Removed unneeded conversion to a slice.

Vec<T> can index now so its a useless conversion.
doc: adding example for type definitions
Add comment on why `RefCell::unwrap` is safe
Implement BorrowFrom<&'a T> for T (with mutable variants)
libs: make Cow usable, improve documentation

This commit makes `Cow` more usable by allowing it to be applied to
unsized types (as was intended) and providing some basic `ToOwned`
implementations on slice types. It also corrects the documentation for
`Cow` to no longer mention `DerefMut`, and adds an example.
Fix an ICE on diagnostics originating in external macros
rustc: Unify fat pointer ABI constants.

This merges the `trt_field_*`, `fn_field_*` and `slice_elt_*` constants
into two `FAT_PTR_*` constants. This resolves the first part of #18590.
rustc: Change all ABI constants to all caps.

This also removes `box_field_refcnt`. It was not used anywhere.
std: Align `raw` modules with unsafe conventions

This commit is an implementation of [RFC 240][rfc] when applied to the standard
library. It primarily deprecates the entirety of `string::raw`, `vec::raw`,
`slice::raw`, and `str::raw` in favor of associated functions, methods, and
other free functions. The detailed renaming is:

* slice::raw::buf_as_slice => slice::with_raw_buf
* slice::raw::mut_buf_as_slice => slice::with_raw_mut_buf
* slice::shift_ptr => deprecated with no replacement
* slice::pop_ptr => deprecated with no replacement
* str::raw::from_utf8 => str::from_utf8_unchecked
* str::raw::c_str_to_static_slice => str::from_c_str
* str::raw::slice_bytes => deprecated for slice_unchecked (slight semantic diff)
* str::raw::slice_unchecked => str.slice_unchecked
* string::raw::from_parts => String::from_raw_parts
* string::raw::from_buf_len => String::from_raw_buf_len
* string::raw::from_buf => String::from_raw_buf
* string::raw::from_utf8 => String::from_utf8_unchecked
* vec::raw::from_buf => Vec::from_raw_buf

All previous functions exist in their `#[deprecated]` form, and the deprecation
messages indicate how to migrate to the newer variants.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0240-unsafe-api-location.md
[breaking-change]

Closes #17863
auto merge of #19134 : sinistersnare/rust/patch-2, r=alexcrichton

Vec<T> can index now so its a useless conversion.
auto merge of #19136 : alfie/rust/master, r=steveklabnik

An example of how type definitions work would be handy
auto merge of #19137 : tbu-/rust/pr_refcell_unsafety, r=huonw
auto merge of #19146 : gereeter/rust/reference-borrow, r=aturon

This should be a more general version of #19131.
auto merge of #19152 : alexcrichton/rust/issue-17863, r=aturon

This commit is an implementation of [RFC 240][rfc] when applied to the standard
library. It primarily deprecates the entirety of `string::raw`, `vec::raw`,
`slice::raw`, and `str::raw` in favor of associated functions, methods, and
other free functions. The detailed renaming is:

* slice::raw::buf_as_slice => slice::from_raw_buf
* slice::raw::mut_buf_as_slice => slice::from_raw_mut_buf
* slice::shift_ptr => deprecated with no replacement
* slice::pop_ptr => deprecated with no replacement
* str::raw::from_utf8 => str::from_utf8_unchecked
* str::raw::c_str_to_static_slice => str::from_c_str
* str::raw::slice_bytes => deprecated for slice_unchecked (slight semantic diff)
* str::raw::slice_unchecked => str.slice_unchecked
* string::raw::from_parts => String::from_raw_parts
* string::raw::from_buf_len => String::from_raw_buf_len
* string::raw::from_buf => String::from_raw_buf
* string::raw::from_utf8 => String::from_utf8_unchecked
* vec::raw::from_buf => Vec::from_raw_buf

All previous functions exist in their `#[deprecated]` form, and the deprecation
messages indicate how to migrate to the newer variants.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0240-unsafe-api-location.md
[breaking-change]

Closes #17863
auto merge of #18856 : ruud-v-a/rust/fatptrs, r=cmr

This merges the `trt_field_*`, `fn_field_*` and `slice_elt_*` constants into two `fat_ptr_*` constants. This resolves the first part of #18590.
auto merge of #19157 : aturon/rust/cow-doc, r=alexcrichton

This commit makes `Cow` more usable by allowing it to be applied to
unsized types (as was intended) and providing some basic `ToOwned`
implementations on slice types. It also corrects the documentation for
`Cow` to no longer mention `DerefMut`, and adds an example.

Closes #19123
auto merge of #19158 : jakub-/rust/issue-14091, r=alexcrichton

Closes #14091.
Closes #19195.