Sema: Teach the compiler to refine VarDecl initializer expressions using the deployment target when the init would not be exposed to module clients. Without this, the initializers of public properties in API modules could be misdiagnosed as potentially unavailable to clients of the module, even though the expression will only ever execute on the deployment target or higher.

While developing this fix I discovered a few other bugs that will need to be addressed separately:
- The availability of the inferred type of a property is not diagnosed. Previously this mattered less since the initializer expression and the var declaration itself would both be diagnosed with the same availability.
- The availability of the property wrapper type of a `VarDecl` appears to not be diagnosed at all.
- Members of frozen structs with property wrappers result in broken swiftinterfaces because the synthesized storage is printed explicitly, confusing the diagnostics.

Resolves rdar://92713589

Author: tshortli

lib/Sema/TypeCheckAvailability.cpp

@@ -24,6 +24,7 @@
 #include "swift/AST/Initializer.h"
 #include "swift/AST/NameLookup.h"
 #include "swift/AST/Pattern.h"
+#include "swift/AST/PrettyStackTrace.h"
 #include "swift/AST/ProtocolConformance.h"
 #include "swift/AST/SourceFile.h"
 #include "swift/AST/TypeDeclFinder.h"
@@ -320,21 +321,6 @@ static bool hasActiveAvailableAttribute(Decl *D,
   return getActiveAvailableAttribute(D, AC);
 }
 
-static bool shouldConstrainBodyToDeploymentTarget(Decl *D) {
-  // The declaration contains code...
-  if (auto afd = dyn_cast<AbstractFunctionDecl>(D)) {
-    // And it has a location so we can check it...
-    if (!afd->isImplicit() && afd->getBodySourceRange().isValid()) {
-      // And the code is within our resilience domain, so it should be
-      // compiled with the minimum deployment target, not the minimum inlining
-      // target.
-      return afd->getResilienceExpansion() != ResilienceExpansion::Minimal;
-    }
-  }
-
-  return false;
-}
-
 static bool computeContainedByDeploymentTarget(TypeRefinementContext *TRC,
                                                ASTContext &ctx) {
   return TRC->getAvailabilityInfo()
@@ -418,6 +404,10 @@ class TypeRefinementContextBuilder : private ASTWalker {
     ContextStack.push_back(Info);
   }
 
+  const char *stackTraceAction() const {
+    return "building type refinement context for";
+  }
+
 public:
   TypeRefinementContextBuilder(TypeRefinementContext *TRC, ASTContext &Context)
       : Context(Context) {
@@ -426,20 +416,23 @@ class TypeRefinementContextBuilder : private ASTWalker {
   }
 
   void build(Decl *D) {
+    PrettyStackTraceDecl trace(stackTraceAction(), D);
     unsigned StackHeight = ContextStack.size();
     D->walk(*this);
     assert(ContextStack.size() == StackHeight);
     (void)StackHeight;
   }
 
   void build(Stmt *S) {
+    PrettyStackTraceStmt trace(Context, stackTraceAction(), S);
     unsigned StackHeight = ContextStack.size();
     S->walk(*this);
     assert(ContextStack.size() == StackHeight);
     (void)StackHeight;
   }
 
   void build(Expr *E) {
+    PrettyStackTraceExpr trace(Context, stackTraceAction(), E);
     unsigned StackHeight = ContextStack.size();
     E->walk(*this);
     assert(ContextStack.size() == StackHeight);
@@ -448,6 +441,8 @@ class TypeRefinementContextBuilder : private ASTWalker {
 
 private:
   bool walkToDeclPre(Decl *D) override {
+    PrettyStackTraceDecl trace(stackTraceAction(), D);
+
     // Adds in a parent TRC for decls which are syntactically nested but are not
     // represented that way in the AST. (Particularly, AbstractStorageDecl
     // parents for AccessorDecl children.)
@@ -460,17 +455,12 @@ class TypeRefinementContextBuilder : private ASTWalker {
       pushContext(DeclTRC, D);
     }
 
-    // Adds in a TRC that covers only the body of the declaration.
-    if (auto BodyTRC = getNewContextForBodyOfDecl(D)) {
-      pushContext(BodyTRC, D);
-    }
-
+    // Create TRCs that cover only the body of the declaration.
+    buildContextsForBodyOfDecl(D);
     return true;
   }
 
   bool walkToDeclPost(Decl *D) override {
-    // As seen above, we could have up to three TRCs in the stack for a single
-    // declaration.
     while (ContextStack.back().ScopeNode.getAsDecl() == D) {
       ContextStack.pop_back();
     }
@@ -515,7 +505,7 @@ class TypeRefinementContextBuilder : private ASTWalker {
     return nullptr;
   }
 
-  /// Builds the type refinement hierarchy for the body of the function.
+  /// Builds the type refinement hierarchy for the signature of the declaration.
   TypeRefinementContext *buildDeclarationRefinementContext(Decl *D) {
     // We require a valid range in order to be able to query for the TRC
     // corresponding to a given SourceLoc.
@@ -611,15 +601,19 @@ class TypeRefinementContextBuilder : private ASTWalker {
     if (auto *storageDecl = dyn_cast<AbstractStorageDecl>(D)) {
       // Use the declaration's availability for the context when checking
       // the bodies of its accessors.
+      SourceRange Range = storageDecl->getSourceRange();
 
-      Decl *locDecl = D;
       // For a variable declaration (without accessors) we use the range of the
       // containing pattern binding declaration to make sure that we include
-      // any type annotation in the type refinement context range.
-      if (auto varDecl = dyn_cast<VarDecl>(storageDecl)) {
-        auto *PBD = varDecl->getParentPatternBinding();
-        if (PBD)
-          locDecl = PBD;
+      // any type annotation in the type refinement context range. We also
+      // need to include any attached property wrappers.
+      if (auto *varDecl = dyn_cast<VarDecl>(storageDecl)) {
+        if (auto *PBD = varDecl->getParentPatternBinding())
+          Range = PBD->getSourceRange();
+
+        for (auto *propertyWrapper : varDecl->getAttachedPropertyWrappers()) {
+          Range.widen(propertyWrapper->getRange());
+        }
       }
 
       // HACK: For synthesized trivial accessors we may have not a valid
@@ -628,58 +622,92 @@ class TypeRefinementContextBuilder : private ASTWalker {
       // to update AbstractStorageDecl::addTrivialAccessors() to take brace
       // locations and have callers of that method provide appropriate source
       // locations.
-      SourceLoc BracesEnd = storageDecl->getBracesRange().End;
-      if (storageDecl->hasParsedAccessors() && BracesEnd.isValid()) {
-        return SourceRange(locDecl->getStartLoc(),
-                           BracesEnd);
+      SourceRange BracesRange = storageDecl->getBracesRange();
+      if (storageDecl->hasParsedAccessors() && BracesRange.isValid()) {
+        Range.widen(BracesRange);
       }
-      
-      return locDecl->getSourceRange();
+
+      return Range;
     }
 
     return D->getSourceRange();
   }
 
-  TypeRefinementContext *getNewContextForBodyOfDecl(Decl *D) {
-    if (bodyIntroducesNewContext(D))
-      return buildBodyRefinementContext(D);
+  TypeRefinementContext *createAPIBoundaryContext(Decl *D, SourceRange range) {
+    AvailabilityContext DeploymentTargetInfo =
+        AvailabilityContext::forDeploymentTarget(Context);
+    DeploymentTargetInfo.intersectWith(getCurrentTRC()->getAvailabilityInfo());
 
-    return nullptr;
+    return TypeRefinementContext::createForAPIBoundary(
+        Context, D, getCurrentTRC(), DeploymentTargetInfo, range);
   }
 
-  bool bodyIntroducesNewContext(Decl *D) {
-    // Are we already constrained by the deployment target? If not, adding a
-    // new context wouldn't change availability.
+  void buildContextsForBodyOfDecl(Decl *D) {
+    // Are we already constrained by the deployment target? If not, adding
+    // new contexts won't change availability.
     if (isCurrentTRCContainedByDeploymentTarget())
-      return false;
-
-    // If we're in a function, check if it ought to use the deployment target.
-    if (auto afd = dyn_cast<AbstractFunctionDecl>(D))
-      return shouldConstrainBodyToDeploymentTarget(afd);
-
-    // The only other case we care about is top-level code.
-    return isa<TopLevelCodeDecl>(D);
-  }
+      return;
 
-  TypeRefinementContext *buildBodyRefinementContext(Decl *D) {
-    SourceRange range;
+    // Top level code always uses the deployment target.
     if (auto tlcd = dyn_cast<TopLevelCodeDecl>(D)) {
-      range = tlcd->getSourceRange();
-    } else if (auto afd = dyn_cast<AbstractFunctionDecl>(D)) {
-      range = afd->getBodySourceRange();
-    } else {
-      llvm_unreachable("unknown decl");
+      pushContext(createAPIBoundaryContext(tlcd, tlcd->getSourceRange()), D);
+      return;
     }
 
-    AvailabilityContext DeploymentTargetInfo =
-        AvailabilityContext::forDeploymentTarget(Context);
-    DeploymentTargetInfo.intersectWith(getCurrentTRC()->getAvailabilityInfo());
+    // Function bodies use the deployment target if they are within the module's
+    // resilience domain.
+    if (auto afd = dyn_cast<AbstractFunctionDecl>(D)) {
+      if (!afd->isImplicit() && afd->getBodySourceRange().isValid() &&
+          afd->getResilienceExpansion() != ResilienceExpansion::Minimal) {
+        pushContext(createAPIBoundaryContext(afd, afd->getBodySourceRange()),
+                    D);
+      }
+      return;
+    }
 
-    return TypeRefinementContext::createForAPIBoundary(
-        Context, D, getCurrentTRC(), DeploymentTargetInfo, range);
+    // Var decls may have associated pattern binding decls or property wrappers
+    // with init expressions. Those expressions need to be constrained to the
+    // deployment target unless they are exposed to clients.
+    if (auto vd = dyn_cast<VarDecl>(D)) {
+      if (!vd->hasInitialValue() || vd->isInitExposedToClients())
+        return;
+
+      if (auto *pbd = vd->getParentPatternBinding()) {
+        int idx = pbd->getPatternEntryIndexForVarDecl(vd);
+        auto *initExpr = pbd->getInit(idx);
+        if (initExpr && !initExpr->isImplicit()) {
+          assert(initExpr->getSourceRange().isValid());
+
+          // Create a TRC for the init written in the source. The ASTWalker
+          // won't visit these expressions so instead of pushing these onto the
+          // stack we build them directly.
+          auto *initTRC =
+              createAPIBoundaryContext(vd, initExpr->getSourceRange());
+          TypeRefinementContextBuilder(initTRC, Context).build(initExpr);
+        }
+
+        // Ideally any init expression would be returned by `getInit()` above.
+        // However, for property wrappers it doesn't get populated until
+        // typechecking completes (which is too late). Instead, we find the
+        // the property wrapper attribute and use its source range to create a
+        // TRC for the initializer expression.
+        //
+        // FIXME: Since we don't have an expression here, we can't build out its
+        // TRC. Currently that doesn't matter because expression contents don't
+        // ever contribute to the TRC, but it could become relevant if we ever
+        // get control flow expressions.
+        for (auto *wrapper : vd->getAttachedPropertyWrappers()) {
+          createAPIBoundaryContext(vd, wrapper->getRange());
+        }
+      }
+
+      return;
+    }
   }
 
   std::pair<bool, Stmt *> walkToStmtPre(Stmt *S) override {
+    PrettyStackTraceStmt trace(Context, stackTraceAction(), S);
+
     if (auto *IS = dyn_cast<IfStmt>(S)) {
       buildIfStmtRefinementContext(IS);
       return std::make_pair(false, S);