From 645f5bd16e65a0137b756d526725d3ea01cba114 Mon Sep 17 00:00:00 2001
From: Florian Hammerschmidt <florianh89@gmail.com>
Date: Thu, 10 Sep 2026 10:38:34 +0200
Subject: [PATCH 01/10] Fix GenType path resolution on Windows

Signed-off-by: Florian Hammerschmidt <florianh89@gmail.com>
---
 compiler/gentype/module_resolver.ml |  9 +++----
 rewatch/src/build/parse.rs          |  9 ++++++-
 rewatch/src/config.rs               | 38 ++++++++++++++++++++++++++---
 3 files changed, 46 insertions(+), 10 deletions(-)

diff --git a/compiler/gentype/module_resolver.ml b/compiler/gentype/module_resolver.ml
index 698cb75d72d..7e42e5034b8 100644
--- a/compiler/gentype/module_resolver.ml
+++ b/compiler/gentype/module_resolver.ml
@@ -133,11 +133,10 @@ let resolve_module ~(config : Config.t) ~import_extension ~output_file_relative
   if Sys.file_exists module_name_res_file then candidate
   else
     let rec path_to_list path =
-      let is_root = path |> Filename.basename = path in
-      match is_root with
-      | true -> [path]
-      | false ->
-        (path |> Filename.basename) :: (path |> Filename.dirname |> path_to_list)
+      let parent = Filename.dirname path in
+      (* On Windows, the basename of a drive root omits the drive prefix. *)
+      if Filename.basename path = path || parent = path then [path]
+      else (path |> Filename.basename) :: path_to_list parent
     in
     match module_name |> apply ~resolver ~use_bs_dependencies with
     | None -> candidate
diff --git a/rewatch/src/build/parse.rs b/rewatch/src/build/parse.rs
index 87944965ae3..9d4e71533af 100644
--- a/rewatch/src/build/parse.rs
+++ b/rewatch/src/build/parse.rs
@@ -436,11 +436,18 @@ fn generate_ast(
     // generate the dir of the ast_path (it mirrors the source file dir)
     let ast_parent_path = package.get_build_path().join(ast_path.parent().unwrap());
     helpers::create_path(&ast_parent_path);
+    // Keep absolute source locations in the AST in the same path form as the
+    // canonical project root passed during compilation. This matters for
+    // Windows paths that have both short and expanded representations.
+    let parse_working_dir = build_path_abs
+        .canonicalize()
+        .map(helpers::StrippedVerbatimPath::to_stripped_verbatim_path)
+        .unwrap_or_else(|_| build_path_abs.clone());
 
     /* Create .ast */
     let result = match Some(
         Command::new(&build_state.compiler_info.bsc_path)
-            .current_dir(&build_path_abs)
+            .current_dir(&parse_working_dir)
             .args(parser_args)
             .output()
             .map_err(|e| {
diff --git a/rewatch/src/config.rs b/rewatch/src/config.rs
index dfdf67dcb05..b01935e14c6 100644
--- a/rewatch/src/config.rs
+++ b/rewatch/src/config.rs
@@ -1029,10 +1029,15 @@ impl Config {
     }
 
     pub fn get_project_root_args(&self) -> Vec<String> {
-        vec![
-            "-bs-project-root".to_string(),
-            self.get_package_root().to_string_lossy().to_string(),
-        ]
+        // The parser records locations from a canonical working directory.
+        // Use the same representation here so GenType can strip this prefix,
+        // including when Windows supplied the project through an 8.3 path.
+        let root = self
+            .get_package_root()
+            .canonicalize()
+            .map(helpers::StrippedVerbatimPath::to_stripped_verbatim_path)
+            .unwrap_or_else(|_| self.get_package_root().to_path_buf());
+        vec!["-bs-project-root".to_string(), root.to_string_lossy().to_string()]
     }
 
     pub fn get_warning_args(&self, is_local_dep: bool, warn_error_override: Option<String>) -> Vec<String> {
@@ -1599,6 +1604,31 @@ pub mod tests {
         assert!(args.contains(&"@teamwalnut/app".to_string()));
     }
 
+    #[test]
+    fn test_compiler_project_root_is_canonicalized() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let nested_dir = temp_dir.path().join("nested");
+        fs::create_dir(&nested_dir).unwrap();
+        let config_path = nested_dir.join("..").join("rescript.json");
+        fs::write(
+            temp_dir.path().join("rescript.json"),
+            r#"{"name":"test","gentypeconfig":{}}"#,
+        )
+        .unwrap();
+
+        let config = Config::new(&config_path).unwrap();
+        let expected_root = temp_dir
+            .path()
+            .canonicalize()
+            .map(helpers::StrippedVerbatimPath::to_stripped_verbatim_path)
+            .unwrap()
+            .to_string_lossy()
+            .to_string();
+
+        let project_root_args = config.get_project_root_args();
+        assert_eq!(project_root_args, ["-bs-project-root", &expected_root]);
+    }
+
     #[test]
     fn test_gentype_shims_object_and_array_forms() {
         let object_form = serde_json::from_str::<GenTypeShims>(r#"{"From": "To"}"#).unwrap();

From a803304bd920cdab025f992733e3f9fae63cb980 Mon Sep 17 00:00:00 2001
From: Florian Hammerschmidt <florianh89@gmail.com>
Date: Thu, 10 Sep 2026 11:25:10 +0200
Subject: [PATCH 02/10] Add changelog entry

Signed-off-by: Florian Hammerschmidt <florianh89@gmail.com>
---
 CHANGELOG.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7dfb72da74f..3c5fddcca9d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -38,6 +38,7 @@
 
