These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 0.83 s (backtrack: 0/20).


!!! All ebuilds that could satisfy "=sys-devel/rust-std-1.99.0" have been masked.
!!! One of the following masked packages is required to complete your request:
- sys-devel/rust-std-1.99.0::gentoo (masked by: missing keyword)

For more information, see the MASKED PACKAGES section in the emerge
man page or refer to the Gentoo Handbook.