Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces

Packages describing “debug” as local USE flag

Package “debug” Flag Description
dev-lang/crystal Build each module as a separate object file. Speeds build up.
games-action/dxx-rebirth Enable various debugging code paths and sanity checks, not just output. Some of these checks date back to the retail build, and choke on some custom levels because the game was never coded to sufficiently distrust inputs
app-arch/innoextract Enable debug logging and the --debug command-line option
app-editors/nano Enable debug messages and assert warnings. Note that these will all be sent straight to stderr rather than some logging facility.
dev-libs/libevent Support for running in debug mode
app-portage/eix Build with upstream's CXXFLAGS/LDFLAGS for debugging support; not recommended for normal use.
x11-misc/albert Enable debugging informations
dev-util/google-perftools Build a set of libraries with debug support (so-called debugalloc). These are available by default but are not needed unless you're actually developing using tcmalloc.
app-emulation/ski enable extra binary emulation debugging via sys-libs/binutils-libs
app-i18n/librime Enable debugging (including logging of input of RIME input methods)
app-misc/rlwrap Enable debug
dev-haskell/chaselev-deque Enable the extra internal checks.
net-misc/stargazer Enable extra debug codepaths, like asserts and extra output.
dev-haskell/pandoc-citeproc Turn on debug tracing.
dev-lang/spidermonkey Enable assertions to allow for easier debugging of programs that link to spidermonkey -- note this will often crash software on regular end-user systems
net-vpn/wireguard-modules Enable verbose debug reporting in dmesg of various WireGuard peer and device information.
dev-java/jfreesvg Enable debugging during compilation.
sci-libs/dealii Build an additional debug library with extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see <http://www.gentoo.org/proj/en/qa/backtraces.xml>
dev-tcltk/expect-lite pull in packages needed for runtime interactive debugger
dev-libs/boost Build and install only the debug version of the Boost libraries. Only enable this flag if you're developing against boost.
games-emulation/zsnes Enable the builtin debugger
sci-mathematics/agda Enable debugging features that may slow Agda down.
dev-perl/perl-headers Add source locations in generated .ph files to assist with debugging when errors occur
dev-libs/openct Add debug output to the driver library for pcsc-lite.
net-misc/ptpd Enable debugging informations
net-wireless/spectools Build the raw output client for spectools. (for developers)
sys-auth/pam_u2f Enable debug messages using the pam logging macros.
sys-block/parted Enable debugging as encouraged by upstream: [The default configuration] includes --enable-debug (by default), which contains many assertions. Obviously, these "waste" space, but in the past, they have caught potentially dangerous bugs before they would have done damage, so we think it's worth it. Also, it means we get more bug reports ;)
media-tv/mythtv Instructs Qt to use the 'debug' target
sys-process/fcron Enable debug code and output. Since version 3.0.5 this will no longer force foreground execution, and fcron will be able to run as a service properly.
net-misc/chrony Get DEBUG_LOG output from chronyd when passing -dd parameter
net-mail/vchkuser Enables debug messages to syslog
app-cdr/dvdisaster Enable memory debugging.
x11-wm/e16 Build libtrip debugging library
sys-apps/timer_entropyd Enable printf debugging and logging to the syslog at DEBUG level
sys-auth/pambase Enable debug information logging on syslog(3) for all the modules supporting this in the system authentication and system login stacks.
sys-kernel/dracut Module installing additional tools like strace, file editor, ssh and more
sys-libs/glibc When USE=hardened, allow fortify/stack violations to dump core (SIGABRT) and not kill self (SIGKILL)
www-servers/monkeyd Enable lots of debugging info

All packages providing a “debug” USE flag (1392)