Get a better error when check_copies fails (#7457)

* Get a better error when check_copies fails

* Fix tests

utils/check_copies.py

@@ -96,7 +96,7 @@ def is_copy_consistent(filename, overwrite=False):
     """
     with open(filename, "r", encoding="utf-8") as f:
         lines = f.readlines()
-    found_diff = False
+    diffs = []
     line_index = 0
     # Not a foor loop cause `lines` is going to change (if `overwrite=True`).
     while line_index < len(lines):
@@ -140,30 +140,29 @@ def is_copy_consistent(filename, overwrite=False):
 
         # Test for a diff and act accordingly.
         if observed_code != theoretical_code:
-            found_diff = True
+            diffs.append([object_name, start_index])
             if overwrite:
                 lines = lines[:start_index] + [theoretical_code] + lines[line_index:]
                 line_index = start_index + 1
 
-    if overwrite and found_diff:
+    if overwrite and len(diffs) > 0:
         # Warn the user a file has been modified.
         print(f"Detected changes, rewriting {filename}.")
         with open(filename, "w", encoding="utf-8") as f:
             f.writelines(lines)
-    return not found_diff
+    return diffs
 
 
 def check_copies(overwrite: bool = False):
     all_files = glob.glob(os.path.join(TRANSFORMERS_PATH, "**/*.py"), recursive=True)
     diffs = []
     for filename in all_files:
-        consistent = is_copy_consistent(filename, overwrite)
-        if not consistent:
-            diffs.append(filename)
+        new_diffs = is_copy_consistent(filename, overwrite)
+        diffs += [f"- {filename}: copy does not match {d[0]} at line {d[1]}" for d in new_diffs]
     if not overwrite and len(diffs) > 0:
         diff = "\n".join(diffs)
         raise Exception(
-            "Found copy inconsistencies in the following files:\n"
+            "Found the follwing copy inconsistencies:\n"
             + diff
             + "\nRun `make fix-copies` or `python utils/check_copies --fix_and_overwrite` to fix them."
         )