This is a partial revert of #42588. There is a usability concern
reported in #44294 that was not considered in the discussion of the PR,
so I would like to back this out of 1.21. As is, I think users would
have a worse and more confusing experience with this lint enabled by
default. We can re-enabled once there are better diagnostics or the case
in #44294 does not trigger the lint.

Move effect-checking to MIR

This allows emitting lints from MIR and moves the effect-checking pass to work on it.

I'll make `repr(packed)` misuse unsafe in a separate PR.

r? @EddyB

put empty generic lists behind a pointer

This reduces the size of hir::Expr from 128 to 88 bytes (!) and shaves
200MB out of #36799.

This is a performance-sensitive PR so please don't roll it up.

r? @EddyB

typeck::check::coercion - roll back failed unsizing type vars

This wraps unsizing coercions within an additional level of
`commit_if_ok`, which rolls back type variables if the unsizing coercion
fails. This prevents a large amount of type-variables from accumulating
while type-checking a large function, e.g. shaving 2GB off one of the
4GB peaks in #36799.

This is a performance-sensitive PR so please don't roll it up.

r? @EddyB
cc @nikomatsakis

Rollup of 4 pull requests

- Successful merges: #44103, #44625, #44789, #44795
- Failed merges:

MIR borrowck: Add span labels for E0381 and E0505

Corresponds to `report_use_of_moved` and `report_move_out_when_borrowed`.

Part of #44596.

Fix warning position in rustdoc code blocks

Before:

<img width="1440" alt="screen shot 2017-09-23 at 14 07 08" src="https://user-images.githubusercontent.com/3050060/30773382-b9649288-a06f-11e7-94ec-faa3c3ed999b.png">

After:

<img width="1440" alt="screen shot 2017-09-23 at 14 58 31" src="https://user-images.githubusercontent.com/3050060/30773384-bdfc9f3e-a06f-11e7-9030-9fb8a5308668.png">

r? @QuietMisdreavus

Indicate how ChildStd{in,out,err} FDs are closed.

Fixes https://github.com/rust-lang/rust/issues/41452.

add comparison operators to must-use lint (under `fn_must_use` feature)

Although RFC 1940 is about annotating functions with `#[must_use]`, a
key part of the motivation was linting unused equality operators.

(See
https://github.com/rust-lang/rfcs/pull/1812#issuecomment-265695898—it
seems to have not been clear to discussants at the time that marking the
comparison methods as `must_use` would not give us the lints on
comparison operators, at least in (what the present author understood
as) the most straightforward implementation, as landed in #43728
(3645b062).)

To rectify the situation, we here lint unused comparison operators as
part of the unused-must-use lint (feature gated by the `fn_must_use`
feature flag, which now arguably becomes a slight (tolerable in the
opinion of the present author) misnomer).

This is in the matter of #43302.

cc @crumblingstatue

this should produce more error stability

No functional changes intended.

This wraps unsizing coercions within an additional level of
`commit_if_ok`, which rolls back type variables if the unsizing coercion
fails. This prevents a large amount of type-variables from accumulating
while type-checking a large function, e.g. shaving 2GB off one of the
4GB peaks in #36799.

This reduces the size of hir::Expr from 128 to 88 bytes (!) and shaves
200MB out of #36799.

Improve diagnostics when attempting to match tuple enum variant with struct pattern

Adds an extra note as below to explain that a tuple pattern was probably intended.

```
error[E0026]: variant `X::Y` does not have a field named `data`
  --> src/main.rs:18:16
   |
18 |         X::Y { data } => println!("The data is {}", data)
   |                ^^^^ variant `X::Y` does not have field `data`

error[E0027]: pattern does not mention field `0`
  --> src/main.rs:18:9
   |
18 |         X::Y { data } => println!("The data is {}", data)
   |         ^^^^^^^^^^^^^ missing field `0`
   |
   = note: trying to match a tuple variant with a struct variant pattern
```

Fixes #41314.

incr.comp.: Add new DepGraph implementation.

This commits does a few things:
1. It adds the new dep-graph implementation -- *in addition* to the old one. This way we can start testing the new implementation without switching all tests at once.
2. It persists the new dep-graph (which includes query result fingerprints) to the incr. comp. caching directory and also loads this data.
3. It removes support for loading fingerprints of metadata imported from other crates (except for when running autotests). This is not needed anymore with red/green. It could provide a performance advantage but that's yet to be determined. For now, as red/green is not fully implemented yet, the cross-crate incremental tests are disabled.

Note, this PR is based on top of soon-to-be-merged #44696 and only the last 4 commits are new:
```
- incr.comp.: Initial implemenation of append-only dep-graph. (c90147c)
- incr.comp.: Do some various cleanup. (8ce20c5)
- incr.comp.: Serialize and deserialize new DepGraph. (0e13c1a)
- incr.comp.: Remove support for loading metadata fingerprints. (270a134)
EDIT 2:
- incr.comp.: Make #[rustc_dirty/clean] test for fingerprint equality ... (d8f7ff9)
```
(EDIT: GH displays the commits in the wrong order for some reason)

Also note that this PR is expected to certainly result in performance regressions in the incr. comp. test cases, since we are adding quite a few things (a whole additional dep-graph, for example) without removing anything. End-to-end performance measurements will only make sense again after red/green is enabled and all the legacy tracking has been turned off.

EDIT 2: Pushed another commit that makes the `#[rustc_dirty]`/`#[rustc_clean]` based autotests compared query result fingerprints instead of testing `DepNode` existence.

Add Duration::from_micros

This fixes #44400 that explains why it could be useful for embedded designs timing.

Trim and document libc shim

(hopefully) easy part of #44515.

r? @alexcrichton

Add deref suggestion

Fixes #34562.

Make the fallback of generator resumption be unreachable instead of using return

Record semantic types for all syntactic types in bodies

... and use recorded types in type privacy checking (types are recorded after inference, so there are no `_`s left).
Also use `hir_ty_to_ty` for types in signatures in type privacy checking.

This could also be potentially useful for save-analysis and diagnostics.

Fixes https://github.com/rust-lang/rust/pull/42125#issuecomment-305987755
r? @EddyB

Rollup of 14 pull requests

- Successful merges: #44554, #44648, #44658, #44712, #44717, #44726, #44745, #44746, #44749, #44759, #44770, #44773, #44776, #44778
- Failed merges:

std::sync::RwLock docs improvement

Addresses the `RwLock` part of #29377.
r? @steveklabnik

Added examples, links to types, and a small comparison between RwLock and Mutex.

Link to Rust forge from CONTRIBUTING.md