Lower patterns before using the bound variable

So as to avoid having to lower patterns twice.

Move lev_distance to rustc_ast, make non-generic

rustc_ast currently has a few dependencies on rustc_lexer. Ideally, an AST
would not have any dependency its lexer, for minimizing
design-time dependencies. Breaking this dependency would also have practical
benefits, since modifying rustc_lexer would not trigger a rebuild of rustc_ast.

This commit does not remove the rustc_ast --> rustc_lexer dependency,
but it does remove one of the sources of this dependency, which is the
code that handles fuzzy matching between symbol names for making suggestions
in diagnostics. Since that code depends only on Symbol, it is easy to move
it to rustc_span. It might even be best to move it to a separate crate,
since other tools such as Cargo use the same algorithm, and have simply
contain a duplicate of the code.

This changes the signature of find_best_match_for_name so that it is no
longer generic over its input. I checked the optimized binaries, and this
function was duplicated for nearly every call site, because most call sites
used short-lived iterator chains, generic over Map and such. But there's
no good reason for a function like this to be generic, since all it does
is immediately convert the generic input (the Iterator impl) to a concrete
Vec<Symbol>. This has all of the costs of generics (duplicated method bodies)
with no benefit.

Changing find_best_match_for_name to be non-generic removed about 10KB of
code from the optimized binary. I know it's a drop in the bucket, but we have
to start reducing binary size, and beginning to tame over-use of generics
is part of that.

suggest turbofish syntax for uninferred const arguments

When not providing a const generic value, and it can not be inferred, the following suggestion is suggested:
![image](https://user-images.githubusercontent.com/39732259/95616180-af127b80-0a69-11eb-8877-551c815f9627.png)

Resolves #76737

r? ``@varkor``

Mirror centos vault to S3

Validate use of parameters in naked functions

* Reject use of parameters inside naked function body.
* Reject use of patterns inside function parameters, to emphasize role
  of parameters a signature declaration (mirroring existing behaviour
  for function declarations) and avoid generating code introducing
  specified bindings.

Closes issues below by considering input to be ill-formed.

Closes #75922.
Closes #77848.
Closes #79350.

Always invoke statement attributes on the statement itself

This is preparation for PR #78296, which will require us to handle
statement items in addition to normal items.

Only create `OnDiskCache` in incremental compilation mode

This lets us skip doing useless work when we're not in incremental
compilation mode.

Add `summary_opts()` for Markdown summary rendering options

We have a similar function `opts()` that is for rendering the main body
of the documentation, but until now we just constructed the options for
rendering summaries on the fly. This is a problem if/when we change the
enabled options since the different places can get out-of-sync.

TRACK '-Z polonius' flag

Fixes #79316.

r? `@jonas-schievink`

Validate that `#[naked]` is applied to a function definition

Rename `optin_builtin_traits` to `auto_traits`

They were originally called "opt-in, built-in traits" (OIBITs), but
people realized that the name was too confusing and a mouthful, and so
they were renamed to just "auto traits". The feature flag's name wasn't
updated, though, so that's what this PR does.

There are some other spots in the compiler that still refer to OIBITs,
but I don't think changing those now is worth it since they are internal
and not particularly relevant to this PR.

Also see <https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/opt-in.2C.20built-in.20traits.20(auto.20traits).20feature.20name>.

r? `@oli-obk` (feel free to re-assign if you're not the right reviewer for this)

Proposal to add Peekable::peek_mut

A "peekable" iterator has a `peek()`-method which provides an immutable reference to the next item. We currently do not have a method to modify that item, which we could easily add via a `peek_mut()`. See the test for a use-case (alike to my original use case), where a "pristine" iterator is passed on after modifying its state via `peek_mut()`.

If there is interest in this, I can expand on the tests and docs.

We have a similar function `opts()` that is for rendering the main body
of the documentation, but until now we just constructed the options for
rendering summaries on the fly. This is a problem if/when we change the
enabled options since the different places can get out-of-sync.

Document unsafety in core::slice::memchr

Contributes to #66219

Note sure if that's good enough, especially for the `align_to` call.
The docs only mention transmuting and I don't think that everything related to reference lifetimes and state validity mentioned in the [nomicon](https://doc.rust-lang.org/nomicon/transmutes.html) are relevant here.

Goodbye, OIBIT!

rustc_ast currently has a few dependencies on rustc_lexer. Ideally, an AST
would not have any dependency its lexer, for minimizing unnecessarily
design-time dependencies. Breaking this dependency would also have practical
benefits, since modifying rustc_lexer would not trigger a rebuild of rustc_ast.

This commit does not remove the rustc_ast --> rustc_lexer dependency,
but it does remove one of the sources of this dependency, which is the
code that handles fuzzy matching between symbol names for making suggestions
in diagnostics. Since that code depends only on Symbol, it is easy to move
it to rustc_span. It might even be best to move it to a separate crate,
since other tools such as Cargo use the same algorithm, and have simply
contain a duplicate of the code.

This changes the signature of find_best_match_for_name so that it is no
longer generic over its input. I checked the optimized binaries, and this
function was duplicated at nearly every call site, because most call sites
used short-lived iterator chains, generic over Map and such. But there's
no good reason for a function like this to be generic, since all it does
is immediately convert the generic input (the Iterator impl) to a concrete
Vec<Symbol>. This has all of the costs of generics (duplicated method bodies)
with no benefit.

