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
(cherry picked from commit 7ffe96a)

Author: kavon

include/swift/AST/DiagnosticsSema.def

@@ -7665,7 +7665,7 @@ 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 +4770,20 @@ 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
+            && !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 +5788,9 @@ 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/Parse/inverses_legacy.swift

@@ -5,7 +5,7 @@
 protocol Sando { func make() } // expected-note {{protocol requires function 'make()'}}
                                // expected-note@-1 {{type 'U' does not conform to inherited protocol 'Copyable'}}
 
-struct BuggerView: ~Escapable {} // expected-error {{can only suppress 'Copyable'}}
+struct BuggerView: ~Escapable {} // expected-error {{can only suppress 'Copyable'}} // expected-error {{type 'Escapable' requires -enable-experimental-feature NonescapableTypes}}
 
 struct S: ~U, // expected-error {{can only suppress 'Copyable'}}
               // expected-error@-1 {{inheritance from non-protocol type 'U'}}