Merge pull request #86337 from kavon/noncopyable-opaque-read-ownership

introduce UnderscoreOwned feature

Author: kavon

docs/ReferenceGuides/UnderscoredAttributes.md

@@ -946,6 +946,44 @@ More generally, multiple availabilities can be specified, like so:
 enum Toast { ... }
 ```
 
+## `@_owned`
+
+Indicates that the [conservative access pattern](/docs/Lexicon.md#access-pattern)
+for some storage (a subscript or a property) should use the `get` accessor instead of `_read`.
+
+This attribute is particularly useful for accessors returning noncopyable values.
+By default, all explicitly-declared `get` accessors that return a noncopyable value and are declared in
+resilient libraries, or accessed opaquely via a protocol, are treated as if they return a borrowed value, 
+rather than one that is valid to consume:
+
+```swift
+// MutableSpan is noncopyable
+
+public protocol Giver {
+  var mutableSpan: MutableSpan { get } // has 'yielding borrow' semantics
+} 
+
+func example(_ s: some Giver) {
+  let x = s.mutableSpan // error
+}
+```
+
+This `@_owned` attribute allows you to override that behavior, so that the `get` requirement (or accessor) is truly
+exposed in a resilient interface, yielding an owned value of noncopyable type:
+
+```swift
+public protocol Giver {
+  @_owned
+  var mutableSpan: MutableSpan { get } // has 'get' semantics
+}
+
+func example(_ s: some Giver) {
+  let x = s.mutableSpan // ok
+}
+```
+
+Adding or removing this attribute is potentially an ABI and Source breaking change.
+
 ## `@_preInverseGenerics`
 
 By default when mangling a generic signature, the presence of a conformance 

include/swift/AST/DeclAttr.def

@@ -608,7 +608,10 @@ SIMPLE_DECL_ATTR(reasync, AtReasync,
   ConcurrencyOnly | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnreachableInABIAttr,
   110)
 
-// Unused '111'
+SIMPLE_DECL_ATTR(_owned, Owned,
+  OnVar | OnSubscript,
+  NotSerialized | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
+  111)
 
 CONTEXTUAL_DECL_ATTR(nonisolated, Nonisolated,
   OnFunc | OnConstructor | OnDestructor | OnVar | OnSubscript | OnProtocol | OnExtension | OnClass | OnStruct | OnEnum,

include/swift/AST/DiagnosticsSema.def

@@ -6929,7 +6929,7 @@ ERROR(exclusivity_on_computed_property,none,
       ())
 
 //------------------------------------------------------------------------------
-// MARK: @_borrowed
+// MARK: @_borrowed and @_owned
 //------------------------------------------------------------------------------
 ERROR(borrowed_with_objc_dynamic,none,
       "%kindonly0 cannot be '@_borrowed' if it is '@objc dynamic'",
@@ -6941,6 +6941,12 @@ ERROR(borrowed_on_objc_protocol_requirement,none,
 ERROR(borrowed_with_effect,none,
       "%kindonly0 cannot be '@_borrowed' if it is 'async' or 'throws'",
       (const AccessorDecl *))
+ERROR(borrowed_and_owned,none,
+      "%kindonly0 cannot be '@_borrowed' and '@_owned' at the same time",
+      (const AbstractStorageDecl *))
+ERROR(owned_non_getter,none,
+      "%kindonly0 must define a 'get' to support '@_owned'",
+      (const AbstractStorageDecl *))
 
 //------------------------------------------------------------------------------
 // MARK: dynamic

include/swift/Basic/Features.def

@@ -578,6 +578,9 @@ EXPERIMENTAL_FEATURE(EnforceSPIOperatorGroup, true)
 /// Enable source-level warning control with `@warn`
 EXPERIMENTAL_FEATURE(SourceWarningControl, true)
 
