[gcc r17-356] libstdc++: Format all contiguous ranges usign spoan.

[Tomasz Kaminski]

https://gcc.gnu.org/g:9b076412e6aecac70504607b5181483ef3e6d754

commit r17-356-g9b076412e6aecac70504607b5181483ef3e6d754
Author: Tomasz Kamiński <tkaminsk@redhat.com>
Date:   Wed Apr 29 16:06:16 2026 +0200

    libstdc++: Format all contiguous ranges usign spoan.
    
    We convert all contiguous ranges into span of the possibly-const qualified
    _Tp (__format::__maybe_const<_Tp, _CharT>). The span object is const qualifed,
    to trigger _M_format<const span<T>> specialization, that is already used by
    formatter specialization for ranges.
    
    This conversion is applied regardless if range is sized, and ranges::distance
    to compute the size. Even for user-defined iterators, they will observe range
    being iterated only once.
    
    Finally, using raw pointers to iterate range during formatting guarantees,
    that no stdio lock will be taken. In consequence it would be now possible
    to enable_nonlocking_formatter_optimization for all contiguous ranges.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/format (range_formatter::format): Format all
            contiguous ranges as span<__format::__maybe_const<_Tp, _CharT>>.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
    Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/include/std/format | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 4627381fa9e7..60adca93b5d8 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -6175,7 +6175,13 @@ namespace __format
 	format(_Rg&& __rg, basic_format_context<_Out, _CharT>& __fc) const
 	{
 	  using _Range = remove_reference_t<_Rg>;
-	  if constexpr (__format::__simply_formattable_range<_Range, _CharT>)
+	  if constexpr (ranges::contiguous_range<_Rg>)
+	    {
+	      const span<__format::__maybe_const<_Tp, _CharT>>
+		__spn(ranges::data(__rg), size_t(ranges::distance(__rg)));
+	      return _M_format(__spn, __fc);
+	    }
+	  else if constexpr (__format::__simply_formattable_range<_Range, _CharT>)
 	    return _M_format<const _Range>(__rg, __fc);
 	  else
 	    return _M_format(__rg, __fc);