Allow tweaking of the compiler artifacts to emit in the profile.

[cgaebel]

It's really frustrating being unable to easily dump the assembly
and llvm ir generated by rustc through cargo. This patch provides
a way.

[alexcrichton]

I'm somewhat wary of doing this as this is breaching into the territory of invoking unintended behavior out of the compiler. Extreme level of customization in running the compiler is definitely something that falls under Cargo's umbrella, but this falls more under a subcommand than configuration in a profile.

Profiles are largely intended for long-term configuration for a project whereas this is quite often a temporary configuration desired, in which case there should be no need to edit a file to quickly get the disassembly.

As an implementation, I think that this may be buggy because if any of my dependencies have an emit key then they won't actually emit rlibs but rather whatever that emit key entails.

[cgaebel]

Good point. I've added this as flags passed to the various subcommands where it makes sense, instead of via the manifest.

I've also decreased the level of customization. When you want to emit one extra build artifact, it's usually no big deal to emit a few extras. There's also less that can go wrong. Therefore, it's now just one flag passed to cargo build: --emit-intermediate

[alexcrichton]

Ah sorry I meant more that any use of --emit should be going through an entirely new subcommand instead of moving out into each of the separate commands that cargo has. Unfortunately this means that at this time we won't be able to add the --emit flags to rustc because the cargo rustc subcommand doesn't exist, but I would hate to start bubbling many of rustc's flags up to the level of cargo for each subcommand.

[cgaebel]

When more flags are requested and/or implemented and it becomes a problem, it can be factored out into a separate subcommand pretty easily.

It'd be a shame to block having this feature on a generic cargo rustc command. It's very ergonomic in its current form and doesn't add much complexity.

[alexcrichton]

One of the problems with a strategy like that, however, is that it's a breaking change to remove flags from commands. We're thinking pretty seriously about the stability of the compiler and Cargo now, and this isn't a flag we'd want to commit to until the end of time which is why I'd like to hold off on it for now.

I'm sorry cargo rustc doesn't exist just yet, and I agree that this is quite simple and doesn't add much, but it's a slippery slope to start including all other flags from the compiler to top-level flags in Cargo. The unambiguous location for this to live is in cargo rustc, and we can always add new subcommands backwards-compatibly, so I'd like to hold off on this until we have a good place to put it.

In the meantime though I'm going to close this, but feel free to reach out to me on IRC if you're interested in working on cargo rustc!