Commits on Nov 6, 2017

1. alloc: Add Borrow impls for &String, &mut String, &Vec, and &mut Vec …

   This patch addresses a small papercut, where the auto-ref/deref
   does not work well with generic types. One small example of this
   is:
   
   ```rust
   use std::borrow::Borrow;
   
   fn foo<T: Borrow<[usize]>>(x: T) {
       println!("{:?}", x.borrow());
   }
   
   fn main() {
       let x = vec![1, 2, 3];
       foo(&x);
   }
   ```
   
   Without this patch, rust will error with:
   
   ```
   error[E0277]: the trait bound `&std::vec::Vec<{integer}>: std::borrow::Borrow<[usize]>` is not satisfied
    --> foo.rs:9:5
     |
   9 |     foo(&x);
     |     ^^^ the trait `std::borrow::Borrow<[usize]>` is not implemented for `&std::vec::Vec<{integer}>`
     |
     = help: the following implementations were found:
               <std::vec::Vec<T> as std::borrow::Borrow<[T]>>
     = note: required by `foo`
   
   error: aborting due to previous error
   ```
   
   This forces users to use `x.as_slice()` to get the code to compile.
   This patch then implements the following to cut down on unnecessary
   line noise:
   
   * `Borrow<str>` for `&String`, `&mut String`
   * `BorrowMut<str>` for `String` and `&mut String`
   * `Borrow<[T]>` for `&Vec<T>` and `&mut Vec<T>`
   * `BorrowMut<[T]>` for `&mut Vec<T>`

   

   erickt committed Nov 6, 2017
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