When trying to compile dev-util/cargo-c on a fchroot environment created from the latest "next release" tarball, I get this error message:

 >>> Compiling source in /var/tmp/portage/dev-util/cargo-c-0.9.15/work/cargo-c-0.9.15 ...  * cargo build --release error: package `cargo-c v0.9.14+cargo-0.67 (/var/tmp/portage/dev-util/cargo-c-0.9.15/work/cargo-c-0.9.15)` cann ot be built because it requires rustc 1.66 or newer, while the currently active rustc version is 1.63.0

It looks like the ebuild should require at least rust version 1.66, but right now it only requires >=virtual/rust-1.51.0

Manually upgrading rust to 1.66 solves the issue, proving that all that needs to be done here is to change the BDEPEND line.