[Exclusivity] Put suppression for free function swap() behind a flag

And leave suppression off by default for now. We'll use this to evaluate
how often swap() causes exclusivity conflicts to be reported.

include/swift/AST/SILOptions.h

@@ -143,6 +143,10 @@ class SILOptions {
   /// Emit compile-time diagnostics when the law of exclusivity is violated.
   bool EnforceExclusivityStatic = false;
 
+  /// Suppress static diagnostics for violations of exclusive access for calls
+  /// to the Standard Library's swap() free function.
+  bool SuppressStaticExclusivitySwap = false;
+
   /// Emit checks to trap at run time when the law of exclusivity is violated.
   bool EnforceExclusivityDynamic = false;
 

include/swift/Option/FrontendOptions.td

@@ -256,6 +256,9 @@ def disable_mandatory_semantic_arc_opts : Flag<["-"], "disable-mandatory-semanti
 def assume_parsing_unqualified_ownership_sil : Flag<["-"], "assume-parsing-unqualified-ownership-sil">,
   HelpText<"Assume unqualified SIL ownership when parsing SIL">;
 
+def suppress_static_exclusivity_swap : Flag<["-"], "suppress-static-exclusivity-swap">,
+  HelpText<"Suppress static violations of exclusive access with swap()">;
+
 def enable_sil_opaque_values : Flag<["-"], "enable-sil-opaque-values">,
   HelpText<"Enable SIL Opaque Values">;
 

lib/Frontend/CompilerInvocation.cpp

@@ -1324,6 +1324,8 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
     |= Args.hasArg(OPT_assume_parsing_unqualified_ownership_sil);
   Opts.EnableMandatorySemanticARCOpts |=
       !Args.hasArg(OPT_disable_mandatory_semantic_arc_opts);
+  Opts.SuppressStaticExclusivitySwap |=
+      Args.hasArg(OPT_suppress_static_exclusivity_swap);
 
   if (Args.hasArg(OPT_debug_on_sil)) {
     // Derive the name of the SIL file for debugging from

lib/SILOptimizer/Mandatory/DiagnoseStaticExclusivity.cpp

@@ -431,7 +431,8 @@ static void checkStaticExclusivity(SILFunction &Fn, PostOrderFunctionInfo *PO) {
       if (auto *AI = dyn_cast<ApplyInst>(&I)) {
         // Suppress for the arguments to the Standard Library's swap()
         // function until we can recommend a safe alternative.
-        if (isCallToStandardLibrarySwap(AI, Fn.getASTContext()))
+        if (Fn.getModule().getOptions().SuppressStaticExclusivitySwap &&
+            isCallToStandardLibrarySwap(AI, Fn.getASTContext()))
           CallsToSuppress.push_back(AI);
       }
 

test/SILOptimizer/exclusivity_static_diagnostics.swift

@@ -13,36 +13,10 @@ func simpleInoutDiagnostic() {
 }
 
 
-func swapSuppression(_ i: Int, _ j: Int) {
+func swapNoSuppression(_ i: Int, _ j: Int) {
   var a: [Int] = [1, 2, 3]
 
-  swap(&a[i], &a[j]) // no-warning
-
-  // expected-warning@+2{{modification requires exclusive access}}
-  // expected-note@+1{{conflicting modification requires exclusive access}}
-  takesTwoInouts(&a[i], &a[j])
-}
-
-func missedSwapSuppression(_ i: Int, _ j: Int) {
-  var a: [Int] = [1, 2, 3]
-
-  // We don't suppress when swap() is used as a value
-  let mySwap: (inout Int, inout Int) -> () = swap
-
   // expected-warning@+2{{modification requires exclusive access}}
   // expected-note@+1{{conflicting modification requires exclusive access}}
-  mySwap(&a[i], &a[j])
-}
-
-func dontSuppressUserSwap(_ i: Int, _ j: Int) {
-  var a: [Int] = [1, 2, 3]
-
-  // Don't suppress on user-defined swap.
-  func swap<T>(_ p1: inout T, _ p2: inout T) {
-    return (p1, p2) = (p2, p1)
-  }
-
-  // expected-warning@+2{{modification requires exclusive access}}
-  // expected-note@+1{{conflicting modification requires exclusive access}}
-  swap(&a[i], &a[j])
+  swap(&a[i], &a[j]) // no-warning
 }

test/SILOptimizer/exclusivity_suppress_swap.swift

@@ -0,0 +1,39 @@
+// RUN: %target-swift-frontend -enforce-exclusivity=checked -suppress-static-exclusivity-swap -emit-sil -primary-file %s -o /dev/null -verify
+
+import Swift
+
+func takesTwoInouts(_ p1: inout Int, _ p2: inout Int) { }
+
+func swapSuppression(_ i: Int, _ j: Int) {
+  var a: [Int] = [1, 2, 3]
+
+  swap(&a[i], &a[j]) // no-warning
+
+  // expected-warning@+2{{modification requires exclusive access}}
+  // expected-note@+1{{conflicting modification requires exclusive access}}
+  takesTwoInouts(&a[i], &a[j])
+}
+
+func missedSwapSuppression(_ i: Int, _ j: Int) {
+  var a: [Int] = [1, 2, 3]
+
+  // We don't suppress when swap() is used as a value
+  let mySwap: (inout Int, inout Int) -> () = swap
+
+  // expected-warning@+2{{modification requires exclusive access}}
+  // expected-note@+1{{conflicting modification requires exclusive access}}
+  mySwap(&a[i], &a[j])
+}
+
+func dontSuppressUserSwap(_ i: Int, _ j: Int) {
+  var a: [Int] = [1, 2, 3]
+
+  // Don't suppress on user-defined swap.
+  func swap<T>(_ p1: inout T, _ p2: inout T) {
+    return (p1, p2) = (p2, p1)
+  }
+
+  // expected-warning@+2{{modification requires exclusive access}}
+  // expected-note@+1{{conflicting modification requires exclusive access}}
+  swap(&a[i], &a[j])
+}