Remove outdated info about nested block comments. Fixes #14767
Removed doubled wording.

The guide previously stated:

> The compiler will automatically convert a box box point to a reference like &amp;point.

This fixes the doubled word `box`, so the statement reads

> The compiler will automatically convert a box point to a reference like &amp;point.

The code it is referring to is `compute_distance(&on_the_stack, on_the_heap);`, so a single `box` is appropriate.
Update description to reflect language changes

Previously, the type system's restrictions on borrowing were summarized as

> The previous example showed that the type system forbids any borrowing of owned boxes found in aliasable, mutable memory

This did not jive with the example, which allowed mutations so long as the borrowed reference had been returned. Also, the language has changed to no longer allow aliasable mutable locations. This changes the summary to read

> The previous example showed that the type system forbids mutations of owned boxed values while they are being borrowed. In general, the type system also forbids borrowing a value as mutable if it is already being borrowed - either as a mutable reference or an immutable one.

This adds more general information for the experienced reader as well, to offer a more complete understanding.
sync: Once is no longer unsafe, update docs
Show impl for Ref & RefMut
Fix unclear wording of comment
librustc: Forbid identifiers that shadow in the same pattern in let
bindings and function arguments.

Issue #14581.

To fix code that this breaks, give the pattern identifiers different names.

[breaking-change]
mk: Allow usage of a local jemalloc install

This adds a new configure option, --jemalloc-root, which will specify a location
at which libjemalloc_pic.a must live. This library is then used for the build
triple as the jemalloc library to link.
travis: Use a pre-installed jemalloc
mk: Allow using a locally compiled libuv.a

Closes #5563
auto merge of #14792 : alexcrichton/rust/local-jemalloc, r=brson

This configures the makefiles to copy a local jemalloc/libuv library into place instead of building the local copy of one. Additionally, this switches our travis builds to using the system-provided jemalloc instead of a custom-built jemalloc to exercise this functionality.
auto merge of #14800 : reem/rust/patch-1, r=alexcrichton

According to #14767 and the grammar right above this documentation, nested comments are supported.
auto merge of #14801 : pcwalton/rust/name-shadowing-in-one-pattern, r=alexcrichton

bindings and function arguments.

Issue #14581.

To fix code that this breaks, give the pattern identifiers different names.

[breaking-change]

r? @brson
auto merge of #14805 : zzmp/rust/patch-1, r=alexcrichton

The guide previously stated:

> The compiler will automatically convert a box box point to a reference like &amp;point.

This fixes the doubled word `box`, so the statement reads

> The compiler will automatically convert a box point to a reference like &amp;point.

The code it is referring to is `compute_distance(&on_the_stack, on_the_heap);`, so a single `box` is appropriate.
auto merge of #14809 : zzmp/rust/patch-2, r=alexcrichton

Previously, the type system's restrictions on borrowing were summarized as

> The previous example showed that the type system forbids any borrowing of owned boxes found in aliasable, mutable memory.

This did not jive with the example, which allowed mutations so long as the borrowed reference had been returned. Also, the language has changed to no longer allow aliasable mutable locations. This changes the summary to read

> The previous example showed that the type system forbids mutations of owned boxed values while they are being borrowed. In general, the type system also forbids borrowing a value as mutable if it is already being borrowed - either as a mutable reference or an immutable one.

This adds more general information for the experienced reader as well, to offer a more complete understanding.
auto merge of #14811 : forticulous/rust/refcell-show, r=alexcrichton

Show impl for RefCell and friends
auto merge of #14813 : cmr/rust/once-docs-unsafe, r=alexcrichton
auto merge of #14816 : theptrk/rust/unclear-comment, r=huonw

The old comment left it unclear if this is creating a random value or doing a check as to whether or not the generator is available or some other operation.

See: http://stackoverflow.com/questions/24153311/when-is-rng-gen-not-true
rustc: Allow safe access of shareable static muts

Taking a shareable loan of a `static mut` is safe if the type contained in the
location is ascribes to `Share`. This commit removes the need to have an
`unsafe` block in these situations.

Unsafe blocks are still required to write to or take mutable loans out of
statics. An example of code that no longer requires unsafe code is:

    use std::sync::atomics;

    static mut CNT: atomics::AtomicUint = atomics::INIT_ATOMIC_UINT;

    let cnt = CNT.fetch_add(1, atomics::SeqCst);

As a consequence of this change, this code is not permitted:

    static mut FOO: uint = 30;

    println!("{}", FOO);

The type `uint` is `Share`, and the static only has a shareable loan taken out
on it, so the access does not require an unsafe block. Writes to the static are
still unsafe, however, as they can invoke undefined behavior if not properly
synchronized.

Closes #13232
log: Avoid undefined behavior with LOG_LEVEL

As I have recently learned from aturon, it is undefined behavior to have any
unsynchronized access exected in parallel with an unsynchronized write. This
sort of behavior was possibly invoked by liblog during initialization. The log
level constant has been altered to become an atomic variable which prevents
undefined behavior.