 - Fix `reanalyze` reporting labels and variant cases of a re-exported type (`type y = x = {...}`) as dead in the editor. The re-export linking existed only in the batch pipeline, while the editor runs the reactive one; both are now the same pipeline. The `reanalyze -reactive` flag is gone with it, since analysis is always reactive. https://github.com/rescript-lang/rescript/issues/8647
 - Fix `reanalyze` reporting record labels reached through a record coercion as dead. The typed tree now keeps the source type of a coercion, so reading a label on the target counts as reading the source label of the same name. https://github.com/rescript-lang/rescript/issues/8643
+- Fix GenType path resolution hanging on Windows when project paths use different short and canonical forms. https://github.com/rescript-lang/rescript/pull/8639
 - Fix constant folding of pattern matches on unboxed variants whose payload overlaps a literal constructor, so inlined calls agree with runtime matching. Reject multi-argument unboxed constructors instead of crashing. https://github.com/rescript-lang/rescript/pull/8631
 - Fix escaped backticks and interpolation openers in backquoted `%raw`, `%ffi`, and `%re` payloads leaking into emitted JavaScript. https://github.com/rescript-lang/rescript/pull/8630
 - Fix the side-effect analysis treating bigint exponentiation and bounds-checked array and string reads as pure, which let dead-code elimination drop an unused one that throws: `let _ = 2n ** -1n` no longer raised. https://github.com/rescript-lang/rescript/pull/8617

From 6480b16ed311cdbc19d3e9a96fc1cad1661e4b84 Mon Sep 17 00:00:00 2001
From: Florian Hammerschmidt <florianh89@gmail.com>
Date: Thu, 10 Sep 2026 11:30:41 +0200
Subject: [PATCH 03/10] Test GenType project roots

Signed-off-by: Florian Hammerschmidt <florianh89@gmail.com>
---
 rewatch/tests/gentype/project-root.test.mjs | 36 +++++++++++++++++++++
 rewatch/tests/suite.sh                      |  1 +
 2 files changed, 37 insertions(+)
 create mode 100644 rewatch/tests/gentype/project-root.test.mjs

diff --git a/rewatch/tests/gentype/project-root.test.mjs b/rewatch/tests/gentype/project-root.test.mjs
new file mode 100644
index 00000000000..af29c542a07
--- /dev/null
+++ b/rewatch/tests/gentype/project-root.test.mjs
@@ -0,0 +1,36 @@
+import assert from "node:assert/strict";
+import {spawnSync} from "node:child_process";
+import {mkdtempSync, mkdirSync, readFileSync, rmSync, writeFileSync} from "node:fs";
+import {tmpdir} from "node:os";
+import {join} from "node:path";
+
+const executable = process.env.REWATCH_EXECUTABLE;
+assert(executable, "REWATCH_EXECUTABLE must be set");
+
+const projectDir = mkdtempSync(join(tmpdir(), "rescript-gentype-root-"));
+
+try {
+  mkdirSync(join(projectDir, "src"));
+  writeFileSync(
+    join(projectDir, "rescript.json"),
+    `${JSON.stringify({name: "gentype-project-root", sources: ["src"], gentypeconfig: {}}, null, 2)}\n`,
+  );
+  writeFileSync(join(projectDir, "src", "A.res"), "@genType\ntype t = {value: int}\n");
+  writeFileSync(
+    join(projectDir, "src", "B.res"),
+    "@genType\nlet getValue = (record: A.t) => record.value\n",
+  );
+
+  const result = spawnSync(executable, ["build"], {
+    cwd: projectDir,
+    encoding: "utf8",
+    env: process.env,
+    timeout: 30_000,
+  });
+
+  assert.ifError(result.error);
+  assert.equal(result.status, 0, `${result.stdout}\n${result.stderr}`);
+  assert.match(readFileSync(join(projectDir, "src", "B.gen.tsx"), "utf8"), /A\.gen/);
+} finally {
+  rmSync(projectDir, {recursive: true, force: true});
+}
diff --git a/rewatch/tests/suite.sh b/rewatch/tests/suite.sh
index 22cc77935d7..335cae6f544 100755
--- a/rewatch/tests/suite.sh
+++ b/rewatch/tests/suite.sh
@@ -132,6 +132,7 @@ fi
 ./compile/17-prod-flag.sh &&
 ./compile/18-external-dep-uncurried-dot.sh &&
 ./compile/19-utf8-warning.sh &&
+node ./gentype/project-root.test.mjs &&
 ./compile/14-no-testrepo-changes.sh &&
 ./compile/15-no-new-files.sh &&
 ./compile/16-snapshots-unchanged.sh &&

From f98eabe12ee20998879fe5f4dc177b7c44e12ba7 Mon Sep 17 00:00:00 2001
From: Florian Hammerschmidt <florianh89@gmail.com>
Date: Thu, 10 Sep 2026 13:15:52 +0200
Subject: [PATCH 04/10] Run GenType path test through Windows shim

