Disable destructuring initializations in struct let stored properties.

This looks like it was never properly implemented, since when we generate the
memberwise initializer for the struct in SILGen, it incorrectly tries to apply
the entire initializer expression to each variable binding in the pattern,
rather than destructuring the result and pattern-matching it to the variables.
Since it never worked it doesn't look like anyone is using this, so let's
put up an error saying it's unsupported until we can implement it properly.
Add `StructLetDestructuring` as an experimental feature flag so that tests around
the feature for things like module interface printing can still work.

include/swift/AST/DiagnosticsSema.def

@@ -2175,6 +2175,9 @@ ERROR(pattern_binds_no_variables,none,
 ERROR(variable_bound_by_no_pattern,none,
       "variable %0 is not bound by any pattern",
       (const VarDecl *))
+ERROR(destructuring_let_struct_stored_property_unsupported,none,
+      "binding multiple 'let' stored properties from a single initializer expression in a struct is unsupported",
+      ())
 
 WARNING(optional_ambiguous_case_ref,none,
         "assuming you mean '%0.%2'; did you mean '%1.%2' instead?",

include/swift/Basic/Features.def

@@ -242,6 +242,9 @@ EXPERIMENTAL_FEATURE(NoncopyableGenerics, false)
 /// Enables typed throws.
 EXPERIMENTAL_FEATURE(TypedThrows, true)
 
+/// Allow destructuring stored `let` bindings in structs.
+EXPERIMENTAL_FEATURE(StructLetDestructuring, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/ASTPrinter.cpp

@@ -3488,6 +3488,26 @@ static bool usesFeatureRawLayout(Decl *decl) {
   return decl->getAttrs().hasAttribute<RawLayoutAttr>();
 }
 
+static bool usesFeatureStructLetDestructuring(Decl *decl) {
+  auto sd = dyn_cast<StructDecl>(decl);
+  if (!sd)
+    return false;
+    
+  for (auto member : sd->getStoredProperties()) {
+    if (!member->isLet())
+      continue;
+    
+    auto init = member->getParentPattern();
+    if (!init)
+      continue;
+      
+    if (!init->getSingleVar())
+      return true;
+  }
+  
+  return false;
+}
+
 static bool hasParameterPacks(Decl *decl) {
   if (auto genericContext = decl->getAsGenericContext()) {
     auto sig = genericContext->getGenericSignature();

lib/SILGen/SILGenConstructor.cpp

@@ -470,6 +470,13 @@ static void emitImplicitValueConstructor(SILGenFunction &SGF,
           .forwardInto(SGF, Loc, init.get());
         ++elti;
       } else {
+        // TODO: This doesn't correctly take into account destructuring
+        // pattern bindings on `let`s, for example `let (a, b) = foo()`. In
+        // cases like that, we ought to evaluate the initializer expression once
+        // and then do a pattern assignment to the variables in the pattern.
+        // That case is currently forbidden with an "unsupported" error message
+        // in Sema.
+        
         assert(field->getTypeInContext()->getReferenceStorageReferent()->isEqual(
                    field->getParentExecutableInitializer()->getType()) &&
                "Initialization of field with mismatched type!");
@@ -534,6 +541,13 @@ static void emitImplicitValueConstructor(SILGenFunction &SGF,
       ++elti;
     } else {
       // Otherwise, use its initializer.
+      // TODO: This doesn't correctly take into account destructuring
+      // pattern bindings on `let`s, for example `let (a, b) = foo()`. In
+      // cases like that, we ought to evaluate the initializer expression once
+      // and then do a pattern assignment to the variables in the pattern.
+      // That case is currently forbidden with an "unsupported" error message
+      // in Sema.
+
       assert(field->isParentExecutabledInitialized());
       Expr *init = field->getParentExecutableInitializer();
 

lib/Sema/TypeCheckConstraints.cpp

@@ -882,6 +882,7 @@ bool TypeChecker::typeCheckPatternBinding(PatternBindingDecl *PBD,
   if (hadError)
     PBD->setInvalid();
   PBD->setInitializerChecked(patternNumber);
+  
   return hadError;
 }
 

