Bump cargo-vendor version

Currently we pin `cargo-vendor` to 0.1.4, which doesn't set the `User-Agent` HTTP header. crates.io is going to require that header in the near future, so this PR bumps the pinned version of the crate to the latest one (which correctly sets the header).

r? @Mark-Simulacrum
cc @sgrif

Take 2: Implement object-safety and dynamic dispatch for arbitrary_self_types

This replaces #50173. Over the months that that PR was open, we made a lot of changes to the way this was going to be implemented, and the long, meandering comment thread and commit history would have been confusing to people reading it in the future. So I decided to package everything up with new, straighforward commits and open a new PR.

Here are the main points. Please read the commit messages for details.

- To simplify codegen, we only support receivers that have the ABI of a pointer. That means they are builtin pointer types, or newtypes thereof.
- We introduce a new trait: `DispatchFromDyn<T>`, similar to `CoerceUnsized<T>`. `DispatchFromDyn` has extra requirements that `CoerceUnsized` does not: when you implement `DispatchFromDyn` for a struct, there cannot be any extra fields besides the field being coerced and `PhantomData` fields. This ensures that the struct's ABI is the same as a pointer.
- For a method's receiver (e.g. `self: Rc<Self>`) to be object-safe, it needs to have the following property:
    - let `DynReceiver` be the receiver when `Self = dyn Trait`
    - let `ConcreteReceiver` be the receiver when `Self = T`, where `T` is some unknown `Sized` type that implements `Trait`, and is the erased type of the trait object.
    - `ConcreteReceiver` must implement `DispatchFromDyn<DynReceiver>`

In the case of `Rc<Self>`, this requires `Rc<T>: DispatchFromDyn<Rc<dyn Trait>>`

These rules are explained more thoroughly in the doc comment on `receiver_is_dispatchable` in object_safety.rs.

r? @nikomatsakis and @EddyB

cc @arielb1 @cramertj @withoutboats

Special thanks to @nikomatsakis for getting me un-stuck when implementing the object-safety checks, and @EddyB for helping with the codegen parts.

EDIT 2018-11-01: updated because CoerceSized has been replaced with DispatchFromDyn

rustc: improve E0669 span

E0669 refers to an operand that cannot be coerced into a single LLVM
value, unfortunately right now this uses the Span for the entire inline
assembly statement, which is less than ideal.

This commit preserves the Span from HIR, which lets us emit the error
using the Span for the operand itself in MIR.

r? @nagisa
cc/ @parched

Add index page argument

@Mark-Simulacrum: I might need some help from you: in bootstrap, I want to add an argument (a new flag added into `rustdoc`) in order to generate the current index directly when `rustdoc` is documenting the `std` lib. However, my change in `bootstrap` didn't do it and I assume it must be moved inside the `Std` struct. But there, I don't see how to pass it to `rustdoc` through `cargo`. Did I miss anything?

r? @QuietMisdreavus

Add Retagging statements

This adds a `Retag` statement kind to MIR, used to perform the retagging operation from [Stacked Borrows](https://www.ralfj.de/blog/2018/08/07/stacked-borrows.html). It also kills the old `Validate` statements that I added last year.

NOTE: This includes https://github.com/rust-lang/rust/pull/55270. Only [these commits are new](https://github.com/RalfJung/rust/compare/stacked-borrows-ng...RalfJung:retagging).

add an appveyor config for aarch64-pc-windows-msvc

This is purely a cargo-cult of things to solicit feedback from humans and/or automation failures.  Not sure that the build artifacts would get packaged properly to start providing nightly tarballs for `libstd`, but this is at least a start.

Fixes #53864.

Add raw_entry API to HashMap

This is a continuation of #50821.

Fixes #46775 -- don't mutate the process's environment in Command::exec

Instead, pass the environment to execvpe, so the kernel can apply it directly to the new process. This avoids a use-after-free in the case where exec'ing the new process fails for any reason, as well as a race condition if there are other threads alive during the exec.