Signed-off-by: Florian Hammerschmidt <florianh89@gmail.com>
---
 rewatch/tests/gentype/project-root.test.mjs | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/rewatch/tests/gentype/project-root.test.mjs b/rewatch/tests/gentype/project-root.test.mjs
index af29c542a07..65ad845513c 100644
--- a/rewatch/tests/gentype/project-root.test.mjs
+++ b/rewatch/tests/gentype/project-root.test.mjs
@@ -1,11 +1,13 @@
 import assert from "node:assert/strict";
 import {spawnSync} from "node:child_process";
-import {mkdtempSync, mkdirSync, readFileSync, rmSync, writeFileSync} from "node:fs";
+import {existsSync, mkdtempSync, mkdirSync, readFileSync, rmSync, writeFileSync} from "node:fs";
 import {tmpdir} from "node:os";
 import {join} from "node:path";
 
 const executable = process.env.REWATCH_EXECUTABLE;
 assert(executable, "REWATCH_EXECUTABLE must be set");
+const executableForNode =
+  process.platform === "win32" && existsSync(`${executable}.cmd`) ? `${executable}.cmd` : executable;
 
 const projectDir = mkdtempSync(join(tmpdir(), "rescript-gentype-root-"));
 
@@ -21,7 +23,7 @@ try {
     "@genType\nlet getValue = (record: A.t) => record.value\n",
   );
 
-  const result = spawnSync(executable, ["build"], {
+  const result = spawnSync(executableForNode, ["build"], {
     cwd: projectDir,
     encoding: "utf8",
     env: process.env,

From 0c3f35a410538b5ebb4ab3c7c8f34c6538818d4f Mon Sep 17 00:00:00 2001
From: Florian Hammerschmidt <florianh89@gmail.com>
Date: Thu, 10 Sep 2026 13:20:16 +0200
Subject: [PATCH 05/10] Exercise GenType through a Windows short path

Signed-off-by: Florian Hammerschmidt <florianh89@gmail.com>
---
 rewatch/tests/gentype/project-root.test.mjs | 49 ++++++++++++++++++---
 1 file changed, 42 insertions(+), 7 deletions(-)

diff --git a/rewatch/tests/gentype/project-root.test.mjs b/rewatch/tests/gentype/project-root.test.mjs
index 65ad845513c..fa8001bec62 100644
--- a/rewatch/tests/gentype/project-root.test.mjs
+++ b/rewatch/tests/gentype/project-root.test.mjs
@@ -1,6 +1,6 @@
 import assert from "node:assert/strict";
 import {spawnSync} from "node:child_process";
-import {existsSync, mkdtempSync, mkdirSync, readFileSync, rmSync, writeFileSync} from "node:fs";
+import {existsSync, mkdtempSync, mkdirSync, readFileSync, realpathSync, rmSync, writeFileSync} from "node:fs";
 import {tmpdir} from "node:os";
 import {join} from "node:path";
 
@@ -9,22 +9,54 @@ assert(executable, "REWATCH_EXECUTABLE must be set");
 const executableForNode =
   process.platform === "win32" && existsSync(`${executable}.cmd`) ? `${executable}.cmd` : executable;
 
+function getBuildDirectory(projectDir) {
+  if (process.platform !== "win32") {
+    return projectDir;
+  }
+
+  const expandedPath = realpathSync.native(projectDir);
+  const result = spawnSync(process.env.ComSpec ?? "cmd.exe", ["/d", "/s", "/c", `for %I in ("${expandedPath}") do @echo %~sI`], {
+    encoding: "utf8",
+  });
+  assert.ifError(result.error);
+  assert.equal(result.status, 0, result.stderr);
+
+  const shortPath = result.stdout.trim();
+  assert(shortPath, "Windows did not return a short path for the temporary project");
+  assert.notEqual(
+    shortPath.toLowerCase(),
+    expandedPath.toLowerCase(),
+    "Windows short and expanded project paths must differ for this regression",
+  );
+  return shortPath;
+}
+
 const projectDir = mkdtempSync(join(tmpdir(), "rescript-gentype-root-"));
 
 try {
-  mkdirSync(join(projectDir, "src"));
+  mkdirSync(join(projectDir, "src-a"));
+  mkdirSync(join(projectDir, "src-b", "nested"), {recursive: true});
   writeFileSync(
     join(projectDir, "rescript.json"),
-    `${JSON.stringify({name: "gentype-project-root", sources: ["src"], gentypeconfig: {}}, null, 2)}\n`,
+    `${JSON.stringify(
+      {
+        name: "gentype-project-root",
+        sources: ["src-a", {dir: "src-b", subdirs: true}],
+        gentypeconfig: {},
+      },
+      null,
+      2,
+    )}\n`,
   );
-  writeFileSync(join(projectDir, "src", "A.res"), "@genType\ntype t = {value: int}\n");
+  writeFileSync(join(projectDir, "src-a", "A.res"), "@genType\ntype t = {value: int}\n");
   writeFileSync(
-    join(projectDir, "src", "B.res"),
+    join(projectDir, "src-b", "nested", "B.res"),
     "@genType\nlet getValue = (record: A.t) => record.value\n",
   );
 
+  const buildDirectory = getBuildDirectory(projectDir);
   const result = spawnSync(executableForNode, ["build"], {
-    cwd: projectDir,
+    cwd: buildDirectory,
     encoding: "utf8",
     env: process.env,
     timeout: 30_000,
@@ -32,7 +64,10 @@ try {
 
   assert.ifError(result.error);
   assert.equal(result.status, 0, `${result.stdout}\n${result.stderr}`);
-  assert.match(readFileSync(join(projectDir, "src", "B.gen.tsx"), "utf8"), /A\.gen/);
+  assert.match(
+    readFileSync(join(projectDir, "src-b", "nested", "B.gen.tsx"), "utf8"),
+    /\.\.\/\.\.\/src-a\/A\.gen/,
+  );
 } finally {
   rmSync(projectDir, {recursive: true, force: true});
 }

From 6ecf20e83380467027db4cdcf498bdd0e047d2a3 Mon Sep 17 00:00:00 2001
From: Florian Hammerschmidt <florianh89@gmail.com>
Date: Thu, 10 Sep 2026 13:36:56 +0200
Subject: [PATCH 06/10] Use a numbered wrapper for the GenType test

