This might spare someone else a little time searching the stdlib for unicode/grapheme support.

s/for reads and writes/for both ...

Added missing condition:
`dst` must be readable

For all methods which read a value of type T, `read`, `read_unaligned`,
`read_volatile` and `replace`, added missing
constraint:
The value they point to must be properly initialized

Added missing conditions:
- Valid for writes
- Valid for destructing

Updating str.chars docs to mention crates.io.

This might spare someone else a little time searching the stdlib for unicode/grapheme support.

Improve #Safety of various methods in core::ptr

For `read`, `read_unaligned`,`read_volatile`, `replace`, and `drop_in_place`:

- The value they point to must be properly initialized

For `replace`, additionally:

- The pointer must be readable

…thewjasper

Don't print block exit state in dataflow graphviz if unchanged

A small quality-of-life improvement I was using while working on rust-lang#68528. It's pretty common to have a lot of zero-statement basic blocks, especially before a `SimplifyCfg` pass is run. When the dataflow state was dense, these blocks could take up a lot of vertical space since the full flow state was printed on both entry and exit. After this PR, we only print a block's exit state if it differs from that block's entry state. Take a look at the two basic blocks on the left.

Before:

![image](https://user-images.githubusercontent.com/29463364/74505395-e2d1dd00-4eab-11ea-8006-ec8f0dc9d1b6.png)

After:
![image](https://user-images.githubusercontent.com/29463364/74505277-98506080-4eab-11ea-8d95-5190bc378331.png)

Rename `FunctionRetTy` to `FnRetTy`

As per FIXME comment

r? @Centril

[tiny] parser: `macro_rules` is a weak keyword

r? @Centril

…lan-DPC

Clean up E0309 explanation

r? @Dylan-DPC