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

@@ -73,6 +73,10 @@
 
   Else any functions which rely on this will not be compiled into Javascript.
 
+- 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-core/src/codegen.rs

@@ -192,11 +192,19 @@ 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");
+
+        if !prelude_path.exists() {
+            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");
+
+            if !prelude_declaration_path.exists() {
+                writer.write(prelude_declaration_path, &rexport)?;
+            }
         }
 
         Ok(())