Signed-off-by: Florian Hammerschmidt <florianh89@gmail.com>
---
 rewatch/tests/gentype/01-gentype-project-root.sh          | 8 ++++++++
 .../gentype/{project-root.test.mjs => project-root.mjs}   | 0
 rewatch/tests/suite.sh                                    | 2 +-
 3 files changed, 9 insertions(+), 1 deletion(-)
 create mode 100755 rewatch/tests/gentype/01-gentype-project-root.sh
 rename rewatch/tests/gentype/{project-root.test.mjs => project-root.mjs} (100%)

diff --git a/rewatch/tests/gentype/01-gentype-project-root.sh b/rewatch/tests/gentype/01-gentype-project-root.sh
new file mode 100755
index 00000000000..365c787530c
--- /dev/null
+++ b/rewatch/tests/gentype/01-gentype-project-root.sh
@@ -0,0 +1,8 @@
+#!/bin/bash
+set -euo pipefail
+cd "$(dirname "$0")"
+source "../utils.sh"
+
+bold "Test: GenType resolves modules through a Windows short project path"
+node ./project-root.mjs
+success "GenType resolved the short project path"
diff --git a/rewatch/tests/gentype/project-root.test.mjs b/rewatch/tests/gentype/project-root.mjs
similarity index 100%
rename from rewatch/tests/gentype/project-root.test.mjs
rename to rewatch/tests/gentype/project-root.mjs
diff --git a/rewatch/tests/suite.sh b/rewatch/tests/suite.sh
index 335cae6f544..5fccd7cf4b1 100755
--- a/rewatch/tests/suite.sh
+++ b/rewatch/tests/suite.sh
@@ -132,7 +132,7 @@ fi
 ./compile/17-prod-flag.sh &&
 ./compile/18-external-dep-uncurried-dot.sh &&
 ./compile/19-utf8-warning.sh &&
-node ./gentype/project-root.test.mjs &&
+./gentype/01-gentype-project-root.sh &&
 ./compile/14-no-testrepo-changes.sh &&
 ./compile/15-no-new-files.sh &&
 ./compile/16-snapshots-unchanged.sh &&

From a45573b38228111d80fb889be54b8df5a6807294 Mon Sep 17 00:00:00 2001
From: Florian Hammerschmidt <florianh89@gmail.com>
Date: Thu, 10 Sep 2026 13:43:22 +0200
Subject: [PATCH 07/10] Group the GenType integration test

Signed-off-by: Florian Hammerschmidt <florianh89@gmail.com>
---
 rewatch/tests/suite.sh | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/rewatch/tests/suite.sh b/rewatch/tests/suite.sh
index 5fccd7cf4b1..039684456d3 100755
--- a/rewatch/tests/suite.sh
+++ b/rewatch/tests/suite.sh
@@ -132,11 +132,13 @@ fi
 ./compile/17-prod-flag.sh &&
 ./compile/18-external-dep-uncurried-dot.sh &&
 ./compile/19-utf8-warning.sh &&
-./gentype/01-gentype-project-root.sh &&
 ./compile/14-no-testrepo-changes.sh &&
 ./compile/15-no-new-files.sh &&
 ./compile/16-snapshots-unchanged.sh &&
 
+# GenType tests
+./gentype/01-gentype-project-root.sh &&
+
 # Watch tests
 ./watch/01-watch-recompile.sh &&
 ./watch/02-watch-warnings-persist.sh &&

From 3102f42d77c5fb29e7a91112f12a3291d10ef414 Mon Sep 17 00:00:00 2001
From: Florian Hammerschmidt <florianh89@gmail.com>
Date: Fri, 11 Sep 2026 12:24:16 +0200
Subject: [PATCH 08/10] Fix GenType path test on CI

Signed-off-by: Florian Hammerschmidt <florianh89@gmail.com>
---
 rewatch/src/build/parse.rs             |  9 +---
 rewatch/src/config.rs                  |  2 +-
 rewatch/tests/gentype/project-root.mjs | 60 +++++++++++++++++++++-----
 3 files changed, 51 insertions(+), 20 deletions(-)

diff --git a/rewatch/src/build/parse.rs b/rewatch/src/build/parse.rs
index 9d4e71533af..87944965ae3 100644
--- a/rewatch/src/build/parse.rs
+++ b/rewatch/src/build/parse.rs
@@ -436,18 +436,11 @@ fn generate_ast(
     // generate the dir of the ast_path (it mirrors the source file dir)
     let ast_parent_path = package.get_build_path().join(ast_path.parent().unwrap());
     helpers::create_path(&ast_parent_path);
-    // Keep absolute source locations in the AST in the same path form as the
-    // canonical project root passed during compilation. This matters for
-    // Windows paths that have both short and expanded representations.
-    let parse_working_dir = build_path_abs
-        .canonicalize()
-        .map(helpers::StrippedVerbatimPath::to_stripped_verbatim_path)
-        .unwrap_or_else(|_| build_path_abs.clone());
 
