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

[rriddle]

Summary

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

[jpienaar]

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

119

Is this change related? (I like the shorthand of ; if there is no body a la TableGen, but independently not sure if this is related to the code change).

[rriddle]

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

119

semi related, as these are native constraints that return results (which aren't supported). Seemed better to express on a constraint that actually supports results (just in case error verification order changed).

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

if (curToken.isString()) {
     consumeToken();
   } else if (isInline) {
     return emitError(name.getLoc(),
                      "external declarations must be declared in global scope");
   }
   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);
 }
 
 LogicalResult Parser::parseUserConstraintOrRewriteSignature(
     SmallVectorImpl<ast::VariableDecl *> &arguments,
     SmallVectorImpl<ast::VariableDecl *> &results,
     ast::DeclScope *&argumentScope, ast::Type &resultType) {
   // Parse the argument list of the decl.

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

 //===----------------------------------------------------------------------===//
 
 // CHECK: expected identifier constraint
 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 {};
 
 // -----
 
 // CHECK: expected `)` to end result list
 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>) {}
 
 // -----
 
 //===----------------------------------------------------------------------===//
 // Native Constraints
 //===----------------------------------------------------------------------===//
 
 Pattern {
   // CHECK: external declarations must be declared in global scope
   Constraint ExternalConstraint();
 }
 
 // -----
 
 // CHECK: expected `;` after native declaration
 Constraint Foo() [{}]
+
+// -----
+
+// CHECK: native Constraints currently do not support returning results
+Constraint Foo() -> Op;