[no-implicit-copy] Emit an error diagnostic if we find a mark_must_check [no_implicit_copy] we don't know how to check.

We do the same thing already with the move function. If someone is using this
functionality and the compiler fails, we don't want to succeed the compilation
since the user is relying on the compiler for safety!

Author: gottesmm

include/swift/AST/DiagnosticsSIL.def

@@ -729,6 +729,9 @@ ERROR(sil_moveonlychecker_value_consumed_more_than_once, none,
       "'%0' consumed more than once", (StringRef))
 NOTE(sil_moveonlychecker_consuming_use_here, none,
      "consuming use", ())
+ERROR(sil_moveonlychecker_not_understand_mark_move, none,
+      "Usage of @noImplicitCopy that the move checker does not know how to "
+      "check!", ())
 
 // move kills copyable values checker diagnostics
 ERROR(sil_movekillscopyablevalue_value_consumed_more_than_once, none,

lib/SILOptimizer/Mandatory/MoveOnlyChecker.cpp

@@ -484,9 +484,13 @@ struct MoveOnlyChecker {
 
 bool MoveOnlyChecker::check(NonLocalAccessBlockAnalysis *accessBlockAnalysis,
                             DominanceInfo *domTree) {
+  bool changed = false;
+
   for (auto &block : *fn) {
-    for (auto &ii : block) {
-      auto *mmci = dyn_cast<MarkMustCheckInst>(&ii);
+    for (auto ii = block.begin(), ie = block.end(); ii != ie;) {
+      auto *mmci = dyn_cast<MarkMustCheckInst>(&*ii);
+      ++ii;
+
       if (!mmci || !mmci->isNoImplicitCopy())
         continue;
 
@@ -496,6 +500,7 @@ bool MoveOnlyChecker::check(NonLocalAccessBlockAnalysis *accessBlockAnalysis,
           if (auto *bbi = dyn_cast<BeginBorrowInst>(cvi->getOperand())) {
             if (bbi->isLexical()) {
               moveIntroducersToProcess.insert(mmci);
+              continue;
             }
           }
         }
@@ -505,9 +510,24 @@ bool MoveOnlyChecker::check(NonLocalAccessBlockAnalysis *accessBlockAnalysis,
         if (auto *bbi = dyn_cast<BeginBorrowInst>(cvi->getOperand())) {
           if (bbi->isLexical()) {
             moveIntroducersToProcess.insert(mmci);
+            continue;
           }
         }
       }
+
+      // If we see a mark_must_check that is marked no implicit copy that we
+      // don't understand, emit a diagnostic to fail the compilation. This
+      // ensures that if someone marks something no implicit copy and we fail to
+      // check it, we fail the compilation.
+      //
+      // We then RAUW the mark_must_check once we have emitted the error since
+      // later passes expect that mark_must_check has been eliminated by
+      // us. Since we are failing already, this is ok to do.
+      diagnose(fn->getASTContext(), mmci->getLoc().getSourceLoc(),
+               diag::sil_moveonlychecker_not_understand_mark_move);
+      mmci->replaceAllUsesWith(mmci->getOperand());
+      mmci->eraseFromParent();
+      changed = true;
     }
   }
 
@@ -518,7 +538,6 @@ bool MoveOnlyChecker::check(NonLocalAccessBlockAnalysis *accessBlockAnalysis,
         instToDelete->eraseFromParent();
       });
   InstructionDeleter deleter(std::move(callbacks));
-  bool changed = false;
 
   SmallVector<Operand *, 32> consumingUsesNeedingCopy;
   auto foundConsumingUseNeedingCopy = [&](Operand *use) {