Use rescript.json dir as working dir for js-post-build (rescript-lang#8195)

* Use rescript.json working dir.

* Add changelog

Author: nojaf

CHANGELOG.md

@@ -19,6 +19,7 @@
 - `Int.fromString` and `Float.fromString` use stricter number parsing and no longer uses an explicit radix argument, but instead supports parsing hexadecimal, binary and exponential notation.
 - Remove `external-stdlib` configuration option from `rescript.json`. This option was rarely used and is no longer supported.
 - `js-post-build` now passes the correct output file path based on `in-source` configuration: when `in-source: true`, the path next to the source file is passed; when `in-source: false`, the path in the `lib/<module>/` directory is passed. Additionally, stdout and stderr from the post-build command are now logged. https://github.com/rescript-lang/rescript/pull/8190
+- `js-post-build` command now runs in the directory containing the `rescript.json` where it is defined, instead of the unpredictable build invocation directory. This provides consistent behavior in monorepos. https://github.com/rescript-lang/rescript/pull/8195
 
 #### :eyeglasses: Spec Compliance
 

docs/docson/build-schema.json

@@ -125,7 +125,7 @@
       "properties": {
         "cmd": {
           "type": "string",
-          "description": "Shell command to run after each JS file is compiled. The absolute path to the output JS file is appended as an argument. The path respects the `in-source` setting."
+          "description": "Shell command to run after each JS file is compiled. The absolute path to the output JS file is appended as an argument. The path respects the `in-source` setting. The command runs in the directory containing the rescript.json where it is defined."
         }
       }
     },
@@ -454,7 +454,7 @@
     },
     "js-post-build": {
       "$ref": "#/definitions/js-post-build",
-      "description": "Post-processing hook. The build system will invoke `cmd <absolute-path-to-js-file>` after each JS file is compiled. The path respects the `in-source` setting: when true, the path is next to the source file; when false, the path is in the `lib/<module>/` directory. The command runs with the same working directory as the build process (typically the project root)."
+      "description": "Post-processing hook. The build system will invoke `cmd <absolute-path-to-js-file>` after each JS file is compiled. The path respects the `in-source` setting: when true, the path is next to the source file; when false, the path is in the `lib/<module>/` directory. The command runs in the directory containing the rescript.json where it is defined."
     },
     "package-specs": {
       "$ref": "#/definitions/package-specs",

rewatch/src/build/compile.rs

@@ -22,16 +22,27 @@ use std::sync::OnceLock;
 use std::time::SystemTime;
 
 /// Execute js-post-build command for a compiled JavaScript file.
-/// Unlike bsb which passes relative paths, rewatch passes absolute paths for clarity.
-fn execute_post_build_command(cmd: &str, js_file_path: &Path) -> Result<()> {
+/// The command runs in the directory containing the rescript.json that defines it.
+/// The absolute path to the JS file is passed as an argument.
+fn execute_post_build_command(cmd: &str, js_file_path: &Path, working_dir: &Path) -> Result<()> {
     let full_command = format!("{} {}", cmd, js_file_path.display());
 
-    debug!("Executing js-post-build: {}", full_command);
+    debug!(
+        "Executing js-post-build: {} (in {})",
+        full_command,
+        working_dir.display()
+    );
 
     let output = if cfg!(target_os = "windows") {
-        Command::new("cmd").args(["/C", &full_command]).output()
+        Command::new("cmd")
+            .args(["/C", &full_command])
+            .current_dir(working_dir)
+            .output()
     } else {
-        Command::new("sh").args(["-c", &full_command]).output()
+        Command::new("sh")
+            .args(["-c", &full_command])
+            .current_dir(working_dir)
+            .output()
     };
 
     match output {
@@ -886,7 +897,8 @@ fn compile_file(
 
                     if js_file.exists() {
                         // Fail the build if post-build command fails (matches bsb behavior with &&)
-                        execute_post_build_command(&js_post_build.cmd, &js_file)?;
+                        // Run in the package's directory (where rescript.json is defined)
+                        execute_post_build_command(&js_post_build.cmd, &js_file, &package.path)?;
                     }
                 }
             }