[clang] [llvm] Reland "[Utils] Add new --update-tests flag to llvm-lit" (PR #153821)

[Min-Yih Hsu via cfe-commits]

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@@ -0,0 +1,39 @@
+import shutil
+
+
+def get_source_and_target(a, b):
+    """
+    Try to figure out which file is the test output and which is the reference.
+    """
+    expected_suffix = ".expected"
+    if a.endswith(expected_suffix) and not b.endswith(expected_suffix):
+        return b, a
+    if b.endswith(expected_suffix) and not a.endswith(expected_suffix):
+        return a, b
+
+    tmp_substr = ".tmp"
+    if tmp_substr in a and not tmp_substr in b:
+        return a, b
+    if tmp_substr in b and not tmp_substr in a:
+        return b, a
+
+    return None
+
+
+def filter_flags(args):
+    return [arg for arg in args if not arg.startswith("-")]
+
+
+def diff_test_updater(result, test):
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mshockwave wrote:

I think it's clever to catch failed diff commands and relay it to this plugin (which update the expected output file), but it wasn't so straightforward to me at first -- perhaps adding some comments at the beginning of this file?

https://github.com/llvm/llvm-project/pull/153821