[clang] [BoundsSafety][Sema] Allow counted_by and counted_by_or_null on pointers where the pointee type is incomplete but potentially completable (PR #106321)

[via cfe-commits]

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@@ -186,4 +218,216 @@ bool Sema::CheckCountedByAttrOnField(FieldDecl *FD, Expr *E, bool CountInBytes,
   return false;
 }
 
+SourceRange Sema::BoundsSafetySourceRangeFor(const CountAttributedType *CATy) {
+  // This is an approximation that's not quite right. This points to the
+  // the expression inside the attribute rather than the attribute itself.
+  //
+  // TODO: Implement logic to find the relevant TypeLoc for the attribute and
+  // get the SourceRange from that (#113582).
+  return CATy->getCountExpr()->getSourceRange();
+}
+
+static void EmitIncompleteCountedByPointeeNotes(Sema &S,
+                                                const CountAttributedType *CATy,
+                                                NamedDecl *IncompleteTyDecl,
+                                                bool NoteAttrLocation = true) {
+  assert(IncompleteTyDecl == nullptr || isa<TypeDecl>(IncompleteTyDecl));
+
+  if (NoteAttrLocation) {
+    // Note where the attribute is declared
+    auto AttrSrcRange = S.BoundsSafetySourceRangeFor(CATy);
+    S.Diag(AttrSrcRange.getBegin(), diag::note_named_attribute)
+        << CATy->getAttributeName(/*WithMacroPrefix=*/true) << AttrSrcRange;
+  }
+
+  if (!IncompleteTyDecl)
+    return;
+
+  // If there's an associated forward declaration display it to emphasize
+  // why the type is incomplete (all we have is a forward declaration).
+
+  // Note the `IncompleteTyDecl` type is the underlying type which might not
+  // be the same as `CATy->getPointeeType()` which could be a typedef.
+  //
+  // The diagnostic printed will be at the location of the underlying type but
+  // the diagnostic text will print the type of `CATy->getPointeeType()` which
+  // could be a typedef name rather than the underlying type. This is ok
+  // though because the diagnostic will print the underlying type name too.
+  // E.g:
+  //
+  // `forward declaration of 'Incomplete_Struct_t'
+  //  (aka 'struct IncompleteStructTy')`
+  //
+  // If this ends up being confusing we could emit a second diagnostic (one
+  // explaining where the typedef is) but that seems overly verbose.
+
+  S.Diag(IncompleteTyDecl->getBeginLoc(), diag::note_forward_declaration)
+      << CATy->getPointeeType();
+}
+
+static bool
+HasCountedByAttrOnIncompletePointee(QualType Ty, NamedDecl **ND,
+                                    const CountAttributedType **CATyOut,
+                                    QualType *PointeeTyOut) {
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Sirraide wrote:

We could change this to return a tuple instead of taking three out-parameters. All but the last `return` here could just be `return {}`, and we could destructure the return value at the call site (and check if the pointee type is non-null to see if it succeeded), which imo is a bit more ergonomic.

https://github.com/llvm/llvm-project/pull/106321