macros: fix inert attributes from `proc_macro_derives` with `#![feature(proc_macro)]`

This PR refactors collection of `proc_macro_derive` invocations to fix #39347.

After this PR, the input to a `#[proc_macro_derive]` function no longer sees `#[derive]`s on the underlying item. For example, consider:
```rust
extern crate my_derives;
use my_derives::{Trait, Trait2};

#[derive(Copy, Clone)]
#[derive(Trait)]
#[derive(Trait2)]
struct S;
```

Today, the input to the `Trait` derive is `#[derive(Copy, Clone, Trait2)] struct S;`, and the input to the `Trait2` derive is `#[derive(Copy, Clone)] struct S;`. More generally, a `proc_macro_derive` sees all builtin derives, as well as all `proc_macro_derive`s listed *after* the one being invoked.

After this PR, both `Trait` and `Trait2` will see `struct S;`.
This is a [breaking-change], but I believe it is highly unlikely to cause breakage in practice.

r? @nrc

Rollup of 7 pull requests

- Successful merges: #39654, #39662, #39697, #39740, #39743, #39756, #39760
- Failed merges:

Improve grammar on field init docs

Sustitutes try! for ? in the Result documentation

Hopefully newcomers will go strait to use `?`

Add tested item in the rustdoc --test output

r? @alexcrichton

cc @SergioBenitez

rustdoc: Only include a stability span if needed.

This patch gets rid of the empty stability boxes in docs by only including the span that creates it when the item actually has a stability class.

Here are images of the issue on `std::process::Output`:

Before:

<img width="340" alt="before" src="https://cloud.githubusercontent.com/assets/122457/22853638/ff88d1b2-f010-11e6-90d6-bf3d10e2fffa.png">

After:

<img width="333" alt="after" src="https://cloud.githubusercontent.com/assets/122457/22853639/06bfe7cc-f011-11e6-9892-f0ea2cc6ec90.png">

This is my first non-trivial patch to Rust, so I'm sure some of my approach is not idiomatic. Let me know how you'd like me to adjust!

Add the item type to the tooltip

See:
https://users.rust-lang.org/t/seeking-opinions-from-colorblind-rustaceans-coloring-in-rustdoc-code-blocks

Fixes #39661

Clarifies the potential ambiguity.

rustdoc: Show attributes on all item types

Currently attributes are only shown for structs, unions and enums but
they should be shown for all items. For example it is useful to know if a
function is `#[no_mangle]`.

Add recursion limit to inhabitedness check

Fixes #39489.
Add test aswell.

Auto merge of #39554 - zackmdavis:assert_eq_has_a_terrible_error_message_when_given_a_trailing_comma, r=BurntSushi

improve error message when two-arg assert_eq! receives a trailing comma

Previously, `assert_eq!(left, right,)` (respectively, `assert_ne!(left,
right,)`; note the trailing comma) would result in a confusing "requires
at least a format string argument" error. In reality, a format string is
optional, but the trailing comma puts us into the "match a token tree of
zero or more tokens" branch of the macro (in order to support the
optional format string), and passing the empty token tree into
`format_args!` results in the confusing error. If instead we match a
token tree of one or more tokens, we get a much more sensible
"unexpected end of macro invocation" error.

While we're here, fix up a stray space before a comma in the match
guards.

Resolves #39369.

-----

**Before:**
```
$ rustc scratch.rs
error: requires at least a format string argument
 --> scratch.rs:2:5
  |
2 |     assert_eq!(1, 2,);
  |     ^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in a macro outside of the current crate

error: aborting due to previous error
```

**After:**
```
$ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc scratch.rs
error: unexpected end of macro invocation
 --> scratch.rs:2:20
  |
2 |     assert_eq!(1, 2,);
  |                    ^
```

treat setting the number of test-threads to 0 as an error

It is currently possible to call `cargo test -- --test-threads=0` which will cause cargo to hang until aborted. This change will fix that and will report an appropriate error to the user.

[LLVM 4.0] Fix CreateCompileUnit

This is largely identical to @dylanmckay's [patch](https://github.com/dylanmckay), except that it doesn't try to use `file_metadata()`. I don't think that is necessary because we don't want the compile unit to be added to  `debug_context.created_files`, though I'd like confirmation from someone who knows for sure. If that is needed, I can modify `file_metadata_()` so that it can be used from `compile_unit_metadata()`.

