This change allows parallelism of test runs to be specified by a
command line flag names --test-threads in addition to the existing
environment variable RUST_TEST_THREADS. Fixes rust-lang#25636.

…e more efficient tokenstream usages.

The previous implementation of this function was overly conservative with
liberal usage of `Option` and `.unwrap()` which in theory never triggers. This
commit essentially removes the `Option`s in favor of unsafe implementations,
improving the code generation of the fast path for LLVM to see through what's
happening more clearly.

cc rust-lang#34727

Similar to the `as_slice` method on `core::slice::Iter` struct.

…akis

[MIR] Add explicit SetDiscriminant StatementKind for deaggregating enums

cc rust-lang#35186

To deaggregate enums, we need to be able to explicitly set the discriminant.  This PR implements a new StatementKind that does that.

I think some of the places that have `panics!` now could maybe do something smarter.

Implement From for Cell, RefCell and UnsafeCell

Considering that `From` is implemented for `Box`, `Rc` and `Arc`, it seems [reasonable](https://internals.rust-lang.org/t/implementing-from-t-for-other-std-types/3744) to implement it for `Cell`, `RefCell` and `UnsafeCell`.

…=alexcrichton

Add --test-threads option to test binaries

This change allows parallelism of test runs to be specified by a
command line flag names --test-threads in addition to the existing
environment variable RUST_TEST_THREADS. Fixes rust-lang#25636.

… r=brson

std: Optimize panic::catch_unwind slightly

The previous implementation of this function was overly conservative with
liberal usage of `Option` and `.unwrap()` which in theory never triggers. This
commit essentially removes the `Option`s in favor of unsafe implementations,
improving the code generation of the fast path for LLVM to see through what's
happening more clearly.

cc rust-lang#34727

…alexcrichton

Introduce `as_slice`/`as_mut_slice` methods on `std::vec::IntoIter` struct.

Similar to the `as_slice` method on `core::slice::Iter` struct.

…matsakis

Correct span for pub_restricted field

Fix rust-lang#35435.

Add regression test for rust-lang#22894.

None

Kicking off libproc_macro

This PR introduces `libproc_macro`, which is currently quite bare-bones (just a few macro construction tools and an initial `quote!` macro).

This PR also introduces a few test cases for it, and an additional `shim` file (at `src/libsyntax/ext/proc_macro_shim.rs` to allow a facsimile usage of Macros 2.0 *today*!

Implemented a smarter TokenStream concatenation system

The new algorithm performs 'aggressive compacting' during concatenation as follows:

- If the nodes' combined total total length is less than 32, we copy both of
   them into a new vector and build a new leaf node.
- If one node is an internal node and the other is a 'small' leaf (length<32),
   we recur down the internal node on the appropriate side.
 - Otherwise, we construct a new internal node that points to them as left and
 right.

This should produce notably better behavior than the current concatenation implementation.