[libc++][format] Implement format-string.

Implements the compile-time checking of the formatting arguments.

Completes:
- P2216 std::format improvements

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D121530

Author: mordante

libcxx/benchmarks/formatter_float.bench.cpp

@@ -216,17 +216,27 @@ struct FloatingPoint {
   void run(benchmark::State& state) const {
     std::array<F, 1000> data{Value<V::value>::template make_data<F>()};
     std::array<char, 20'000> output;
-    std::string fmt{std::string("{:") + Alignment<A::value>::fmt + Precision<P::value>::fmt +
-                    Localization<L::value>::fmt + DisplayType<DT::value>::fmt + "}"};
 
     while (state.KeepRunningBatch(1000))
       for (F value : data)
-        benchmark::DoNotOptimize(std::format_to(output.begin(), fmt, value));
+        benchmark::DoNotOptimize(std::format_to(output.begin(), std::string_view{fmt.data(), fmt.size()}, value));
   }
 
   std::string name() const {
     return "FloatingPoint" + L::name() + DT::name() + T::name() + V::name() + A::name() + P::name();
   }
+
+  static constexpr std::string make_fmt() {
+    return std::string("{:") + Alignment<A::value>::fmt + Precision<P::value>::fmt + Localization<L::value>::fmt +
+           DisplayType<DT::value>::fmt + "}";
+  }
+
+  static constexpr auto fmt = []() {
+    constexpr size_t s = make_fmt().size();
+    std::array<char, s> r;
+    std::ranges::copy(make_fmt(), r.begin());
+    return r;
+  }();
 };
 
 int main(int argc, char** argv) {

libcxx/docs/ReleaseNotes.rst

@@ -73,6 +73,14 @@ New Features
   moved from the dylib to the header. This means the function no longer has a
   minimum deployment target.
 
+- Implemented P2216R3 (std::format improvements). The format functions
+  (``std::format``, ``std::format_to``, ``std::format_to_n``, and
+  ``std::formatted_size``) now validate the format string at compile time.
+  When the format string is invalid this will make the code ill-formed instead
+  of throwing an exception at run-time.  (This does not affect the ``v``
+  functions.)
+
+
 API Changes
 -----------
 

libcxx/docs/Status/Cxx20Papers.csv

@@ -196,7 +196,7 @@
 "`P2231R1 <https://wg21.link/P2231R1>`__","LWG","Missing constexpr in std::optional and std::variant","June 2021","|Partial| [#note-P2231]_","13.0"
 "`P2325R3 <https://wg21.link/P2325R3>`__","LWG","Views should not be required to be default constructible","June 2021","|In progress|",""
 "`P2210R2 <https://wg21.link/P2210R2>`__","LWG",Superior String Splitting,"June 2021","",""
-"`P2216R3 <https://wg21.link/P2216R3>`__","LWG",std::format improvements,"June 2021","|Partial|",""
+"`P2216R3 <https://wg21.link/P2216R3>`__","LWG",std::format improvements,"June 2021","|Complete|","15.0"
 "`P2281R1 <https://wg21.link/P2281R1>`__","LWG",Clarifying range adaptor objects,"June 2021","|Complete|","14.0"
 "`P2328R1 <https://wg21.link/P2328R1>`__","LWG",join_view should join all views of ranges,"June 2021","",""
 "`P2367R0 <https://wg21.link/P2367R0>`__","LWG",Remove misuses of list-initialization from Clause 24,"June 2021","",""

libcxx/include/__format/parser_std_format_spec.h

@@ -252,25 +252,21 @@ class _LIBCPP_TYPE_VIS __parser_width {
   /** Determines whether the value stored is a width or an arg-id. */
   uint32_t __width_as_arg : 1 {0};
 
-protected:
   /**
-   * Does the supplied std-format-spec contain a width field?
+   * Does the supplied width field contain an arg-id?
    *
-   * When the field isn't present there's no padding required. This can be used
-   * to optimize the formatting.
+   * If @c true the formatter needs to call @ref __substitute_width_arg_id.
    */
-  constexpr bool __has_width_field() const noexcept {
-    return __width_as_arg || __width;
-  }
+  constexpr bool __width_needs_substitution() const noexcept { return __width_as_arg; }
 
+protected:
   /**
-   * Does the supplied width field contain an arg-id?
+   * Does the supplied std-format-spec contain a width field?
    *
-   * If @c true the formatter needs to call @ref __substitute_width_arg_id.
+   * When the field isn't present there's no padding required. This can be used
+   * to optimize the formatting.
    */
-  constexpr bool __width_needs_substitution() const noexcept {
-    return __width_as_arg;
-  }
+  constexpr bool __has_width_field() const noexcept { return __width_as_arg || __width; }
 
   template <class _CharT>
   _LIBCPP_HIDE_FROM_ABI constexpr const _CharT*
@@ -327,6 +323,15 @@ class _LIBCPP_TYPE_VIS __parser_precision {
    */
   uint32_t __precision_as_arg : 1 {1};
 
+  /**
+   * Does the supplied precision field contain an arg-id?
+   *
+   * If @c true the formatter needs to call @ref __substitute_precision_arg_id.
+   */
+  constexpr bool __precision_needs_substitution() const noexcept {
+    return __precision_as_arg && __precision != __format::__number_max;
+  }
+
 protected:
   /**
    * Does the supplied std-format-spec contain a precision field?
@@ -340,15 +345,6 @@ class _LIBCPP_TYPE_VIS __parser_precision {
            __precision != __format::__number_max; // The arg-id is valid?
   }
 
-  /**
-   * Does the supplied precision field contain an arg-id?
-   *
-   * If @c true the formatter needs to call @ref __substitute_precision_arg_id.
-   */
-  constexpr bool __precision_needs_substitution() const noexcept {
-    return __precision_as_arg && __precision != __format::__number_max;
-  }
-
   template <class _CharT>
   _LIBCPP_HIDE_FROM_ABI constexpr const _CharT*
   __parse(const _CharT* __begin, const _CharT* __end, auto& __parse_ctx) {