Add RST220 for redefining an anonymous link

Closes #94.

Author: peterjc

README.rst

@@ -109,6 +109,7 @@ RST216 Multiple roles in interpreted text (both prefix and suffix present; only
 RST217 Mismatch: both interpreted text role suffix and reference suffix.
 RST218 Literal block expected; none found.
 RST219 Inline substitution_reference start-string without end-string.
+RST220 
 RST299 Previously unseen warning, not yet assigned a unique code.
 ====== =======================================================================
 
@@ -245,7 +246,8 @@ Version History
 ======= ========== ===========================================================
 Version Released   Changes
 ------- ---------- -----------------------------------------------------------
-v0.3.1  *Pending*  - Requires Python 3.8 or later (no code changes).
+v0.3.1  *Pending*  - Adds ``RST220`` for redefined anonymous links.
+                   - Requires Python 3.8 or later (no code changes).
 v0.3.0  2022-11-16 - Replaced ``setup.py`` with ``pyproject.toml``.
 v0.2.7  2022-07-15 - Fix where function signature occurred in docstring body.
 v0.2.6  2022-06-07 - Configuration option to define additional substitutions

flake8_rst_docstrings.py

@@ -63,6 +63,7 @@
     "Mismatch: both interpreted text role suffix and reference suffix.": 17,
     "Literal block expected; none found.": 18,
     "Inline substitution_reference start-string without end-string.": 19,
+    'Duplicate explicit target name: "*".': 20,
 }
 
 # Level 3 - error
@@ -224,11 +225,12 @@ def run(self):
                             - ast.get_docstring(node, clean=False).count("\n")
                             - 1
                         )
-                    assert (
-                        node.body[0].lineno >= 1 and start >= 0
-                    ), "Bad start line, node line number %i for: %s\n" % (
-                        node.body[0].lineno,
-                        docstring,
+                    assert node.body[0].lineno >= 1 and start >= 0, (
+                        "Bad start line, node line number %i for: %s\n"
+                        % (
+                            node.body[0].lineno,
+                            docstring,
+                        )
                     )
                 for rst_error in rst_errors:
                     # TODO - make this a configuration option?

tests/RST220/dup_explicit_target.py

@@ -0,0 +1,25 @@
+"""Print 'Hello world' to the terminal.
+
+It is common in RST to use unnamed or anonymous links in text (like
+`here <https://example.com/t-and-c/>`__ and `here
+<https://example.org/co-op-rules.html>`__), but they should have two trailing
+underscores!
+
+You *can* use a `single trailing underscore <https://example.com/links.html>`_
+but it is treated as an implicitly named link. You can use the exact same link
+text and URL again too (`single trailing underscore
+<https://example.com/links.html>`_).
+
+However, the missing second underscore becomes a problem if the same link text
+is used again for another implicitly named link weth a different URL (e.g.
+missing `single trailing underscore <https://example.com/bad.html>`_ *again*).
+
+What happens here?
+
+    $ flake8 --select RST RST220/dup_explicit_target.py
+    RST220/dup_explicit_target.py:15:1: RST220 Duplicate explicit target name: "single trailing underscore".
+
+This file triggers an error when the implicitly named link is redefined.
+"""
+
+print("Hello world")