     /* Create .ast */
     let result = match Some(
         Command::new(&build_state.compiler_info.bsc_path)
-            .current_dir(&parse_working_dir)
+            .current_dir(&build_path_abs)
             .args(parser_args)
             .output()
             .map_err(|e| {
diff --git a/rewatch/src/config.rs b/rewatch/src/config.rs
index b01935e14c6..44304ae1009 100644
--- a/rewatch/src/config.rs
+++ b/rewatch/src/config.rs
@@ -1029,7 +1029,7 @@ impl Config {
     }
 
     pub fn get_project_root_args(&self) -> Vec<String> {
-        // The parser records locations from a canonical working directory.
+        // Package discovery gives the parser a canonical working directory.
         // Use the same representation here so GenType can strip this prefix,
         // including when Windows supplied the project through an 8.3 path.
         let root = self
diff --git a/rewatch/tests/gentype/project-root.mjs b/rewatch/tests/gentype/project-root.mjs
index fa8001bec62..a3a1cf80a9e 100644
--- a/rewatch/tests/gentype/project-root.mjs
+++ b/rewatch/tests/gentype/project-root.mjs
@@ -6,8 +6,44 @@ import {join} from "node:path";
 
 const executable = process.env.REWATCH_EXECUTABLE;
 assert(executable, "REWATCH_EXECUTABLE must be set");
-const executableForNode =
-  process.platform === "win32" && existsSync(`${executable}.cmd`) ? `${executable}.cmd` : executable;
+
+function runBuild(buildDirectory) {
+  if (process.platform !== "win32") {
+    return spawnSync(executable, ["build"], {
+      cwd: buildDirectory,
+      encoding: "utf8",
+      env: process.env,
+      timeout: 30_000,
+    });
+  }
+
+  const executablePath = executable.endsWith(".exe") ? executable : `${executable}.exe`;
+  const commandShim = executable.endsWith(".cmd") ? executable : `${executable}.cmd`;
+  const resolvedExecutable = existsSync(executablePath)
+    ? executablePath
+    : existsSync(commandShim)
+      ? commandShim
+      : executable;
+
+  return spawnSync(
+    process.env.ComSpec ?? "cmd.exe",
+    [
+      "/d",
+      "/c",
+      'cd /d "%REWATCH_TEST_PROJECT%" && "%REWATCH_TEST_EXECUTABLE%" build',
+    ],
+    {
+      encoding: "utf8",
+      env: {
+        ...process.env,
+        REWATCH_TEST_EXECUTABLE: resolvedExecutable,
+        REWATCH_TEST_PROJECT: buildDirectory,
+      },
+      timeout: 30_000,
+      windowsVerbatimArguments: true,
+    },
+  );
+}
 
 function getBuildDirectory(projectDir) {
   if (process.platform !== "win32") {
@@ -15,9 +51,15 @@ function getBuildDirectory(projectDir) {
   }
 
   const expandedPath = realpathSync.native(projectDir);
-  const result = spawnSync(process.env.ComSpec ?? "cmd.exe", ["/d", "/s", "/c", `for %I in ("${expandedPath}") do @echo %~sI`], {
-    encoding: "utf8",
-  });
+  const result = spawnSync(
+    process.env.ComSpec ?? "cmd.exe",
+    ["/d", "/c", 'for %I in ("%REWATCH_TEST_PROJECT%") do @echo %~sI'],
+    {
+      encoding: "utf8",
+      env: {...process.env, REWATCH_TEST_PROJECT: expandedPath},
+      windowsVerbatimArguments: true,
+    },
+  );
   assert.ifError(result.error);
   assert.equal(result.status, 0, result.stderr);
 
@@ -55,12 +97,8 @@ try {
   );
 
   const buildDirectory = getBuildDirectory(projectDir);
-  const result = spawnSync(executableForNode, ["build"], {
-    cwd: buildDirectory,
-    encoding: "utf8",
-    env: process.env,
-    timeout: 30_000,
-  });
+  assert(existsSync(buildDirectory), `Build directory does not exist: ${buildDirectory}`);
+  const result = runBuild(buildDirectory);
 
   assert.ifError(result.error);
   assert.equal(result.status, 0, `${result.stdout}\n${result.stderr}`);

From c286781887265583729bdac3a56c65d777d090ca Mon Sep 17 00:00:00 2001
From: Florian Hammerschmidt <florianh89@gmail.com>
Date: Tue, 13 Oct 2026 14:13:36 +0200
Subject: [PATCH 09/10] Tighten GenType project path regression coverage

Signed-off-by: Florian Hammerschmidt <florianh89@gmail.com>
---
 changes/gentype-windows-paths.fix.md          |  2 +-
 .../tests/gentype/01-gentype-project-root.sh  |  4 +-
 rewatch/tests/gentype/project-root.mjs        | 38 ++++++++++---------
 3 files changed, 24 insertions(+), 20 deletions(-)

diff --git a/changes/gentype-windows-paths.fix.md b/changes/gentype-windows-paths.fix.md
index d3873b38431..e826291c3c4 100644
--- a/changes/gentype-windows-paths.fix.md
+++ b/changes/gentype-windows-paths.fix.md
@@ -1 +1 @@
-- Fix GenType path resolution hanging on Windows when project paths use different short and canonical forms. https://github.com/rescript-lang/rescript/pull/8639
+- Fix GenType path resolution for Windows short paths and POSIX symlinked project paths, and prevent traversal from hanging at a filesystem root. https://github.com/rescript-lang/rescript/pull/8639
diff --git a/rewatch/tests/gentype/01-gentype-project-root.sh b/rewatch/tests/gentype/01-gentype-project-root.sh
index 365c787530c..61d278e2319 100755
--- a/rewatch/tests/gentype/01-gentype-project-root.sh
+++ b/rewatch/tests/gentype/01-gentype-project-root.sh
@@ -3,6 +3,6 @@ set -euo pipefail
 cd "$(dirname "$0")"
 source "../utils.sh"
 
