{compiler-builtins,libc} shim cleanup

~~Depends on https://github.com/rust-lang/libc/pull/764; opening early for feedback.~~ r? @alexcrichton

implement pattern-binding-modes RFC

See the [RFC] and [tracking issue].

[tracking issue]: #42640
[RFC]: https://github.com/rust-lang/rfcs/blob/491e0af/text/2005-match-ergonomics.md

See the [RFC] and [tracking issue].

[tracking issue]: https://github.com/rust-lang/rust/issues/42640
[RFC]: https://github.com/rust-lang/rfcs/blob/491e0af/text/2005-match-ergonomics.md

Improvements to `proc_macro::Span` API

Motivation: https://internals.rust-lang.org/t/better-panic-location-reporting-for-unwrap-and-friends/5042/12?u=logician

TODO:
- [x] Bikeshedding/complete API
- [x] Implement tests/verify return values

cc @jseyfried @nrc

fix logic error in #44269's `prune_cache_value_obligations`

We want to retain obligations that *contain* inference variables, not
obligations that *don't contain* them, in order to fix #43132. Because
of surrounding changes to inference, the ICE doesn't occur in its
original case, but I believe it could still be made to occur on master.

Maybe I should try to write a new test case? Certainly not right now
(I'm mainly trying to get us a beta that we can ship) but maybe before
we land this PR on nightly?

This seems to cause a 10% performance regression in my imprecise
attempt to benchmark item-body checking for #43613, but it's better to
be slow and right than fast and wrong. If we want to recover that, I
think we can change the constrained-type-parameter code to actually
give a list of projections that are important for resolving inference
variables and filter everything else out.

We want to retain obligations that *contain* inference variables, not
obligations that *don't contain* them, in order to fix #43132. Because
of surrounding changes to inference, the ICE doesn't occur in its
original case, but I believe it could still be made to occur on master.

Maybe I should try to write a new test case? Certainly not right now
(I'm mainly trying to get us a beta that we can ship) but maybe before
we land this PR on nightly?

This seems to cause a 10% performance regression in my imprecise
attempt to benchmark item-body checking for #43613, but it's better to
be slow and right than fast and wrong. If we want to recover that, I
think we can change the constrained-type-parameter code to actually
give a list of projections that are important for resolving inference
variables and filter everything else out.

Implement `and_modify` on `Entry`

## Motivation

`Entry`s are useful for allowing access to existing values in a map while also allowing default values to be inserted for absent keys. The existing API is similar to that of `Option`, where `or` and `or_with` can be used if the option variant is `None`.

The `Entry` API is, however, missing an equivalent of `Option`'s `and_then` method. If it were present it would be possible to modify an existing entry before calling `or_insert` without resorting to matching on the entry variant.

Tracking issue: https://github.com/rust-lang/rust/issues/44733.

replace libc::res_init with res_init_if_glibc_before_2_26

The previous workaround for gibc's res_init bug is not thread-safe on
other implementations of libc, and it can cause crashes. Use a runtime
check to make sure we only call res_init when we need to, which is also
when it's safe. See https://github.com/rust-lang/rust/issues/43592.

~This PR is returning an InvalidData IO error if the glibc version string fails to parse. We could also have treated that case as "not glibc", and gotten rid of the idea that these functions could return an error. (Though I'm not a huge fan of ignoring error returns from `res_init` in any case.) Do other folks agree with these design choices?~

I'm pretty new to hacking on libstd. Is there an easy way to build a toy rust program against my changes to test this, other than doing an entire `sudo make install` on my system? What's the usual workflow?

Improve resolution of associated types in declarative macros 2.0

Make various identifier comparisons for associated types (and sometimes other associated items) hygienic.
Now declarative macros 2.0 can use `Self::AssocTy`, `TyParam::AssocTy`, `Trait<AssocTy = u8>` where `AssocTy` is an associated type of a trait `Trait` visible from the macro. Also, `Trait` can now be implemented inside the macro and specialization should work properly (fixes https://github.com/rust-lang/rust/pull/40847#issuecomment-310867299).

r? @jseyfried or @EddyB

incr compilation struct_defs.rs

I am prematurely openeing this as I need mentoring help from @michaelwoerister (also pinged @nikomatsakis)

First, is this the right approach for these changes?

Second, I'm a bit confused by the results so far.

- Changing `TupleStructFieldType(i32)` -> `...(u32)` changes only Hir and HirBody, not TypeOfItem
- Chaning `TupleStructAddField(i32)` -> `...(i32, u32)` *does* change TypeOfItem

This seems wrong. I feel like it should change TypeOfItem in both cases. Is this a bug in incr compilation or is it expected?

Faster compile times for release builds with llvm fix

Run global optimizations after the inliner to avoid spending time on optimizing dead code.

fixes #44655

The previous workaround for gibc's res_init bug is not thread-safe on
other implementations of libc, and it can cause crashes. Use a runtime
check to make sure we only call res_init when we need to, which is also
when it's safe. See https://github.com/rust-lang/rust/issues/43592.

address some FIXME whose associated issues were marked as closed

part of #44366

Store a new Region value every time we create a new region variable

Paired with @spastorino to walk through this and implement #44870.

Rollup of 9 pull requests

- Successful merges: #44664, #44935, #44972, #44980, #44987, #44997, #45006, #45017, #45024
- Failed merges:

add the issue number to doc_masked's feature gate

Whoops, missed this in the original `#[doc(masked)]` PR.

Add missing urls for Mutex

r? @rust-lang/docs

Typo in `librustc/README.md`

Use HTTPS protocol for "chat.mibbit.com"

I changed the `http://` protocol to `https://` for the `chat.mibbit.com` website. 📝

--[**Suriyaa**](https://mozillians.org/de/u/suriyaakudo/) 🦊

(*PS: Is somebody interested to vouch me at https://mozillians.org/de/u/suriyaakudo/?*)

`EndRegion` do not always correspond to borrow-data entries

Remove assertion that the argument to every `EndRegion` correspond to some dataflow-tracked borrow-data entry.

Fix #44828

(The comment thread on the aforementioned issue discusses why its best to just remove this assertion.)

update books for next release

Also fixes some stdlib links to the reference which have changed.

First step in https://forge.rust-lang.org/release-process.html, doing it a few days early 😄

fix ItemKind::DefaultImpl doc comment

Upgrade comment to doc comment.

...Is this actually used? If so, why does the `Impl` variant right below have a `Defaultness`?

Add links to headers in README and CONTRIBUTING

this also adds dependencies to CONTRIBUTING

I'm just getting started building the rust compiler and noticed this information/ability was missing.

It was also missing the gdb dependency for running tests. I pulled the information out of `appveyor.yml` and recommended later than 7.1 because that is what [apt ships](https://packages.ubuntu.com/search?suite=trusty&keywords=gdb). Feel free to tell me something different!

Corrected the CONTRIBUTING.md "External Dependencies" section

The "External Dependencies" section is a little outdated.
Please see following comments https://github.com/rust-lang/rust/pull/44567#issuecomment-329797331 https://github.com/rust-lang/rust/pull/44567#issuecomment-329799655 for rationale.

Don't unwrap work item results as the panic trace is useless

Fixes #43402 now there's no multithreaded panic printouts

Also update a comment

--------

Likely regressed in #43506, where the code was changed to panic in worker threads on error.

Unwrapping gives zero extra information since the stack trace is so short, so we may as well just surface that there was an error and exit the thread properly. Because there are then no multithreaded printouts, I think it should mean the output of the test for #26199 is deterministic and not interleaved (thanks to @philipc https://github.com/rust-lang/rust/issues/43402#issuecomment-333835271 for a hint).

Sadly the output is now:
```
thread '<unnamed>' panicked at 'aborting due to worker thread panic', src/librustc_trans/back/write.rs:1643:20
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: could not write output to : No such file or directory

error: aborting due to previous error
```
but it's an improvement over the multi-panic situation before.

r? @alexcrichton

Don't use remapped path when loading modules and include files

Fixes bug reported in https://github.com/rust-lang/rust/issues/41555#issuecomment-327866056.

cc @michaelwoerister

Updated RELEASES.md for 1.21.0

[Rendered](https://github.com/Aaronepower/rust/blob/master/RELEASES.md)