Sema: Suppress explicit availability diagnostics on non-Darwin targets.

Suggesting the addition of an `@available` attribute on Linux and Windows is
inapprorpriate given that Swift is not ABI stable on those platforms.

Resolves rdar://107387133

include/swift/AST/ASTContext.h

@@ -970,6 +970,9 @@ public:
   /// Swift compiler for future versions of the target platform.
   AvailabilityContext getSwiftFutureAvailability();
 
+  /// Returns `true` if versioned availability annotations are supported for the
+  /// target triple.
+  bool supportsVersionedAvailability() const;
 
   //===--------------------------------------------------------------------===//
   // Diagnostics Helper functions

lib/AST/Availability.cpp

@@ -22,6 +22,7 @@
 #include "swift/AST/TypeCheckRequests.h"
 #include "swift/AST/TypeWalker.h"
 #include "swift/AST/Types.h"
+#include "swift/Basic/Platform.h"
 #include <map>
 
 using namespace swift;
@@ -706,3 +707,7 @@ ASTContext::getSwift5PlusAvailability(llvm::VersionTuple swiftVersion) {
       Twine("Missing call to getSwiftXYAvailability for Swift ") +
       swiftVersion.getAsString());
 }
+
+bool ASTContext::supportsVersionedAvailability() const {
+  return minimumAvailableOSVersionForTriple(LangOpts.Target).has_value();
+}

lib/Sema/TypeCheckAvailability.cpp

@@ -4260,6 +4260,13 @@ swift::diagnoseSubstitutionMapAvailability(SourceLoc loc,
 /// Should we warn that \p decl needs an explicit availability annotation
 /// in -require-explicit-availability mode?
 static bool declNeedsExplicitAvailability(const Decl *decl) {
+  auto &ctx = decl->getASTContext();
+
+  // Don't require an introduced version on platforms that don't support
+  // versioned availability.
+  if (!ctx.supportsVersionedAvailability())
+    return false;
+
   // Skip non-public decls.
   if (auto valueDecl = dyn_cast<const ValueDecl>(decl)) {
     AccessScope scope =
@@ -4280,7 +4287,6 @@ static bool declNeedsExplicitAvailability(const Decl *decl) {
     return false;
 
   // Warn on decls without an introduction version.
-  auto &ctx = decl->getASTContext();
   auto safeRangeUnderApprox = AvailabilityInference::availableRange(decl, ctx);
   return !safeRangeUnderApprox.getOSVersion().hasLowerEndpoint();
 }
@@ -4288,9 +4294,9 @@ static bool declNeedsExplicitAvailability(const Decl *decl) {
 void swift::checkExplicitAvailability(Decl *decl) {
   // Skip if the command line option was not set and
   // accessors as we check the pattern binding decl instead.
-  auto DiagLevel = decl->getASTContext().LangOpts.RequireExplicitAvailability;
-  if (!DiagLevel ||
-      isa<AccessorDecl>(decl))
+  auto &ctx = decl->getASTContext();
+  auto DiagLevel = ctx.LangOpts.RequireExplicitAvailability;
+  if (!DiagLevel || isa<AccessorDecl>(decl))
     return;
 
   // Only look at decls at module level or in extensions.
@@ -4335,8 +4341,7 @@ void swift::checkExplicitAvailability(Decl *decl) {
     auto diag = decl->diagnose(diag::public_decl_needs_availability);
     diag.limitBehavior(*DiagLevel);
 
-    auto suggestPlatform =
-      decl->getASTContext().LangOpts.RequireExplicitAvailabilityTarget;
+    auto suggestPlatform = ctx.LangOpts.RequireExplicitAvailabilityTarget;
     if (!suggestPlatform.empty()) {
       auto InsertLoc = decl->getAttrs().getStartLoc(/*forModifiers=*/false);
       if (InsertLoc.isInvalid())
@@ -4349,7 +4354,6 @@ void swift::checkExplicitAvailability(Decl *decl) {
       {
          llvm::raw_string_ostream Out(AttrText);
 
-         auto &ctx = decl->getASTContext();
          StringRef OriginalIndent = Lexer::getIndentationForLine(
            ctx.SourceMgr, InsertLoc);
          Out << "@available(" << suggestPlatform << ", *)\n"

test/SPI/spi-only-and-library-level.swift

@@ -7,7 +7,8 @@
 // RUN: %target-swift-frontend -emit-module %t/APILib.swift -I %t \
 // RUN:   -swift-version 5 -verify \
 // RUN:   -experimental-spi-only-imports \
-// RUN:   -library-level api
+// RUN:   -library-level api \
+// RUN:   -require-explicit-availability=ignore
 // RUN: %target-swift-frontend -emit-module %t/SPILib.swift -I %t \
 // RUN:   -swift-version 5 -verify \
 // RUN:   -experimental-spi-only-imports \
@@ -25,7 +26,6 @@ public struct LibStruct {}
 @_spiOnly import Lib
 
 public func publicClient() -> LibStruct { fatalError() } // expected-error {{cannot use struct 'LibStruct' here; 'Lib' was imported for SPI only}}
-// expected-warning @-1 {{public declarations should have an availability attribute with an introduction version}}
 @_spi(X) public func spiClient() -> LibStruct { fatalError() }
 
 //--- SPILib.swift

test/attr/require_explicit_availability_non_darwin.swift

@@ -0,0 +1,22 @@
+// Test that the -require-explicit-availability flag does not cause diagnostics
+// to be emitted on platforms where versioned availability annotations are not
+// meaningful.
+
+// RUN: %target-swift-frontend -typecheck -parse-as-library -verify %s -require-explicit-availability=error
+
+// Currently versioned availability should only be required on Apple platforms.
+// UNSUPPORTED: VENDOR=apple
+
+public struct NoAvailabilityStruct {
+  public func method() { }
+}
+
+@available(*, unavailable)
+public struct UnavailableStruct {
+  public func okMethod() { }
+}
+
+public func noAvailabilityFunc() { }
+
+@available(SwiftStdlib 5.9, *)
+public func stdlib5_9AvailabilityFunc() { }