Allow Strings to be created from borrowed Strings. This is mostly
to make things like passing &String to an `impl Into<String>`
parameter frictionless.

LLVM will soon require 2017+ [1] (and our in-tree version just rejected the version of 2015 I was using), so update the mention and provide a link.

[1]: https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library

Fixes rust-lang#60849

string: implement From<&String> for String

Allow Strings to be created from borrowed Strings. This is mostly
to make things like passing `&String` to an `impl Into<String>`
parameter frictionless.

Fixes rust-lang#59827.

…teveklabnik

Fix convert module's documentation links

r? @steveklabnik

…tril

Include expression to wait for to the span of Await

Currently the span of `await!` only includes itself:

```rust
    await!(3);
//  ^^^^^
```

This PR changes it so that the span holds the whole `await!` expression:

```rust
    await!(3);
//  ^^^^^^^^^

Update rustc book CLI docs.

This adds a little detail (and missing flags) to the rustc book. There is still a lot of information missing, but this seemed like a good step to expanding it.

…crichton

README.md: Mention MSVC 2017+, not 2013(!)

LLVM will soon require 2017+ (and our in-tree version just rejected the version of 2015 I was using), so update the mention and provide a link.

Ref: https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library

Move `box` from the stable keyword to unstable keywords list

Fixes rust-lang#60849