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[libc++][string_view] Applied [[nodiscard]] #169010

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Merged
merged 4 commits into from
Nov 24, 2025
Merged

[libc++][string_view] Applied [[nodiscard]] #169010

merged 4 commits into from
Nov 24, 2025

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@H-G-Hristov
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@H-G-Hristov H-G-Hristov commented Nov 21, 2025

[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue.

@H-G-Hristov H-G-Hristov requested a review from a team as a code owner November 21, 2025 07:14
@llvmbot llvmbot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Nov 21, 2025
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llvmbot commented Nov 21, 2025

@llvm/pr-subscribers-libcxx

Author: Hristo Hristov (H-G-Hristov)

Changes

[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue.

Patch is 27.40 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/169010.diff

2 Files Affected:

  • (modified) libcxx/include/string_view (+67-56)
  • (modified) libcxx/test/libcxx/diagnostics/string_view.nodiscard.verify.cpp (+101-3)
diff --git a/libcxx/include/string_view b/libcxx/include/string_view
index 5ecaa3de7deba..72612a3c1e856 100644
--- a/libcxx/include/string_view
+++ b/libcxx/include/string_view
@@ -362,11 +362,11 @@ public:
 #  endif
 
   // [string.view.iterators], iterators
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_iterator begin() const _NOEXCEPT { return cbegin(); }
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_iterator begin() const _NOEXCEPT { return cbegin(); }
 
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_iterator end() const _NOEXCEPT { return cend(); }
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_iterator end() const _NOEXCEPT { return cend(); }
 
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_iterator cbegin() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_iterator cbegin() const _NOEXCEPT {
 #  ifdef _LIBCPP_ABI_BOUNDED_ITERATORS
     return std::__make_bounded_iter(data(), data(), data() + size());
 #  else
@@ -374,7 +374,7 @@ public:
 #  endif
   }
 
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_iterator cend() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_iterator cend() const _NOEXCEPT {
 #  ifdef _LIBCPP_ABI_BOUNDED_ITERATORS
     return std::__make_bounded_iter(data() + size(), data(), data() + size());
 #  else
@@ -382,51 +382,54 @@ public:
 #  endif
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX17 _LIBCPP_HIDE_FROM_ABI const_reverse_iterator rbegin() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX17 _LIBCPP_HIDE_FROM_ABI const_reverse_iterator
+  rbegin() const _NOEXCEPT {
     return const_reverse_iterator(cend());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX17 _LIBCPP_HIDE_FROM_ABI const_reverse_iterator rend() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX17 _LIBCPP_HIDE_FROM_ABI const_reverse_iterator rend() const _NOEXCEPT {
     return const_reverse_iterator(cbegin());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX17 _LIBCPP_HIDE_FROM_ABI const_reverse_iterator crbegin() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX17 _LIBCPP_HIDE_FROM_ABI const_reverse_iterator
+  crbegin() const _NOEXCEPT {
     return const_reverse_iterator(cend());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX17 _LIBCPP_HIDE_FROM_ABI const_reverse_iterator crend() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX17 _LIBCPP_HIDE_FROM_ABI const_reverse_iterator crend() const _NOEXCEPT {
     return const_reverse_iterator(cbegin());
   }
 
   // [string.view.capacity], capacity
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI size_type size() const _NOEXCEPT { return __size_; }
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI size_type size() const _NOEXCEPT { return __size_; }
 
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI size_type length() const _NOEXCEPT { return __size_; }
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI size_type length() const _NOEXCEPT { return __size_; }
 
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI size_type max_size() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI size_type max_size() const _NOEXCEPT {
     return numeric_limits<size_type>::max() / sizeof(value_type);
   }
 
   [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool empty() const _NOEXCEPT { return __size_ == 0; }
 
   // [string.view.access], element access
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_reference operator[](size_type __pos) const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_reference
+  operator[](size_type __pos) const _NOEXCEPT {
     return _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__pos < size(), "string_view[] index out of bounds"), __data_[__pos];
   }
 
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_reference at(size_type __pos) const {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_reference at(size_type __pos) const {
     return __pos >= size() ? (__throw_out_of_range("string_view::at"), __data_[0]) : __data_[__pos];
   }
 
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_reference front() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_reference front() const _NOEXCEPT {
     return _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "string_view::front(): string is empty"), __data_[0];
   }
 
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_reference back() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_reference back() const _NOEXCEPT {
     return _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "string_view::back(): string is empty"), __data_[__size_ - 1];
   }
 
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_pointer data() const _NOEXCEPT { return __data_; }
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI const_pointer data() const _NOEXCEPT { return __data_; }
 
   // [string.view.modifiers], modifiers:
   _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI void remove_prefix(size_type __n) _NOEXCEPT {
@@ -459,7 +462,8 @@ public:
     return __rlen;
   }
 
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI basic_string_view substr(size_type __pos = 0, size_type __n = npos) const {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI basic_string_view
+  substr(size_type __pos = 0, size_type __n = npos) const {
     // Use the `__assume_valid` form of the constructor to avoid an unnecessary check. Any substring of a view is a
     // valid view. In particular, `size()` is known to be smaller than `numeric_limits<difference_type>::max()`, so the
     // new size is also smaller. See also https://llvm.org/PR91634.
@@ -474,7 +478,7 @@ public:
   }
 #  endif
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 int compare(basic_string_view __sv) const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 int compare(basic_string_view __sv) const _NOEXCEPT {
     size_type __rlen = std::min(size(), __sv.size());
     int __retval     = _Traits::compare(data(), __sv.data(), __rlen);
     if (__retval == 0) // first __rlen chars matched
@@ -482,50 +486,51 @@ public:
     return __retval;
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI int
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI int
   compare(size_type __pos1, size_type __n1, basic_string_view __sv) const {
     return substr(__pos1, __n1).compare(__sv);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI int
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI int
   compare(size_type __pos1, size_type __n1, basic_string_view __sv, size_type __pos2, size_type __n2) const {
     return substr(__pos1, __n1).compare(__sv.substr(__pos2, __n2));
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI int
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI int
   compare(const _CharT* _LIBCPP_DIAGNOSE_NULLPTR __s) const _NOEXCEPT {
     return compare(basic_string_view(__s));
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI int
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI int
   compare(size_type __pos1, size_type __n1, const _CharT* _LIBCPP_DIAGNOSE_NULLPTR __s) const {
     return substr(__pos1, __n1).compare(basic_string_view(__s));
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI int
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI int
   compare(size_type __pos1, size_type __n1, const _CharT* __s, size_type __n2) const
       _LIBCPP_DIAGNOSE_NULLPTR_IF(__n2 != 0 && __s == nullptr, " if n2 is not zero") {
     return substr(__pos1, __n1).compare(basic_string_view(__s, __n2));
   }
 
   // find
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find(basic_string_view __s, size_type __pos = 0) const _NOEXCEPT {
     return std::__str_find<value_type, size_type, traits_type, npos>(data(), size(), __s.data(), __pos, __s.size());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type find(_CharT __c, size_type __pos = 0) const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  find(_CharT __c, size_type __pos = 0) const _NOEXCEPT {
     return std::__str_find<value_type, size_type, traits_type, npos>(data(), size(), __c, __pos);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find(const _CharT* __s, size_type __pos, size_type __n) const _NOEXCEPT
       _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero") {
     _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "string_view::find(): received nullptr");
     return std::__str_find<value_type, size_type, traits_type, npos>(data(), size(), __s, __pos, __n);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find(const _CharT* _LIBCPP_DIAGNOSE_NULLPTR __s, size_type __pos = 0) const _NOEXCEPT {
     _LIBCPP_ASSERT_NON_NULL(__s != nullptr, "string_view::find(): received nullptr");
     return std::__str_find<value_type, size_type, traits_type, npos>(
@@ -533,24 +538,24 @@ public:
   }
 
   // rfind
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   rfind(basic_string_view __s, size_type __pos = npos) const _NOEXCEPT {
     return std::__str_rfind<value_type, size_type, traits_type, npos>(data(), size(), __s.data(), __pos, __s.size());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   rfind(_CharT __c, size_type __pos = npos) const _NOEXCEPT {
     return std::__str_rfind<value_type, size_type, traits_type, npos>(data(), size(), __c, __pos);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   rfind(const _CharT* __s, size_type __pos, size_type __n) const _NOEXCEPT
       _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero") {
     _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "string_view::rfind(): received nullptr");
     return std::__str_rfind<value_type, size_type, traits_type, npos>(data(), size(), __s, __pos, __n);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   rfind(const _CharT* _LIBCPP_DIAGNOSE_NULLPTR __s, size_type __pos = npos) const _NOEXCEPT {
     _LIBCPP_ASSERT_NON_NULL(__s != nullptr, "string_view::rfind(): received nullptr");
     return std::__str_rfind<value_type, size_type, traits_type, npos>(
@@ -558,25 +563,25 @@ public:
   }
 
   // find_first_of
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_first_of(basic_string_view __s, size_type __pos = 0) const _NOEXCEPT {
     return std::__str_find_first_of<value_type, size_type, traits_type, npos>(
         data(), size(), __s.data(), __pos, __s.size());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_first_of(_CharT __c, size_type __pos = 0) const _NOEXCEPT {
     return find(__c, __pos);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_first_of(const _CharT* __s, size_type __pos, size_type __n) const _NOEXCEPT
       _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero") {
     _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "string_view::find_first_of(): received nullptr");
     return std::__str_find_first_of<value_type, size_type, traits_type, npos>(data(), size(), __s, __pos, __n);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_first_of(const _CharT* _LIBCPP_DIAGNOSE_NULLPTR __s, size_type __pos = 0) const _NOEXCEPT {
     _LIBCPP_ASSERT_NON_NULL(__s != nullptr, "string_view::find_first_of(): received nullptr");
     return std::__str_find_first_of<value_type, size_type, traits_type, npos>(
@@ -584,25 +589,25 @@ public:
   }
 
   // find_last_of
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_last_of(basic_string_view __s, size_type __pos = npos) const _NOEXCEPT {
     return std::__str_find_last_of<value_type, size_type, traits_type, npos>(
         data(), size(), __s.data(), __pos, __s.size());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_last_of(_CharT __c, size_type __pos = npos) const _NOEXCEPT {
     return rfind(__c, __pos);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_last_of(const _CharT* __s, size_type __pos, size_type __n) const _NOEXCEPT
       _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero") {
     _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "string_view::find_last_of(): received nullptr");
     return std::__str_find_last_of<value_type, size_type, traits_type, npos>(data(), size(), __s, __pos, __n);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_last_of(const _CharT* _LIBCPP_DIAGNOSE_NULLPTR __s, size_type __pos = npos) const _NOEXCEPT {
     _LIBCPP_ASSERT_NON_NULL(__s != nullptr, "string_view::find_last_of(): received nullptr");
     return std::__str_find_last_of<value_type, size_type, traits_type, npos>(
@@ -610,25 +615,25 @@ public:
   }
 
   // find_first_not_of
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_first_not_of(basic_string_view __s, size_type __pos = 0) const _NOEXCEPT {
     return std::__str_find_first_not_of<value_type, size_type, traits_type, npos>(
         data(), size(), __s.data(), __pos, __s.size());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_first_not_of(_CharT __c, size_type __pos = 0) const _NOEXCEPT {
     return std::__str_find_first_not_of<value_type, size_type, traits_type, npos>(data(), size(), __c, __pos);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_first_not_of(const _CharT* __s, size_type __pos, size_type __n) const _NOEXCEPT
       _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero") {
     _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "string_view::find_first_not_of(): received nullptr");
     return std::__str_find_first_not_of<value_type, size_type, traits_type, npos>(data(), size(), __s, __pos, __n);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_first_not_of(const _CharT* _LIBCPP_DIAGNOSE_NULLPTR __s, size_type __pos = 0) const _NOEXCEPT {
     _LIBCPP_ASSERT_NON_NULL(__s != nullptr, "string_view::find_first_not_of(): received nullptr");
     return std::__str_find_first_not_of<value_type, size_type, traits_type, npos>(
@@ -636,25 +641,25 @@ public:
   }
 
   // find_last_not_of
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_last_not_of(basic_string_view __s, size_type __pos = npos) const _NOEXCEPT {
     return std::__str_find_last_not_of<value_type, size_type, traits_type, npos>(
         data(), size(), __s.data(), __pos, __s.size());
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_last_not_of(_CharT __c, size_type __pos = npos) const _NOEXCEPT {
     return std::__str_find_last_not_of<value_type, size_type, traits_type, npos>(data(), size(), __c, __pos);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_last_not_of(const _CharT* __s, size_type __pos, size_type __n) const _NOEXCEPT
       _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero") {
     _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "string_view::find_last_not_of(): received nullptr");
     return std::__str_find_last_not_of<value_type, size_type, traits_type, npos>(data(), size(), __s, __pos, __n);
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI size_type
   find_last_not_of(const _CharT* _LIBCPP_DIAGNOSE_NULLPTR __s, size_type __pos = npos) const _NOEXCEPT {
     _LIBCPP_ASSERT_NON_NULL(__s != nullptr, "string_view::find_last_not_of(): received nullptr");
     return std::__str_find_last_not_of<value_type, size_type, traits_type, npos>(
@@ -662,37 +667,43 @@ public:
   }
 
 #  if _LIBCPP_STD_VER >= 20
-  constexpr _LIBCPP_HIDE_FROM_ABI bool starts_with(basic_string_view __s) const noexcept {
+  [[nodiscard]] constexpr _LIBCPP_HIDE_FROM_ABI bool starts_with(basic_string_view __s) const noexcept {
     return size() >= __s.size() && compare(0, __s.size(), __s) == 0;
   }
 
