[libc++] Apply `[[nodiscard]]` to the Standard Library

[H-G-Hristov]

Applying [[nodiscard]] to libc++ is in progress in compliance with the Coding Guidelines:

• Apply [[nodiscard]] where relevant - https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant

Large parts of libc++ are already annotated. To request additional changes, add to the list bellow alphabetically or post a comment:

References:

• libc++ Coding Guidelines
• C++ Standard

Known status:

• 17 Language support library [support]
  • [libc++] Applied [[nodiscard]] to Language Support (partially) #169611
  • Exceptions [libc++] Applied [[nodiscard]] to <exception>, <stdexcept> and <system_error> #170837
• 20 Memory management library [mem]
  • Allocators [libc+] Applied [[nodiscard]] to Allocators #172131
  • in/out_ptr [libc++] Apply [[nodiscard]] to in/out_ptr #167097
  • memory_resource [libc++][memory_resource] Applied [[nodiscard]] #172134
  • unique/shared/weak_ptr [libc++][memory] Applied [[nodiscard]] to smart pointers #168483
• 22 General utilities library [utilities]
  • any [libc++][any] Applied [[nodiscard]] #168826
  • bind/_front/_back, function, mem_fn, reference_wrapper [libc++] Applied [[nodiscard]] to some general utilities #169322
  • bitset [libc++][bitset] Applied [[nodiscard]] #170623
  • expected [libc++][expected] Applied [[nodiscard]] #170245
  • hash [libc++] Applied [[nodiscard]] to hash<shared_ptr>, hash<unique_ptr>, etc. #170674
  • optional [libc++][optional] Applied [[nodiscard]] #170045
  • pair [libc++][pair] Applied [[nodiscard]] #171999
  • tuple [libc++][tuple] Applied [[nodiscard]] #172008
  • variant [libc++][variant] Applied [[nodiscard]] #172058
• 23 Containers library [containers]
  • array [libc++][array] Applied [[nodiscard]] #168829
  • deque [libc++][deque] Applied [[nodiscard]] #169745
  • flat_map [libc++][flat_map] Applied [[nodiscard]] #169453
  • flat_multimap [libc++][flat_multimap] Applied [[nodiscard]] #169986
  • flat_multiset [libc++][flat_multiset] Applied [[nodiscard]] #169984
  • flat_set [libc++][flat_set] Applied [[nodiscard]] #169739
  • forward_list [libc++][forward_list] Applied [[nodiscard]] #169019
  • list [libc++][list] Applied [[nodiscard]] #169015
  • mdspan [libc++][mdspan] Applied [[nodiscard]] #169326
  • map [libc++][map] Applied [[nodiscard]] #169971
  • multimap [libc++][multimap] Applied [[nodiscard]] #171644
  • multiset [libc++][multiset] Applied [[nodiscard]] #171654
  • queue [libc++][queue] Applied [[nodiscard]] #169469
  • span [libc++][span] Mark functions as [[nodiscard]] #168033
  • set [libc++][set] Applied [[nodiscard]] #169982
  • stack [libc++][stack] Applied [[nodiscard]] #169468
  • unordered_map [libc++][unordered_map] Applied [[nodiscard]] #170423
  • unordered_multimap [libc++][unordered_multimap] Applied [[nodiscard]] #171659
  • unordered_set [libc++][unordered_set] Applied [[nodiscard]] #170435
  • unordered_multiset [libc++][unordered_multiset] Applied [[nodiscard]] #171664
• 24 Iterators library [iterators]
• 25 Ranges library [ranges]
  • views::as_rvalue
  • views::common
  • views::drop
  • views::drop_while
  • views::empty
• 26 Algorithms library [algorithms]
• 27 Strings library [strings]
  • string [libc++] Mark string functions as [[nodiscard]] #166524, [libc++][string] Applied [[nodiscard]] to non-member functions #169330
  • string_view [libc++][string_view] Applied [[nodiscard]] #169010
• 28 Text processing library [text]
• format [libc++][format] Applied [[nodiscard]] to more classes #170808
• regex [libc++][regex] Applied [[nodiscard]] #170974
• 29 Numerics library [numerics]
  • complex [libc++][complex] Applied [[nodiscard]] #171027
  • cmath [libc++][cmath] Adding [[nodicard]] #171763
  • Saturation Arithmetic [libc++][numeric] Marked saturation artithmetic functions as [[nodiscard]] #166898
  • valarray [libc++][valarray] Applied [[nodiscard]] #170996
• 30 Time library [time]
• 31 Input/output library [input.output]
  • filesystem [libc++][filesystem] Applied [[nodiscard]] #171085
  • Streams
    • basic_ios
    • basic_iostream
    • basic_istream
    • basic_ostream
    • basic_streambuf
    • ios_base
• 32 Concurrency support library [thread]
  • barrier [libc++] Applied [[nodiscard]] to concurrency (partially) #169463
  • condition_variable [libc++] Applied [[nodiscard]] to concurrency (partially) #169463
  • latch [libc++] Applied [[nodiscard]] to concurrency (partially) #169463
  • mutex [libc++] Applied [[nodiscard]] to concurrency (partially) #169463
  • semaphore [libc++] Applied [[nodiscard]] to concurrency (partially) #169463
  • thread [libc++] Applied [[nodiscard]] to concurrency (partially) #169463