Add @Spell clusters to Vim syntax file

Add @Spell clusters to Vim syntax highlighting file to do spell checking
only inside comments and strings
doc: Boxes diagram shows enum value inside box.
doc: add note below diagram about memory layout.
Fixed bug when initialising bitv from bool=true
Fixed typos in comments of librustc backend.
rustc: Feature gate `log_syntax!`. Closes #11602
handle zero-size allocations correctly

The `malloc` family of functions may return a null pointer for a
zero-size allocation, which should not be interpreted as an
out-of-memory error.

If the implementation does not return a null pointer, then handling
this will result in memory savings for zero-size types.

This also switches some code to `malloc_raw` in order to maintain a
centralized point for handling out-of-memory in `rt::global_heap`.

Closes #11634
Actually force a / in the path for ext crates

If the library is in the working directory, its path won't have a "/"
which will cause dlopen to search /usr/lib etc. It turns out that Path
auto-normalizes during joins so Path::new(".").join(path) is actually a
no-op.
Expose platform independent path separators
rustc: Clean up error reporting

This commit re-works how the monitor() function works and how it both receives
and transmits errors. There are a few cases in which the compiler can abort:

1. A normal compiler error. In this case, the compiler raises a FatalError as
   the failure value of the task. If this happens, then the monitor task does
   nothing. It ignores all stderr output of the child task and it also
   suppresses the failure message of the main task itself. This means that on a
   normal compiler error just the error message itself is printed.

2. A normal internal compiler error. These are invoked from sess.span_bug() and
   friends. In these cases, they follow the same path (raising a FatalError),
   but they will also print an ICE message which has a URL to go report a bug.

3. An actual compiler bug. This happens whenever anything calls fail!() instead
   of going through the session itself. In this case, we print out stuff about
   RUST_LOG=2 and we by default capture all stderr and print via warn!() so it's
   only printed out with the RUST_LOG var set.
Disallow implementation of cross-crate priv traits

Turns out we were just forgetting to encode the privacy for trais, and
everything without privacy defaults to public!

Closes #11593
auto merge of #11620 : alexcrichton/rust/rustc-silent, r=brson

This commit re-works how the monitor() function works and how it both receives
and transmits errors. There are a few cases in which the compiler can abort:

1. A normal compiler error. In this case, the compiler raises a FatalError as
   the failure value of the task. If this happens, then the monitor task does
   nothing. It ignores all stderr output of the child task and it also
   suppresses the failure message of the main task itself. This means that on a
   normal compiler error just the error message itself is printed.

2. A normal internal compiler error. These are invoked from sess.span_bug() and
   friends. In these cases, they follow the same path (raising a FatalError),
   but they will also print an ICE message which has a URL to go report a bug.

3. An actual compiler bug. This happens whenever anything calls fail!() instead
   of going through the session itself. In this case, we print out stuff about
   RUST_LOG=2 and we by default capture all stderr and print via warn!() so it's
   only printed out with the RUST_LOG var set.
auto merge of #11632 : brson/rust/issue-11602, r=huonw
syntax: convert ast_map to use a SmallIntMap.

NodeIds are sequential integers starting at zero, so we can achieve some
memory savings by just storing the items all in a line in a vector.

The occupancy for typical crates seems to be 75-80%, so we're already
more efficient than a HashMap (maximum occupancy 75%), not even counting
the extra book-keeping that HashMap does.
auto merge of #11311 : hdima/rust/vim-syntax-spell-check, r=cmr

Add `@Spell` clusters to Vim syntax highlighting file to do spell checking only inside comments and strings
auto merge of #11567 : divtxt/rust/master, r=cmr

I found the boxes diagram in the tutorial misleading about how the enum worked.

The current diagram makes it seem that there is a separate Cons struct when there is only one type of struct for the  List type, and Nil is drawn almost as if it's not consuming memory.

I'm aware of the optimization that happens for this enum which takes advantage of the fact that pointer cannot be null but this is an implementation detail and not the only one that applies here.  I can add a note below the diagram mentioning this if you like.
auto merge of #11615 : adwhit/rust/master, r=cmr

This is my first patch so feedback appreciated!

Bug when initialising `bitv:Bitv::new(int,bool)` when `bool=true`. It created a `Bitv` with underlying representation `!0u` rather than the actual desired bit layout ( e.g. `11111111` instead of `00001111`). This works OK because a size attribute is included which keeps access to legal bounds.  However when using `BitvSet::from_bitv(Bitv)`, we then find that `bitvset.contains(i)` can return true when `i` should not in fact be in the set.

```
let bs = BitvSet::from_bitv(Bitv::new(100, true));
assert!(!bs.contains(&127)) //fails
```

The fix is to create the correct representation by treating various cases separately and using a bitshift `(1<<nbits) - 1` to generate correct number of `1`s where necessary.
auto merge of #11616 : huonw/rust/ast_map, r=pnkfelix

NodeIds are sequential integers starting at zero, so we can achieve some
memory savings by just storing the items all in a line in a vector.

The occupancy for typical crates seems to be 75-80%, so we're already
more efficient than a HashMap (maximum occupancy 75%), not even counting
the extra book-keeping that HashMap does.
auto merge of #11628 : alexcrichton/rust/issue-11593, r=brson

Turns out we were just forgetting to encode the privacy for trais, and
everything without privacy defaults to public!

Closes #11593
auto merge of #11633 : chromatic/rust/master, r=alexcrichton