Add c alias for check

[ordovicia]

This PR adds cargo c alias for cargo check.

I believe that one of the most frequently used subcommands is check.
Adding this alias would save much time.

Currently, there are three aliases: b for build, r for run, t for test.
I think that adding out-of-the-box c alias is natural for many developers, and these aliases would cover most of the use cases.
We can add aliases via a configuration file, but I guess people would expect built-in c alias along with b and others.

One problem I have come up with is that the clean subcommand also starts with the letter c.
But I believe that running check subcommand by mistake would not hurt developers so much.

Fixes #6215

[rust-highfive]

r? @matklad

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[dwijnand]

I like it.

[ehuss]

Just a note of caution on this, this takes precedence over user aliases, so this would (silently) disable them.

[ordovicia]

Good point.
Currently, Cargo warns if an alias is shadowed by a built-in command here:

cargo/src/bin/cargo/cli.rs

Lines 121 to 126 in efb7972

	(Some(_), Some(_)) => {
	config.shell().warn(format!(
	"alias `{}` is ignored, because it is shadowed by a built in command",
	cmd
	))?;
	}

... but Cargo checks only after subcommands are expanded by clap.
So the c alias (as well as b, r, and t) silently override user-defined aliases.

I tried to fix Cargo to warn if built-in aliases override user-defined ones, but failed to resolve.
Does anyone have a solution?

EDIT: I have filed a separate issue for this shadowing (#6221).

[ehuss]

One idea is to not use clap aliases. Just hardcode the aliases inside expand_aliases, and it can handle the precedence logic (base > user-alias > builtin-alias). One drawback is that the help subcommand would not know about the aliases, but I think that's fine.

[ordovicia]

Thank you for your advice!
I will try to fix it this weekend today.

[ordovicia]

Now that #6259 is merged, we can add built-in aliases without interfering existing user config.
r? @ehuss

[dwijnand]

LGTM! Thanks for the two PRs, @ordovicia!

@bors: r+

[dwijnand]

@bors: r+

[bors]

📌 Commit 2a66732 has been approved by dwijnand

[bors]

⌛ Testing commit 2a66732 with merge 2ad447f...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: dwijnand
Pushing 2ad447f to master...

[pickfire]

Would it be better if we bold or underline the b for build, c for check and t for test for help? That would make it a lot discoverable.