Single integrated UI for all rust tooling

[brson]

As discussed in this week's meeting, let's create a single command-line interface that encompasses all of the rust tooling, and presents a simple and consistent experience for all aspects of rust programming.

We currently have three tools, rustc, cargo and rustdoc, and we will likely soon bee writing a bunch of new build logic for #2237.

Go has a go command that is a one-stop shop for working on go projects. We should learn from this. I thought this description of their motivation rang a bell:

An explicit goal for Go from the beginning was to be able to build Go code using only the information found in the
source itself, not needing to write a makefile or one of the many modern replacements for makefiles. If Go needed
a configuration file to explain how to build your program, then Go would have failed.

[brson]

@pcwalton suggested we do something like git with git-foo subcommands and I agree. Furthermore, I suggest implementing it with dynamically loaded crates for each subcommand so that folks can use cargo to write and install new ones.

[graydon]

See also https://github.com/mozilla/rust/wiki/Meeting-weekly-2012-11-13

[Kimundi]

I implemented a first version (#5014). It just forwards arguments to rustc, rusti, rustdoc and rustpkg and adds a few convenient shortcuts.

[catamorphism]

FIXMEs from the code for the rust tool:

// FIXME Make commands run and test emit proper file endings on winds
// FIXME Make run only accept source that emits an executable

[kud1ing]

There is a rust tool now. Should this issue be closed in the tradition of opening follow-up issues for outstanding details?

[thestinger]

@kud1ing: closing, thanks

[emberian]

Reopening, since rust is removed (unmaintained).