lib/Sema/TypeCheckStorage.cpp

@@ -567,6 +567,21 @@ const PatternBindingEntry *PatternBindingEntryRequest::evaluate(
                                  : GlobalVariable);
     }
   }
+  
+  // If the pattern binding appears as a compound stored `let` property with an
+  // initializer inside of a struct type, diagnose it as unsupported.
+  // This hasn't ever been implemented properly.
+  if (!Context.LangOpts.hasFeature(Feature::StructLetDestructuring)
+      && !binding->isStatic()
+      && binding->isInitialized(entryNumber)
+      && isa<StructDecl>(binding->getDeclContext())
+      && !pattern->getSingleVar()
+      && !vars.empty()
+      && vars[0]->isLet()) {
+    Context.Diags.diagnose(binding->getPattern(entryNumber)->getLoc(),
+                           diag::destructuring_let_struct_stored_property_unsupported);
+  }
+  
   return &pbe;
 }
 

test/ModuleInterface/stored-properties-client.swift

@@ -4,12 +4,12 @@
 
 // 1. Build ../stored-properties.swift to a dylib and emit its interface in %t
 
-// RUN: %target-build-swift-dylib(%t/%target-library-name(StoredProperties)) -emit-module-interface-path %t/StoredProperties.swiftinterface %S/stored-properties.swift -module-name StoredProperties -swift-version 5
+// RUN: %target-build-swift-dylib(%t/%target-library-name(StoredProperties)) -enable-experimental-feature StructLetDestructuring -emit-module-interface-path %t/StoredProperties.swiftinterface %S/stored-properties.swift -module-name StoredProperties -swift-version 5
 // RUN: %target-swift-typecheck-module-from-interface(%t/StoredProperties.swiftinterface) -module-name StoredProperties
 
 // 2. Build this file and link with StoredProperties
 
-// RUN: %target-build-swift %s -I %t -L %t -lStoredProperties -o %t/stored-properties-client %target-rpath(%t)
+// RUN: %target-build-swift -enable-experimental-feature StructLetDestructuring %s -I %t -L %t -lStoredProperties -o %t/stored-properties-client %target-rpath(%t)
 
 // 3. Codesign and run this, and ensure it exits successfully.
 
@@ -20,9 +20,9 @@
 
 // RUN: %empty-directory(%t)
 
-// RUN: %target-build-swift-dylib(%t/%target-library-name(StoredProperties)) -emit-module-interface-path %t/StoredProperties.swiftinterface %S/stored-properties.swift -module-name StoredProperties -swift-version 5 -enable-library-evolution
+// RUN: %target-build-swift-dylib(%t/%target-library-name(StoredProperties)) -enable-experimental-feature StructLetDestructuring -emit-module-interface-path %t/StoredProperties.swiftinterface %S/stored-properties.swift -module-name StoredProperties -swift-version 5 -enable-library-evolution
 
-// RUN: %target-build-swift %s -I %t -L %t -lStoredProperties -o %t/stored-properties-client %target-rpath(%t)
+// RUN: %target-build-swift -enable-experimental-feature StructLetDestructuring %s -I %t -L %t -lStoredProperties -o %t/stored-properties-client %target-rpath(%t)
 // RUN: %target-codesign %t/stored-properties-client %t/%target-library-name(StoredProperties)
 // RUN: %target-run %t/stored-properties-client %t/%target-library-name(StoredProperties)
 

test/ModuleInterface/stored-properties.swift

@@ -1,18 +1,18 @@
 // RUN: %empty-directory(%t)
 
-// RUN: %target-swift-frontend -typecheck -emit-module-interface-path %t.swiftinterface -module-name StoredProperties %s
+// RUN: %target-swift-frontend -enable-experimental-feature StructLetDestructuring -typecheck -emit-module-interface-path %t.swiftinterface -module-name StoredProperties %s
 // RUN: %target-swift-typecheck-module-from-interface(%t.swiftinterface) -module-name StoredProperties
 // RUN: %FileCheck %s < %t.swiftinterface --check-prefix CHECK --check-prefix COMMON
 
