Tracking issue for RFC 2128: Nested groups in imports

[aturon]

This is a tracking issue for the RFC "Nested groups in imports" (rust-lang/rfcs#2128).

Steps:

• Implement the RFC (very rough mentoring instructions here and here)
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

[emilyalbini]

I'd like to work on this.

[nikomatsakis]

@pietroalbini great! I think @petrochenkov or @jseyfried would be best equipped to writing up some mentoring notes here. But I think the general idea is going to be something like this:

• Extend the AST to support the new syntax. Generally speaking, we try to keep the structure of the AST as a "close match" to the raw rust input, so I think we would want to mirror the new structure there.
• Update name resolution to handle this. This is the part where I can't help -- at least without doing some digging! Hopefully, there are some intermediate data structures that we can "desugar" these extended-form view items into, and it doesn't operate directly off of the AST.

[petrochenkov]

Update name resolution to handle this.

The desugaring is done here https://github.com/rust-lang/rust/blob/master/src/librustc_resolve/build_reduced_graph.rs#L113

[nikomatsakis]

@pietroalbini just checking in -- any progress? Any questions arisen?