[BoundsSafety][-Wunsafe-buffer-usage] Add a safe pattern of span construction from bounds-attributed pointers (swiftlang#9746) (swiftlang#10027)

For a `__sized_by(n)` pointer `p`, constructing a std::span the way below is safe.
`std::span<char>{(char *)p, n}`

cherry-picked from downstream
(rdar://141103910)

Author: ziqingluo-90

clang/lib/Analysis/UnsafeBufferUsage.cpp

@@ -903,6 +903,7 @@ bool isSinglePointerArgumentSafe(ASTContext &Context, const Expr *Arg) {
 //   `n`
 //   5. `std::span<T>{any, 0}`
 //   6. `std::span<T>{p, n}`, where `p` is a __counted_by(`n`)/__sized_by(`n`)
+//   pointer OR `std::span<char>{(char*)p, n}`, where `p` is a __sized_by(`n`)
 //   pointer.
 AST_MATCHER(CXXConstructExpr, isSafeSpanTwoParamConstruct) {
   assert(Node.getNumArgs() == 2 &&
@@ -972,9 +973,28 @@ AST_MATCHER(CXXConstructExpr, isSafeSpanTwoParamConstruct) {
   }
 
   // Check form 6:
+  bool isArg0CastToBytePtrType = false;
+
+  if (auto *CE = dyn_cast<CastExpr>(Arg0)) {
+    if (auto DestTySize = Finder->getASTContext().getTypeSizeInCharsIfKnown(
+            Arg0Ty->getPointeeType())) {
+      if (!DestTySize->isOne())
+        return false; // If the destination pointee type is NOT of one byte
+                      // size, pattern match fails.
+      Arg0 = CE->getSubExpr()->IgnoreParenImpCasts();
+      Arg0Ty = Arg0->getType();
+      isArg0CastToBytePtrType = true;
+    }
+  }
+  // Check pointer and count/size with respect to the count-attribute:
   if (const auto *CAT = Arg0Ty->getAs<CountAttributedType>()) {
-    // Accept __sized_by() if the size of the pointee type is 1.
-    if (CAT->isCountInBytes()) {
+    // For the pattern of `std::span<char>{(char *) p, n}`, p must NOT be a
+    // __counted_by pointer.
+    if (!CAT->isCountInBytes() && isArg0CastToBytePtrType)
+      return false;
+    // If `Arg0` is not a cast and is a sized_by pointer, its pointee type size
+    // must be one byte:
+    if (CAT->isCountInBytes() && !isArg0CastToBytePtrType) {
       std::optional<CharUnits> SizeOpt =
           Finder->getASTContext().getTypeSizeInCharsIfKnown(
               CAT->getPointeeType());

clang/test/SemaCXX/warn-unsafe-buffer-usage-in-container-span-construct-count-attributed-pointer.cpp

@@ -102,6 +102,32 @@ void span_from_sb_int(sb_int *i, sb_char *c) {
   std::span<char>{c->p, c->size};
 }
 
+void span_from_sb_void(void * __sized_by(n) p, size_t n) {
+  std::span<char>{(char *) p, n};
+  std::span<unsigned char>{(unsigned char *) p, n};
+  std::span<int>{(int *) p, n}; // expected-warning{{the two-parameter std::span construction is unsafe}}
+  std::span<char>{(char *) p, n+1}; // expected-warning{{the two-parameter std::span construction is unsafe}}
+}
+
+class SpanFromSbVoidMemberTest {
+  void * __sized_by(n) p;
+  size_t n;
+
+  void test() {
+    std::span<char>{(char *) p, n};
+    std::span<unsigned char>{(unsigned char *) p, n};
+    std::span<int>{(int *) p, n}; // expected-warning{{the two-parameter std::span construction is unsafe}}
+    std::span<char>{(char *) p, n+1}; // expected-warning{{the two-parameter std::span construction is unsafe}}
+  }
+
+  void test(SpanFromSbVoidMemberTest t) {
+    std::span<char>{(char *) t.p, t.n};
+    std::span<unsigned char>{(unsigned char *) t.p, t.n};
+    std::span<int>{(int *) t.p, t.n}; // expected-warning{{the two-parameter std::span construction is unsafe}}
+    std::span<char>{(char *) t.p, t.n+1}; // expected-warning{{the two-parameter std::span construction is unsafe}}
+  }
+};
+
 void span_from_output_parm(int * __counted_by(n)  *cb_p, size_t n,
                            int * __counted_by(*m) *cb_q, size_t *m,
                            int * __counted_by(*l) cb_z,  size_t *l,