Correct span in privacy error
[collections] Adds `drain`: a way to sneak out the elements while clearing.

It is useful to move all the elements out of some collections without
deallocating the underlying buffer. It came up in IRC, and this patch
implements it as `drain`. This has been discussed as part of RFC 509.

r? @Gankro
cc: @frankmcsherry
Add privacy tests
std: Second pass stabilization of Result<T, E>

This commit, like the second pass of `Option`, largely just stablizes the
existing functionality after renaming a few iterators.

The specific actions taken were:

* The `Ok` and `Err` variants were marked `#[stable]` as the stability
  inheritance was since removed.
* The `as_mut` method is now stable.
* The `map` method is now stable
* The `map_err` method is now stable
* The `iter`, `iter_mut`, and `into_iter` methods now returned structures named
  after the method of iteration. The methods are also now all stable.
* The `and_then` method is now stable.
* The `or_else` method is now stable.
* The `unwrap` family of functions are now all stable: `unwrap_or`,
  `unwrap_or_else`, `unwrap`, and `unwrap_err`.

There is a possible open extension to `Result::{and, and_then}` to make the
return type further generic over `FromError` (as proposed in #19078), but this
is a backwards compatible change due to the usage of default type parameters,
which makes the two functions safe to stabilize now regardless of the outcome of
that issue.
auto merge of #19946 : cgaebel/rust/hashmap-drain-iter, r=gankro

It is useful to move all the elements out of a hashmap without deallocating
the underlying buffer. It came up in IRC, and this patch implements it as
`drain`.

r? @Gankro
cc: @frankmcsherry
auto merge of #19953 : sanxiyn/rust/privacy-span, r=alexcrichton

Fix #19062.
auto merge of #19961 : alexcrichton/rust/second-pass-result, r=aturon

This commit, like the second pass of `Option`, largely just stablizes the
existing functionality after renaming a few iterators.

The specific actions taken were:

* The `Ok` and `Err` variants were marked `#[stable]` as the stability
  inheritance was since removed.
* The `as_mut` method is now stable.
* The `map` method is now stable
* The `map_err` method is now stable
* The `iter`, `iter_mut`, and `into_iter` methods now returned structures named
  after the method of iteration. The methods are also now all stable.
* The `and_then` method is now stable.
* The `or_else` method is now stable.
* The `unwrap` family of functions are now all stable: `unwrap_or`,
  `unwrap_or_else`, `unwrap`, and `unwrap_err`.

There is a possible open extension to `Result::{and, and_then}` to make the
return type further generic over `FromError` (as proposed in #19078), but this
is a backwards compatible change due to the usage of default type parameters,
which makes the two functions safe to stabilize now regardless of the outcome of
that issue.
Introduce a tcx() helper method to cleanup this mess.
Make ty_bare_fn carry an optional def-id indicating whether it is the
type of a fn item or a fn pointer, which are now differentiated.
Introduce coercion from fn item to fn pointer.
Rote changes that don't care to distinguish between a fn pointer and a fn item.
Adjust metadata for new fields and enum variants. Yawn.
Insert FIXME links to issue #19925: fn item types should be zero-sized.
Perform coercions on LHS assignments.
Fix fallout from changes. In cases where stage0 compiler is needed, we
cannot use an `as` expression to coerce, so I used a one-off function
instead (this is a no-op in stage0, but in stage1+ it triggers
coercion from the fn pointer to the fn item type).
Insert coercions to fn pointer types required for the new types
post-unboxed-closure-conversion. This requires a fair amount of
annoying coercions because all the `map` etc types are defined
generically over the `F`, so the automatic coercions don't propagate;
this is compounded by the need to use `let` and not `as` due to
stage0. That said, this pattern is to a large extent temporary and
unusual.
Various simple tests for fn item type vs fn pointer type and for coercions between them.
Change print out to include the path to the fn, a bit ad-hoc for now .
Address nit.
Correct merge failure around imports in `str.rs`.
Tweak region-related error messages that changed slightly due to coercion
changes, and also stop printing semi-useless inference by-products.