Typecheck noasync attr

This patch adds validation to ensure that the noasync attribute is only
applied to useful declarations.
Specifically, the `noasync` attribute cannot be applied to `deinit`
declarations, asynchronous functions, or asynchronous properties.

Author: etcwilde

lib/Sema/TypeCheckAttr.cpp

@@ -1640,6 +1640,35 @@ void AttributeChecker::visitAvailableAttr(AvailableAttr *attr) {
     }
   }
 
+  if (attr->isNoAsync()) {
+    const DeclContext * dctx = dyn_cast<DeclContext>(D);
+    bool isAsyncDeclContext = dctx && dctx->isAsyncContext();
+
+    if (const AbstractStorageDecl *decl = dyn_cast<AbstractStorageDecl>(D)) {
+      const AccessorDecl * accessor = decl->getEffectfulGetAccessor();
+      isAsyncDeclContext |= accessor && accessor->isAsyncContext();
+    }
+
+    if (isAsyncDeclContext) {
+      if (const ValueDecl *vd = dyn_cast<ValueDecl>(D)) {
+        D->getASTContext().Diags.diagnose(
+            D->getLoc(), diag::async_named_decl_must_be_available_from_async,
+            D->getDescriptiveKind(), vd->getName());
+      } else {
+        D->getASTContext().Diags.diagnose(
+            D->getLoc(), diag::async_decl_must_be_available_from_async,
+            D->getDescriptiveKind());
+      }
+    }
+
+    // deinit's may not be unavailable from async contexts
+    if (isa<DestructorDecl>(D)) {
+      D->getASTContext().Diags.diagnose(
+          D->getLoc(), diag::invalid_decl_attribute, attr);
+    }
+
+  }
+
   if (!attr->hasPlatform() || !attr->isActivePlatform(Ctx) ||
       !attr->Introduced.hasValue()) {
     return;

test/attr/attr_availability_noasync.swift

@@ -39,3 +39,24 @@ func asyncFunc() async {
     // expected-error@+1{{global function 'readStringFromIO' is unavailable from asynchronous contexts}}{{13-29=IOActor.readString}}
     let _ = readStringFromIO()
 }
+
+// expected-error@+2{{asynchronous global function 'unavailableAsyncFunc()' must be available from asynchronous contexts}}
+@available(*, noasync)
+func unavailableAsyncFunc() async {
+}
+
+protocol BadSyncable {
+    // expected-error@+2{{asynchronous property 'isSyncd' must be available from asynchronous contexts}}
+    @available(*, noasync)
+    var isSyncd: Bool { get async }
+
+    // expected-error@+2{{asynchronous instance method 'sync' must be available from asynchronous contexts}}
+    @available(*, noasync)
+    func sync(_ str: String) async
+}
+
+class TestClass {
+    // expected-error@+2{{'@available' attribute cannot be applied to this declaration}}
+    @available(*, noasync)
+    deinit { }
+}