Fast compile-check of one C++ file in a meson+nix flake project (skip the full nix build)

A full nix build of a large meson C++ project (~790 steps, -O3) kept getting killed by the session timeout before it reached the changed file. Faster: open the flake dev shell and build only that object.

nix develop "git+file://$PWD" -c bash -c 'meson setup build-check -Dcpp_std=c++23; obj=$(ninja -C build-check -t targets all | grep -o "^[^:]*my_file.cpp.o" | head -1); ninja -C build-check "$obj"'

ninja -t targets all lists the object target names (for example libfoo.a.p/src_dir_my_file.cpp.o). Building that one target also builds its generated dependencies (Wayland protocol headers and so on) and nothing else, so it finishes in about a minute. Expect a harmless _FORTIFY_SOURCE requires compiling with optimization warning in the debug setup.

Gotchas: nix build path:$PWD failed with "attribute 'dirtyShortRev' missing" because the flake reads self.shortRev or self.dirtyShortRev. Use git+file://$PWD so git rev attrs exist. Delete build-check/ afterwards. It only proves that one translation unit compiles, not that the final link succeeds.