Use defvar for rust-top-item-beg-re instead

Since rust-top-item-beg-re hasn't been defined yet, using defvar instead of
setq is more appropriate here (and also silences compilation warnings).
Implement fallbacks for functions unavailable in older versions of Windows
etc: Add box as a keyword to editor configs

This keyword is about to be used much more often than it was a few weeks ago,
it's time to tell editors about the new keyword.
Refactor writeback code. cc #5527
Remove error message that refactor suppresses
Revise doc-comments for graphviz to avoid generating files from rustdoc runs.

Fix #13965.

There is a dance here between the `main` that actually runs versus the
`main` that is printed in the output documentation.  We don't run the
latter `main`, but we do at least compile (and thus type-check) it.
It is still the responsibility of the documenter to ensure that the
signatures of `fn render` are kept in sync across the blocks.
rustc: Fix enum variant privacy across crates

The code in resolve erroneously assumed that private enums weren't visited, so
the logic was adjusted to check to see if the enum definition itself was public.

Closes #11680
Add documentation for Windows compatibility layer and remove unneeded attributes
rustc: Enable writing "unsafe extern fn() {}"

Previously, the parser would not allow you to simultaneously implement a
function with a different abi as well as being unsafe at the same time. This
extends the parser to allow functions of the form:

    unsafe extern fn foo() {
        // ...
    }

The closure type grammar was also changed to reflect this reversal, types
previously written as "extern unsafe fn()" must now be written as
"unsafe extern fn()". The parser currently has a hack which allows the old
style, but this will go away once a snapshot has landed.

Closes #10025

[breaking-change]
Add testcase for #11493
std: Implement the Buffer trait for some wrappers

This will allow methods like read_line() on RefReader, LimitReader, etc.
mk: Add libcore
core: Inherit the intrinsics module
core: Inherit the mem module
core: Inherit the ptr module
core: Inherit the cast module
core: Inherit the kinds module
core: Inherit the ops module
core: Inherit the ty module
core: Inherit the container module