Introduce the notion of a warning that is ignored by default, but enabled by -Wwarning/-Werror

As an example, use this for the "`@preconcurrency` on import has no
effect" warning, which is not yet working correctly. This disables it
by default but leaves it in place for our testing.

include/swift/AST/DiagnosticGroups.def

@@ -25,6 +25,7 @@ GROUP(no_group, "")
 GROUP(DeprecatedDeclaration, "DeprecatedDeclaration.md")
 GROUP(Unsafe, "Unsafe.md")
 GROUP(UnknownWarningGroup, "UnknownWarningGroup.md")
+GROUP(PreconcurrencyImport, "PreconcurrencyImport.md")
 
 #define UNDEFINE_DIAGNOSTIC_GROUPS_MACROS
 #include "swift/AST/DefineDiagnosticGroupsMacros.h"

include/swift/AST/DiagnosticsSema.def

@@ -2710,8 +2710,9 @@ WARNING(add_predates_concurrency_import,none,
      "add '@preconcurrency' to %select{suppress|treat}0 "
      "'Sendable'-related %select{warnings|errors}0 from module %1"
      "%select{| as warnings}0", (bool, Identifier))
-WARNING(remove_predates_concurrency_import,none,
-     "'@preconcurrency' attribute on module %0 has no effect", (Identifier))
+GROUPED_WARNING(remove_predates_concurrency_import,PreconcurrencyImport,
+                DefaultIgnore,
+                "'@preconcurrency' attribute on module %0 has no effect", (Identifier))
 NOTE(add_preconcurrency_to_conformance,none,
      "add '@preconcurrency' to the %0 conformance to defer isolation checking to run time", (DeclName))
 WARNING(remove_public_import,none,

lib/AST/DiagnosticEngine.cpp

@@ -74,6 +74,11 @@ enum class DiagnosticOptions {
 
   /// A diagnostic warning about an unused element.
   NoUsage,
+
+  /// The diagnostic should be ignored by default, but will be re-enabled
+  /// by various warning options (-Wwarning, -Werror). This only makes sense
+  /// for warnings.
+  DefaultIgnore,
 };
 struct StoredDiagnosticInfo {
   DiagnosticKind kind : 2;
@@ -82,15 +87,17 @@ struct StoredDiagnosticInfo {
   bool isAPIDigesterBreakage : 1;
   bool isDeprecation : 1;
   bool isNoUsage : 1;
+  bool defaultIgnore : 1;
   DiagGroupID groupID;
 
   constexpr StoredDiagnosticInfo(DiagnosticKind k, bool firstBadToken,
                                  bool fatal, bool isAPIDigesterBreakage,
                                  bool deprecation, bool noUsage,
-                                 DiagGroupID groupID)
+                                 bool defaultIgnore, DiagGroupID groupID)
       : kind(k), pointsToFirstBadToken(firstBadToken), isFatal(fatal),
         isAPIDigesterBreakage(isAPIDigesterBreakage),
-        isDeprecation(deprecation), isNoUsage(noUsage), groupID(groupID) {}
+        isDeprecation(deprecation), isNoUsage(noUsage),
+        defaultIgnore(defaultIgnore), groupID(groupID) {}
   constexpr StoredDiagnosticInfo(DiagnosticKind k, DiagnosticOptions opts,
                                  DiagGroupID groupID)
       : StoredDiagnosticInfo(k,
@@ -98,7 +105,9 @@ struct StoredDiagnosticInfo {
                              opts == DiagnosticOptions::Fatal,
                              opts == DiagnosticOptions::APIDigesterBreakage,
                              opts == DiagnosticOptions::Deprecation,
-                             opts == DiagnosticOptions::NoUsage, groupID) {}
+                             opts == DiagnosticOptions::NoUsage,
+                             opts == DiagnosticOptions::DefaultIgnore,
+                             groupID) {}
 };
 } // end anonymous namespace
 
