Use mktemp for temporary download directory

Using the current directory may not always be appropriate, for example in
the case where it will unnecessarily trigger a backup to be made.

The only risk with this change is that systems might not have a mktemp.
I am not aware of such a system, but have not tested on Windows. It is
working on a basic Ubuntu and OS X installation.
Fall back to hard coded download directory

If mktemp fails, fall back to a hard coded directory, per @nodakai's feedback.
Deprecate `DList::ListInsertion`
Merge branch 'master' into cfg_tmp_dir

Conflicts:
	src/etc/rustup.sh
Better temporary directory name
Prepared most `StrExt` pattern using methods for stabilization
Made iterator-returning methods return newtypes
Adjusted some docs to be forwards compatible with a generic pattern API
syntax: format: remove unused method_statics field.
syntax: format: put static arrays in their own blocks to avoid needing a wrapper block.
syntax: turn the match-call generated by format_args inside-out.
syntax: use std::string::String unqualified in format.
syntax: change format_args! to produce fmt::Arguments instead of calling a function with them.
Fallout of changing format_args!(f, args) to f(format_args!(args)).
auto merge of #20136 : eddyb/rust/format-args, r=alexcrichton

We have the technology: no longer do you need to write closures to use `format_args!`.
This is a `[breaking-change]`, as it forces you to clean up old hacks - if you had code like this:
```rust
format_args!(fmt::format, "{} {} {}", a, b, c)
format_args!(|args| { w.write_fmt(args) }, "{} {} {}", x, y, z)
```
change it to this: 
```rust
fmt::format(format_args!("{} {} {}", a, b, c))
w.write_fmt(format_args!("{} {} {}", x, y, z))
```
To allow them to be called with `format_args!(...)` directly, several functions were modified to
take `fmt::Arguments` by value instead of by reference. Also, `fmt::Arguments` derives `Copy`
now in order to preserve all usecases that were previously possible.
Split overly long line
auto merge of #20185 : csouth3/rust/dlist-deprecate, r=alexcrichton

This PR deprecates the `DList::ListInsertion` trait, in accordance with rust-lang/rfcs#509.  The functions which were previously part of the ListInsertion impl for `DList::IterMut` have been moved to be inherent methods on the iterator itself, and appropriate doctests have been added.
mk: Stop generating docs for deprecated crates

These crates are all deprecated for their rust-lang/$crate equivalents and by
generating docs we're generating broken links. The documentation for these
crates are generated out-of-tree and are managed separately, so we're not losing
the documentation altogether, just the links from the main distribution's docs.

Closes #20096
librustc: Traverse arbitrarily deep for nullable enum opt.
librustc: Try looking in tuple fields for nullable enum opt.
librustc: Try looking in fixed sized arrays for nullable enum opt.
Add tests.