libtest: DSTify `Stats`
libsyntax: DSTify `ToSource` and `ToSourceWithHygiene`
Disallow coercions from [T, ..n] to &[T]or *[T]

[breaking-change]

Insert an `&` to fix
Regression test
librustc: DSTify `ClassList`, `LlvmRepr` and `Repr`
auto merge of #19050 : japaric/rust/moar-dst, r=aturon

r? @aturon 
cc #16918
implement Writer for Vec<u8>

The trait has an obvious, sensible implementation directly on vectors so
the MemWriter wrapper is unnecessary. This will halt the trend towards
providing all of the vector methods on MemWriter along with eliminating
the noise caused by conversions between the two types. It also provides
the useful default Writer methods on Vec<u8>.

After the type is removed and code has been migrated, it would make
sense to add a new implementation of MemWriter with seeking support. The
simple use cases can be covered with vectors alone, and ones with the
need for seeks can use a new MemWriter implementation.
Windows and OS X fallout
auto merge of #18885 : thestinger/rust/writer, r=aturon

The trait has an obvious, sensible implementation directly on vectors so
the MemWriter wrapper is unnecessary. This will halt the trend towards
providing all of the vector methods on MemWriter along with eliminating
the noise caused by conversions between the two types. It also provides
the useful default Writer methods on Vec<u8>.

After the type is removed and code has been migrated, it would make
sense to add a new implementation of MemWriter with seeking support. The
simple use cases can be covered with vectors alone, and ones with the
need for seeks can use a new MemWriter implementation.
auto merge of #18645 : nick29581/rust/coercions-1, r=alexcrichton

r?

(I realise this needs a rebase, but I will probably have to chop it up in order to land and I'd like to get r+ first so I can do that quicker)