I have been using argmin-rs-0.2.6 in a project I'm working on and after updating to cargo 1.42.0-nightly (86134e766 2019-12-23) the build fails because of a dependency for argmin-rs, namely matrixmultiply-0.2.3.
By looking at your Travis status, it seems that builds should be passing with nightly rust, but then looking at the output logs it seems your building with cargo 1.41.0-nightly (626f0f40e 2019-12-03).
I'm pretty new to rust, so I'm not actually sure what nightly is meant to signify - i.e. is it reasonable that we both are using nightly but at different versions? It seems so...
Anyway, I don't know exactly what my question is, but it's somewhere in the realm of: do you know when argmin-rs will be compatible with 1.42.0-nightly?
I did some investigation and it seems that this is actually a bug in the nightly compiler: rust-lang/rust#67743. Apart from waiting for a fix you could also try to roll back to an older nightly compiler (I think this should work:
rustup install nightly-2019-12-03). If possible, I would suggest to use the stable compiler. You can give stable a try with cargo +stable build. To entirely switch to the stable compiler you can do rustup default stable.