use the actual line contents when processing physical lines

src/flake8/checker.py

@@ -581,7 +581,7 @@ class FileChecker:
             line_no = token[2][0]
             with self.processor.inside_multiline(line_number=line_no):
                 for line in self.processor.split_line(token):
-                    self.run_physical_checks(line + "\n")
+                    self.run_physical_checks(line)
 
 
 def _pool_init() -> None:

src/flake8/processor.py

@@ -233,8 +233,10 @@ class FileProcessor:
 
         This also auto-increments the line number for the caller.
         """
-        for line in token[1].split("\n")[:-1]:
+        # intentionally don't include the last line, that line will be
-            yield line
+        # terminated later by a future end-of-line
+        for line_no in range(token.start[0], token.end[0]):
+            yield self.lines[line_no - 1]
             self.line_number += 1
 
     def keyword_arguments_for(

tests/integration/test_plugins.py

@@ -160,3 +160,37 @@ extension =
     out, err = capsys.readouterr()
     assert out == f"{t_py}:1:1: ABC123 error\n"
     assert err == ""
+
+
+def yields_physical_line(physical_line):
+    yield 0, f"T001 {physical_line!r}"
+
+
+def test_physical_line_plugin_multiline_string(tmpdir, capsys):
+    cfg_s = f"""\
+[flake8:local-plugins]
+extension =
+    T = {yields_physical_line.__module__}:{yields_physical_line.__name__}
+"""
+
+    cfg = tmpdir.join("tox.ini")
+    cfg.write(cfg_s)
+
+    src = '''\
+x = "foo" + """
+bar
+"""
+'''
+    t_py = tmpdir.join("t.py")
+    t_py.write_binary(src.encode())
+
+    with tmpdir.as_cwd():
+        assert main(("t.py", "--config", str(cfg))) == 1
+
+    expected = '''\
+t.py:1:1: T001 'x = "foo" + """\\n'
+t.py:2:1: T001 'bar\\n'
+t.py:3:1: T001 '"""\\n'
+'''
+    out, err = capsys.readouterr()
+    assert out == expected

tests/unit/test_file_processor.py

@@ -256,30 +256,30 @@ def test_keyword_arguments_for_does_not_handle_attribute_errors(
         file_processor.keyword_arguments_for({"fake": True})
 
 
-@pytest.mark.parametrize(
+def test_processor_split_line(default_options):
-    "unsplit_line, expected_lines",
-    [
-        ("line", []),
-        ("line 1\n", ["line 1"]),
-        ("line 1\nline 2\n", ["line 1", "line 2"]),
-        ("line 1\n\nline 2\n", ["line 1", "", "line 2"]),
-    ],
-)
-def test_split_line(unsplit_line, expected_lines, default_options):
-    """Verify the token line splitting."""
     file_processor = processor.FileProcessor(
         "-",
         default_options,
         lines=[
-            "Line 1",
+            'x = """\n',
+            "contents\n",
+            '"""\n',
         ],
     )
-
+    token = tokenize.TokenInfo(
-    token = tokenize.TokenInfo(1, unsplit_line, (0, 0), (0, 0), "")
+        3,
-    actual_lines = list(file_processor.split_line(token))
+        '"""\ncontents\n"""',
-    assert expected_lines == actual_lines
+        (1, 4),
-
+        (3, 3),
-    assert len(actual_lines) == file_processor.line_number
+        'x = """\ncontents\n"""\n',
+    )
+    expected = [('x = """\n', 0), ("contents\n", 1)]
+    actual = [
+        (line, file_processor.line_number)
+        for line in file_processor.split_line(token)
+    ]
+    assert expected == actual
+    assert file_processor.line_number == 2
 
 
 def test_build_ast(default_options):