Use an unavailable extension to suppress "missing explicit Sendable" warning

Becca noticed that one can already write an unavailable `Sendable`
conformance in language:

    @available(*, unavailable)
    extension MyType: Sendable { ... }

Add tests to verify that this suppresses the "missing Sendable
annotation" warning (it does) and suggest this spelling in a Fix-It to
help users find it.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -756,8 +756,6 @@ void swift::diagnoseMissingExplicitSendable(NominalTypeDecl *nominal) {
         /*treatUsableFromInlineAsPublic=*/true).isPublic())
     return;
 
-  // FIXME: Check for explicit "do not conform to Sendable" marker.
-
   // Look for any conformance to `Sendable`.
   auto proto = ctx.getProtocol(KnownProtocolKind::Sendable);
   if (!proto)
@@ -797,11 +795,16 @@ void swift::diagnoseMissingExplicitSendable(NominalTypeDecl *nominal) {
   }
 
   // Note to disable the warning.
-  nominal->diagnose(
-      diag::explicit_disable_sendable, nominal->getDescriptiveKind(),
-      nominal->getName())
-    .fixItInsert(nominal->getAttributeInsertionLoc(/*forModifier=*/false),
-                 "@_nonSendable ");
+  {
+    auto note = nominal->diagnose(
+        diag::explicit_disable_sendable, nominal->getDescriptiveKind(),
+        nominal->getName());
+    auto insertionLoc = nominal->getBraces().End;
+    note.fixItInsertAfter(
+        insertionLoc,
+        ("\n\n@available(*, unavailable)\nextension " + nominal->getName().str()
+         + ": Sendable { }\n").str());
+  }
 }
 
 /// Determine whether this is the main actor type.

test/Concurrency/require-explicit-sendable.swift

@@ -0,0 +1,56 @@
+// RUN: %target-typecheck-verify-swift -require-explicit-sendable
+
+public protocol P { }
+
+public struct S1 { // expected-warning{{public struct 'S1' does not specify whether it is 'Sendable' or not}}
+  // expected-note@-1{{consider making struct 'S1' conform to the 'Sendable' protocol}}{{18-18=: Sendable}}
+  // expected-note@-2{{make struct 'S1' explicitly non-Sendable to suppress this warning}}{{2-2=\n\n@available(*, unavailable)\nextension S1: Sendable { \}\n}}
+  var str: String
+}
+
+class C { }
+
+public struct S2 { // expected-warning{{public struct 'S2' does not specify whether it is 'Sendable' or not}}
+  // expected-note@-1{{add '@unchecked Sendable' conformance to struct 'S2' if this type manually implements concurrency safety}}{{18-18=: @unchecked Sendable}}
+  // expected-note@-2{{make struct 'S2' explicitly non-Sendable to suppress this warning}}{{2-2=\n\n@available(*, unavailable)\nextension S2: Sendable { \}\n}}
+  var c: C
+}
+
+final public class C1: P { // expected-warning{{public class 'C1' does not specify whether it is 'Sendable' or not}}
+  // expected-note@-1{{consider making class 'C1' conform to the 'Sendable' protocol}}{{25-25=, Sendable}}
+  // expected-note@-2{{make class 'C1' explicitly non-Sendable to suppress this warning}}{{2-2=\n\n@available(*, unavailable)\nextension C1: Sendable { \}\n}}
+  let str: String = ""
+}
+
+public class C2 { // expected-warning{{public class 'C2' does not specify whether it is 'Sendable' or not}}
+  // expected-note@-1{{add '@unchecked Sendable' conformance to class 'C2' if this type manually implements concurrency safety}}{{17-17=: @unchecked Sendable}}
+  // expected-note@-2{{make class 'C2' explicitly non-Sendable to suppress this warning}}{{2-2=\n\n@available(*, unavailable)\nextension C2: Sendable { \}\n}}
+  var str: String = ""
+}
+
+public struct S3<T> { // expected-warning{{public generic struct 'S3' does not specify whether it is 'Sendable' or not}}
+  // expected-note@-1{{add '@unchecked Sendable' conformance to generic struct 'S3' if this type manually implements concurrency safety}}{{21-21=: @unchecked Sendable}}
+  // expected-note@-2{{make generic struct 'S3' explicitly non-Sendable to suppress this warning}}{{2-2=\n\n@available(*, unavailable)\nextension S3: Sendable { \}\n}}
+  var t: T
+}
+
+public struct S4<T> { // expected-warning{{public generic struct 'S4' does not specify whether it is 'Sendable' or not}}
+  // expected-note@-1{{add '@unchecked Sendable' conformance to generic struct 'S4' if this type manually implements concurrency safety}}{{21-21=: @unchecked Sendable}}
+  // expected-note@-2{{make generic struct 'S4' explicitly non-Sendable to suppress this warning}}{{2-2=\n\n@available(*, unavailable)\nextension S4: Sendable { \}\n}}
+  var t: T
+  var c: C
+}
+
+public struct S5 { } // no diagnostic: S5 is not Sendable
+
+@available(*, unavailable)
+extension S5: Sendable { }
+
+// Public type with a conditional conformance, so don't complain
+public struct S6<T, U> {
+  var t: T
+  var u: U
+}
+
+extension S6: Sendable where T: Sendable, U: Sendable { }
+