Make future-compat lint match_of_unit_variant_via_paren_dotdot deny by default

[petrochenkov]

This warning was introduced on Nov 28, 2015 and got into 1.6 stable, it was later requalified from a hardwired warning to a warn-by-default lint.
If this patch is landed soon enough, then match_of_unit_variant_via_paren_dotdot will get into 1.8 stable as a deny-by-default lint.
My intention is to turn it into a hard error after March 3, 2016, then it will hit stable at 1.9.

r? @nikomatsakis
cc @pnkfelix

[nikomatsakis]

Nominating for discussion in the @rust-lang/lang team mtg today. This makes sense, but it always seems like a good idea to double check before we go on making our deprecations more aggressive. This is also a @rust-lang/core issue (@brson may have particular opinions here).

[nikomatsakis]

We discussed this in @rust-lang/lang and decided it makes sense to go forward with this as proposed. There is still some general teeth gnashing (at least on my part) that we now lack any generic way to write a pattern that says "this variant and I don't care what fields are there", but the feeling is that the distinctions around () vs {} can be confused, and so being more specific should help. And anyway we already started down this road. (@pnkfelix and I have a secret plan to introduce Foo { ... } as the "one true" pattern that can be used with any declaration style, but that'd clearly be the topic of an RFC.)

[nikomatsakis]

@bors r+

[bors]

📌 Commit e950659 has been approved by nikomatsakis

[petrochenkov]

@pnkfelix and I have a secret plan to introduce Foo { ... } as the "one true" pattern that can be used with any declaration style, but that'd clearly be the topic of an RFC

FYI, I started writing such an RFC about an hour ago.
Coincidence? I don't think so!

[brson]

+1

[bors]

⌛ Testing commit e950659 with merge c894ff6...

[alexcrichton]

@bors: retry force

[bors]

⚡ Previous build results for auto-mac-ios-opt are reusable. Rebuilding only auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt