show up some extra info when t!() fails

src/bootstrap/lib.rs

@@ -1126,7 +1126,7 @@ impl Build {
         }
 
         let mut paths = Vec::new();
-        let contents = t!(fs::read(stamp));
+        let contents = t!(fs::read(stamp), &stamp);
         // This is the method we use for extracting paths from the stamp file passed to us. See
         // run_cargo for more information (in compile.rs).
         for part in contents.split(|b| *b == 0) {

src/build_helper/lib.rs

@@ -21,6 +21,13 @@ macro_rules! t {
             Err(e) => panic!("{} failed with {}", stringify!($e), e),
         }
     };
+    // it can show extra info in the second parameter
+    ($e:expr, $extra:expr) => {
+        match $e {
+            Ok(e) => e,
+            Err(e) => panic!("{} failed with {} ({:?})", stringify!($e), e, $extra),
+        }
+    };
 }
 
 // Because Cargo adds the compiler's dylib path to our library search path, llvm-config may