Builder: Warn if test file does not exist

src/bootstrap/test.rs

@@ -1126,7 +1126,19 @@ note: if you're sure you want to do this, please open an issue as to why. In the
                 Ok(path) => path,
                 Err(_) => p,
             })
-            .filter(|p| p.starts_with(suite_path) && (p.is_dir() || p.is_file()))
+            .filter(|p| p.starts_with(suite_path))
+            .filter(|p| {
+                let exists = p.is_dir() || p.is_file();
+                if !exists {
+                    if let Some(p) = p.to_str() {
+                        builder.info(&format!(
+                            "Warning: Skipping \"{}\": not a regular file or directory",
+                            p
+                        ));
+                    }
+                }
+                exists
+            })
             .filter_map(|p| {
                 // Since test suite paths are themselves directories, if we don't
                 // specify a directory or file, we'll get an empty string here