[libc] Add `-Wno-sign-conversion` & re-attempt `-Wconversion` (#129811)

Relates to
https://github.com/llvm/llvm-project/issues/119281#issuecomment-2699470459

2 files changed, 12 insertions, 9 deletions

diff --git a/libc/src/string/memory_utils/utils.h b/libc/src/string/memory_utils/utils.h
index 5c9bc72208f8..bdf0b8652188 100644
--- a/libc/src/string/memory_utils/utils.h
+++ b/libc/src/string/memory_utils/utils.h
@@ -263,7 +263,7 @@ LIBC_INLINE void store_aligned(ValueType value, Ptr dst) {
   static_assert(sizeof(ValueType) >= (sizeof(T) + ... + sizeof(TS)));
   constexpr size_t SHIFT = sizeof(T) * 8;
   if constexpr (Endian::IS_LITTLE) {
-    store<T>(assume_aligned<sizeof(T)>(dst), value & ~T(0));
+    store<T>(assume_aligned<sizeof(T)>(dst), T(value & T(~0)));
     if constexpr (sizeof...(TS) > 0)
       store_aligned<ValueType, TS...>(value >> SHIFT, dst + sizeof(T));
   } else if constexpr (Endian::IS_BIG) {
@@ -297,7 +297,7 @@ LIBC_INLINE void adjust(ptrdiff_t offset, T1 *__restrict &p1,
                         T2 *__restrict &p2, size_t &count) {
   p1 += offset;
   p2 += offset;
-  count -= offset;
+  count -= static_cast<size_t>(offset);
 }
 
 // Advances p1 and p2 so p1 gets aligned to the next SIZE bytes boundary
@@ -306,7 +306,8 @@ LIBC_INLINE void adjust(ptrdiff_t offset, T1 *__restrict &p1,
 template <size_t SIZE, typename T1, typename T2>
 void align_p1_to_next_boundary(T1 *__restrict &p1, T2 *__restrict &p2,
                                size_t &count) {
-  adjust(distance_to_next_aligned<SIZE>(p1), p1, p2, count);
+  adjust(static_cast<ptrdiff_t>(distance_to_next_aligned<SIZE>(p1)), p1, p2,
+         count);
   p1 = assume_aligned<SIZE>(p1);
 }
 
diff --git a/libc/src/string/string_utils.h b/libc/src/string/string_utils.h
index 583d35014d39..e4659f65c93e 100644
--- a/libc/src/string/string_utils.h
+++ b/libc/src/string/string_utils.h
@@ -66,7 +66,7 @@ LIBC_INLINE size_t string_length_wide_read(const char *src) {
   for (; reinterpret_cast<uintptr_t>(char_ptr) % sizeof(Word) != 0;
        ++char_ptr) {
     if (*char_ptr == '\0')
-      return char_ptr - src;
+      return static_cast<size_t>(char_ptr - src);
   }
   // Step 2: read blocks
   for (const Word *block_ptr = reinterpret_cast<const Word *>(char_ptr);
@@ -77,7 +77,7 @@ LIBC_INLINE size_t string_length_wide_read(const char *src) {
   for (; *char_ptr != '\0'; ++char_ptr) {
     ;
   }
-  return char_ptr - src;
+  return static_cast<size_t>(char_ptr - src);
 }
 
 // Returns the length of a string, denoted by the first occurrence
@@ -169,7 +169,7 @@ LIBC_INLINE size_t complementary_span(const char *src, const char *segment) {
   for (; *src && !bitset.test(*reinterpret_cast<const unsigned char *>(src));
        ++src)
     ;
-  return src - initial;
+  return static_cast<size_t>(src - initial);
 }
 
 // Given the similarities between strtok and strtok_r, we can implement both
@@ -189,12 +189,14 @@ LIBC_INLINE char *string_token(char *__restrict src,
   if (LIBC_UNLIKELY(src == nullptr && ((src = *saveptr) == nullptr)))
     return nullptr;
 
+  static_assert(sizeof(char) == sizeof(cpp::byte),
+                "bitset of 256 assumes char is 8 bits");
   cpp::bitset<256> delimiter_set;
   for (; *delimiter_string != '\0'; ++delimiter_string)
-    delimiter_set.set(*delimiter_string);
+    delimiter_set.set(static_cast<size_t>(*delimiter_string));
 
   if constexpr (SkipDelim)
-    for (; *src != '\0' && delimiter_set.test(*src); ++src)
+    for (; *src != '\0' && delimiter_set.test(static_cast<size_t>(*src)); ++src)
       ;
   if (*src == '\0') {
     *saveptr = src;
@@ -202,7 +204,7 @@ LIBC_INLINE char *string_token(char *__restrict src,
   }
   char *token = src;
   for (; *src != '\0'; ++src) {
-    if (delimiter_set.test(*src)) {
+    if (delimiter_set.test(static_cast<size_t>(*src))) {
       *src = '\0';
       ++src;
       break;