support rustdoc test from stdin to rustc

src/librustc_driver/lib.rs

@@ -439,6 +439,15 @@ fn make_input(free_matches: &[String]) -> Option<(Input, Option<PathBuf>, Option
             } else {
                 None
             };
+            if let Ok(path) = env::var("UNSTABLE_RUSTDOC_TEST_PATH") {
+                let line = env::var("UNSTABLE_RUSTDOC_TEST_LINE").
+                            expect("when UNSTABLE_RUSTDOC_TEST_PATH is set \
+                                    UNSTABLE_RUSTDOC_TEST_LINE also needs to be set");
+                let line = isize::from_str_radix(&line, 10).
+                            expect("UNSTABLE_RUSTDOC_TEST_LINE needs to be an number");
+                let file_name = FileName::doc_test_source_code(PathBuf::from(path), line);
+                return Some((Input::Str { name: file_name, input: src }, None, err));
+            }
             Some((Input::Str { name: FileName::anon_source_code(&src), input: src },
                   None, err))
         } else {