+/// Allows a `get` returning a noncopyable type have owned semantics in opaque interfaces.
+EXPERIMENTAL_FEATURE(UnderscoreOwned, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/ASTDumper.cpp

@@ -5106,6 +5106,7 @@ class PrintAttribute : public AttributeVisitor<PrintAttribute, void, Label>,
   TRIVIAL_ATTR_PRINTER(ObjCNonLazyRealization, objc_non_lazy_realization)
   TRIVIAL_ATTR_PRINTER(Optional, optional)
   TRIVIAL_ATTR_PRINTER(Override, override)
+  TRIVIAL_ATTR_PRINTER(Owned, owned)
   TRIVIAL_ATTR_PRINTER(Postfix, postfix)
   TRIVIAL_ATTR_PRINTER(PreInverseGenerics, pre_inverse_generics)
   TRIVIAL_ATTR_PRINTER(Preconcurrency, preconcurrency)

lib/AST/FeatureSet.cpp

@@ -131,6 +131,10 @@ UNINTERESTING_FEATURE(ImportMacroAliases)
 UNINTERESTING_FEATURE(NoExplicitNonIsolated)
 UNINTERESTING_FEATURE(EmbeddedExistentials)
 
+static bool usesFeatureUnderscoreOwned(Decl *D) {
+  return D->getAttrs().hasAttribute<OwnedAttr>();
+}
+
 // TODO: Return true for inlinable function bodies with module selectors in them
 UNINTERESTING_FEATURE(ModuleSelector)
 

lib/ASTGen/Sources/ASTGen/DeclAttrs.swift

@@ -291,6 +291,7 @@ extension ASTGenVisitor {
         .NSManaged,
         .ObjCMembers,
         .ObjCNonLazyRealization,
+        .Owned,
         .Preconcurrency,
         .PreInverseGenerics,
         .PropertyWrapper,

lib/Sema/TypeCheckAttr.cpp

@@ -304,6 +304,32 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
       D->getAttrs().removeAttribute(attr);
       return;
     }
+
+    // Can't have both @_owned and @_borrwed on the same decl.
+    if (D->getAttrs().hasAttribute<OwnedAttr>()) {
+      diagnose(attr->getLocation(), diag::borrowed_and_owned,
+               ASD)
+          .fixItRemove(attr->getRange());
+      D->getAttrs().removeAttribute(attr);
+    }
+  }
+
+  void visitOwnedAttr(OwnedAttr *attr) {
+    assert(!D->hasClangNode() && "@_owned on imported declaration?");
+
+    auto *ASD = cast<AbstractStorageDecl>(D);
+
+    // Ensure it defines a 'get' for read accesses.
+    //
+    // The only known use-case for this attribute is for getters returning
+    // noncopyable types that want to override the resilient '_read' that is
+    // always generated, exposing 'get' that returns an owned value.
+    if (!ASD->getAccessor(AccessorKind::Get)) {
+      diagnose(attr->getLocation(), diag::owned_non_getter,
+               ASD)
+          .fixItRemove(attr->getRange());
+      D->getAttrs().removeAttribute(attr);
+    }
   }
 
   void visitTransparentAttr(TransparentAttr *attr);

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1615,6 +1615,7 @@ namespace  {
     UNINTERESTING_ATTR(Inline)
     UNINTERESTING_ATTR(Isolated)
     UNINTERESTING_ATTR(Optimize)
+    UNINTERESTING_ATTR(Owned)
     UNINTERESTING_ATTR(Exclusivity)
     UNINTERESTING_ATTR(Nonexhaustive)
     UNINTERESTING_ATTR(NoLocks)

lib/Sema/TypeCheckStorage.cpp

@@ -1074,6 +1074,9 @@ OpaqueReadOwnershipRequest::evaluate(Evaluator &evaluator,
   if (storage->getAttrs().hasAttribute<BorrowedAttr>())
     return usesBorrowed(DiagKind::BorrowedAttr);
 
+  if (storage->getAttrs().hasAttribute<OwnedAttr>())
+    return OpaqueReadOwnership::Owned;
+
   if (storage->getInnermostDeclContext()->mapTypeIntoEnvironment(
         storage->getValueInterfaceType())->isNoncopyable())
     return usesBorrowed(DiagKind::NoncopyableType);