we were using interior mutability (RefCells, TyIvar), add some reads/writes.

random tables. The old code was weird anyway because it would
potentially walk traits from other crates etc. The new code fits
seamlessly with the dependency tracking.

along with a README explaining how they are to be used

r? @nikomatsakis

cc @EddyB @nagisa

This PR changes most of the MIR graphviz debug output, making it smaller and more consistent. Also, it changes all fonts to monospace and adds a graph label containing the type of the `fn` the MIR is for and all the values (arguments, named bindings, and compiler temporaries).

I chose to re-write the graphviz output code instead of using the existing libgraphviz API because I found it much easier to prototype usage of various graphviz features when I had full control of the text output. It also makes the code simpler, I think.

Below are a bunch of example functions and links to their output images on the current nightly vs. this PR. File names starting with numbers (e.g. `80-factorial_fold-new.png`) are for closures. There's still a bunch of low hanging fruit to make it even better, particularly around aggregates and references.

I also imagine the textual output for MIR will be able to closely match the graphviz output. The list of statements should look identical and the terminators will be the same except that the text form will have a list of target blocks (potentially using the same edge labels as the graphviz does). I can PR a simple text output right after this PR.

This is my first large change to the compiler, so if anything should be reorganized/renamed/etc, let me know! Also, feel free to bikeshed the details of the output, though any minor changes can come in future PRs.

```rust
fn empty() {}
```

http://vps.solson.me/mir-graphviz/empty-new.png
http://vps.solson.me/mir-graphviz/empty-old.png

```rust
fn constant() -> i32 {
    42
}
```

http://vps.solson.me/mir-graphviz/constant-new.png
http://vps.solson.me/mir-graphviz/constant-old.png

```rust
fn increment(x: i32) -> i32 {
    x + 1
}
```

http://vps.solson.me/mir-graphviz/increment-new.png
http://vps.solson.me/mir-graphviz/increment-old.png

```rust
fn factorial_recursive(n: usize) -> usize {
    if n == 0 {
        1
    } else {
        n * factorial_recursive(n - 1)
    }
}
```

http://vps.solson.me/mir-graphviz/factorial_recursive-new.png
http://vps.solson.me/mir-graphviz/factorial_recursive-old.png

```rust
fn factorial_iterative(n: usize) -> usize {
    let mut prod = 1;
    for x in 1..n {
        prod *= x;
    }
    prod
}
```

http://vps.solson.me/mir-graphviz/factorial_iterative-new.png
http://vps.solson.me/mir-graphviz/factorial_iterative-old.png

```rust
fn factorial_fold(n: usize) -> usize {
    (1..n).fold(1, |prod, x| prod * x)
}
```

http://vps.solson.me/mir-graphviz/factorial_fold-new.png
http://vps.solson.me/mir-graphviz/factorial_fold-old.png
http://vps.solson.me/mir-graphviz/80-factorial_fold-new.png
http://vps.solson.me/mir-graphviz/80-factorial_fold-old.png

```rust
fn collatz(mut n: usize) {
    while n != 1 {
        if n % 2 == 0 {
            n /= 2;
        } else {
            n = 3 * n + 1;
        }
    }
}
```

http://vps.solson.me/mir-graphviz/collatz-new.png
http://vps.solson.me/mir-graphviz/collatz-old.png

```rust
fn multi_switch(n: usize) -> usize {
    match n {
        5 | 10 | 15 => 3,
        20 | 30 => 2,
        _ => 1,
    }
}
```

http://vps.solson.me/mir-graphviz/multi_switch-new.png
http://vps.solson.me/mir-graphviz/multi_switch-old.png

Add OpAssign to Wrapping<T>, plus fix some problems in core::num::wrapping

including, but not limited to:

* Testing Wrapping<T>
* Pull out a lot of broken code that doesn't need to be there with the new stage0 compiler
* Adding Rem and RemAssign to Wrapping<T>
* Removed 3 (assumed accidental) re-exports, which is a minor [breaking-change].
* Change shl and shr to take all integer types, instead of a usize; this is a more major [breaking-change], because of values that were inferred before, but brings us in line with the integer shifts.

Fixes #30524 and #30523

Fixes #30527.

```Rust

fn main() {
    let _abc = match Some(101i8) {
        Some(xyz) if xyz > 100 => xyz,
        Some(_) => -1,
        None => -2
    };
}
```

Resulting MIR now includes the `Some(xyz)` arm, guard and all:
![match.dot](https://cloud.githubusercontent.com/assets/287063/11999413/066f7610-aa8b-11e5-927b-24215af57fc4.png)

~~Not quite sure how to write a test for this.~~ Thinking too hard, just tested the end result.

r? @nikomatsakis

`fs::File` was being referenced without either calling via `std::fs::File` or by using `File` after having used `std::fs::File`.  Also `Path` was being referenced without first having used `std::path::Path`.

fs::File was being referenced without either calling via std::fs::File or by using File after having used fs::File.  Also Path was being referenced without first having used std::path::Path.

This is not a fix to checks themselves per se (though we still use `Eq` MIR test instead of calling `PartialEq::eq`), but rather how we handle items we encounter in pattern position.

Previously we would just call `PartialEq` with the constant and the matchee, but now we essentially inline the constant instead. E.g. these two snippets are functionally equivalent at MIR level:

```
match val { Some(42) => true, _ => false }
```
and
```
const SECRET: Option<u8> = Some(42);
match val { SECRET => true, _ => false }
```

This approach also allows for more optimizations of matches. I.e. It can now exploit `SwitchInt` to switch on number inside a `Some` regardless of whether the value being an item or not.

This is based on @tsion’s already approved PR so I could reuse the file for more tests.

r? @EddyB
cc @nikomatsakis @tsion

Obviously we can't remove the character one past the end of the String. And we can't today either - we'll just panic at char_at() instead - but if we're going to keep that assertion, we should at least have a correct assertion.

This PR for #21659 uses `DefId.for_each_relevant_impl()` to show other possible implementations in the "trait not implemented" message.

Obviously we can't remove the character one past the end of the String. And we can't today either - we'll just panic at char_at() instead - but if we're going to keep that assertion, we should at least have a correct assertion.

r? @Manishearth

Also: should I merged both commits? Not sure if it's really useful to keep the first one.

f64 methods have been stable since rust 1.0, but f32 never got stabilised.

I suggest backporting this to beta as well (needs changing stablilisation version then).

r? @aturon

Fixes https://github.com/rust-lang/rfcs/issues/1438

f64 methods have been stable since rust 1.0, but f32 never got stabilised.

closes #21659

When looking in the documentation I often scan the examples the first thing I do. In these 3 cases it's not obvious which direction the operation happens by adding this comment it makes it more obvious.

r? @steveklabnik

CC #30642

r? @Gankro