-  constexpr _LIBCPP_HIDE_FROM_ABI bool starts_with(value_type __c) const noexcept {
+  [[nodiscard]] constexpr _LIBCPP_HIDE_FROM_ABI bool starts_with(value_type __c) const noexcept {
     return !empty() && _Traits::eq(front(), __c);
   }
 
-  constexpr _LIBCPP_HIDE_FROM_ABI bool starts_with(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s) const noexcept {
+  [[nodiscard]] constexpr _LIBCPP_HIDE_FROM_ABI bool
+  starts_with(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s) const noexcept {
     return starts_with(basic_string_view(__s));
   }
 
-  constexpr _LIBCPP_HIDE_FROM_ABI bool ends_with(basic_string_view __s) const noexcept {
+  [[nodiscard]] constexpr _LIBCPP_HIDE_FROM_ABI bool ends_with(basic_string_view __s) const noexcept {
     return size() >= __s.size() && compare(size() - __s.size(), npos, __s) == 0;
   }
 
-  constexpr _LIBCPP_HIDE_FROM_ABI bool ends_with(value_type __c) const noexcept {
+  [[nodiscard]] constexpr _LIBCPP_HIDE_FROM_ABI bool ends_with(value_type __c) const noexcept {
     return !empty() && _Traits::eq(back(), __c);
   }
 
-  constexpr _LIBCPP_HIDE_FROM_ABI bool ends_with(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s) const noexcept {
+  [[nodiscard]] constexpr _LIBCPP_HIDE_FROM_ABI bool
+  ends_with(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s) const noexcept {
     return ends_with(basic_string_view(__s));
   }
 #  endif
 
 #  if _LIBCPP_STD_VER >= 23
-  constexpr _LIBCPP_HIDE_FROM_ABI bool contains(basic_string_view __sv) const noexcept { return find(__sv) != npos; }
+  [[nodiscard]] constexpr _LIBCPP_HIDE_FROM_ABI bool contains(basic_string_view __sv) const noexcept {
+    return find(__sv) != npos;
+  }
 
-  constexpr _LIBCPP_HIDE_FROM_ABI bool contains(value_type __c) const noexcept { return find(__c) != npos; }
+  [[nodiscard]] constexpr _LIBCPP_HIDE_FROM_ABI bool contains(value_type __c) const noexcept {
+    return find(__c) != npos;
+  }
 
-  constexpr _LIBCPP_HIDE_FROM_ABI bool contains(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s) const {
+  [[nodiscard]] constexpr _LIBCPP_HIDE_FROM_ABI bool contains(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s) const {
     return find(__s) != npos;
   }
 #  endif
diff --git a/libcxx/test/libcxx/diagnostics/string_view.nodiscard.verify.cpp b/libcxx/test/libcxx/diagnostics/string_view....
[truncated]

@H-G-Hristov H-G-Hristov force-pushed the hgh/libcxx/nodiscard-to-string_view branch from 06ba8c1 to 12fdd84 Compare November 21, 2025 07:37
philnik777
philnik777 reviewed Nov 21, 2025
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@H-G-Hristov H-G-Hristov force-pushed the hgh/libcxx/nodiscard-to-string_view branch 4 times, most recently from 34b90fb to b479d4d Compare November 21, 2025 20:35
@H-G-Hristov
[libc++][string_view] Applied [[nodiscard]]
ec331c6 
`[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue.
- https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant
@H-G-Hristov H-G-Hristov force-pushed the hgh/libcxx/nodiscard-to-string_view branch from b479d4d to ec331c6 Compare November 21, 2025 20:38
philnik777
philnik777 approved these changes Nov 24, 2025
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libcxx/test/libcxx/diagnostics/string_view.nodiscard.verify.cpp
Comment on lines +143 to +144
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
""sv; // const char*
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@philnik777 philnik777 Nov 24, 2025

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I wonder whether it would make sense for Clang to warn by default if the return value is discarded. I can't really think of a case where it makes sense to have a UDL that is discarded. Could you file a bug against Clang for this?

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@H-G-Hristov H-G-Hristov Nov 24, 2025

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Filed an issue: #169315

Do you want me to remove them?

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@philnik777 philnik777 Nov 24, 2025

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Not until Clang actually issues a diagnostic.

@H-G-Hristov H-G-Hristov mentioned this pull request Nov 24, 2025
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@Zingam Zingam merged commit e3d0ac1 into llvm:main Nov 24, 2025
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@H-G-Hristov H-G-Hristov deleted the hgh/libcxx/nodiscard-to-string_view branch November 24, 2025 17:19
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