adding iterator benches to tree-based maps for comparison purposes
flesh out BTree docs
doc fixups
librustdoc/html: recognize slices not to nest A tags.

1. A slice of parametrized type, say
   BorrowedRef { ... Vector(Generic(T)) }, is rendered as
   "<a href='primitive.slice.html'>&amp;[T]</a>"
2. A slice of other types, say
   BorrowedRef { ... Vector(int) }, is rendered as
   "<a href='primitive.slice.html'>&amp;[</a>
    <a href='primitive.int.html'>int</a>
    <a href='primitive.slice.html'>]</a>"
3. Other cases, say BorrowedRef { ... int }, are
   rendered as same as before:
   "&<a href='primitive.int.html'>int</a>"

Relevant W3C specs:
- http://www.w3.org/TR/html401/struct/links.html#h-12.2.2
  12.2.2 Nested links are illegal
- http://www.w3.org/TR/html5/text-level-semantics.html#the-a-element
  states A tag must not enclose any "interactive contents"
  which include A tags themselves.
compiletest needs to link to native crate, or at least the `rt` library.

(I tried using a dependency on `rustrt` instead, and that did not
resolve the problem.  But this does.)

Fix #17883
Remove into_vec method from &[T]

[breaking-change]
Closes #17916
Fix cfg warnings in compiletest
auto merge of #17940 : pnkfelix/rust/fix-check-stage1-on-clean-dir, r=huonw

compiletest needs to link to native crate, or at least the `rt` library.

(I tried using a dependency on `rustrt` instead, and that did not resolve the problem.  But this does.)

Partially addresses #17883
Fix cfg warnings in libcoretest
Fix cfg warnings in librustc_back
Remove `virtual` structs from the language
Remove `virtual` struct tests
Remove `virtual` structs from the reference
auto merge of #17801 : Gankro/rust/collections-stuff, r=sfackler

I previously avoided `#[inline]`ing anything assuming someone would come in and explain to me where this would be appropriate. Apparently no one *really* knows, so I'll just go the opposite way an inline everything assuming someone will come in and yell at me that such-and-such shouldn't be `#[inline]`.

==================

For posterity, iteration comparisons:

```
test btree::map::bench::iter_20                            ... bench:       971 ns/iter (+/- 30)
test btree::map::bench::iter_1000                          ... bench:     29445 ns/iter (+/- 480)
test btree::map::bench::iter_100000                        ... bench:   2929035 ns/iter (+/- 21551)

test treemap::bench::iter_20                               ... bench:       530 ns/iter (+/- 66)
test treemap::bench::iter_1000                             ... bench:     26287 ns/iter (+/- 825)
test treemap::bench::iter_100000                           ... bench:   7650084 ns/iter (+/- 356711)

test trie::bench_map::iter_20                              ... bench:       646 ns/iter (+/- 265)
test trie::bench_map::iter_1000                            ... bench:     43556 ns/iter (+/- 5014)
test trie::bench_map::iter_100000                          ... bench:  12988002 ns/iter (+/- 139676)
```

As you can see `btree` "scales" much better than `treemap`. `triemap` scales quite poorly.

Note that *completely* different results are given if the elements are inserted in order from the range [0, size]. In particular, TrieMap *completely* dominates in the sorted case. This suggests adding benches for both might be worthwhile. However unsorted is *probably* the more "normal" case, so I consider this "good enough" for now.
auto merge of #17892 : nodakai/rust/rustdoc-dont-nest-a, r=alexcrichton

Some examples:

- `std::slice::slice`: [before](http://validator.w3.org/check?uri=http%3A%2F%2Fdoc.rust-lang.org%2Fstd%2Fslice%2Fprimitive.slice.html&charset=%28detect+automatically%29&doctype=Inline&group=0) and [after](http://validator.w3.org/check?uri=http%3A%2F%2Fnodakai.github.io%2Frust-f40b60b-doc%2Fstd%2Fslice%2Fprimitive.slice.html&charset=%28detect+automatically%29&doctype=Inline&group=0)
- `core::char::char`: [before](http://validator.w3.org/check?uri=http%3A%2F%2Fdoc.rust-lang.org%2Fcore%2Fchar%2Fprimitive.char.html&charset=%28detect+automatically%29&doctype=Inline&group=0) and [after](http://validator.w3.org/check?uri=http%3A%2F%2Fnodakai.github.io%2Frust-f40b60b-doc%2Fcore%2Fchar%2Fprimitive.char.html&charset=%28detect+automatically%29&doctype=Inline&group=0)
- `hexfloat::expand_syntax_ext`: [before](http://validator.w3.org/check?uri=http%3A%2F%2Fdoc.rust-lang.org%2Fhexfloat%2Ffn.expand_syntax_ext.html&charset=%28detect+automatically%29&doctype=Inline&group=0) and [after](http://validator.w3.org/check?uri=http%3A%2F%2Fnodakai.github.io%2Frust-f40b60b-doc%2Fhexfloat%2Ffn.expand_syntax_ext.html&charset=%28detect+automatically%29&doctype=Inline&group=0)

TODO: do the same for tuples.
auto merge of #17942 : JIghtuse/rust/master, r=alexcrichton

[breaking-change]
Closes #17916
auto merge of #17949 : ebfe/rust/cfg_warn, r=sfackler

This converts old style cfgs that generate warnings during `make check` to the new syntax.
auto merge of #17952 : jakub-/rust/remove-virtual-structs, r=eddyb

Closes #17861.
Never expand specialized columns that only contain wild patterns in them

Doing so would incur deeply nested expansion of the tree with no useful
side effects. This is problematic for "wide" data types such as structs
with dozens of fields but where only a few are actually being matched or bound.
Most notably, matching a fixed slice would use a number of stack frames that
grows with the number of elements in the slice.

Fixes #17877.
auto merge of #17944 : jakub-/rust/issue-17877, r=alexcrichton

Fixes #17877.