Add option to show links to error code docs (once per code)

mypy/errors.py

@@ -22,6 +22,8 @@
 SHOW_NOTE_CODES: Final = {codes.ANNOTATION_UNCHECKED}
 allowed_duplicates: Final = ["@overload", "Got:", "Expected:"]
 
+BASE_RTD_URL: Final = "https://mypy.rtfd.io/en/stable/_refs.html#code"
+
 # Keep track of the original error code when the error code of a message is changed.
 # This is used to give notes about out-of-date "type: ignore" comments.
 original_error_codes: Final = {codes.LITERAL_REQ: codes.MISC, codes.TYPE_ABSTRACT: codes.MISC}
@@ -434,6 +436,34 @@ def report(
             target=self.current_target(),
         )
         self.add_error_info(info)
+        if (
+            self.options.show_error_code_links
+            and not self.options.hide_error_codes
+            and code is not None
+        ):
+            message = f"See {BASE_RTD_URL}-{code.code} for information about this error"
+            if offset:
+                message = " " * offset + message
+            info = ErrorInfo(
+                self.import_context(),
+                file,
+                self.current_module(),
+                type,
+                function,
+                line,
+                column,
+                end_line,
+                end_column,
+                "note",
+                message,
+                code,
+                blocker=False,
+                only_once=True,
+                allow_dups=False,
+                origin=(self.file, origin_span),
+                target=self.current_target(),
+            )
+            self.add_error_info(info)
 
     def _add_error_info(self, file: str, info: ErrorInfo) -> None:
         assert file not in self.flushed_files

mypy/main.py

@@ -885,6 +885,12 @@ def add_invertible_flag(
         help="Hide error codes in error messages",
         group=error_group,
     )
+    add_invertible_flag(
+        "--show-error-code-links",
+        default=False,
+        help="Show links to error code documentation",
+        group=error_group,
+    )
     add_invertible_flag(
         "--pretty",
         default=False,

mypy/options.py

@@ -309,6 +309,7 @@ def __init__(self) -> None:
         self.show_column_numbers: bool = False
         self.show_error_end: bool = False
         self.hide_error_codes = False
+        self.show_error_code_links = False
         # Use soft word wrap and show trimmed source snippets with error location markers.
         self.pretty = False
         self.dump_graph = False

test-data/unit/check-flags.test

@@ -2195,3 +2195,10 @@ cb(lambda x: a)  # OK
 
 fn = lambda x: a
 cb(fn)
+
+[case testShowErrorCodeLinks]
+# flags: --show-error-codes --show-error-code-links
+
+x: int = ""  # E: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment] \
+             # N: See https://mypy.rtfd.io/en/stable/_refs.html#code-assignment for information about this error
+y: int = ""  # E: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment]