Failed to emerge chromeos-base/crosvm-0.0.1-r93 

......
                                
error: `std::mem::size_of` is not yet stable as a const fn
   --> kvm/src/lib.rs:975:32    
    |                                                                                             
975 |         const _ASSERT: usize = size_of::<sigset_t>() - 8 as usize;
    |                                ^^^^^^^^^^^^^^^^^^^^^                                               
    |                                                                                                   
    = help: in Nightly builds, add `#![feature(const_size_of)]` to the crate attributes to enable

Apparently is from this change:

https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/944849/

The solution is to be running 
rustc --version = 1.24.0 or better

I currently have 1.23.0, but if I do sudo emerge rust I get an error
"couldn't find required command: "armv7a-cros-linux-gnueabi-ar""

	
Zach Reizner suggested "
you can also mask out kvm_host is the USE flags for your overlay
I see moblab-generic-vm inherits the generic amd64 board, which has kvm_host, but you almost certainly don't need it.
Without kvm_host, crosvm is not built, and you don't have to worry about the rust toolchain"