[PDLL] Properly error out on returning results from native constraints

PDL currently doesn't support result values from constraints, meaning we need
to error out until this is actually supported to avoid crashes.

Differential Revision: https://reviews.llvm.org/D119782

mlir/lib/Tools/PDLL/Parser/Parser.cpp

@@ -955,6 +955,12 @@ FailureOr<T *> Parser::parseUserNativeConstraintOrRewriteDecl(
   if (failed(parseToken(Token::semicolon,
                         "expected `;` after native declaration")))
     return failure();
+  // TODO: PDL should be able to support constraint results in certain
+  // situations, we should revise this.
+  if (std::is_same<ast::UserConstraintDecl, T>::value && !results.empty()) {
+    return emitError(
+        "native Constraints currently do not support returning results");
+  }
   return T::createNative(ctx, name, arguments, results, optCodeStr, resultType);
 }
 

mlir/test/mlir-pdll/Parser/constraint-failure.pdll

@@ -116,22 +116,22 @@ Constraint Foo() -> {}
 // -----
 
 // CHECK: cannot create a single-element tuple with an element label
-Constraint Foo() -> result: Value;
+Constraint Foo() -> result: Value {}
 
 // -----
 
 // CHECK: cannot create a single-element tuple with an element label
-Constraint Foo() -> (result: Value);
+Constraint Foo() -> (result: Value) {}
 
 // -----
 
 // CHECK: expected identifier constraint
-Constraint Foo() -> ();
+Constraint Foo() -> () {}
 
 // -----
 
 // CHECK: expected `:` before result constraint
-Constraint Foo() -> (result{};
+Constraint Foo() -> (result {};
 
 // -----
 
@@ -141,7 +141,7 @@ Constraint Foo() -> (Op{};
 // -----
 
 // CHECK: inline `Attr`, `Value`, and `ValueRange` type constraints are not permitted on arguments or results
-Constraint Foo() -> Value<type>){}
+Constraint Foo() -> Value<type>) {}
 
 // -----
 
@@ -158,3 +158,8 @@ Pattern {
 
 // CHECK: expected `;` after native declaration
 Constraint Foo() [{}]
+
+// -----
+
+// CHECK: native Constraints currently do not support returning results
+Constraint Foo() -> Op;