[BoundsSafety][Sema] Allow counted_by and counted_by_or_null on pointers where the pointee type is incomplete but potentially completable

Previously using the `counted_by` or `counted_by_or_null` attribute on a
pointer with an incomplete pointee type was forbidden. Unfortunately
this prevented a situation like the following from being allowed.

Header file:

```
struct EltTy; // Incomplete type
struct Buffer {
  size_t count;
  struct EltTy* __counted_by(count) buffer; // error before this patch
};
```

Implementation file:

```
struct EltTy {
  // definition
};

void allocBuffer(struct Buffer* b) {
  b->buffer = malloc(sizeof(EltTy)* b->count);
}
```

To allow code like the above but still enforce that the pointee
type size is known in locations where `-fbounds-safety` needs to
emit bounds checks the following scheme is used.

* For incomplete pointee types that can never be completed (e.g. `void`)
  these are treated as error where the attribute is written (just like
  before this patch).
* For incomplete pointee types that might be completable later on
  (struct, union, and enum forward declarations)
  in the translation unit, writing the attribute on the incomplete
  pointee type is allowed on the FieldDecl declaration but "uses" of the
  declared pointer are forbidden if at the point of "use" the pointee
  type is still incomplete.

For this patch a "use" of a FieldDecl covers:

* Explicit and Implicit initialization (note see **Tentative Definition
  Initialization** for an exception to this)
* Assignment
* Conversion to an lvalue (e.g. for use in an expression)

In Apple's internal fork of Clang the `counted_by` and
`counted_by_or_null` attribute are allowed in many more contexts. That
isn't the case for upstream Clang so the "use" checks for the attribute
on VarDecl, ParamVarDecl, and function return type have been omitted
from this patch because they can't be tested. However, the
`BoundsSafetyCheckAssignmentToCountAttrPtrWithIncompletePointeeTy` and
`BoundsSafetyCheckUseOfCountAttrPtrWithIncompletePointeeTy` functions
retain the ability to emit diagnostics for these other contexts to avoid
unnecessary divergence between upstream Clang and Apple's internal fork.
Support for checking "uses" will be upstreamed when upstream Clang
allows the `counted_by` and `counted_by_or_null` attribute in additional
contexts.

This patch also includes the new `Sema::BoundsSafetySourceRangeFor`
method which allows a more accurate SourceRange for the `counted_by`
and `counted_by_or_null` attributes to be shown in diagnostics. This
is used by the new `diag::note_named_attribute` diagnostic introduced in
this patch. This method has several shortcomings but it didn't seem
worth covering them given this is essentially a workaround for the
fact `CountAttributedType` doesn't store the SourceLocation for its
attribute.

This patch has a few limitations:

** 1. Tentative Defition Initialization **

This patch currently allows something like:

```
struct IncompleteTy;
struct Buffer {
  int count;
  struct IncompleteTy* __counted_by(count) buf;
};

// Tentative definition
struct Buffer GlobalBuf;
```

The Tentative definition in this example becomes an actual definition
whose initialization **should be checked** but it currently isn't.
Addressing this problem will be done in a subseqent patch.

** 2. When the incomplete pointee type is a typedef diagnostics are slightly misleading **

For this situation:

```

struct IncompleteTy;
typedef struct IncompleteTy Incomplete_t;

struct Buffer {
  int count;
  struct IncompleteTy* __counted_by(count) buf;
};

void use(struct Buffer b) {
  b.buf = 0x0;
}
```

This code emits `note: forward declaration of 'Incomplete_t' (aka
'struct IncompleteTy')` but the location is on the `struct
IncompleteTy;` forward declaration.  This is misleading because
`Incomplete_t` isn't actually forward declared there (instead the
underlying type is). This could be resolved by additional diagnostics
that walk the chain of typedefs and explain each step of the walk.
However, that would be very verbose and didn't seem like a direction
worth pursuing.

rdar://133600117

Author: delcypher

clang/include/clang/AST/Type.h

@@ -2440,6 +2440,26 @@ class alignas(TypeAlignment) Type : public ExtQualsTypeCommonBase {
     return !isFunctionType();
   }
 
