ci: minimal version check

[Darksonn]

This checks that crates compile with the minimum allowed version of each dependency.

[taiki-e]

If I remember correctly, cargo check -Z minimal-versions doesn't really do anything. It needs to be separated into cargo update -Z minimal-versions and cargo check.
Also, dev-dependencies may raise version requirements. Ideally, remove them before run cargo update -Z minimal-versions. (Also, note that Cargo.lock is actually shared within the workspace. However as far as I know, there is no workaround for this yet.)

So, I prefer to use commands like:

# Remove dev-dependencies from Cargo.toml to prevent the next `cargo update`
# from determining minimal versions based on dev-dependencies.
cargo hack --remove-dev-deps --workspace
# Update Cargo.lock to minimal version dependencies.
cargo update -Z minimal-versions
cargo check --all-features

(See also pin-project's check-minimal-version script)

[Darksonn]

I'll try that.

[taiki-e]

error: no such subcommand: hack

Did you mean check?

Sorry, I forgot to mention the install of cargo-hack...

[taiki-e]

error: use of deprecated constant std::sync::ONCE_INIT: the new function is now preferred
--> tokio/src/process/unix/mod.rs:65:1
|
65 | / lazy_static::lazy_static! {
66 | | static ref ORPHAN_QUEUE: OrphanQueueImpl = OrphanQueueImpl::new();
67 | | }
| |_^
|
= note: -D deprecated implied by -D warnings
= note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

It seems lazy_static's requirement needs to be raised to 1.4.
rust-lang-nursery/lazy-static.rs@1dbd5ae