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.
Add cargo.vim compiler file.
Link libgcc statically on Win64.
Allow linking it statically on Win32 with an override.
Use locale 'C' for running tests. Closes #17423
collapse setting and exporting RUST_BENCH into one line
Check for null bytes before calling with_c_str on link_section and export_name value strings.
Fixed: iOS build was broken because of deprecated APIs
Cleanup the check_null function.
rustdoc: Prevent infinite recursion when inlining

Cyclic pub-use chains triggered infinite recursion, and this commit adds a hash
set to guard against cyclic recursion. This will cause one of the reexports to
render as a `pub use` instead of inlining the documentation.

Closes #16274
rustdoc: Don't try to inline the crate root

Fixes other test cases found in #16274
Deal with the fallout of string stabilization
auto merge of #17459 : dradtke/rust/master, r=brson

This PR adds a new Vim compiler file specifically for use with Cargo. It passes all arguments through, so commands like `:make build`, `:make clean`, and `:make run` all work as expected.

It also adds a quickfix autocommand for fixing the paths before populating the error list. `cargo build` reports errors with file paths that are relative to Cargo.toml, so if you're further down in the project tree, then trying to open the error will result in a blank buffer because Vim treats that path as relative to the working directory instead. With this fix, the paths work properly no matter where you are in the project.
auto merge of #17463 : oskchaitanya/rust/master, r=alexcrichton

Setting LC_ALL to C helps keep gdb's output consistent ('print' gives us expected output). This fixes #17423. I do not have access to a windows/mac machines to test this. I've only tested it on an x86_64 linux box.
auto merge of #17471 : vadimcn/rust/link-libgcc, r=alexcrichton

Closes #17271
Closes #15420
auto merge of #17438 : alexcrichton/rust/string-stable, r=aturon

# 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]: rust-lang/rfcs#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
auto merge of #17472 : kaseyc/rust/ICE_fix, r=aturon

Add checks for null bytes in the value strings for the export_name and link_section attributes, reporting an error if any are found, before calling with_c_str on them.

Fixes #16478
auto merge of #17477 : vhbit/rust/ios-deprecation-fix, r=alexcrichton
Add detection of unused enum variants
Use more descriptive names in dead code messages