Include ChaCha pseudorandom generator
rust.css: Make it more clear when code spans are links
Add some notes about macro scoping
Use relative doc URLs in top-level guides
Update some old references to rust.md
Add a red-box warning to the macros guide
Add a guide to compiler plugins

Fixes #16983.
Link plugins guide from elsewhere
Disable the Roman numerals test on Android due to cross-compile issues
test: Don't depend on /bin/bash

Our FreeBSD bots apparently don't have bash installed, and it's ok to run with
sh anyway!

Unblocks a snapshot
syntax: Enable parsing of `const` globals

This rewrites them to the current `ItemStatic` production of the compiler, but I
want to get this into a snapshot. It will be illegal to use a `static` in a
pattern of a `match` statement, so all those current uses will need to be
rewritten to `const` once it's implemented. This requires that the stage0
snapshot is able to parse `const`.

cc #17718
Fix missing entries in upvar borrows map for capture-by-ref unboxed closures

This prevents a later ICE in borrowck.

Closes issue #17655
Fix categorization of upvars of capture-by-reference unboxed closures

In particular, this causes mutation of an upvar to correctly mark
it as mutable during adjustment.  This makes borrowck correctly
flag conflicting borrows, etc.

We still seem to generate incorrect code in trans which copies the upvar
by value into the closure.  This remains to be fixed.
Return correct types for capture-by-ref unboxed closure upvars

Treat upvars of capture-by-reference unboxed closures as references
with appropriate regions and mutability.
Correctly trans capture-by-ref unboxed closures

Store references to the freevars instead of copies when constructing
the environment and insert an additional load when reading them from
the environment.
Rehabilitate an unboxed closure test

This test works as a regression test for issue #17655.  It also
exercises mutation of by-ref upvars.
auto merge of #16995 : kmcallister/rust/plugin-tutorial, r=alexcrichton

@steveklabnik, are you interested in looking this over?
Set the `non_uppercase_statics` lint to warn by default
Update the `unused` lint group to include more lints
Improve the `non_snake_case` lint to give better suggestions