Add Feature: NonescapableAccessorOnTrivial

To guard the new UnsafeMutablePointer.mutableSpan APIs.

This allows older compilers to ignore the new APIs. Otherwise, the type checker
will crash on the synthesized _read accessor for a non-Escapable type:

    error: cannot infer lifetime dependence on the '_read' accessor because 'self'
    is BitwiseCopyable, specify '@Lifetime(borrow self)'

I don't know why the _read is synthesized in these cases, but apparently it's
always been that way.

Fixes: rdar://153773093 ([nonescapable] add a compiler feature to guard
~Escapable accessors when self is trivial)

Author: atrick

include/swift/AST/Types.h

@@ -700,6 +700,13 @@ class alignas(1 << TypeAlignInBits) TypeBase
   /// Returns true if this contextual type is (Escapable && !isNoEscape).
   bool mayEscape() { return !isNoEscape() && isEscapable(); }
 
+  /// Returns true if this contextual type satisfies a conformance to Escapable.
+  bool isBitwiseCopyable();
+
+  /// Returns true if this type satisfies a conformance to Escapable in the
+  /// given generic signature.
+  bool isBitwiseCopyable(GenericSignature sig);
+
   /// Are values of this type essentially just class references,
   /// possibly with some sort of additional information?
   ///

include/swift/Basic/Features.def

@@ -530,6 +530,9 @@ EXPERIMENTAL_FEATURE(DefaultIsolationPerFile, false)
 /// Enable @_lifetime attribute
 SUPPRESSIBLE_EXPERIMENTAL_FEATURE(Lifetimes, true)
 
+/// Enable UnsafeMutablePointer.mutableSpan
+EXPERIMENTAL_FEATURE(NonescapableAccessorOnTrivial, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/ConformanceLookup.cpp

@@ -963,3 +963,21 @@ bool TypeBase::isEscapable(GenericSignature sig) {
   }
   return contextTy->isEscapable();
 }
+
+bool TypeBase::isBitwiseCopyable() {
+  auto &ctx = getASTContext();
+  auto *bitwiseCopyableProtocol =
+    ctx.getProtocol(KnownProtocolKind::BitwiseCopyable);
+  if (!bitwiseCopyableProtocol) {
+    return false;
+  }
+  return (bool)checkConformance(this, bitwiseCopyableProtocol);
+}
+
+bool TypeBase::isBitwiseCopyable(GenericSignature sig) {
+  Type contextTy = this;
+  if (sig) {
+    contextTy = sig.getGenericEnvironment()->mapTypeIntoContext(contextTy);
+  }
+  return contextTy->isBitwiseCopyable();
+}

lib/AST/FeatureSet.cpp

@@ -335,9 +335,26 @@ static bool usesFeatureLifetimeDependenceMutableAccessors(Decl *decl) {
     return false;
   }
   if (auto dc = var->getDeclContext()) {
-    if (auto nominal = dc->getSelfNominalTypeDecl()) {
-      auto sig = nominal->getGenericSignature();
-      return !var->getInterfaceType()->isEscapable(sig);
+    auto sig = dc->getGenericSignatureOfContext();
+    return !var->getInterfaceType()->isEscapable(sig);
+  }
+  return false;
+}
+
+static bool usesFeatureNonescapableAccessorOnTrivial(Decl *decl) {
+  if (!isa<VarDecl>(decl)) {
+    return false;
+  }
+  auto var = cast<VarDecl>(decl);
+  if (!var->hasParsedAccessors()) {
+    return false;
+  }
+  if (auto dc = var->getDeclContext()) {
+    auto sig = dc->getGenericSignatureOfContext();
+    if (!var->getInterfaceType()->isEscapable(sig)) {
+      auto selfTy = dc->getSelfTypeInContext();
+      // Consider 'self' trivial if it is BitwiseCopyable and Escapable.
+      return selfTy->isBitwiseCopyable() && selfTy->isEscapable();
     }
   }
   return false;