Improve MappingPattern rest location (#20407)

The `rest` attribute doesn't have any line or column information. Fall
back to the `MappingPattern` info.
Closes #20259

---------

Co-authored-by: Shantanu Jain <hauntsaninja@gmail.com>

Author: cdce8p

mypy/fastparse.py

@@ -1799,7 +1799,7 @@ def visit_MatchMapping(self, n: ast3.MatchMapping) -> MappingPattern:
         if n.rest is None:
             rest = None
         else:
-            rest = NameExpr(n.rest)
+            rest = self.set_line(NameExpr(n.rest), n)
 
         node = MappingPattern(keys, values, rest)
         return self.set_line(node, n)

test-data/unit/check-python310.test

@@ -3194,3 +3194,15 @@ def fun(val: str | None):
         case types.NoneType():
             reveal_type(val)  # N: Revealed type is "None"
 [builtins fixtures/tuple.pyi]
+
+[case testMatchMappingRestErrorLocation]
+rest: list[int]
+data: list[int]
+match data:
+    case [1, *rest]:
+        ...
+config: dict[str, object]
+match config:
+    case {"a": 1, **rest}:  # E: Incompatible types in capture pattern (pattern captures type "dict[str, object]", variable has type "list[int]")
+        ...
+[builtins fixtures/dict.pyi]