Codex, etc.)working in this repository. The files `CLAUDE.md` and `GEMINI.md` are symlinks to this document. For Codex‑specific workflow details also see `CODEX.md`.

		## Project Overview

Expand DownExpand Up		@@ -136,4 +136,24 @@ The compiler is designed for fast feedback loops and scales to large codebases.
		- Avoid introducing meaningless symbols
		- Maintain readable JavaScript output
		- Consider compilation speed impact
		- Use appropriate optimization passes in the Lambda and JS IRs
		- Use appropriate optimization passes in the Lambda and JS IRs

		## Agent Notes and Gotchas

		- Node version features: Runtime docstring tests conditionally enable features by Node major version:
		- 20+: array `toReversed`/`toSorted`
		- 22+: new `Set` APIs and `Promise.withResolvers`
		- 24+: `RegExp.escape`
		Tests auto‑skip unsupported features based on `process.version`.
		- CPU count in sandboxes: Some CI/sandboxed environments report `os.cpus().length === 0`.
		- Clamp concurrency/batch size to at least 1 when using `os.cpus()`.
		- This pattern is used in `tests/docstring_tests/DocTest.res` and `cli/rescript-legacy/format.js`.
		- Formatting in tests: `make test` checks formatting (OCaml, ReScript, JS, Rust). If it fails locally, run `make format` and re‑run tests.
		- Executables location: Build copies platform binaries into `packages/@rescript/<platform>/bin/` and convenience folders like `darwinarm64/`.
		- Direct dune usage: You can use `dune build`/`dune build -w`, but prefer `make` targets which also copy executables.

		## References

		- `CODEX.md`: detailed setup, build, testing, and workflows for agented development
		- `README.md`: high‑level repo overview and usage
		- `Makefile`: authoritative list of build/test targets
Original file line number	Diff line number	Diff line change
Expand Up		@@ -25,6 +25,7 @@
		- Fix generation of interfaces for module types containing multiple type constraints. https://github.com/rescript-lang/rescript/pull/7825
		- JSX preserve mode: fix "make is not a valid component name". https://github.com/rescript-lang/rescript/pull/7831
		- Rewatch: include parser arguments of experimental features. https://github.com/rescript-lang/rescript/pull/7836
		- Suppress spurious “constructor … is never used” warnings when constructors are introduced via variant coercions; still warn for constructors only present on the target side. https://github.com/rescript-lang/rescript/pull/7839

		#### :memo: Documentation

Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -3018,6 +3018,36 @@ and type_expect_ ~context ?in_function ?(recarg = Rejected) env sexp ty_expected
		raise (Error (loc, env, Not_subtype (tr1, tr2, ctx))));
		(arg, ty', cty')
		in
		(* After a successful coercion, if we coerced between concrete variant
		types, mark the intersection of constructors as positively used for the
		target type to avoid spurious "constructor ... is never used" warnings
		when values are introduced via coercions. *)
		(try
		let _p_src, _p_src_conc, src_decl =
		Ctype.extract_concrete_typedecl env arg.exp_type
		in
		let _, p_tgt_conc, tgt_decl = Ctype.extract_concrete_typedecl env ty' in
		match (src_decl.type_kind, tgt_decl.type_kind) with
		\| Type_variant src_cons, Type_variant tgt_cons ->
		let module StringSet = Set.Make (String) in
		let src_set =
		List.fold_left
		(fun acc (c : Types.constructor_declaration) ->
		StringSet.add (Ident.name c.cd_id) acc)
		StringSet.empty src_cons
		in
		let has_src name = StringSet.mem name src_set in
		let tgt_ty_name_conc = Path.last p_tgt_conc in
		(* Mark usage for the concrete target decl (implementation view). *)
		List.iter
		(fun (c : Types.constructor_declaration) ->
		let cname = Ident.name c.cd_id in
		if has_src cname then
		Env.mark_constructor_used Env.Positive env tgt_ty_name_conc
		tgt_decl cname)
		tgt_cons
		\| _ -> ()
		with Not_found -> ());
		rue
		{
		exp_desc = arg.exp_desc;
Expand Down
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,13 @@

		[1;33mWarning number 37[0m
		[36m/.../fixtures/VariantCoercionWarnOnC.res[0m:[2m9:3-11:7[0m

		7 [2m│[0m let log: a => unit
		8 [2m│[0m } = {
		[1;33m9[0m [2m│[0m [1;33mtype b =[0m
		[1;33m10[0m [2m│[0m [1;33m \| ...a[0m
		[1;33m11[0m [2m│[0m [1;33m \| C[0m
		12 [2m│[0m
		13 [2m│[0m let log = (x: a) => Js.log((x :> b))

		unused constructor C.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,16 @@
		// Ensure unused-constructor warning still fires for new members introduced
		// only on the target side of a coercion.

		type a = A \| B

		module M: {
		let log: a => unit
		} = {
		type b =
		\| ...a
		\| C

		let log = (x: a) => Js.log((x :> b))
		}

		let _ = M.log(A)
Original file line number	Diff line number	Diff line change
Expand Up		@@ -62,7 +62,11 @@ let extractDocFromFile = async file => {
		}
		}

		let batchSize = OS.cpus()->Array.length
		// Some environments may report 0 CPUs; clamp to at least 1 to avoid zero-sized batches
		let batchSize = switch OS.cpus()->Array.length {
		\| n if n > 0 => n
		\| _ => 1
		}
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		let runtimePath = Path.join(["packages", "@rescript", "runtime"])

Expand Down
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,17 @@
		// Generated by ReScript, PLEASE EDIT WITH CARE


		function log(x) {
		console.log(x);
		}

		let DoNotWarn = {
		log: log
		};

		console.log("A");

		export {
		DoNotWarn,
		}
		/* Not a pure module */
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,15 @@
		// Repro for incorrect "constructor ... is never used" warning with coercions
		// This should compile cleanly without warnings when coercing from a -> b.

		type a = A \| B
		module DoNotWarn: {
		let log: a => unit
		} = {
		type b =
		\| ...a
		\| C

		let log = (x: a) => Js.log((x :> b))
		}

		let _ = DoNotWarn.log(A)