prevent more synthesized conformances for move-only types

enums with only cases that have no associated values
automatically get Equatable and Hashable synthesized.

That's not valid for move-only enums, so we were
just getting errors about broken conformances when
we hadn't specified any explicitly.

This PR just prevents the synthesis from the start
so we don't get any errors.

fixes rdar://104986597

Author: kavon

lib/AST/ProtocolConformance.cpp

@@ -1056,6 +1056,13 @@ void NominalTypeDecl::prepareConformanceTable() const {
     if (!proto)
       return;
 
+    // No synthesized conformances for move-only nominals.
+    if (isMoveOnly()) {
+      // assumption is Sendable gets synthesized elsewhere.
+      assert(!proto->isSpecificProtocol(KnownProtocolKind::Sendable));
+      return;
+    }
+
     if (protocols.count(proto) == 0) {
       ConformanceTable->addSynthesizedConformance(
           mutableThis, proto, mutableThis);

test/Sema/moveonly_restrictions.swift

@@ -182,3 +182,33 @@ struct MoveOnlyStructP : P { // expected-error {{move-only struct 'MoveOnlyStruc
 }
 @_moveOnly
 enum MoveOnlyEnumP : P {} // expected-error {{move-only enum 'MoveOnlyEnumP' cannot conform to 'P'}}
+
+// ensure there is no auto-synthesis of Equatable, Hashable, etc, for this move-only enum,
+// because it normally would be synthesized since it only has cases without associated values.
+@_moveOnly
+enum Color {
+    case red
+    case green
+    case blue
+
+    static func same(_ c1: __shared Color, c2: __shared Color) -> Bool {
+        return c1 == c2
+        // expected-error@-1 {{binary operator '==' cannot be applied to two 'Color' operands}}
+    }
+}
+
+@_moveOnly
+enum StrengthLevel: Int { // ensure move-only raw enums do not conform to RawRepresentable
+    case none = 0
+    case low
+    case high
+
+    static func lowEnergy() {
+        _ = StrengthLevel(rawValue: 1)
+        // expected-error@-1 {{'StrengthLevel' cannot be constructed because it has no accessible initializers}}
+    }
+}
+
+
+
+