Implement fmt::Pointer for pointers to unsized types

[kamalmarhubi]

This allows printing pointers to unsized types with the {:p} formatting
directive. The following impls are extended to unsized types:

• impl<'a, T: ?Sized> Pointer for &'a T
• impl<'a, T: ?Sized> Pointer for &'a mut T
• impl<T: ?Sized> Pointer for *const T
• impl<T: ?Sized> Pointer for *mut T
• impl<T: ?Sized> fmt::Pointer for Box
• impl<T: ?Sized> fmt::Pointer for Rc
• impl<T: ?Sized> fmt::Pointer for Arc

[rust-highfive]

r? @nikomatsakis

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[sfackler]

Which half of the fat pointer will this print?

[bluss]

The cast to thin pointer should give you the data pointer. However it doesn't matter which — Rust has already decided that you can cast a fat pointer to a thin pointer, and whatever it thinks the result of that is, it's the one we should print too 😄

[kamalmarhubi]

I added a test to ensure you get the data pointer for &str and &[u8].

[alexcrichton]

Thanks @kamalmarhubi! I think @bluss's reasoning is sound, so these all likely look good to me. I'd like to run it by the libs team, however, just to make sure as these'll be insta-stable once landed.

cc @rust-lang/libs

[kamalmarhubi]

Thanks @alexcrichton! In the meantime, squashed and rebased.

[alexcrichton]

The libs team discussed this during triage today and the discussion was to merge. Thanks again for the PR @kamalmarhubi!

@bors: r+ c5f73ed

[bors]

⌛ Testing commit c5f73ed with merge 106070b...

[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