+  /// \returns True if the type is incomplete and it is also a type that
+  /// cannot be completed by a later type definition.
+  ///
+  /// E.g. For `void` this is true but for `struct ForwardDecl;` this is false
+  /// because a definition for `ForwardDecl` could be provided later on in the
+  /// translation unit.
+  ///
+  /// Note even for types that this function returns true for it is still
+  /// possible for the declarations that contain this type to later have a
+  /// complete type in a translation unit. E.g.:
+  ///
+  /// \code{.c}
+  /// // This decl has type 'char[]' which is incomplete and cannot be later
+  /// // completed by another by another type declaration.
+  /// extern char foo[];
+  /// // This decl how has complete type 'char[5]'.
+  /// char foo[5]; // foo has a complete type
+  /// \endcode
+  bool isIncompletableIncompleteType() const;
+
   /// Determine whether this type is an object type.
   bool isObjectType() const {
     // C++ [basic.types]p8:
@@ -3343,6 +3363,8 @@ class CountAttributedType final
   static bool classof(const Type *T) {
     return T->getTypeClass() == CountAttributed;
   }
+
+  StringRef GetAttributeName(bool WithMacroPrefix) const;
 };
 
 /// Represents a type which was implicitly adjusted by the semantic

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -4376,6 +4376,7 @@ def err_mismatched_visibility: Error<"visibility does not match previous declara
 def note_previous_attribute : Note<"previous attribute is here">;
 def note_conflicting_attribute : Note<"conflicting attribute is here">;
 def note_attribute : Note<"attribute is here">;
+def note_named_attribute : Note<"%0 attribute is here">;
 def err_mismatched_ms_inheritance : Error<
   "inheritance model does not match %select{definition|previous declaration}0">;
 def warn_ignored_ms_inheritance : Warning<
@@ -6628,6 +6629,42 @@ def err_counted_by_attr_pointee_unknown_size : Error<
     "%select{|. This will be an error in a future compiler version}3"
     ""
   "}2">;
+def err_counted_by_on_incomplete_type_on_assign : Error <
+  "cannot %select{"
+    "assign to %select{object|'%1'}2 that has|" // AA_Assigning,
+    "pass argument to %select{parameter|parameter '%1'}2 that has|" // AA_Passing,
+    "return|" // AA_Returning,
+    "convert to|" // AA_Converting (UNUSED)
+    "%select{|implicitly }3initialize %select{object|'%1'}2 that has|" // AA_Initializing,
+    "pass argument to parameter that has|" // AA_Sending (UNUSED)
+    "cast to|" // AA_Casting (UNUSED)
+    "pass argument to parameter that has" // AA_Passing_CFAudited (UNUSED)
+  "}0 type %4 because the pointee type %6 is incomplete and the '%5' attribute "
+  "requires the pointee type be complete when %select{"
+    "assigning|" // AA_Assigning,
+    "passing|" // AA_Passing,
+    "returning|" // AA_Returning,
+    "converting|" // AA_Converting (UNUSED)
+    "initializing|" // AA_Initializing,
+    "passing|" // AA_Sending (UNUSED)
+    "casting|" // AA_Casting (UNUSED)
+    "passing" // AA_Passing_CFAudited (UNUSED)
+  "}0; "
+  "consider providing a complete definition for %6 or using the "
+  "'__sized_by%select{|_or_null}7' attribute"
+>;
+def err_counted_by_on_incomplete_type_on_use : Error <
+  "cannot %select{"
+    "use '%1'|" // Generic expr
+    "call '%1'" // CallExpr
+  "}0 with%select{"
+    "|"       // Generic expr
+    " return" // CallExpr
+  "}0 type %2 because the pointee type %3 is incomplete "
+  "and the '%4' attribute requires the pointee type be complete in this context; "
+  "consider providing a complete definition for %3 or using the "
+  "'__sized_by%select{|_or_null}5' attribute"
+>;
 
 def warn_counted_by_attr_elt_type_unknown_size :
   Warning<err_counted_by_attr_pointee_unknown_size.Summary>,

clang/include/clang/Sema/Sema.h

@@ -2099,6 +2099,70 @@ class Sema final : public SemaBase {
   bool CheckCountedByAttrOnField(FieldDecl *FD, Expr *E, bool CountInBytes,
                                  bool OrNull);
 
+  // AssignmentAction - This is used by all the assignment diagnostic functions
+  // to represent what is actually causing the operation
+  enum AssignmentAction {
+    AA_Assigning,
+    AA_Passing,
+    AA_Returning,
+    AA_Converting,
+    AA_Initializing,
+    AA_Sending,
+    AA_Casting,
+    AA_Passing_CFAudited
+  };
+
+  /// Perform Bounds Safety Semantic checks for assigning to a `__counted_by` or
+  /// `__counted_by_or_null` pointer type \param LHSTy.
+  ///
+  /// \param LHSTy The type being assigned to. Checks will only be performed if
+  ///              the type is a `counted_by` or `counted_by_or_null ` pointer.
+  /// \param RHSExpr The expression being assigned from.
+  /// \param Action The type assignment being performed
+  /// \param Loc The SourceLocation to use for error diagnostics
+  /// \param ComputeAssignee If provided this function will be called before
+  ///        emitting a diagnostic. The function should return the name of
+  ///        entity being assigned to or an empty string if this cannot be
+  ///        determined.
+  ///
+  /// \returns True iff no diagnostic where emitted, false otherwise.
+  bool BoundsSafetyCheckAssignmentToCountAttrPtr(
+      QualType LHSTy, Expr *RHSExpr, Sema::AssignmentAction Action,
+      SourceLocation Loc,
+      std::function<std::string()> ComputeAssignee = nullptr);
+
+  /// Perform Bounds Safety Semantic checks for initializing a Bounds Safety
+  /// pointer.
+  ///
+  /// \param Entity The entity being initialized
+  /// \param Kind The kind of initialization being performed
+  /// \param Action The type assignment being performed
+  /// \param LHSTy The type being assigned to. Checks will only be performed if
+  ///              the type is a `counted_by` or `counted_by_or_null ` pointer.
+  /// \param RHSExpr The expression being used for initialization.
+  ///
+  /// \returns True iff no diagnostic where emitted, false otherwise.
+  bool BoundsSafetyCheckInitialization(const InitializedEntity &Entity,
+                                       const InitializationKind &Kind,
+                                       Sema::AssignmentAction Action,
+                                       QualType LHSType, Expr *RHSExpr);
+
+  /// Perform Bounds Safety semantic checks for uses of invalid uses counted_by
+  /// or counted_by_or_null pointers in \param E.
+  ///
+  /// \param E the expression to check
+  ///
+  /// \returns True iff no diagnostic where emitted, false otherwise.
+  bool BoundsSafetyCheckUseOfCountAttrPtr(Expr *E);
+
+  /// \returns A SourceRange for \param CATy.
+  ///
+  /// This method tries to compute the most useful SourceRange for diagnostics.
+  /// If this fails the returned SourceRange will be the same as
+  /// `CATy->getCountExpr()->getSourceRange()`.
+  ///
+  SourceRange BoundsSafetySourceRangeFor(const CountAttributedType *CATy);
+
   ///@}
 
   //
@@ -6490,19 +6554,6 @@ class Sema final : public SemaBase {
   /// cleanup that are created by the current full expression.
   SmallVector<ExprWithCleanups::CleanupObject, 8> ExprCleanupObjects;
 
-  // AssignmentAction - This is used by all the assignment diagnostic functions
-  // to represent what is actually causing the operation
-  enum AssignmentAction {
-    AA_Assigning,
-    AA_Passing,
-    AA_Returning,
-    AA_Converting,
-    AA_Initializing,
-    AA_Sending,
-    AA_Casting,
-    AA_Passing_CFAudited
-  };
-
   /// Determine whether the use of this declaration is valid, without
   /// emitting diagnostics.
   bool CanUseDecl(NamedDecl *D, bool TreatUnavailableAsInvalid);

clang/lib/AST/Type.cpp

@@ -2438,6 +2438,22 @@ bool Type::isIncompleteType(NamedDecl **Def) const {
   }
 }
 
+bool Type::isIncompletableIncompleteType() const {
+  if (!isIncompleteType())
+    return false;
+
+  // Forward declarations of structs, classes, enums, and unions could be later
+  // completed in a compilation unit by providing a type definition.
+  if (isStructureOrClassType() || isEnumeralType() || isUnionType())
+    return false;
+
+  // Other types are incompletable.
+  //
+  // E.g. `char[]` and `void`. The type is incomplete and no future
+  // type declarations can make the type complete.
+  return true;
+}
+
 bool Type::isSizelessBuiltinType() const {
   if (isSizelessVectorType())
     return true;
@@ -3862,6 +3878,27 @@ CountAttributedType::CountAttributedType(
     DeclSlot[i] = CoupledDecls[i];
 }
 
+StringRef CountAttributedType::GetAttributeName(bool WithMacroPrefix) const {
+#define ENUMERATE_ATTRS(PREFIX)                                                \
+  do {                                                                         \
+    if (isCountInBytes()) {                                                    \
+      if (isOrNull())                                                          \
+        return PREFIX "sized_by_or_null";                                      \
+      return PREFIX "sized_by";                                                \
+    }                                                                          \
+    if (isOrNull())                                                            \
+      return PREFIX "counted_by_or_null";                                      \
+    return PREFIX "counted_by";                                                \
+  } while (0)
+
+  if (WithMacroPrefix)
+    ENUMERATE_ATTRS("__");
+  else
+    ENUMERATE_ATTRS("");
+
+#undef ENUMERATE_ATTRS
+}
+
 TypedefType::TypedefType(TypeClass tc, const TypedefNameDecl *D,
                          QualType Underlying, QualType can)
     : Type(tc, can, toSemanticDependence(can->getDependence())),