Source location in Debug

[mosra]

Similarly to the new dbg! macro in Rust, this implements source location for Debug using std::experiemntal::source_location. At the moment this feature is present only in GCC 8.1 / libstdc++ and up and a builtin in Clang 9:

• no traces of std::source_location in libc++ yet as of November: https://github.com/llvm-mirror/libcxx/tree/master/include/experimental using the builtins instead
• feature request for MSVC (it apparently won't be in C++20?): https://developercommunity.visualstudio.com/idea/354069/implement-c-library-fundamentals-ts-v2.html, tracking issue: P1208R6 <source_location> microsoft/STL#54 -- builtins implemented for MSVC 2019 16.4 it seems

This is an experiment that needs further design iterations:

• At the moment, all Debug{}, Error{} etc. constructors are acquiring the source location and saving it even if it might not be used, which means extreme increase in binary sizes. While this is convenient and doesn't require changes on the user side (it's just enabling a flag and then all following debug statements in the same scope get source location), it's not efficient. Some ideas:
  • Provide a dedicated function such as debug() that instantiates Debug and with source location (bad, it requires users to learn a new thing, also we would need to do an error(), fatal() etc. and there's a high chance these names will clash with user code)
  • Provide a constructor tag such as Debug{Debug::SourceLocation} (again kinda bad, since it's verbose to say every time)
  • Save source file name only at the point of enabling the debug output but save line in every constructor (would need some way to detect what file we are in in order to not print file info from unrelated files, maybe abusing unnamed namespaces?)
  • Provide a constructor tag that explicitly does not use source location and then use that in all the library internals to minimize the impact (goes against "don't pay for what you don't use", doesn't really help users)
  • Use !Debug{}
• The Rust dbg! macro works inside if() as well, provide something like that too? not sure how
• The GCC implementation requires C++14, I'm working around that by #undef constexpr for C++11 (which is insane), submit a PR to libstdc++ that uses some C++14 constexpr instead? -- using the builtins directly
• The file+line info output format is probably not the best (use colors? wrap in [] like Rust does? make it format()able, allowing stuff like column, date and other things there?) postponed, will be done together with arbitrary line prefixing
• Some way to print just file & line without anything else like Rust 1.35 has

Obligatory docs:

Further design ideas welcome! :)

[codecov-io]

Codecov Report

Merging #55 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #55      +/-   ##
=========================================
+ Coverage   97.49%   97.5%   +<.01%     
=========================================
  Files          87      81       -6     
  Lines        5796    5442     -354     
=========================================
- Hits         5651    5306     -345     
+ Misses        145     136       -9

Impacted Files	Coverage Δ
src/Corrade/Utility/Debug.cpp	96.75% <ø> (+0.96%)	⬆️
src/Corrade/Utility/Debug.h	100% <100%> (ø)	⬆️
src/Corrade/Interconnect/Connection.cpp	36.36% <0%> (-63.64%)	⬇️
...rrade/TestSuite/Implementation/BenchmarkCounters.h	88.88% <0%> (-11.12%)	⬇️
src/Corrade/Utility/FileWatcher.cpp	96.66% <0%> (-3.34%)	⬇️
src/Corrade/Utility/Arguments.cpp	98.06% <0%> (-1.94%)	⬇️
src/Corrade/Utility/Unicode.cpp	98.09% <0%> (-1.91%)	⬇️
src/Corrade/Utility/Directory.cpp	88.46% <0%> (-0.47%)	⬇️
src/Corrade/TestSuite/Tester.cpp	93.36% <0%> (-0.31%)	⬇️
src/Corrade/PluginManager/AbstractManager.cpp	97.66% <0%> (-0.25%)	⬇️
... and 50 more

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[mosra]

Rust 1.35 has an empty dbg! macro to print just the file & line. Added that to the list above.

[mosra]

Invented a much better version that's opt-in, easy to use and doesn't bloat the binary when not used. The last remaining blocker is #define constexpr, but I might just ignore that problem for now.

[mosra]

To have this working on both Clang and GCC, I'll probably change to using the compiler builtins, and that'll solve the #define constexpr as well.

[mosra]

MSVC builtins seem to be implemented in upcoming 16.6: microsoft/STL#54 (comment)