Add test for errors triggered on parts of decomposed index

Author: dtolnay

src/test/ui/tuple/index-invalid.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let _ = (((),),).1.0; //~ ERROR no field `1` on type `(((),),)`
+
+    let _ = (((),),).0.1; //~ ERROR no field `1` on type `((),)`
+
+    let _ = (((),),).000.000; //~ ERROR no field `000` on type `(((),),)`
+}

src/test/ui/tuple/index-invalid.stderr

@@ -0,0 +1,21 @@
+error[E0609]: no field `1` on type `(((),),)`
+  --> $DIR/index-invalid.rs:2:22
+   |
+LL |     let _ = (((),),).1.0;
+   |                      ^
+
+error[E0609]: no field `1` on type `((),)`
+  --> $DIR/index-invalid.rs:4:24
+   |
+LL |     let _ = (((),),).0.1;
+   |                        ^
+
+error[E0609]: no field `000` on type `(((),),)`
+  --> $DIR/index-invalid.rs:6:22
+   |
+LL |     let _ = (((),),).000.000;
+   |                      ^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0609`.