Rollup of 9 pull requests

- Successful merges: #39174, #39660, #39676, #39692, #39701, #39710, #39721, #39724, #39725
- Failed merges:

Updated nightly book with installing nightly instructions

Allow rustc data structures compile to android

flock structure is defined in asm*/fcntl.h. This file on android is
generated from the linux kernel source, so they are the same.

Change std::panicking::try::Data into a union

No longer potentially call `mem::uninitialized::<!>()`

Fixes #39432

Update 1.15.1 relnotes

Matching what is on stable.

Explicitly mention that `Vec::reserve` is based on len not capacity

I spent a good chunk of time tracking down a buffer overrun bug that
resulted from me mistakenly thinking that `reserve` was based on the
current capacity not the current length. It would be helpful if this
were called out explicitly in the docs.

remove wrong packed struct test

This UB was found by running the test under [Miri](https://github.com/solson/miri) which rejects these unsafe unaligned loads. 😄

Actually fix manifest generation

The previous fix contained an error where `toml::encode` returned a runtime
error, so this version just constructs a literal `toml::Value`.

Don't include directory names in shasums

Right now we just run `shasum` on an absolute path but right now the shasum
files only include filenames, so let's use `current_dir` and just the file name
to only have the file name emitted.

Fix a misleading statement in `Iterator.nth()`

The `Iterator.nth()` documentation says "Note that all preceding elements will be consumed". I assumed from that that the preceding elements would be the *only* ones that were consumed, but in fact the returned element is consumed as well.

The way I read the documentation, I assumed that `nth(0)` would not discard anything (there are 0 preceding elements, and maybe it just peeks at the start of the iterator somehow), so I added a sentence clarifying that it does. I also rephrased it to avoid the stunted "i.e." phrasing.

Specialize `PartialOrd<A> for [A] where A: Ord`

This way we can call `cmp` instead of `partial_cmp` in the loop, removing some burden of optimizing `Option`s away from the compiler.

PR #39538 introduced a regression where sorting slices suddenly became slower, since `slice1.lt(slice2)` was much slower than `slice1.cmp(slice2) == Less`. This problem is now fixed.

To verify, I benchmarked this simple program:
```rust
fn main() {
    let mut v = (0..2_000_000).map(|x| x * x * x * 18913515181).map(|x| vec![x, x ^ 3137831591]).collect::<Vec<_>>();
    v.sort();
}
```

Before this PR, it would take 0.95 sec, and now it takes 0.58 sec.
I also tried changing the `is_less` lambda to use `cmp` and `partial_cmp`. Now all three versions (`lt`, `cmp`, `partial_cmp`) are equally performant for sorting slices - all of them take 0.58 sec on the
benchmark.

Tangentially, as soon as we get `default impl`, it might be a good idea to implement a blanket default impl for `lt`, `gt`, `le`, `ge` in terms of `cmp` whenever possible. Today, those four functions by default are only implemented in terms of `partial_cmp`.

r? @alexcrichton

Add Emscripten-specific linker

Emscripten claims to accept most GNU linker options, but in fact most of `-Wl,...` are useless for it and instead it requires some additional special options which are easier to handle in a separate trait.

Currently added:
 - `export_symbols`: works on executables as special Emscripten case since staticlibs/dylibs aren't compiled to JS, while exports are required to be accessible from JS.
Fixes #39171.
 - `optimize` - translates Rust's optimization level to Emscripten optimization level (whether passed via `-C opt-level=...` or `-O...`).
Fixes #36899.
 - `debuginfo` - translates debug info; Emscripten has 5 debug levels while Rust has 3, so chose to translate `-C debuginfo=1` to `-g3` (preserves whitespace, variable and function names for easy debugging).
Fixes #36901.
 - `no_default_libraries` - tells Emscripten to exclude `memcpy` and co.

TODO (in future PR): dynamic linking via `SIDE_MODULE` / `MAIN_MODULE` mechanism.

Conversions between slices and boxes

This allows conversion for `Copy` slices, `str`, and `CStr` into their boxed counterparts.

This also adds the method `CString::into_boxed_c_str`.

I would like to add similar implementations for `OsStr` as well, but I have not figured out how.

No longer potentially call `mem::uninitialized::<!>()`

Fixes #39432

flock structure is defined in asm*/fcntl.h. This file on android is
generated from the linux kernel source, so they are the same.