Diagnostics such as the following

```
mismatched types: expected `core::result::Result<uint,()>`, found `core::option::Option<<generic #1>>`
<anon>:6     let a: Result<uint, ()> = None;
                                       ^~~~
mismatched types: expected `&mut <generic #2>`, found `uint`
<anon>:7     f(42u);
               ^~~
```

tend to be fairly unappealing to new users. While specific type var IDs are valuable in
diagnostics that deal with more than one such variable, in practice many messages
only mention one. In those cases, leaving out the specific number makes the messages
slightly less terrifying.

In addition, type variables have been changed to use the type hole syntax `_` in diagnostics.
With a variable ID, they're printed as `_#id` (e.g. `_#1`). In cases where the ID is left out,
it's simply `_`. Integer and float variables have an additional suffix after the number, e.g.
`_#1i` or `_#3f`.

This in-progress PR implements https://github.com/rust-lang/rust/issues/17489.

I made the code changes in this commit, next is to go through alllllllll the documentation and fix various things.

- Rename column headings as appropriate, `# Panics` for panic conditions and `# Errors` for `Result`s.
- clean up usage of words like 'fail' in error messages

Anything else to add to the list, @aturon ? I think I should leave the actual functions with names like `slice_or_fail` alone, since you'll get to those in your conventions work?

I'm submitting just the code bits now so that we can see it separately, and I also don't want to have to keep re-building rust over and over again if I don't have to 😉 

Listing all the bits so I can remember as I go:

- [x] compiler-rt
- [x] compiletest
- [x] doc
- [x] driver
- [x] etc
- [x] grammar
- [x] jemalloc
- [x] liballoc
- [x] libarena
- [x] libbacktrace
- [x] libcollections
- [x] libcore
- [x] libcoretest
- [x] libdebug
- [x] libflate
- [x] libfmt_macros
- [x] libfourcc
- [x] libgetopts
- [x] libglob
- [x] libgraphviz
- [x] libgreen
- [x] libhexfloat
- [x] liblibc
- [x] liblog
- [x] libnative
- [x] libnum
- [x] librand
- [x] librbml
- [x] libregex
- [x] libregex_macros
- [x] librlibc
- [x] librustc
- [x] librustc_back
- [x] librustc_llvm
- [x] librustdoc
- [x] librustrt
- [x] libsemver
- [x] libserialize
- [x] libstd
- [x] libsync
- [x] libsyntax
- [x] libterm
- [x] libtest
- [x] libtime
- [x] libunicode
- [x] liburl
- [x] libuuid
- [x] llvm
- [x] rt
- [x] test

This includes updating the language items and marking what needs to
change after a snapshot.

If you do not use the standard library, the language items you need to
implement have changed. For example:

```rust
 #[lang = "fail_fmt"] fn fail_fmt() -> ! { loop {} }
```

is now

```rust
 #[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} }
```

Related, lesser-implemented language items `fail` and
`fail_bounds_check` have become `panic` and `panic_bounds_check`, as
well. These are implemented by `libcore`, so it is unlikely (though
possible!) that these two renamings will affect you.

[breaking-change]

Fix test suite

Fixes #18034

3 bugs fixed.

https://github.com/rust-lang/rfcs/pull/221

The current terminology of "task failure" often causes problems when
writing or speaking about code. You often want to talk about the
possibility of an operation that returns a Result "failing", but cannot
because of the ambiguity with task failure. Instead, you have to speak
of "the failing case" or "when the operation does not succeed" or other
circumlocutions.

Likewise, we use a "Failure" header in rustdoc to describe when
operations may fail the task, but it would often be helpful to separate
out a section describing the "Err-producing" case.

We have been steadily moving away from task failure and toward Result as
an error-handling mechanism, so we should optimize our terminology
accordingly: Result-producing functions should be easy to describe.

To update your code, rename any call to `fail!` to `panic!` instead.
Assuming you have not created your own macro named `panic!`, this
will work on UNIX based systems:

    grep -lZR 'fail!' . | xargs -0 -l sed -i -e 's/fail!/panic!/g'

You can of course also do this by hand.

[breaking-change]

Fixes #17969

[breaking-change]

(for syntax-extensions)

- Token variant identifiers have been converted to PascalCase for consistency with Rust coding standards
- Some free-functions in `syntax::token` have been converted to methods on `syntax::token::Token`:
    - `can_begin_expr`         -> `Token::can_begin_expr`
    - `close_delimiter_for`    -> `Token::get_close_delimiter`
    - `is_lit`                 -> `Token::is_lit`
    - `is_ident`               -> `Token::is_ident`
    - `is_path`                -> `Token::is_path`
    - `is_plain_ident`         -> `Token::is_plain_ident`
    - `is_lifetime`            -> `Token::is_lifetime`
    - `is_mutability`          -> `Token::is_mutability`
    - `to_binop`               -> `Token::to_binop`
    - `is_keyword`             -> `Token::is_keyword`
    - `is_any_keyword`         -> `Token:is_any_keyword`
    - `is_strict_keyword`      -> `Token::is_strict_keyword`
    - `is_reserved_keyword`    -> `Token::is_reserved_keyword`
    - `mtwt_token_eq`          -> `Token::mtwt_eq`
- `token::Ident` now takes an enum instead of a boolean for clarity
- `token::{to_string, binop_to_string}` were moved to `pprust::{token_to_string, binop_to_string}`

Some minor wording fixes to the Closures chapter; my brain tripped a few times when reading it, so I tried to come up with something a bit smoother. I’m not a native speaker, so please do review this critically.

