add a way to set a default --target

[japaric]

For example, if for your project you always cross compile using cargo build --target=thumbv6m-none-eabi. This feature would let you add a key value pair to your cargo config file:

# .cargo/config
[target]
default = "thumbv6m-none-eabi"

That will let you simply call cargo build to cross compile your project for thumbv6m-none-eabi. This would also extend to cargo doc.

Originally reported here.

[alexcrichton]

This seems pretty reasonable to me, although I might go as far as to say that it should be in Cargo.toml as it's likely a per-project thing if it's a thing to start out with.

[japaric]

One downside (?) of making it a project setting is that the setting will always be ignored for dependencies. Example: If my project P depends on crate D and D has set T as its default target, that setting will always be ignored because D will always be compiled for the target I choose for P.

One advantage of using .cargo/config is that if I'm developing several crates locally, then I can have a setup like this:

$ tree .
my-RTOS-project
├── .cargo
│   └── config
├── allocator
│   └── (...)
├── hal
│   └── (...)
├── RusTOS
│   └── (...)
└── scheduler
    └── (...)

And I can jump from one crate directory to another and run cargo test. At some point I can edit a single line in .cargo/config and start testing my crates for a different target.

[alexcrichton]

Aha yes indeed! That somewhat falls under the purview of #2122 where that shouldn't be a downside, but there's still some design work to do there as well.