Add cross subcommands shortcuts

[sigmaSd]

Hi, this pr adds sub-commands shortcuts ['b','c','r', 't'] to the cli.
The main reason is currently the error when using them is misleading.
Example:
this command:
cross b --target arm-linux-androideabi
outputs this error:
error: linking with cc failed: exit code: 1
which took me a while to figure out that cross is not recognizing the sub-command and is falling-back to cargo.
Also they are useful to have.

[Emilgardis]

Thank you for your contribution!

This seems like it should be the default behaviour, but should cross maybe support all aliases, including userdefined aliases?

In cargo, the b,c,r,t aliases are shadowed by user-defined aliases.

[sigmaSd]

I agree it makes sense. The issue here is to handle aliases 100% there are a lot of things to account for https://doc.rust-lang.org/cargo/reference/config.html,
1- handling config file location
2- handling aliases from environment variable
3- parsing the config file
And all of these have a lot of edge cases. As far as I see so far cross tried to avoid this complexity and delegate most of this to cargo
The good thing is -from the docs- Aliases are not allowed to redefine existing built-in commands. which mean that misleading errors when building or running should not happen

[Emilgardis]

bors r+

[bors]

Build failed:

• rust-embedded.cross

[Emilgardis]

bors retry

[bors]

Build failed:

• rust-embedded.cross

[domenicquirl]

Hi! I just spent a good while tearing my hair out over this, because I was trying to run cross b on a system where the corresponding --target was also installed and the command cause a regular build to run and complete without error (context: cross-compiling from a linux system to armv7, having installed the toolchain and a matching linker on the source machine to test the build locally). Got it figured out, but landed here in the process and I think this landing would help avoid such situations for others the future.

[reitermarkus]

bors r+

[bors]

Build succeeded:

• conclusion