-// RUN: %target-swift-frontend -typecheck -emit-module-interface-path %t-resilient.swiftinterface -module-name StoredProperties -enable-library-evolution %s
+// RUN: %target-swift-frontend -enable-experimental-feature StructLetDestructuring -typecheck -emit-module-interface-path %t-resilient.swiftinterface -module-name StoredProperties -enable-library-evolution %s
 // RUN: %target-swift-typecheck-module-from-interface(%t-resilient.swiftinterface) -module-name StoredProperties
 // RUN: %FileCheck %s < %t-resilient.swiftinterface --check-prefix RESILIENT --check-prefix COMMON
 
-// RUN: %target-swift-frontend -emit-module -o %t/Test.swiftmodule -module-name StoredProperties %t.swiftinterface -disable-objc-attr-requires-foundation-module
-// RUN: %target-swift-frontend -emit-module -o /dev/null -merge-modules %t/Test.swiftmodule -module-name StoredProperties -emit-module-interface-path - | %FileCheck %s --check-prefix CHECK --check-prefix COMMON
+// RUN: %target-swift-frontend -enable-experimental-feature StructLetDestructuring -emit-module -o %t/Test.swiftmodule -module-name StoredProperties %t.swiftinterface -disable-objc-attr-requires-foundation-module
+// RUN: %target-swift-frontend -enable-experimental-feature StructLetDestructuring -emit-module -o /dev/null -merge-modules %t/Test.swiftmodule -module-name StoredProperties -emit-module-interface-path - | %FileCheck %s --check-prefix CHECK --check-prefix COMMON
 
-// RUN: %target-swift-frontend -emit-module -o %t/TestResilient.swiftmodule -module-name StoredProperties -enable-library-evolution %t-resilient.swiftinterface -disable-objc-attr-requires-foundation-module
-// RUN: %target-swift-frontend -emit-module -o /dev/null -merge-modules %t/TestResilient.swiftmodule -module-name StoredProperties -enable-library-evolution -emit-module-interface-path - | %FileCheck %s --check-prefix RESILIENT --check-prefix COMMON
+// RUN: %target-swift-frontend -enable-experimental-feature StructLetDestructuring -emit-module -o %t/TestResilient.swiftmodule -module-name StoredProperties -enable-library-evolution %t-resilient.swiftinterface -disable-objc-attr-requires-foundation-module
+// RUN: %target-swift-frontend -enable-experimental-feature StructLetDestructuring -emit-module -o /dev/null -merge-modules %t/TestResilient.swiftmodule -module-name StoredProperties -enable-library-evolution -emit-module-interface-path - | %FileCheck %s --check-prefix RESILIENT --check-prefix COMMON
 
 // COMMON: public struct HasStoredProperties {
 public struct HasStoredProperties {

test/Sema/struct_property_let_destructuring.swift

@@ -0,0 +1,35 @@
+// RUN: %target-swift-frontend -typecheck -verify %s
+
+// https://github.com/apple/swift/issues/68915
+// Destructuring initializations for `let` properties in structs isn't
+// implemented correctly in SILGen, so diagnose it as unsupported for now.
+
+struct Foo {
+    var value: Int = 42
+
+    let (aaa, bbb) = ("aaa", "bbb") // expected-error{{unsupported}}
+
+    let (z1, z2, z3) = ("one", 1, Double.pi) // expected-error{{unsupported}}
+
+
+    func tellMe() {
+        print(foo.aaa)
+        print(foo.bbb)          // output:  aaa
+
+
+        assert(aaa == "aaa")
+        assert(bbb == "bbb", "bbb should be bbb but it's \(bbb)")
+    }
+
+}
+
+
+let foo = Foo(/*value: 1*/)
+
+
+foo.tellMe()
+
+
+
+
+print("Hello")

test/decl/var/properties.swift

@@ -1215,7 +1215,8 @@ _ = r19874152S5()  // ok
 
 
 struct r19874152S6 {
-  let (a,b) = (1,2)   // Cannot handle implicit synth of this yet.
+  // Cannot handle implicit synth of this yet.
+  let (a,b) = (1,2)   // expected-error {{unsupported}}
 }
 _ = r19874152S5()  // ok