fix: Filter out directories that do not contain dylibs

This is an optimization to reduce the amount for paths that get added to
the dylib search path. This is especially important for Windows as it
has issues when PATH gets too long. With the new Cargo build-dir we
increased the number of paths being passed.

Author: ranger-ross

src/bootstrap/src/core/build_steps/tool.rs

@@ -1622,8 +1622,9 @@ impl Builder<'_> {
         //
         // Notably this munges the dynamic library lookup path to point to the
         // right location to run `compiler`.
-        let mut lib_paths: Vec<PathBuf> =
-            discover_out_dirs(self.cargo_out(compiler, Mode::ToolBootstrap, *host).join("build"));
+        let mut lib_paths: Vec<PathBuf> = discover_out_dirs_with_dylibs(
+            self.cargo_out(compiler, Mode::ToolBootstrap, *host).join("build"),
+        );
 
         // On MSVC a tool may invoke a C compiler (e.g., compiletest in run-make
         // mode) and that C compiler may need some extra PATH modification. Do
@@ -1653,18 +1654,21 @@ impl Builder<'_> {
 }
 
 /// Gets all of the `out` dirs in a given Cargo `build-dir/<profile>/build` dir.
-fn discover_out_dirs(dir: PathBuf) -> Vec<PathBuf> {
+fn discover_out_dirs_with_dylibs(dir: PathBuf) -> Vec<PathBuf> {
     if !dir.exists() {
         return Vec::new();
     }
-
     let read_dir = |path: &Path| path.read_dir().ok().into_iter().flatten().filter_map(Result::ok);
+    let has_dylib = |path: &Path| {
+        read_dir(path)
+            .any(|e| e.path().extension().is_some_and(|ext| ext == std::env::consts::DLL_EXTENSION))
+    };
     dir.read_dir()
         .unwrap_or_else(|e| panic!("Couldn't read {}: {}", dir.display(), e))
         .map(|e| e.unwrap())
         .flat_map(|e| read_dir(&e.path()))
         .flat_map(|e| read_dir(&e.path()))
         .map(|e| e.path())
-        .filter(|path| path.ends_with("out"))
+        .filter(|path| path.ends_with("out") && has_dylib(path))
         .collect::<Vec<_>>()
 }