Enable rustc internal lints

[flip1995]

Closes #3965

I'm not 100% sure if enabling the -Zunstable-options flag in the .cargo/config file is the right place.

[oli-obk]

I'm not 100% sure if enabling the -Zunstable-options flag in the .cargo/config file is the right place.

It seems not :D the stable compiler test doesn't work

[flip1995]

Any idea how we can hack in the -Zunstable-options so it is only set in clippy_lints? Adding a clippy_lints/.cargo/config file doesn't work, since cargo is run from the root of the project and therefore ignores the config file in clippy_lints.

Also adding a clippy_dev/.cargo/config with

[build]
rustflags = []

does not work, since the config file in the root gets unified with this file.

Adding it to the build script also seems not really possible:

rustc-flags=FLAGS is a set of flags passed to the compiler, only -l and -L flags are supported.
https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script

[oli-obk]

Maybe clippy_dev should not be using stable. I mean there's no need to download a full stable compiler in CI just for that crate.

[flip1995]

It was just changed to stable recently: #3885

I think removing the rust-toolchain file again would be the easiest fix.

[oli-obk]

sgtm

[oli-obk]

@bors r+

[bors]

📌 Commit 118f7d5 has been approved by oli-obk

[bors]

⌛ Testing commit 118f7d5 with merge 29893fe...

[bors]

💥 Test timed out

[phansch]

@bors retry

[bors]

⌛ Testing commit 118f7d5 with merge 27d62cf...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: oli-obk
Pushing 27d62cf to master...