[ASan][libc++] Correct (explicit) annotation size (llvm#79292)

A quick examination suggests that the current code in the codebase does
not lead to incorrect annotations. However, the intention is for the
object after the function to be annotated in a way that only its
contents are unpoisoned and the rest is poisoned. This commit makes it
explicit and avoids potential issues in future.

In addition, I have implemented a few tests for a function that helped
me identify the specific argument value.

Notice: there is no known scenario where old code results in incorrect
annotation.

libcxx/include/string

@@ -2380,7 +2380,7 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocator>::__
   __old_sz = __n_copy + __n_add + __sec_cp_sz;
   __set_long_size(__old_sz);
   traits_type::assign(__p[__old_sz], value_type());
-  __annotate_new(__old_cap + __delta_cap);
+  __annotate_new(__old_sz);
 }
 
 // __grow_by is deprecated because it does not set the size. It may not update the size when the size is changed, and it

libcxx/test/std/strings/basic.string/string.modifiers/string_append/pointer_size.pass.cpp

@@ -36,6 +36,7 @@ TEST_CONSTEXPR_CXX20 void test_string() {
   test(S(), "12345678901234567890", 1, S("1"));
   test(S(), "12345678901234567890", 3, S("123"));
   test(S(), "12345678901234567890", 20, S("12345678901234567890"));
+  test(S(), "1234567890123456789012345678901234567890", 40, S("1234567890123456789012345678901234567890"));
 
   test(S("12345"), "", 0, S("12345"));
   test(S("12345"), "12345", 5, S("1234512345"));
@@ -44,6 +45,23 @@ TEST_CONSTEXPR_CXX20 void test_string() {
   test(S("12345678901234567890"), "", 0, S("12345678901234567890"));
   test(S("12345678901234567890"), "12345", 5, S("1234567890123456789012345"));
   test(S("12345678901234567890"), "12345678901234567890", 20, S("1234567890123456789012345678901234567890"));
+
+  // Starting from long string (no SSO)
+  test(S("1234567890123456789012345678901234567890"), "", 0, S("1234567890123456789012345678901234567890"));
+  test(S("1234567890123456789012345678901234567890"), "a", 1, S("1234567890123456789012345678901234567890a"));
+  test(S("1234567890123456789012345678901234567890"),
+       "aaaaaaaaaa",
+       10,
+       S("1234567890123456789012345678901234567890aaaaaaaaaa"));
+  test(S("1234567890123456789012345678901234567890"),
+       "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+       "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+       "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+       300,
+       S("1234567890123456789012345678901234567890aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+         "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+         "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+         "aaaaaaaaaaaaa"));
 }
 
 TEST_CONSTEXPR_CXX20 bool test() {