Avoid building as root with `sudo make install`

When running `sudo make install`, we only want to run the actual install
as root, the building of the documentation and the distribution folder
should happen as the non-root user.

Related to #13728.
Add help diagnostic messages

This adds ‘help’ diagnostic messages to rustc. This is used for anything that
provides help to the user, particularly the `--explain` messages that were
previously integrated into the relevant error message.
from_str has an impl for uint, not Option<uint>
Refactor `Vec::map_in_place` to move code out of `PartialVec`

Additionally, support zero-sized types.
Do not attempt to autoderef `!`

This prevents error spam when attempting to look up overload traits.
Disallow dereference of `!`

Later compiler passes are not prepared to deal with deref of
`ty_bot` and will generate various ICEs, so disallow it outright for now.

Closes issue #17373
Disallow index of `!`

This avoids ICEs in later passes that cannot cope with this construct
Add regression test for issue #17373
Add test that `!` cannot be indexed
Induce an empty loan for the value being matched in match expressions

This is to make sure it hadn't been moved if there are no bindings
in any of the arms.

Fixes #17385.
Update CONTRIBUTING.md with new issues policy
collections: Stabilize String

# Rationale

When dealing with strings, many functions deal with either a `char` (unicode
codepoint) or a byte (utf-8 encoding related). There is often an inconsistent
way in which methods are referred to as to whether they contain "byte", "char",
or nothing in their name.  There are also issues open to rename *all* methods to
reflect that they operate on utf8 encodings or bytes (e.g. utf8_len() or
byte_len()).

The current state of String seems to largely be what is desired, so this PR
proposes the following rationale for methods dealing with bytes or characters:

> When constructing a string, the input encoding *must* be mentioned (e.g.
> from_utf8). This makes it clear what exactly the input type is expected to be
> in terms of encoding.
>
> When a method operates on anything related to an *index* within the string
> such as length, capacity, position, etc, the method *implicitly* operates on
> bytes. It is an understood fact that String is a utf-8 encoded string, and
> burdening all methods with "bytes" would be redundant.
>
> When a method operates on the *contents* of a string, such as push() or pop(),
> then "char" is the default type. A String can loosely be thought of as being a
> collection of unicode codepoints, but not all collection-related operations
> make sense because some can be woefully inefficient.

# Method stabilization

The following methods have been marked #[stable]

* The String type itself
* String::new
* String::with_capacity
* String::from_utf16_lossy
* String::into_bytes
* String::as_bytes
* String::len
* String::clear
* String::as_slice

The following methods have been marked #[unstable]

* String::from_utf8 - The error type in the returned `Result` may change to
                      provide a nicer message when it's `unwrap()`'d
* String::from_utf8_lossy - The returned `MaybeOwned` type still needs
                            stabilization
* String::from_utf16 - The return type may change to become a `Result` which
                       includes more contextual information like where the error
                       occurred.
* String::from_chars - This is equivalent to iter().collect(), but currently not
                       as ergonomic.
* String::from_char - This method is the equivalent of Vec::from_elem, and has
                      been marked #[unstable] becuase it can be seen as a
                      duplicate of iterator-based functionality as well as
                      possibly being renamed.
* String::push_str - This *can* be emulated with .extend(foo.chars()), but is
                     less efficient because of decoding/encoding. Due to the
                     desire to minimize API surface this may be able to be
                     removed in the future for something possibly generic with
                     no loss in performance.
* String::grow - This is a duplicate of iterator-based functionality, which may
                 become more ergonomic in the future.
* String::capacity - This function was just added.
* String::push - This function was just added.
* String::pop - This function was just added.
* String::truncate - The failure conventions around String methods and byte
                     indices isn't totally clear at this time, so the failure
                     semantics and return value of this method are subject to
                     change.
* String::as_mut_vec - the naming of this method may change.
* string::raw::* - these functions are all waiting on [an RFC][2]

[2]: https://github.com/rust-lang/rfcs/pull/240

The following method have been marked #[experimental]

* String::from_str - This function only exists as it's more efficient than
                     to_string(), but having a less ergonomic function for
                     performance reasons isn't the greatest reason to keep it
                     around. Like Vec::push_all, this has been marked
                     experimental for now.

The following methods have been #[deprecated]

* String::append - This method has been deprecated to remain consistent with the
                   deprecation of Vec::append. While convenient, it is one of
                   the only functional-style apis on String, and requires more
                   though as to whether it belongs as a first-class method or
                   now (and how it relates to other collections).
* String::from_byte - This is fairly rare functionality and can be emulated with
                      str::from_utf8 plus an assert plus a call to to_string().
                      Additionally, String::from_char could possibly be used.
* String::byte_capacity - Renamed to String::capacity due to the rationale
                          above.
* String::push_char - Renamed to String::push due to the rationale above.
* String::pop_char - Renamed to String::pop due to the rationale above.
* String::push_bytes - There are a number of `unsafe` functions on the `String`
                       type which allow bypassing utf-8 checks. These have all
                       been deprecated in favor of calling `.as_mut_vec()` and
                       then operating directly on the vector returned. These
                       methods were deprecated because naming them with relation
                       to other methods was difficult to rationalize and it's
                       arguably more composable to call .as_mut_vec().
* String::as_mut_bytes - See push_bytes
* String::push_byte - See push_bytes
* String::pop_byte - See push_bytes
* String::shift_byte - See push_bytes

# Reservation methods

This commit does not yet touch the methods for reserving bytes. The methods on
Vec have also not yet been modified. These methods are discussed in the upcoming
[Collections reform RFC][1]

[1]: https://github.com/aturon/rfcs/blob/collections-conventions/active/0000-collections-conventions.md#implicit-growth
collections: Deprecate shift_char for insert/remove

This commit deprecates the String::shift_char() function in favor of the
addition of an insert()/remove() pair of functions. This aligns the API with Vec
in that characters can be inserted at arbitrary positions.  Additionaly, there
is no `_char` suffix due to the rationaled laid out in the previous commit.

These functions are both introduced as unstable as their failure semantics,
while in line with slices/vectors, are uncertain about whether they should
remain the same.
Fix snapshot builders

The test in question does not pass when cross compiling because the syntax
extension must always be compiled for the host, not the target.
Revert "Prefer bundled gcc.  External gcc can still be used if one provides a full path via -Clinker=..."

This reverts commit 94f05324fee6cd4637adc01f441fafd09e678668.
Lint stability now checks macro arguments.

Closes #17185.
Update calls of deprecated functions in macros.

Fallout of #17185.
Fix deprecation warnings in check-docs.

Fallout of closing #17185.
remove references to owned and managed pointers in clone docs

Fixes #17445.
auto merge of #17440 : alexcrichton/rust/fix-snapshot, r=brson

The test in question does not pass when cross compiling because the syntax
extension must always be compiled for the host, not the target.