rustc: Remove all "consider using an explicit lifetime parameter" suggestions

These give so many incorrect suggestions that having them is
detrimental to the user experience. The compiler should not be
suggesting changes to the code that are wrong - it is infuriating: not
only is the compiler telling you that _you don't understand_ borrowing,
_the compiler itself_ appears to not understand borrowing. It does not
inspire confidence.

r? @nikomatsakis

travis: Turn off core dumps on OSX

I've seen these take up quite a bit of log space and I have the sneaking
suspicion that they're just making our test suite take longer (sometimes timing
out on 32-bit OSX now). In any case the backtraces haven't proven too useful,
unfortunately.

Stabilize Self and associated types in struct expressions and patterns

Rebase of https://github.com/rust-lang/rust/pull/37734
Closes https://github.com/rust-lang/rust/issues/37544
r? @nikomatsakis

Hide a few more standard library symbols

These commits touch up some of the symbol visibility rules for some crates related to the standard library, notably:

* Symbols that are `pub extern` and `#[no_mangle]` which are internal-to-rust ABI things are no longer at the `C` export level, but the `Rust` export level. This includes allocators, panic runtimes, and compiler builtins.
* The libbacktrace library is now compiled with `-fvisibility=hidden` to ensure that we don't export those symbols.

We don't want these symbols exported from the standard library, this is
just an internal implementation detail of the standard library
currently.

Closes #34984

This hides symbols from various unstable and implementation-detail
crates of the standard library. Although typically transitive exported
`pub extern` functions are exported from cdylibs, these crates aren't
necessary as they're all implementation details.

Closes #34493

incr.comp.: Make cross-crate tracking for incr. comp. opt-in.

The current implementation of cross-crate dependency tracking can cause quite long compile times and high memory usage for some crates (see #39208 for example). This PR therefore makes that part of dependency tracking optional. Incremental compilation still works, it will only have very coarse dep-tracking for upstream crates.

r? @nikomatsakis

Point to immutable arg/fields when trying to use as &mut

Present the following output when trying to access an immutable borrow's
field as mutable:

```
error[E0389]: cannot borrow data mutably in a `&` reference
  --> $DIR/issue-38147-1.rs:27:9
   |
26 | fn f(&self) {
   |      -----  use `&mut self` here to make mutable
27 |     f.s.push('x');
   |     ^^^ assignment into an immutable reference
```

And the following when trying to access an immutable struct field as mutable:

```
error: cannot borrow immutable borrowed content `*self.s` as mutable
  --> $DIR/issue-38147-3.rs:17:9
   |
12 |     s: &'a String
   |     ------------- use `&'a mut String` here to make mutable
...|
16 |     fn f(&self) {
   |          -----  use `&mut self` here to make mutable
17 |         self.s.push('x');
   |         ^^^^^^ cannot borrow as mutable
```

Fixes #38147.

Bounds parsing refactoring 2

See https://github.com/rust-lang/rust/pull/37511 for previous discussion.
cc @matklad

Relaxed parsing rules:
 - zero bounds after `:` are allowed in all contexts.
 - zero predicates are allowed after `where`.
- trailing separator `,` is allowed after predicates in `where` clauses not followed by `{`.

Other parsing rules:
 - trailing separator `+` is still allowed in all bound lists.

Code is also cleaned up and tests added.

I haven't touched parsing of trait object types yet, I'll do it later.

These give so many incorrect suggestions that having them is
detrimental to the user experience. The compiler should not be
suggesting changes to the code that are wrong - it is infuriating: not
only is the compiler telling you that _you don't understand_ borrowing,
_the compiler itself_ appears to not understand borrowing. It does not
inspire confidence.

Point to immutable borrow arguments and fields when trying to use them as
mutable borrows. Add label to primary span on "cannot borrow as mutable"
errors.

Present the following output when trying to access an immutable borrow's
field as mutable:

```
error[E0389]: cannot borrow data mutably in a `&` reference
  --> $DIR/issue-38147-1.rs:27:9
   |
26 | fn f(&self) {
   |      -----  use `&mut self` here to make mutable
27 |     f.s.push('x');
   |     ^^^ assignment into an immutable reference
```

And the following when trying to access an immutable struct field as mutable:

```
error: cannot borrow immutable borrowed content `*self.s` as mutable
  --> $DIR/issue-38147-3.rs:17:9
   |
12 |     s: &'a String
   |     ------------- use `&'a mut String` here to make mutable
...|
16 |     fn f(&self) {
   |          -----  use `&mut self` here to make mutable
17 |         self.s.push('x');
   |         ^^^^^^ cannot borrow as mutable
```

End temporary lifetimes being extended by `let X: &_` hints

cc #39283

r? @nikomatsakis

I've seen these take up quite a bit of log space and I have the sneaking
suspicion that they're just making our test suite take longer (sometimes timing
out on 32-bit OSX now). In any case the backtraces haven't proven too useful,
unfortunately.

