'extern location for serde does not exist' error on CI

[phansch]

For some reason, some of our Travis CI builds are failing with this error since today:

error: extern location for serde does not exist: target/debug/deps/libserde-61a62cbc4921f441.rmeta

error: aborting due to previous error

https://travis-ci.com/rust-lang/rust-clippy/jobs/201008591#L882

Clippy is building just fine, it's two tests depending on serde that are failing.

cc #4107, #4100

[phansch]

Notably this didn't happen in the beta backport PR: #4104

[flip1995]

We could test in one of the PRs if it also occurs with the current nightly (Clippy should be buildable with it ATM). If that's the case the fault is with the master toolchain.

[oli-obk]

it's in nightly now

[oli-obk]

I guess rust-lang/rust#60190 looks very much like the cause

Can this issue be closed? It looks like the issue was resolved.