Sema: Introduce an ExportabilityReason for availability attributes.

This allows diagnostics to be more precise and will also support logic that
allows for special cases for `@available` attributes in exportability checking.

Also fixes a bug where the exportability of `@available` attributes attached to
extensions were diagnosed twice for slightly differing reasons.

Author: tshortli

include/swift/AST/DiagnosticsSema.def

@@ -3826,7 +3826,8 @@ ERROR(decl_from_hidden_module,none,
       "as result builder here|"
       "in an extension with public or '@usableFromInline' members|"
       "in an extension with conditional conformances|"
-      "in a public or '@usableFromInline' conformance}1; "
+      "in a public or '@usableFromInline' conformance|"
+      "in an '@available' attribute here}1; "
       "%select{%2 has been imported as implementation-only|"
       "it is an SPI imported from %2|"
       "it is SPI|"
@@ -3841,7 +3842,8 @@ ERROR(typealias_desugars_to_type_from_hidden_module,none,
       "as result builder here|"
       "in an extension with public or '@usableFromInline' members|"
       "in an extension with conditional conformance|"
-      "in a public or '@usableFromInline' conformance}3 "
+      "in a public or '@usableFromInline' conformance|"
+      "<<ERROR>>}3 "
       "because %select{%4 has been imported as implementation-only|"
       "it is an SPI imported from %4|"
       "<<ERROR>>|"
@@ -3855,7 +3857,7 @@ ERROR(conformance_from_implementation_only_module,none,
       "as result builder here|"
       "in an extension with public or '@usableFromInline' members|"
       "in an extension with conditional conformances|"
-      "<<ERROR>>}2; "
+      "<<ERROR>>|<<ERROR>>}2; "
       "%select{%3 has been imported as implementation-only|"
       "the conformance is declared as SPI in %3|"
       "the conformance is declared as SPI|"

lib/Sema/ResilienceDiagnostics.cpp

@@ -257,6 +257,13 @@ static bool diagnoseValueDeclRefExportability(SourceLoc loc, const ValueDecl *D,
           return true;
         case ExportabilityReason::Inheritance:
           return isa<ProtocolDecl>(D);
+        case ExportabilityReason::AvailableAttribute:
+          // If the context is an extension and that extension has an explicit
+          // access level, then access has already been diagnosed for the
+          // @available attribute.
+          if (auto *ED = dyn_cast_or_null<ExtensionDecl>(DC->getAsDecl()))
+            return !ED->getAttrs().getAttribute<AccessControlAttr>();
+          return false;
         default:
           return false;
       }

lib/Sema/TypeCheckAccess.cpp

@@ -2261,11 +2261,13 @@ class DeclAvailabilityChecker : public DeclVisitor<DeclAvailabilityChecker> {
 
   void checkAvailabilityDomains(const Decl *D) {
     D = D->getAbstractSyntaxDeclForAttributes();
+
+    auto where = Where.withReason(ExportabilityReason::AvailableAttribute);
     for (auto attr : D->getSemanticAvailableAttrs()) {
       if (auto *domainDecl = attr.getDomain().getDecl()) {
         diagnoseDeclAvailability(domainDecl,
                                  attr.getParsedAttr()->getDomainLoc(), nullptr,
-                                 Where, std::nullopt);
+                                 where, std::nullopt);
       }
     }
   }
@@ -2557,18 +2559,6 @@ class DeclAvailabilityChecker : public DeclVisitor<DeclAvailabilityChecker> {
             : ExportabilityReason::ExtensionWithConditionalConformances;
     checkConstrainedExtensionRequirements(ED, reason);
 
-    // Diagnose the exportability of the availability domains referenced by the
-    // @available attributes attached to the extension.
-    if (Where.isExported()) {
-      for (auto availableAttr : ED->getSemanticAvailableAttrs()) {
-        if (auto *domainDecl = availableAttr.getDomain().getDecl()) {
-          TypeChecker::diagnoseDeclRefExportability(
-              availableAttr.getParsedAttr()->getDomainLoc(), domainDecl,
-              Where.withReason(reason));
-        }
-      }
-    }
-
     // If we haven't already visited the extended nominal visit it here.
     // This logic is too wide but prevents false reports of an unused public
     // import. We should instead check for public generic requirements

lib/Sema/TypeCheckAvailability.h

@@ -77,7 +77,8 @@ enum class ExportabilityReason : unsigned {
   ResultBuilder,
   ExtensionWithPublicMembers,
   ExtensionWithConditionalConformances,
-  Inheritance
+  Inheritance,
+  AvailableAttribute,
 };
 
 /// A description of the restrictions on what declarations can be referenced