fix shootout-spectralnorm, broken since Arc cannot unwrap.
borrowck: classify expressions as assignees, uses or both

- Repurposes `MoveData.assignee_ids` to mean only `=` but not `+=`, so
  that borrowck effectively classifies all expressions into assignees,
  uses or both.
- Removes two `span_err` in liveness analysis, which are now borrowck's
  responsibilities.

Closes #12527.
syntax: fixed ICEs and incorrect line nums when reporting Spans at the end of the file.

CodeMap.span_to_* perform a lookup of a BytePos(sp.hi), which lands into the next filemap if the last byte of range denoted by Span is also the last byte of the filemap, which results in ICEs or incorrect error reports.

    Example:
        ````

        pub fn main() {
            let mut num = 3;
            let refe = &mut num;
            *refe = 5;
            println!("{}", num);
        }````

(note the empty line in the beginning and the absence of newline at the end)

The above would have caused ICE when trying to report where "refe" borrow ends.
The above without an empty line in the beginning would have reported borrow end to be the first line.

Most probably, this is also responsible for (at least some occurrences of) issue #8256.

The issue is fixed by always adding a newline at the end of non-empty filemaps in case there isn't a new line there already.
auto merge of #12670 : dmski/rust/master, r=nick29581

CodeMap.span_to_* perform a lookup of a BytePos(sp.hi), which lands into the next filemap if the last byte of range denoted by Span is also the last byte of the filemap, which results in ICEs or incorrect error reports.

        Example:
            ````

            pub fn main() {
                let mut num = 3;
                let refe = &mut num;
                *refe = 5;
                println!("{}", num);
            }````

(note the empty line in the beginning and the absence of newline at the end)

The above would have caused ICE when trying to report where "refe" borrow ends.
The above without an empty line in the beginning would have reported borrow end to be the first line.

Most probably, this is also responsible for (at least some occurrences of) issue #8256.

The issue is fixed by always adding a newline at the end of non-empty filemaps in case there isn't a new line there already.
auto merge of #12733 : edwardw/rust/rw-liveness, r=nikomatsakis

- Repurposes `MoveData.assignee_ids` to mean only `=` but not `+=`, so
  that borrowck effectively classifies all expressions into assignees,
  uses or both.
- Removes two `span_err` in liveness analysis, which are now borrowck's
  responsibilities.
    
Closes #12527.
Implement hexadecimal floating point literals via a syntax extension

closes #1433
auto merge of #12652 : rcxdude/rust/hexfloatext, r=alexcrichton

Closes #1433. Implemented after suggestion by @cmr in #12323

This is slightly less flexible than the implementation in #12323 (binary and octal floats aren't supported, nor are underscores in the literal), but is cleaner in that it doesn't modify the core grammar, or require odd syntax for the number itself. The missing features could be added back with relatively little effort (the main awkwardness is parsing the string. Is there a good approach for this in the stdlib currently?)
auto merge of #12766 : TeXitoi/rust/fix-shootout-spectralnorm, r=alexcrichton
Add an ItemModifier syntax extension type

Where ItemDecorator creates new items given a single item, ItemModifier
alters the tagged item in place. The expansion rules for this are a bit
weird, but I think are the most reasonable option available.

When an item is expanded, all ItemModifier attributes are stripped from
it and the item is folded through all ItemModifiers. At that point, the
process repeats until there are no ItemModifiers in the new item.