Set doctest extension in platform-specific way

Author: workingjubilee

src/librustdoc/doctest.rs

@@ -311,11 +311,9 @@ fn run_test(
         make_test(test, Some(crate_name), lang_string.test_harness, opts, edition, Some(test_id));
 
     // Make sure we emit well-formed executable names for our platform.
-    #[cfg(windows)]
-    let out_name = "rust_out.exe";
-    #[cfg(not(windows))]
-    let out_name = "rust_out";
-    let output_file = outdir.path().join(out_name);
+    let mut output_file = outdir.path().join("rust_out");
+    output_file.set_extension(env::consts::EXE_EXTENSION);
+    let output_file = output_file;
 
     let rustc_binary = rustdoc_options
         .test_builder