Currently `TypedArena` allocates its first chunk, which is usually 4096
bytes, as soon as it is created. If no allocations are ever made from
the arena then this allocation (and the corresponding deallocation) is
wasted effort.

This commit changes `TypedArena` so it doesn't allocate the first chunk
until the first allocation is made.

This change speeds up rustc by a non-trivial amount because rustc uses
`TypedArena` heavily: compilation speed (producing debug builds) on
several of the rustc-benchmarks increases by 1.02--1.06x. The change
should never cause a slow-down because the hot `alloc` function is
unchanged. It does increase the size of `TypedArena` by one `usize`
field, however.

The commit also fixes some out-of-date comments.

Fixes rust-lang#29372.

…rner

Updated E0560 to new error format

Added a small bit of code to add a label for E0560.

Also renamed src/test/compile-fail/E560.rs file to E0560.rs (not sure why it was named E560).
Updated all test cases which check this error.

Closes rust-lang#36199

fix top level attr spans

issue rust-lang#36530
?r @jonathandturner

…cs, r=GuillaumeGomez

Tweak std::rc docs

Fixes rust-lang#29372.

r? @GuillaumeGomez

…bnik

Replace 'e.g.' by 'i.e.'

Fixes rust-lang#36577.

r? @steveklabnik

…, r=nrc

Add the ability to merge spans to codemap

This PR adds the ability to merge Spans.  To do so, it builds on the Codemap's ability to verify the locations of spans, namely that following can be verified:

* the expn_id of both spans much match
* the lhs span needs to end on the same line the rhs span begins
* the lhs span must start at or before the rhs span

If all of these are met, a new span is returned that is min(lo), max(hi) of the two spans.

This PR also removes an older Span merge, as this new functionality subsumes it.

r? @nrc

…_fix, r=bluss

fixed the safety header/wording in option.rs

Fixes rust-lang#36581

screenshot of the rendered documentation: http://imgur.com/14kLVrA

r? @steveklabnik

Lazily allocate TypedArena's first chunk

Currently `TypedArena` allocates its first chunk, which is usually 4096
bytes, as soon as it is created. If no allocations are ever made from
the arena then this allocation (and the corresponding deallocation) is
wasted effort.

This commit changes `TypedArena` so it doesn't allocate the first chunk
until the first allocation is made.

This change speeds up rustc by a non-trivial amount because rustc uses
`TypedArena` heavily: compilation speed (producing debug builds) on
several of the rustc-benchmarks increases by 1.02--1.06x. The change
should never cause a slow-down because the hot `alloc` function is
unchanged. It does increase the size of `TypedArena` by one `usize`
field, however.

The commit also fixes some out-of-date comments.

…pasel422

Minor correction in `sort_by_key` doc comment

The parameter to `sort_by_key` is named `f`, not `key`.

Implement Debug for DirEntry.

None