This reverts commit c245c5bb.

Parallel code generation generates invalid code for librand, which is
caught by recent versions of binutils.

This reverts commit c245c5bb.

Parallel code generation generates invalid code for librand, which is
caught by recent versions of binutils.

We now instead use a fresh variable for expressions that diverge.

Closes #14973.
Closes #13847.

[Work in progress]

cc @nikomatsakis

This PR changes the signature of several methods from `foo(self, ...)` to `foo(&self, ...)`/`foo(&mut self, ...)`, but there is no breakage of the usage of these methods due to the autoref nature of `method.call()`s. This PR also removes the lifetime parameter from some traits (`Trait<'a>` -> `Trait`). These changes break any use of the extension traits for generic programming, but those traits are not meant to be used for generic programming in the first place. In the whole rust distribution there was only one misuse of a extension trait as a bound, which got corrected (the bound was unnecessary and got removed) as part of this PR.

I've kept the commits as small and self-contained as possible for reviewing sake, but I can squash them when the review is over.

See this [table] to get an idea of what's left to be done. I've already DSTified [`Show`][show] and I'm working on `Hash`, but bootstrapping those changes seem to require a more recent snapshot (#18259 does the trick)

r? @aturon 
cc #16918 

[show]: https://github.com/japaric/rust/commits/show
[table]: https://docs.google.com/spreadsheets/d/1MZ_iSNuzsoqeS-mtLXnj9m0hBYaH5jI8k9G_Ud8FT5g/edit?usp=sharing

Fixes #17969

Just to have it somewhere to point to. Updating it will not
automatically update the one on static.rust-lang.org.

We now instead use a fresh variable for expressions that diverge.

Closes #18218

Avoid O(n^2) performance by reconsidering the full set of obligations only when we are about to report an error (#18208). I found it is still important to consider the full set in order to make tests like `let x: Vec<_> = obligations.iter().collect()` work.

I think we lack the infrastructure to write a regression test for this, but when I did manual testing I found a massive reduction in type-checking time for extreme examples like those found in #18208 vs stage0.

f? @dotdash

Avoid O(n^2) performance by reconsidering the full set of obligations only when we are about to report an error (#18208). I found it is still important to consider the full set in order to make tests like `let x: Vec<_> = obligations.iter().collect()` work.

- shrink_to_fit example is now more clear by asserting the capacity value
- annotation [0, mid) changed for [0, mid]

This adds impls of Eq/Ord/PartialEq/PartialOrd/Show/Default to Arc<T>, and it
also removes the `Send + Sync` bound on the `Clone` impl of Arc to make it more
deriving-friendly. The `Send + Sync` requirement is still enforce on
construction, of course!

The goal here is to make it easier to add new platform definitions,
especially when the additions are programmatic (ie: in build scripts).

This adds impls of Eq/Ord/PartialEq/PartialOrd/Show/Default to Arc<T>, and it
also removes the `Send + Sync` bound on the `Clone` impl of Arc to make it more
deriving-friendly. The `Send + Sync` requirement is still enforce on
construction, of course!

This allows unboxed closures that reference free type/region parameters to be monomorphized correctly in trans.

It was necessary to make `ty_unboxed_closure` carry around a `Substs` to accomplish this.  Plumbing this through typeck revealed several areas where type/region parameters in unboxed closure types are possibly not being handled correctly.  Since my goal was just to fix trans, I decided to leave FIXME comments on areas that still need attention and seek feedback on the best way to clean them up, possibly as a follow-up PR.

Closes #16791

This adds a `Substs` field to `ty_unboxed_closure` and plumbs basic
handling of it throughout the compiler. trans now correctly
monomorphizes captured free variables and llvm function defs.  This
fixes uses of unboxed closures which reference a free type or region
parameter from their environment in either their signature or free
variables.  Closes #16791

This PR changes the signature of several methods from `foo(self, ...)` to
`foo(&self, ...)`/`foo(&mut self, ...)`, but there is no breakage of the usage
of these methods due to the autoref nature of `method.call()`s. This PR also
removes the lifetime parameter from some traits (`Trait<'a>` -> `Trait`). These
changes break any use of the extension traits for generic programming, but
those traits are not meant to be used for generic programming in the first
place. In the whole rust distribution there was only one misuse of a extension
trait as a bound, which got corrected (the bound was unnecessary and got
removed) as part of this PR.

[breaking-change]