attached is the pstree
Created attachment 525214 [details] 20.20180324-113536 pstree
Err, based on that pstree, it looks like you had MAKEOPTS=-j7. The child of emake is "make -j7".
The tinderbox has this set - works for nearly all packages except few ones, ncurses are among them : MAKEOPTS="-j 1" NINJAFLAGS="-j 1" EGO_BUILD_FLAGS="-p 1" GOMAXPROCS="1" GO19CONCURRENTCOMPILATION=0 RUSTFLAGS="-C codegen-units=1" RUST_TEST_THREADS=1 RUST_TEST_TASKS=1
Please attach your build log that corresponds to the process tree you provided.
Also, I still really think this is a config issue on your end. emake is clearly passing -j7 to make; I don't see anything in the ebuild that would override MAKEOPTS.
Created attachment 525278 [details] sys-libs:ncurses-6.1-r1:20180324-143115.log.gz log from 17.0-desktop-plasma_abi32+64_20180324-084537
BTW have a look at bug 646092 - similar issue there
Maybe something like this is needed? : https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=dca844ed582e369708a7769d4e84abf6e9b5fb0d
Rust uses a completely custom build system. ncurses uses pretty normal looking makefiles along with autoconf. That's why this bug makes no sense to me -- there's no obvious build system that needs fixing here. I did some experiments to try and reproduce the problem myself, and was unable to get anything resembling your pstree with anything other than MAKEOPTS=-j7.
OMG - needed hours here to realize that this is indeed PEBKAC. Sry.