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
app-cdr/dvdisaster Enable memory debugging.
dev-haskell/chaselev-deque Enable the extra internal checks.
dev-java/jfreesvg Enable debugging during compilation.
app-misc/rlwrap Enable debug
app-arch/innoextract Enable debug logging and the --debug command-line option
app-portage/eix Build with upstream's CXXFLAGS/LDFLAGS for debugging support; not recommended for normal use.
dev-lang/crystal Build each module as a separate object file. Speeds build up.
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-perl/perl-headers Add source locations in generated .ph files to assist with debugging when errors occur
games-emulation/zsnes Enable the builtin debugger
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
net-misc/chrony Get DEBUG_LOG output from chronyd when passing -dd parameter
net-misc/ptpd Enable debugging informations
media-tv/mythtv Instructs Qt to use the 'debug' target
dev-haskell/pandoc-citeproc Turn on debug tracing.
dev-libs/libevent Support for running in debug mode
net-vpn/wireguard-modules Enable verbose debug reporting in dmesg of various WireGuard peer and device information.
net-wireless/spectools Build the raw output client for spectools. (for developers)
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 ;)
www-client/chromium Enable DCHECK feature with severity configurable at runtime. Mostly intended for debugging and development, NOT RECOMMENDED for general use.
dev-tcltk/expect-lite pull in packages needed for runtime interactive debugger
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>
sci-mathematics/agda Enable debugging features that may slow Agda down.
net-mail/vchkuser Enables debug messages to syslog
www-servers/monkeyd Enable lots of debugging info
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.
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)
dev-libs/boost Build and install only the debug version of the Boost libraries. Only enable this flag if you're developing against boost.
x11-misc/albert Enable debugging informations
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
dev-libs/openct Add debug output to the driver library for pcsc-lite.
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.
x11-wm/e16 Build libtrip debugging library
sys-auth/pam_u2f Enable debug messages using the pam logging macros.
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-libs/glibc When USE=hardened, allow fortify/stack violations to dump core (SIGABRT) and not kill self (SIGKILL)

All packages providing a “debug” USE flag (1390)