--lf now skips colletion of files without failed tests

Fix #5172

src/_pytest/cacheprovider.py

@@ -158,6 +158,33 @@ class LFPlugin(object):
         self.lastfailed = config.cache.get("cache/lastfailed", {})
         self._previously_failed_count = None
         self._report_status = None
+        self._skipped_files = 0  # count skipped files during collection due to --lf
+
+    def last_failed_paths(self):
+        """Returns a set with all Paths()s of the previously failed nodeids (cached).
+        """
+        result = getattr(self, "_last_failed_paths", None)
+        if result is None:
+            rootpath = Path(self.config.rootdir)
+            result = {rootpath / nodeid.split("::")[0] for nodeid in self.lastfailed}
+            self._last_failed_paths = result
+        return result
+
+    def pytest_ignore_collect(self, path):
+        """
+        Ignore this file path if we are in --lf mode and it is not in the list of
+        previously failed files.
+        """
+        if (
+            self.active
+            and self.config.getoption("lf")
+            and path.isfile()
+            and self.lastfailed
+        ):
+            skip_it = Path(path) not in self.last_failed_paths()
+            if skip_it:
+                self._skipped_files += 1
+            return skip_it
 
     def pytest_report_collectionfinish(self):
         if self.active and self.config.getoption("verbose") >= 0:
@@ -206,9 +233,19 @@ class LFPlugin(object):
                     items[:] = previously_failed + previously_passed
 
                 noun = "failure" if self._previously_failed_count == 1 else "failures"
+                if self._skipped_files > 0:
+                    files_noun = "file" if self._skipped_files == 1 else "files"
+                    skipped_files_msg = " (skipped {files} {files_noun})".format(
+                        files=self._skipped_files, files_noun=files_noun
+                    )
+                else:
+                    skipped_files_msg = ""
                 suffix = " first" if self.config.getoption("failedfirst") else ""
-                self._report_status = "rerun previous {count} {noun}{suffix}".format(
-                    count=self._previously_failed_count, suffix=suffix, noun=noun
+                self._report_status = "rerun previous {count} {noun}{suffix}{skipped_files}".format(
+                    count=self._previously_failed_count,
+                    suffix=suffix,
+                    noun=noun,
+                    skipped_files=skipped_files_msg,
                 )
         else:
             self._report_status = "no previously failed tests, "