Diagnose use of @_silgen_name and @abi together

Author: beccadax

include/swift/AST/DiagnosticsSema.def

@@ -8134,5 +8134,10 @@ ERROR(attr_abi_mismatched_var,none,
       "no match for %select{%kind0 in the ABI|ABI %kind0}1",
       (ValueDecl *, /*isABI=*/bool))
 
+ERROR(attr_abi_incompatible_with_silgen_name,none,
+      "cannot use '@_silgen_name' and '@abi' on the same %0 because they serve "
+      "the same purpose",
+      (DescriptiveDeclKind))
+
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

lib/Sema/TypeCheckAttr.cpp

@@ -2636,6 +2636,11 @@ void AttributeChecker::visitSILGenNameAttr(SILGenNameAttr *A) {
     diagnose(A->getLocation(), diag::reserved_runtime_symbol_name,
              A->Name);
   }
+
+  if (D->getAttrs().hasAttribute<ABIAttr>()) {
+    diagnoseAndRemoveAttr(A, diag::attr_abi_incompatible_with_silgen_name,
+                          D->getDescriptiveKind());
+  }
 }
 
 void AttributeChecker::visitUsedAttr(UsedAttr *attr) {

test/attr/attr_abi.swift

@@ -124,6 +124,14 @@ func async01() async {}
 @abi(func async11() async)
 func async11() async {}
 
+//
+// Miscellaneous function checking
+//
+
+@_silgen_name("conflictingAttrsSilgenName")
+@abi(func conflictingAttrsABI())
+func conflictingAttrsAPI() {} // expected-error@-2 {{cannot use '@_silgen_name' and '@abi' on the same global function because they serve the same purpose}} {{1-44=}}
+
 //
 // PBD shape checking
 //