Fixes #46775

universes refactor 3

Some more refactorings from my universe branch. These are getting a bit more "invasive" -- they start to plumb the universe information through the canonicalization process. As of yet though I don't **believe** this branch changes our behavior in any notable way, though I'm marking the branch as `WIP` to give myself a chance to verify this.

r? @scalexm

Update clippy

r? @kennytm @oli-obk

disallow `#[repr(C)] and `#[repr(packed)]` on structs implementing DispatchFromDyn because they will change the ABI from Scalar/ScalarPair to Aggregrate, resulting in an ICE during object-safety checks or codegen

I don't really understand what it's for, but see the comment here:
https://github.com/rust-lang/rust/pull/50173#discussion_r204222336

where arielb1 said

> Does this check do anything these days? I think `$0: Trait` is always considered ambiguous

and nikomatsakis agreed we may be able to get rid of it

also updated the doc on `receiver_is_dispatchable` to reflect current state of the implementation

Added to `DispatchFromDyn` and `CoerceUnsized` error messages

If object-safety checks succeed for a receiver type, make sure the
receiver’s abi is
a) a Scalar, when Self = ()
b) a ScalarPair, when Self = dyn Trait

Rename `CoerceSized` to `DispatchFromDyn`, and reverse the direction so that, for example, you write

```
impl<T: Unsize<U>, U> DispatchFromDyn<*const U> for *const T {}
```

instead of

```
impl<T: Unsize<U>, U> DispatchFromDyn<*const T> for *const U {}
```

this way the trait is really just a subset of `CoerceUnsized`.

The checks in object_safety.rs are updated for the new trait, and some documentation and method names in there are updated for the new trait name — e.g. `receiver_is_coercible` is now called `receiver_is_dispatchable`. Since the trait now works in the opposite direction, some code had to updated here for that too.

I did not update the error messages for invalid `CoerceSized` (now `DispatchFromDyn`) implementations, except to find/replace `CoerceSized` with `DispatchFromDyn`. Will ask for suggestions in the PR thread.

tests were failing because I didn't wrap code snippets like  in backticks. fixed that now, so hopefully tests will pass on travis

 I’m not sure why these tests have different output now, but they do.
In all cases, the error message that is missing looks like this: “the
trait bound `dyn Trait: Trait` is not satisfied”

My guess is that the error message is going away because object-safety
now involves trait solving, and these extra error messages are no
longer leaking out.

This will make receiver types like `Rc<Self>` and `Pin<&mut Self>`
object-safe.

For now, all of the receivers that we care about are just a newtyped
pointer — i.e. `Box<Self>`, `Rc<Self>`, `Pin<Box<Self>>`, `Pin<&mut
Self>`. This is much simpler to implement in codeine than the more
general case, because the ABI is the same as a pointer. So we add some
checks in typeck/coherence/builtin.rs to make sure that implementors of
CoerceSized are just newtyped pointers. In this commit, we also
implement the codegen bits.

For a trait method to be considered object-safe, the receiver type must
satisfy certain properties: first, we need to be able to get the vtable
to so we can look up the method, and second, we need to convert the
receiver from the version where `Self=dyn Trait`, to the version where
`Self=T`, `T` being some unknown, `Sized` type that implements `Trait`.

To check that the receiver satisfies those properties, we use the
following query:

forall (U) {
if (Self: Unsize<U>) {
Receiver[Self => U]: CoerceSized<Receiver>
}
}

where `Receiver` is the receiver type of the method (e.g. `Rc<Self>`),
and `Receiver[Self => U]` is the receiver type where `Self = U`, e.g.
`Rc<U>`.

forall queries like this aren’t implemented in the trait system yet, so
for now we are using a bit of a hack — see the code for explanation.

This trait is more-or-less the reverse of CoerceUnsized, and will be
used for object-safety checks. Receiver types like `Rc` will have to
implement `CoerceSized` so that methods that use `Rc<Self>` as the
receiver will be considered object-safe.

Rollup of 13 pull requests

Successful merges:

 - #55280 (Add libproc_macro to rust-src distribution)
 - #55469 (Regression tests for issue #54477.)
 - #55504 (Use vec![x; n] instead of iter::repeat(x).take(n).collect())
 - #55522 (use String::from() instead of format!() macro to construct Strings.)
 - #55536 (Pass suggestions as impl Iterator instead of Vec)
 - #55542 (syntax: improve a few allocations)
 - #55558 (Tweak `MatcherPos::matches`)
 - #55561 (Fix double_check tests on big-endian targets)
 - #55573 (Make sure the `aws` executable is in $PATH on macOS)
 - #55574 (Use `SmallVec` within `MoveData`.)
 - #55575 (Fix invalid_const_promotion test on some archs)
 - #55578 (Made doc example of `impl Default for …` use `-> Self` instead of explicit self type)
 - #55582 (Remove unused import copy from publish_toolstate.py)

Made doc example of `impl Default for …` use `-> Self` instead of explicit self type

There is no need to state the explicit type of `self`.

Fix double_check tests on big-endian targets

Since the enums get optimized down to 1 byte long, the bits set in the `usize` member don't align with the `enum` values on big-endian machines. Avoid this issue by shrinking the integer member to the
same size as the enums.

Remove unused import copy from publish_toolstate.py