dialects: (builtin) Fix printing and parsing of UnitAttr

tests/filecheck/parser-printer/builtin_attrs.mlir

@@ -8,6 +8,9 @@
 
   // CHECK: (index)
 
+  "func.func"() ({}) {function_type = () -> (), sym_name = "unit_attr_func", unitarray = [unit]} : () -> ()
+
+  // CHECK: "unitarray" = [unit]
 
   "func.func"() ({
     ^bb0(%arg0: i32, %arg1: i64, %arg2: i1):

xdsl/parser/attribute_parser.py

@@ -595,6 +595,7 @@ def _parse_optional_builtin_attr(self) -> Attribute | None:
             return BytesAttr(bytes_lit)
 
         attrs = (
+            self.parse_optional_unit_attr,
             self.parse_optional_builtin_int_or_float_attr,
             self._parse_optional_array_attr,
             self._parse_optional_symref_attr,
@@ -651,6 +652,22 @@ def _parse_strided_layout_attr(self, name: Span) -> Attribute:
         self._parse_token(Token.Kind.GREATER, "Expected '>' in end of stride attribute")
         return StridedLayoutAttr(strides, None if offset == "?" else offset)
 
+    def parse_optional_unit_attr(self) -> Attribute | None:
+        """
+        Parse a value of `unit` type.
+        unit-attribute ::= `unit`
+        """
+        if self._current_token.kind != Token.Kind.BARE_IDENT:
+            return None
+        name = self._current_token.span.text
+
+        # Unit attribute
+        if name == "unit":
+            self._consume_token()
+            return UnitAttr()
+
+        return None
+
     def _parse_optional_builtin_parametrized_attr(self) -> Attribute | None:
         if self._current_token.kind != Token.Kind.BARE_IDENT:
             return None

xdsl/printer.py

@@ -398,6 +398,7 @@ def print_bytes_literal(self, bytestring: bytes):
 
     def print_attribute(self, attribute: Attribute) -> None:
         if isinstance(attribute, UnitAttr):
+            self.print("unit")
             return
 
         if isinstance(attribute, LocationAttr):