-bold "Test: GenType resolves modules through a Windows short project path"
+bold "Test: GenType resolves modules through a non-canonical project path"
 node ./project-root.mjs
-success "GenType resolved the short project path"
+success "GenType project-root checks completed"
diff --git a/rewatch/tests/gentype/project-root.mjs b/rewatch/tests/gentype/project-root.mjs
index a3a1cf80a9e..86d905d9e6f 100644
--- a/rewatch/tests/gentype/project-root.mjs
+++ b/rewatch/tests/gentype/project-root.mjs
@@ -1,6 +1,6 @@
 import assert from "node:assert/strict";
 import {spawnSync} from "node:child_process";
-import {existsSync, mkdtempSync, mkdirSync, readFileSync, realpathSync, rmSync, writeFileSync} from "node:fs";
+import {existsSync, mkdtempSync, mkdirSync, readFileSync, realpathSync, rmSync, symlinkSync, writeFileSync} from "node:fs";
 import {tmpdir} from "node:os";
 import {join} from "node:path";
 
@@ -9,8 +9,8 @@ assert(executable, "REWATCH_EXECUTABLE must be set");
 
 function runBuild(buildDirectory) {
   if (process.platform !== "win32") {
-    return spawnSync(executable, ["build"], {
-      cwd: buildDirectory,
+    return spawnSync(executable, ["build", buildDirectory], {
+      cwd: tmpdir(),
       encoding: "utf8",
       env: process.env,
       timeout: 30_000,
@@ -47,7 +47,9 @@ function runBuild(buildDirectory) {
 
 function getBuildDirectory(projectDir) {
   if (process.platform !== "win32") {
-    return projectDir;
+    const symlink = `${projectDir}-link`;
+    symlinkSync(projectDir, symlink, "dir");
+    return symlink;
   }
 
   const expandedPath = realpathSync.native(projectDir);
@@ -65,11 +67,10 @@ function getBuildDirectory(projectDir) {
 
   const shortPath = result.stdout.trim();
   assert(shortPath, "Windows did not return a short path for the temporary project");
-  assert.notEqual(
-    shortPath.toLowerCase(),
-    expandedPath.toLowerCase(),
-    "Windows short and expanded project paths must differ for this regression",
-  );
+  if (shortPath.toLowerCase() === expandedPath.toLowerCase()) {
+    console.log("SKIP: Windows 8.3 short names are unavailable on this volume");
+    return null;
+  }
   return shortPath;
 }
 
@@ -97,15 +98,18 @@ try {
   );
 
   const buildDirectory = getBuildDirectory(projectDir);
-  assert(existsSync(buildDirectory), `Build directory does not exist: ${buildDirectory}`);
-  const result = runBuild(buildDirectory);
+  if (buildDirectory !== null) {
+    assert(existsSync(buildDirectory), `Build directory does not exist: ${buildDirectory}`);
+    const result = runBuild(buildDirectory);
 
-  assert.ifError(result.error);
-  assert.equal(result.status, 0, `${result.stdout}\n${result.stderr}`);
-  assert.match(
-    readFileSync(join(projectDir, "src-b", "nested", "B.gen.tsx"), "utf8"),
-    /\.\.\/\.\.\/src-a\/A\.gen/,
-  );
+    assert.ifError(result.error);
+    assert.equal(result.status, 0, `${result.stdout}\n${result.stderr}`);
+    assert.match(
+      readFileSync(join(projectDir, "src-b", "nested", "B.gen.tsx"), "utf8"),
+      /from '\.\.\/\.\.\/src-a\/A\.gen'/,
+    );
+  }
 } finally {
+  rmSync(`${projectDir}-link`, {force: true});
   rmSync(projectDir, {recursive: true, force: true});
 }

From 23fe47e59b9ebe56f6da0437c11841e4e12760e4 Mon Sep 17 00:00:00 2001
From: Florian Hammerschmidt <florianh89@gmail.com>
Date: Wed, 14 Oct 2026 23:27:18 +0200
Subject: [PATCH 10/10] Preserve GenType editor paths and test Windows root
 traversal

Signed-off-by: Florian Hammerschmidt <florianh89@gmail.com>
---
 compiler/gentype/module_resolver.ml      | 12 +++----
 rewatch/src/build.rs                     |  1 +
 rewatch/src/build/compile.rs             |  9 ++++-
 rewatch/src/config.rs                    | 44 -----------------------
 rewatch/tests/gentype/project-root.mjs   | 46 +++++++++++++++++++-----
 scripts/test.js                          | 24 ++++++++++++-
 tests/ounit_tests/ounit_gentype_tests.ml | 14 ++++++++
 7 files changed, 90 insertions(+), 60 deletions(-)

diff --git a/compiler/gentype/module_resolver.ml b/compiler/gentype/module_resolver.ml
index 01af1d499e0..b60ce5c8c52 100644
--- a/compiler/gentype/module_resolver.ml
+++ b/compiler/gentype/module_resolver.ml
@@ -109,6 +109,12 @@ let create_lazy_resolver ~config ~extensions ~exclude_file =
 let apply ~resolver ~use_bs_dependencies module_name =
   module_name |> Lazy.force resolver.lazy_find ~use_bs_dependencies
 
