add an experimental feature DeferSendableChecking to defer the sendable checking of some sites. For now, only diagnostics corresponding to non-sendable arguments passed to calls with unsatisfied isolation are deferred. A SIL pass SendNonSendable is added to emit the deferred diagnostics, and ApplyExpr is appropriately enriched to make that deferral possible.

lib/SILOptimizer/Mandatory/SendNonSendable.cpp

@@ -0,0 +1,34 @@
+#include "../../Sema/TypeCheckConcurrency.h"
+#include "swift/AST/Expr.h"
+#include "swift/SILOptimizer/PassManager/Transforms.h"
+
+using namespace swift;
+
+class SendNonSendable : public SILFunctionTransform {
+
+  // find any ApplyExprs in this function, and check if any of them make an
+  // unsatisfied isolation jump, emitting appropriate diagnostics if so
+  void run() override {
+    SILFunction *function = getFunction();
+
+    if (!function->getASTContext().LangOpts
+        .hasFeature(Feature::DeferredSendableChecking))
+      return;
+
+    DeclContext *declContext = function->getDeclContext();
+
+    for (SILBasicBlock &bb : *function) {
+      for (SILInstruction &instr : bb) {
+        if (ApplyExpr *apply = instr.getLoc().getAsASTNode<ApplyExpr>()) {
+          diagnoseApplyArgSendability(apply, declContext);
+        }
+      }
+    }
+  }
+};
+
+/// This pass is known to depend on the following passes having run before it:
+/// none so far
+SILTransform *swift::createSendNonSendable() {
+  return new SendNonSendable();
+}