prevent uses of Escapable in general

This protocol appears in the stdlib as scaffolding for the
`NonescapableTypes` feature, which is still experimental and not gone
through evolution as an approved addition to the stdlib.

Rather than delete it from the stdlib, because it needs to still remain
to support that feature work, gate references to it behind a feature
flag.

Additionally, prevent documentation from seeing this declaration.

rdar://126705184

Author: kavon

include/swift/AST/DiagnosticsSema.def

@@ -7708,7 +7708,7 @@ NOTE(add_explicit_protocol_for_conformance,none,
      "consider making %kind0 explicitly conform to the '%1' protocol",
      (const ValueDecl *, StringRef))
 ERROR(escapable_requires_feature_flag,none,
-      "type '~Escapable' requires -enable-experimental-feature NonescapableTypes",
+      "type 'Escapable' requires -enable-experimental-feature NonescapableTypes",
       ())
 ERROR(non_bitwise_copyable_type_class,none,
       "class cannot conform to 'BitwiseCopyable'", ())

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -124,10 +124,9 @@ class CheckRepressions {
     auto ipk = getInvertibleProtocolKind(*kp);
     if (ipk) {
       // Gate the '~Escapable' type behind a specific flag for now.
+      // Uses of 'Escapable' itself are already diagnosed; return ErrorType.
       if (*ipk == InvertibleProtocolKind::Escapable &&
           !ctx.LangOpts.hasFeature(Feature::NonescapableTypes)) {
-        diagnoseInvalid(repr, repr.getLoc(),
-                        diag::escapable_requires_feature_flag);
         return ErrorType::get(ctx);
       }
 

lib/Sema/TypeCheckType.cpp

@@ -4770,6 +4770,21 @@ TypeResolver::resolveDeclRefTypeRepr(DeclRefTypeRepr *repr,
     auto *dc = getDeclContext();
     auto &ctx = getASTContext();
 
+    // Gate the 'Escapable' type behind a specific flag for now.
+    //
+    // NOTE: we do not return an ErrorType, though! We're just artificially
+    // preventing people from referring to the type without the feature.
+    if (auto proto = result->getAs<ProtocolType>()) {
+      if (auto known = proto->getKnownProtocol()) {
+        if (*known == KnownProtocolKind::Escapable
+            && !isSILSourceFile()
+            && !ctx.LangOpts.hasFeature(Feature::NonescapableTypes)) {
+          diagnoseInvalid(repr, repr->getLoc(),
+                          diag::escapable_requires_feature_flag);
+        }
+      }
+    }
+
     if (ctx.LangOpts.hasFeature(Feature::ImplicitSome) &&
         options.isConstraintImplicitExistential()) {
       // Check whether this type is an implicit opaque result type.
@@ -5774,10 +5789,9 @@ NeverNullType TypeResolver::resolveInverseType(InverseTypeRepr *repr,
     if (auto kind = getInvertibleProtocolKind(*kp)) {
 
       // Gate the '~Escapable' type behind a specific flag for now.
+      // Uses of 'Escapable' itself are already diagnosed; return ErrorType.
       if (*kind == InvertibleProtocolKind::Escapable &&
           !getASTContext().LangOpts.hasFeature(Feature::NonescapableTypes)) {
-        diagnoseInvalid(repr, repr->getLoc(),
-                        diag::escapable_requires_feature_flag);
         return wrapInExistential(ErrorType::get(getASTContext()));
       }
 

stdlib/public/core/Misc.swift

@@ -171,6 +171,7 @@ func _rethrowsViaClosure(_ fn: () throws -> ()) rethrows {
 
 @_marker public protocol Copyable {}
 
+@_documentation(visibility: internal)
 @_marker public protocol Escapable {}
 
 #if $NoncopyableGenerics && $NonescapableTypes

test/Parse/inverse_escapable_feature.swift

@@ -2,4 +2,8 @@
 
 
 
-struct S: ~Escapable {} // expected-error {{type '~Escapable' requires -enable-experimental-feature NonescapableTypes}}
+struct S: ~Escapable {} // expected-error {{type 'Escapable' requires -enable-experimental-feature NonescapableTypes}}
+
+func hello(_ t: some Escapable, _ u: any Escapable) {} // expected-error 2{{type 'Escapable' requires -enable-experimental-feature NonescapableTypes}}
+
+protocol Whatever: Escapable {} // expected-error {{type 'Escapable' requires -enable-experimental-feature NonescapableTypes}}

test/Sema/invertible_no_stdlib.swift

@@ -14,7 +14,7 @@ func reqCopy2<T: Builtin.Copyable>(_ t: T) {} // expected-note {{generic paramet
 
 protocol P {}
 
-struct DataType: P, Builtin.Escapable {}
+struct DataType: P, Builtin.Escapable {} // expected-error {{type 'Escapable' requires -enable-experimental-feature NonescapableTypes}}
 struct DataTypeNC: ~Builtin.Copyable {}
 
 func main() {

test/decl/ext/extensions.swift

@@ -398,6 +398,7 @@ extension B4 {
 extension Sendable {} // expected-error {{cannot extend protocol 'Sendable'}}
 extension Copyable {} // expected-error {{cannot extend protocol 'Copyable'}}
 extension Escapable {} // expected-error {{cannot extend protocol 'Escapable'}}
+// expected-error@-1 {{type 'Escapable' requires -enable-experimental-feature NonescapableTypes}}
 extension _BitwiseCopyable {} // expected-error {{cannot extend protocol '_BitwiseCopyable'}}
 
 @_marker protocol MyMarkerProto {}