@@ -168,8 +177,12 @@ static constexpr EducationalNotes<NumDiagIDs> _EducationalNotes =
 static constexpr auto educationalNotes = _EducationalNotes.value;
 
 DiagnosticState::DiagnosticState() {
-  // Initialize our ignored diagnostics to default
-  ignoredDiagnostics.resize(NumDiagIDs);
+  // Initialize our ignored diagnostics to defaults
+  ignoredDiagnostics.reserve(NumDiagIDs);
+  for (const auto &info : storedDiagnosticInfos) {
+    ignoredDiagnostics.push_back(info.defaultIgnore);
+  }
+
   // Initialize warningsAsErrors to default
   warningsAsErrors.resize(DiagGroupsCount);
 }
@@ -547,6 +560,9 @@ void DiagnosticEngine::setWarningsAsErrorsRules(
           groupID && *groupID != DiagGroupID::no_group) {
         getDiagGroupInfoByID(*groupID).traverseDepthFirst([&](auto group) {
           state.setWarningsAsErrorsForDiagGroupID(*groupID, isEnabled);
+          for (DiagID diagID : group.diagnostics) {
+            state.setIgnoredDiagnostic(diagID, false);
+          }
         });
       } else {
         unknownGroups.push_back(std::string(name));

test/Concurrency/predates_concurrency_import.swift

@@ -3,9 +3,9 @@
 // RUN: %target-swift-frontend -emit-module -emit-module-path %t/NonStrictModule.swiftmodule -module-name NonStrictModule %S/Inputs/NonStrictModule.swift
 // RUN: %target-swift-frontend -emit-module -emit-module-path %t/OtherActors.swiftmodule -module-name OtherActors %S/Inputs/OtherActors.swift -target %target-swift-5.1-abi-triple
 
-// RUN: %target-swift-frontend  -I %t %s -emit-sil -o /dev/null -verify  -parse-as-library -enable-upcoming-feature GlobalConcurrency
-// RUN: %target-swift-frontend  -I %t %s -emit-sil -o /dev/null -verify -strict-concurrency=targeted -parse-as-library -enable-upcoming-feature GlobalConcurrency
-// RUN: %target-swift-frontend  -I %t %s -emit-sil -o /dev/null -verify -strict-concurrency=complete  -parse-as-library -enable-upcoming-feature GlobalConcurrency
+// RUN: %target-swift-frontend  -I %t %s -emit-sil -o /dev/null -verify  -parse-as-library -enable-upcoming-feature GlobalConcurrency -Wwarning PreconcurrencyImport
+// RUN: %target-swift-frontend  -I %t %s -emit-sil -o /dev/null -verify -strict-concurrency=targeted -parse-as-library -enable-upcoming-feature GlobalConcurrency -Wwarning PreconcurrencyImport
+// RUN: %target-swift-frontend  -I %t %s -emit-sil -o /dev/null -verify -strict-concurrency=complete  -parse-as-library -enable-upcoming-feature GlobalConcurrency -Wwarning PreconcurrencyImport
 
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_GlobalConcurrency

test/Concurrency/predates_concurrency_import_nowarn.swift

@@ -0,0 +1,9 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -emit-module -emit-module-path %t/OtherActors.swiftmodule -module-name OtherActors %S/Inputs/OtherActors.swift -target %target-swift-5.1-abi-triple
+
+// RUN: %target-swift-frontend  -I %t %s -emit-sil -o /dev/null -verify  -parse-as-library
+
+// REQUIRES: concurrency
+
+@preconcurrency import OtherActors

userdocs/diagnostic_groups/PreconcurrencyImport.md

@@ -0,0 +1,6 @@
+# Extraneous @preconcurrency imports
+
+
+This diagnostic group includes warnings that diagnose `@preconcurrency import`
+declarations that don't need `@preconcurrency`. It is an experimental warning
+that is currently disabled.