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-arch/innoextractEnable debug logging and the --debug command-line option
app-cdr/dvdisasterEnable memory debugging.
app-editors/nanoEnable debug messages and assert warnings. Note that these will all be sent straight to stderr rather than some logging facility.
app-emulation/skienable extra binary emulation debugging via sys-libs/binutils-libs
app-i18n/librimeEnable debugging (including logging of input of RIME input methods)
app-misc/rlwrapEnable debug
app-portage/eixBuild with upstream's CXXFLAGS/LDFLAGS for debugging support; not recommended for normal use.
dev-haskell/atomic-primopsEnable extra internal checks.
dev-haskell/text-shortEnable runtime-checks via 'assert'
dev-lang/crystalBuild each module as a separate object file. Speeds build up.
dev-lang/spidermonkeyEnable 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/boostBuild and install only the debug version of the Boost libraries. Only enable this flag if you're developing against boost.
dev-libs/libeventSupport for running in debug mode
dev-libs/openctAdd debug output to the driver library for pcsc-lite.
dev-perl/perl-headersAdd source locations in generated .ph files to assist with debugging when errors occur
dev-tcltk/expect-litepull in packages needed for runtime interactive debugger
dev-util/google-perftoolsBuild 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.
games-action/dxx-rebirthEnable 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
media-tv/mythtvInstructs Qt to use the 'debug' target
net-misc/chronyGet DEBUG_LOG output from chronyd when passing -dd parameter
net-misc/ptpdEnable debugging informations
net-p2p/transmissionEnable assertions
net-vpn/ovpn-dcoEnable debug output in dmesg
net-vpn/wireguard-modulesEnable verbose debug reporting in dmesg of various WireGuard peer and device information.
sys-auth/pambaseEnable debug information logging on syslog(3) for all the modules supporting this in the system authentication and system login stacks.
sys-auth/pam_u2fEnable debug messages using the pam logging macros.
sys-block/partedEnable 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 ;)
sys-devel/gccEnables GCC's 'checking' facility via --enable-checking=yes,extra,rtl. This adds checks to various compiler passes for integrity and input validation. This can help catch possible miscompilations early as well as latent bugs which could become real problems in future, but at the cost of slower compile times when using GCC. Unrelated to backtraces.
sys-libs/glibcWhen USE=hardened, allow fortify/stack violations to dump core (SIGABRT) and not kill self (SIGKILL)
sys-process/fcronEnable 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.
www-client/chromiumEnable DCHECK feature with severity configurable at runtime. Mostly intended for debugging and development, NOT RECOMMENDED for general use.
www-servers/monkeydEnable lots of debugging info
x11-wm/e16Build libtrip debugging library

All packages providing a “debug” USE flag (1735)