Rename tcx.map to the far more descriptive tcx.hir.

Also a bit more renaming because `ast_map` and `'ast` were still used with HIR.
Main motivation is to "free up" `tcx.map`, or rather, `tcx.maps`, to consolidate `ty::maps` there.

r? @nikomatsakis

Add a distinct error code and description for "main function has wron…

…g type"

Remove Reflect

PR for removing the `Reflect` trait. Opened so that a crater run can be done for testing the impact: https://github.com/rust-lang/rust/issues/27749#issuecomment-272665163

Fixes #27749

process trait/impl items directly from the visitor callback

The current setup processes impl/trait items while visiting
the impl/trait. This means we basically have this setup:

    <Lots> -> TypeckItemBody(Impl) -> Tables(ImplItem{0,1,2,3})

But this was largely an artifact of the older code. By moving the
processing of items into method dedicated for their use, we produce this
setup:

    <Little> -> TypeckItemBody(ImplItem0) -> Tables(ImplItem0)
    ...
    <Little> -> TypeckItemBody(ImplItem3) -> Tables(ImplItem3)

r? @michaelwoerister

Also, we might consider removing the `TypeckItemBody` node altogether and just using `Tables` as the task. `Tables` is its primary output, I imagine? That would reduce size of dep-graph somewhat.

cc @EddyB -- perhaps this pattern applies elsewhere?

Fix wording around sort guarantees

Fixes #38524

/cc @rust-lang/libs @stjepang

Partially implement RFC 1647 (`Self` in impl headers)

The name resolution part is easy, but the typeck part contains an unexpected problem.

It turns out that `Self` type *depends* on bounds and `where` clauses, so we need to convert them first to determine what the `Self` type is! If bounds/`where` clauses can refer to `Self` then we have a cyclic dependency.
This is required to support impls like this:
```
// Found in libcollections
impl<I: IntoIterator> SpecExtend<I> for LinkedList<I::Item> { .... }
                                                      ^^^^^ associated type `Item` is found using information from bounds

```
I'm not yet sure how to resolve this issue.
One possible solution (that feels hacky) is to make two passes over generics - first collect predicates ignoring everything involving `Self`, then determine `Self`, then collect predicates again without ignoring anything. (Some kind of lazy on-demand checking or something looks like a proper solution.)

This patch in its current state doesn't solve the problem with `Self` in bounds, so the only observable things it does is improving error messages and supporting `impl Trait<Self> for Type {}`.

There's also a question about feature gating. It's non-trivial to *detect* "newly resolved" `Self`s to feature gate them, but it's simple to *enable* the new resolution behavior when the feature gate is already specified. Alternatively this can be considered a bug fix and merged without a feature gate.

cc https://github.com/rust-lang/rust/issues/38864
r? @nikomatsakis
cc @EddyB
Whitespace ignoring diff https://github.com/rust-lang/rust/pull/38920/files?w=1

The current setup processes impl/trait items while visiting
the impl/trait. This means we basically have this setup:

    <Lots> -> TypeckItemBody(Impl) -> Tables(ImplItem{0,1,2,3})

But this was largely an artifact of the older code. By moving the
processing of items into method dedicated for their use, we produce this
setup:

    <Little> -> TypeckItemBody(ImplItem0) -> Tables(ImplItem0)
    ...
    <Little> -> TypeckItemBody(ImplItem3) -> Tables(ImplItem3)

Add std::process::Command::envs()

`Command::envs()` adds a vector of key-value pairs to the child
process environment all at once.  Suggested in #38526.

This is not fully baked and frankly I'm not sure it even _works_, but I need some help finishing it up, and this is the simplest way to show you what I've got.  The problems I know exist and don't know how to solve, from most to least important, are:

* [ ] I don't know if the type signature of the new function is correct.
* [x] The new test might not be getting run.  I didn't see it go by in the output of `x.py test src/libstd --stage 1`.
* [x] The tidy check says ``process.rs:402: different `since` than before`` which I don't know what it means.

r? @brson