+let rec path_to_list path =
+  let parent = Filename.dirname path in
+  (* On Windows, the basename of a drive root omits the drive prefix. *)
+  if Filename.basename path = path || parent = path then [path]
+  else (path |> Filename.basename) :: path_to_list parent
+
 (** Resolve a reference to ModuleName, and produce a path suitable for require.
    E.g. require "../foo/bar/ModuleName.ext" where ext is ".res" or ".js". *)
 let resolve_module ~(config : Config.t) ~import_extension ~output_file_relative
@@ -132,12 +138,6 @@ let resolve_module ~(config : Config.t) ~import_extension ~output_file_relative
   in
   if Sys.file_exists module_name_res_file then candidate
   else
-    let rec path_to_list path =
-      let parent = Filename.dirname path in
-      (* On Windows, the basename of a drive root omits the drive prefix. *)
-      if Filename.basename path = path || parent = path then [path]
-      else (path |> Filename.basename) :: path_to_list parent
-    in
     match module_name |> apply ~resolver ~use_bs_dependencies with
     | None -> candidate
     | Some (resolved_module_dir, case, bs_dependencies) ->
diff --git a/rewatch/src/build.rs b/rewatch/src/build.rs
index 1db5dbc218a..87ae6e2656d 100644
--- a/rewatch/src/build.rs
+++ b/rewatch/src/build.rs
@@ -134,6 +134,7 @@ pub fn get_compiler_args(rescript_file_path: &Path) -> Result<String> {
     };
     let compiler_args = compile::compiler_args(
         &project_context.current_config,
+        &current_package,
         &ast_path,
         relative_filename,
         is_interface,
diff --git a/rewatch/src/build/compile.rs b/rewatch/src/build/compile.rs
index b7a64fa747b..c4677632cf1 100644
--- a/rewatch/src/build/compile.rs
+++ b/rewatch/src/build/compile.rs
@@ -751,6 +751,9 @@ pub fn get_runtime_path_args(
 
 pub fn compiler_args(
     config: &config::Config,
+    // Match the parser's root spelling: canonical during builds, caller-supplied
+    // for compiler-args consumers such as editor incremental type checking.
+    project_root: &Path,
     ast_path: &Path,
     file_path: &Path,
     is_interface: bool,
@@ -833,7 +836,10 @@ pub fn compiler_args(
     };
 
     let package_name_arg = vec!["-bs-package-name".to_string(), config.name.to_owned()];
-    let project_root_args = config.get_project_root_args();
+    let project_root_args = vec![
+        "-bs-project-root".to_string(),
+        project_root.to_string_lossy().to_string(),
+    ];
 
     let implementation_args = if is_interface {
         debug!("Compiling interface file: {}", &module_name);
@@ -1022,6 +1028,7 @@ fn compile_file(
     let current_package_dirs: &[PathBuf] = package.gentype_dirs.as_deref().unwrap_or(&[]);
     let to_mjs_args = compiler_args(
         &package.config,
+        &package.path,
         ast_path,
         implementation_file_path,
         is_interface,
diff --git a/rewatch/src/config.rs b/rewatch/src/config.rs
index 56e1c5ebb4f..fb0b329737b 100644
--- a/rewatch/src/config.rs
+++ b/rewatch/src/config.rs
@@ -1009,25 +1009,6 @@ impl Config {
         args
     }
 
-    /// Directory containing the `rescript.json` this config was parsed from.
-    fn get_package_root(&self) -> &Path {
-        self.path
-            .parent()
-            .expect("rescript.json path should always have a parent directory")
-    }
-
-    pub fn get_project_root_args(&self) -> Vec<String> {
-        // Package discovery gives the parser a canonical working directory.
-        // Use the same representation here so GenType can strip this prefix,
-        // including when Windows supplied the project through an 8.3 path.
-        let root = self
-            .get_package_root()
-            .canonicalize()
-            .map(helpers::StrippedVerbatimPath::to_stripped_verbatim_path)
-            .unwrap_or_else(|_| self.get_package_root().to_path_buf());
-        vec!["-bs-project-root".to_string(), root.to_string_lossy().to_string()]
-    }
-
     pub fn get_warning_args(&self, is_local_dep: bool, warn_error_override: Option<String>) -> Vec<String> {
         // Ignore warning config for non local dependencies (node_module dependencies)
         if !is_local_dep {
@@ -1592,31 +1573,6 @@ pub mod tests {
         assert!(args.contains(&"@teamwalnut/app".to_string()));
     }
 
-    #[test]
-    fn test_compiler_project_root_is_canonicalized() {
-        let temp_dir = tempfile::tempdir().unwrap();
-        let nested_dir = temp_dir.path().join("nested");
-        fs::create_dir(&nested_dir).unwrap();
-        let config_path = nested_dir.join("..").join("rescript.json");
-        fs::write(
-            temp_dir.path().join("rescript.json"),
-            r#"{"name":"test","gentypeconfig":{}}"#,
-        )
-        .unwrap();
-
-        let config = Config::new(&config_path).unwrap();
-        let expected_root = temp_dir
-            .path()
-            .canonicalize()
-            .map(helpers::StrippedVerbatimPath::to_stripped_verbatim_path)
-            .unwrap()
-            .to_string_lossy()
-            .to_string();
-
-        let project_root_args = config.get_project_root_args();
-        assert_eq!(project_root_args, ["-bs-project-root", &expected_root]);
-    }
-
     #[test]
     fn test_gentype_shims_object_and_array_forms() {
         let object_form = serde_json::from_str::<GenTypeShims>(r#"{"From": "To"}"#).unwrap();
diff --git a/rewatch/tests/gentype/project-root.mjs b/rewatch/tests/gentype/project-root.mjs
index 86d905d9e6f..6e8569df557 100644
--- a/rewatch/tests/gentype/project-root.mjs
+++ b/rewatch/tests/gentype/project-root.mjs
@@ -7,6 +7,14 @@ import {join} from "node:path";
 const executable = process.env.REWATCH_EXECUTABLE;
 assert(executable, "REWATCH_EXECUTABLE must be set");
 
