fix excessive gleam.mjs generation

This patch fixes excessive generation by checking for file existence.

An earlier attempt (gleam-lang#3238) had done this by checking for empty `modules`,
noting in its commit message that file existence checks will cause old
`gleam.mjs` to never be updated when a new version is available. I have
since learned that that was incorrect, as new versions of `gleam` delete
`build` folders created by previous versions.

closes gleam-lang#3178

CHANGELOG.md

@@ -78,6 +78,10 @@
   statements in a function when there is an error in a previous one.
   ([Ameen Radwan](https://github.com/Acepie))
 
+- Fixed a bug where the compiler would unnecessarily generate `gleam.mjs`,
+  confusing build tools like Vite.
+  ([Ofek Doitch](https://github.com/ofekd))
+
 ### Formatter
 
 ### Language Server

compiler-cli/src/fs.rs

@@ -181,6 +181,10 @@ impl FileSystemWriter for ProjectIO {
     fn write_bytes(&self, path: &Utf8Path, content: &[u8]) -> Result<(), Error> {
         write_bytes(path, content)
     }
+
+    fn exists(&self, path: &Utf8Path) -> bool {
+        path.exists()
+    }
 }
 
 impl CommandExecutor for ProjectIO {

compiler-core/src/codegen.rs

@@ -192,11 +192,22 @@ impl<'a> JavaScript<'a> {
 
     fn write_prelude(&self, writer: &impl FileSystemWriter) -> Result<()> {
         let rexport = format!("export * from \"{}\";\n", self.prelude_location);
-        writer.write(&self.output_directory.join("gleam.mjs"), &rexport)?;
+        let prelude_path = &self.output_directory.join("gleam.mjs");
+
+        // This check skips unnecessary `gleam.mjs` writes which confuse
+        // watchers and HMR build tools
+        if !writer.exists(prelude_path) {
+            writer.write(prelude_path, &rexport)?;
+        }
 
         if self.typescript == TypeScriptDeclarations::Emit {
             let rexport = rexport.replace(".mjs", ".d.mts");
-            writer.write(&self.output_directory.join("gleam.d.mts"), &rexport)?;
+            let prelude_declaration_path = &self.output_directory.join("gleam.d.mts");
+
+            // Type decleration may trigger badly configured watchers
+            if !writer.exists(prelude_declaration_path) {
+                writer.write(prelude_declaration_path, &rexport)?;
+            }
         }
 
         Ok(())

compiler-core/src/io.rs

@@ -236,6 +236,7 @@ pub trait FileSystemWriter {
     fn hardlink(&self, from: &Utf8Path, to: &Utf8Path) -> Result<(), Error>;
     fn symlink_dir(&self, from: &Utf8Path, to: &Utf8Path) -> Result<(), Error>;
     fn delete_file(&self, path: &Utf8Path) -> Result<(), Error>;
+    fn exists(&self, path: &Utf8Path) -> bool;
 }
 
 #[derive(Debug)]

compiler-core/src/io/memory.rs

@@ -127,6 +127,10 @@ impl FileSystemWriter for InMemoryFileSystem {
             .insert(path.to_path_buf(), file);
         Ok(())
     }
+
+    fn exists(&self, path: &Utf8Path) -> bool {
+        self.files.deref().borrow().contains_key(path)
+    }
 }
 
 impl FileSystemReader for InMemoryFileSystem {

compiler-core/src/language_server/files.rs

@@ -93,6 +93,10 @@ where
     fn delete_file(&self, path: &Utf8Path) -> Result<()> {
         self.io.delete_file(path)
     }
+
+    fn exists(&self, path: &Utf8Path) -> bool {
+        self.io.exists(path)
+    }
 }
 
 impl<IO> FileSystemReader for FileSystemProxy<IO>

compiler-wasm/src/wasm_filesystem.rs

@@ -70,6 +70,10 @@ impl FileSystemWriter for WasmFileSystem {
         tracing::trace!("write_bytes {:?}", path);
         self.imfs.write_bytes(path, content)
     }
+
+    fn exists(&self, path: &Utf8Path) -> bool {
+        self.imfs.exists(path)
+    }
 }
 
 impl FileSystemReader for WasmFileSystem {