We already disallowed them to be in the arg list, such as
Fn(impl Debug), but now we disallow Fn() -> impl Debug.

Also remove the ImplTraitContext argument from the function
lower_parenthesized_parameter_data as it is now unused.

Comment out part of test run-pass/impl-trait/xcrate.rs that now fails.

Add requested comments, restructure some small bits of code. Fix extern
declarations allowing impl Trait.

Uses Symbol::intern and hir.node_to_pretty_string to create a name for
the impl Trait parameter that is just impl and then a ' + ' separated
list of bounds that the user typed.

It'd be good to have a positive test for each case where it is
allowed, I should think.

also merge disallowed and disallowed-2 into that set

Move feature gate check to inside HIR lowering. Change error messages
and update tests.

First some background:
To the compiler, the following two signatures in the trait vs the impl
are the same.

```rust
trait Foo {
    fn foo(&self, &impl Debug);
}
impl Foo for () {
    fn foo<U: Debug>(&self, x: &U) { ... }
}
```

We do not want to allow this, and so we add a new error and check.

The check just tests that all paramters 'syntheticness' match up. As
during collection, the impl Trait parameters are transformed into
anonymous synthetic generics.

Furthermore, causes a check for unused type parameters to be skipped in
check_bounds_are_used if there is already a TyError. Thus, an unused
input will not trigger `type parameter unused` errors.

Update the one test that checked for this error in the case of
a TyError.

In ast_generics extraction in generics_of and explicit_predicates_of,
also collect inputs if there are any.

Then use a Visitor to extract the necessary information from the
TyImplTraitUniversal types before extending generics and predicates with
the new information.

This is for tracking if an ImplItem is part of a trait impl. Add
a with_trait_impl_ref method to ItemLowerer to appropriately save the
state to allow appropriate nesting of trait and non-trait impls.

Replace hir::TyImplTrait with TyImplTraitUniversal and
TyImplTraitExistential.

Add an ImplTraitContext enum to rustc::hir::lowering to track the kind
and allowedness of an impl Trait.

Significantly alter lowering to thread ImplTraitContext and one other
boolean parameter described below throughought much of lowering.

The other parameter is for tracking if lowering a function is in a trait
impl, as there is not enough information to otherwise know this
information during lowering otherwise.

This change also removes the checks from ast_ty_to_ty for impl trait
allowedness as they are now all taking place in HIR lowering.

With the check for impl trait moving from type checking to HIR lowering
the error needs to move too.

add `StorageDead` handling

fix #45642
r? @arielb1

Reenable clippy testing

fixes #45680

rustc_driver: expose a way to override query providers in CompileController.

This API has been a long-time coming and will probably become the main method for custom drivers (that is, binaries other than `rustc` itself that use `librustc_driver`) to adapt the compiler's behavior.

Fix MIR borrowck EndRegion not found

Fixes #45702

- [x] Add Tests

Handle closures correctly in MIR inlining

Fixes #45894.

always add an unreachable branch on matches to give more info to llvm

As part of https://github.com/djzin/rustc-optimization I discovered that some simple enum optimizations (src/unary/three_valued_enum.rs and src/unary/four_valued_enum.rs in the repo) are not applied - and the reason for this is that we erase the info that the discriminant of an enum is one of the options by putting the last one in an "otherwise" branch. This patch adds an extra branch so that LLVM can know what the possibilities are for the discriminant, which fixes the three- and four- valued cases.

Note that for whatever reason, this doesn't fix the case of 2 variants (most notably `Option` and `Result` have 2 variants) - a pass re-ordering might fix this or we may wish to add "assume" annotations on discriminants to force it to optimize.

Rollup of 7 pull requests

- Successful merges: #45815, #45941, #45950, #45961, #45967, #45970, #45977
- Failed merges:

Fixed several pulldown warnings when documenting libstd.

Add missing links in FromStr docs

r? @QuietMisdreavus

MIR-borrowck: don't ICE for cannot move from array error

Closes #45694
compile-fail test E0508 now gives
```text
error[E0508]: cannot move out of type `[NonCopy; 1]`, a non-copy array (Ast)
  --> .\src\test\compile-fail\E0508.rs:18:18
   |
18 |     let _value = array[0];  //[ast]~ ERROR E0508
   |                  ^^^^^^^^
   |                  |
   |                  cannot move out of here
   |                  help: consider using a reference instead: `&array[0]`

error[E0508]: cannot move out of type `[NonCopy; 1]`, a non-copy array (Mir)
  --> .\src\test\compile-fail\E0508.rs:18:18
   |
18 |     let _value = array[0];  //[ast]~ ERROR E0508
   |                  ^^^^^^^^ cannot move out of here

error: aborting due to 2 previous errors
```

Use #!/usr/bin/env as shebang for Bash scripts

On some systems, the bash command could be available in another
directory than /bin. As such, to offer an env shebang is more
convenient.

This make sense even for docker scripts, as you can use Docker
on FreeBSD or SmartOS for example.

Rollup merge of #45950 - fitzgen:update-unary-and-binary-exprs-test-to-use-incr-except, r=michaelwoerister

incr: Make `unary_and_binary_exprs.rs` use `except`-style incremental checking

Part of #44924

r? @michaelwoerister

update match-expressions.rs with DepNode labels

As a part of #44924, I have updated the match-expressions.rs. The PR has tests verified for the following dependency nodes for let-expressions

- MirValidated
- MirOptimized
- TypeCheckTables
- TypeOfItem
- GenericsOfItem
- PredicatesOfItem
- FnSignature

cc @michaelwoerister
r? @nikomatsakis

rustdoc: tweak notes on ignore/compile_fail examples

Part of https://github.com/rust-lang/rust/issues/44927

This is a softening of these notices to mention *why* a given example has a given callout, rather then telling viewers to be careful with an example. It also changes the character used for these samples from a warning logo to a circle-i/information logo.

![image](https://user-images.githubusercontent.com/5217170/32464361-5fbb5d9e-c305-11e7-8482-ce71b97a54df.png)