+const executablePath = executable.endsWith(".exe") ? executable : `${executable}.exe`;
+const commandShim = executable.endsWith(".cmd") ? executable : `${executable}.cmd`;
+const resolvedExecutable = existsSync(executablePath)
+  ? executablePath
+  : existsSync(commandShim)
+    ? commandShim
+    : executable;
+
 function runBuild(buildDirectory) {
   if (process.platform !== "win32") {
     return spawnSync(executable, ["build", buildDirectory], {
@@ -17,14 +25,6 @@ function runBuild(buildDirectory) {
     });
   }
 
-  const executablePath = executable.endsWith(".exe") ? executable : `${executable}.exe`;
-  const commandShim = executable.endsWith(".cmd") ? executable : `${executable}.cmd`;
-  const resolvedExecutable = existsSync(executablePath)
-    ? executablePath
-    : existsSync(commandShim)
-      ? commandShim
-      : executable;
-
   return spawnSync(
     process.env.ComSpec ?? "cmd.exe",
     [
@@ -74,6 +74,35 @@ function getBuildDirectory(projectDir) {
   return shortPath;
 }
 
+function checkEditorCompilation(buildDirectory) {
+  const options = {encoding: "utf8", timeout: 30_000};
+  const result = spawnSync(
+    resolvedExecutable,
+    ["compiler-args", join(buildDirectory, "src-a", "A.res")],
+    {...options, shell: resolvedExecutable.endsWith(".cmd")},
+  );
+  assert.ifError(result.error);
+  assert.equal(result.status, 0, result.stderr);
+  const {compiler_args: args} = JSON.parse(result.stdout);
+  assert.equal(args[args.indexOf("-bs-project-root") + 1], buildDirectory);
+
+  // The editor compiles an absolute incremental source instead of the AST,
+  // and omits generated-JavaScript output while retaining the GenType flags.
+  args.splice(args.indexOf("-bs-package-output"), 2);
+  args.pop();
+  const incrementalDir = join(buildDirectory, "lib", "bs", "___incremental");
+  mkdirSync(incrementalDir);
+  const source = join(incrementalDir, "Editor.res");
+  writeFileSync(source, "@genType\nlet value = 42\n");
+  const compiled = spawnSync(process.env.RESCRIPT_BSC_EXE, [...args, source], {
+    ...options,
+    cwd: join(buildDirectory, "lib", "bs"),
+  });
+  assert.ifError(compiled.error);
+  assert.equal(compiled.status, 0, `${compiled.stdout}\n${compiled.stderr}`);
+  assert(existsSync(join(incrementalDir, "Editor.gen.tsx")));
+}
+
 const projectDir = mkdtempSync(join(tmpdir(), "rescript-gentype-root-"));
 
 try {
@@ -108,6 +137,7 @@ try {
       readFileSync(join(projectDir, "src-b", "nested", "B.gen.tsx"), "utf8"),
       /from '\.\.\/\.\.\/src-a\/A\.gen'/,
     );
+    checkEditorCompilation(buildDirectory);
   }
 } finally {
   rmSync(`${projectDir}-link`, {force: true});
diff --git a/scripts/test.js b/scripts/test.js
index e5e26839043..79ca4569583 100644
--- a/scripts/test.js
+++ b/scripts/test.js
@@ -65,7 +65,29 @@ if (formatTest) {
 
 if (ounitTest) {
   if (process.platform === "win32") {
-    console.log("Skipping OUnit tests on Windows");
+    // GenType's drive-root regression needs the native Windows Filename module.
+    const binary = path.join(
+      projectDir,
+      "_build/default/tests/ounit_tests/ounit_tests_main.exe",
+    );
+    const { stdout } = await execBin(binary, ["-list-test"], {
+      shell: false,
+      throwOnFail: true,
+    });
+    const tests = stdout
+      .split(/\r?\n/)
+      .filter(test => test.includes(":gentype:"));
+    if (tests.length === 0) {
+      throw new Error("No GenType unit tests found");
+    }
+    await execBin(
+      binary,
+      tests.flatMap(test => ["-only-test", test]),
+      {
+        stdio: "inherit",
+        shell: false,
+      },
+    );
   } else {
     await execBin(ounitTestBin, [], {
       stdio: "inherit",
diff --git a/tests/ounit_tests/ounit_gentype_tests.ml b/tests/ounit_tests/ounit_gentype_tests.ml
index 2b3e363efbc..eed2cb147e3 100644
--- a/tests/ounit_tests/ounit_gentype_tests.ml
+++ b/tests/ounit_tests/ounit_gentype_tests.ml
@@ -3,6 +3,20 @@ open OUnit
 let suites =
   "gentype"
   >::: [
+         ( "split relative module paths" >:: fun _ ->
+           assert_equal ["."] (Module_resolver.path_to_list ".");
+           assert_equal ["src"] (Module_resolver.path_to_list "src");
+           assert_equal ["nested"; "src"]
+             (Module_resolver.path_to_list (Filename.concat "src" "nested")) );
+         ( "stop at Windows drive roots" >:: fun _ ->
+           skip_if (not Sys.win32) "Windows drive roots require Windows";
+           List.iter
+             (fun root ->
+               assert_equal [root] (Module_resolver.path_to_list root);
+               assert_equal ["nested"; "src"; root]
+                 (Module_resolver.path_to_list
+                    (Filename.concat root (Filename.concat "src" "nested"))))
+             ["C:\\"; "D:\\"] );
          ( "escape semantic import paths" >:: fun _ ->
            let emit path =
              path |> Import_path.from_string_unsafe |> Import_path.emit