Changing find_best_match_for_name to be non-generic removed about 10KB of
code from the optimized binary. I know it's a drop in the bucket, but we have
to start reducing binary size, and beginning to tame over-use of generics
is part of that.

* Reject use of parameters inside naked function body.
* Reject use of patterns inside function parameters, to emphasize role
  of parameters a signature declaration (mirroring existing behaviour
  for function declarations) and avoid generating code introducing
  specified bindings.

driver: Only output ANSI logging if connected to a terminal

Fixes #78435.

See #78435 for more.

Cc `@RalfJung` `@oli-obk`

This is preparation for PR #78296, which will require us to handle
statement items in addition to normal items.

Update cargo

10 commits in 2af662e22177a839763ac8fb70d245a680b15214..bfca1cd22bf514d5f2b6c1089b0ded0ba7dfaa6e
2020-11-12 19:04:56 +0000 to 2020-11-24 16:33:21 +0000
- Shrink the progress bar, to give more space after it. (rust-lang/cargo#8892)
- Add some comments to the toml code (rust-lang/cargo#8887)
- Start searching git config at new path (rust-lang/cargo#8886)
- Fix documentation for CARGO_PRIMARY_PACKAGE. (rust-lang/cargo#8891)
- Bump to 0.51.0, update changelog (rust-lang/cargo#8894)
- Publish target's "doc" setting when emitting metadata (rust-lang/cargo#8869)
- Relaxes expectation of `cargo test` tests to accept test execution time (rust-lang/cargo#8884)
- Finish implementation of `-Zextra-link-arg`. (rust-lang/cargo#8441)
- Reproducible crate builds (rust-lang/cargo#8864)
- Allow resolver="1" to explicitly use the old resolver behavior. (rust-lang/cargo#8857)

Get rid of doctree::{ExternalCrate, ForeignItem, Trait, Function}

Closes #79314, closes #79331, closes #79332. Follow-up to #79264 and #79312, continues breaking up #78082.

r? `@GuillaumeGomez`

Rollup of 10 pull requests

Successful merges:

 - #76858 (Add exploit mitigations chapter to the rustc book)
 - #79310 (Make `fold_item_recur` non-nullable)
 - #79312 (Get rid of `doctree::Impl`)
 - #79321 (Accept '!' in intra-doc links)
 - #79346 (Allow using `-Z fewer-names=no` to retain value names)
 - #79351 (Fix typo in `keyword` docs for traits)
 - #79354 (BTreeMap: cut out the ceremony around BoxedNode)
 - #79358 (BTreeMap/BTreeSet: make public doc more consistent)
 - #79367 (Allow disabling TrapUnreachable via -Ztrap-unreachable=no)
 - #79374 (Add note to use nightly when using expr in const generics)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Add note to use nightly when using expr in const generics

As recommended by `@Icnr` in #73899 and in zulip, I've added a note saying that const expressions can be used in nightly.

```
error: generic parameters may not be used in const operations
  --> $DIR/issue-61935.rs:10:23
   |
 6 |         Self:FooImpl<{N==0}>
   |                       ^ cannot perform const operation using `N`
   |
   = help: const parameters may only be used as standalone arguments, i.e. `N`
   = note: use feature(const_generics) and feature(const_evaluatable_checked) to enable this

error: aborting due to previous error
```

I hope the note is well written 😅

Allow disabling TrapUnreachable via -Ztrap-unreachable=no

Currently this is only possible by defining a custom target, which is quite unwieldy.

This is useful for embedded targets where small code size is desired. For example, on my project (thumbv7em-none-eabi) this yields a 0.6% code size reduction: 132892 bytes -> 132122 bytes (770 bytes down).

BTreeMap/BTreeSet: make public doc more consistent

Tweaks #72876 and #73667 and propagate them to `BTreeSet`.

BTreeMap: cut out the ceremony around BoxedNode

The opposite direction of #79093.

r? ``@Mark-Simulacrum``

Fix typo in `keyword` docs for traits

This PR fixes a small typo in the `keyword_docs.rs` file, describing the differences between the 2015 and 2018 editions of traits.

Allow using `-Z fewer-names=no` to retain value names

Change `-Z fewer-names` into an optional boolean flag and allow using it
to either discard value names when true or retain them when false,
regardless of other settings.

Accept '!' in intra-doc links

This will allow linking to things like `Result<T, !>`.

*See <https://github.com/rust-lang/rust/pull/77832#discussion_r528409079>.*

r? ``@jyn514``

Get rid of `doctree::Impl`

Follow-up to https://github.com/rust-lang/rust/pull/79264, continues breaking up https://github.com/rust-lang/rust/pull/78082. At some point I want to introduce `MaybeInlined`, but I think I'll wait until I need `MaybeInlined::InlinedWithOriginal` because it's not very useful in other situations.

r? ``@GuillaumeGomez``

Make `fold_item_recur` non-nullable

This gets rid of a bunch of `unwrap()`s and makes it a little more clear
what's going on.

Originally I wanted to make `fold_item` non-nullable too, which would
have been a lot nicer to work with, but unfortunately `stripper` does
actually return `None` in some places. I might make a follow-up moving
stripper to be special and not a pass so that passes can be
non-nullable.

Found while working on https://github.com/rust-lang/rust/issues/76998.