Define a new setting g:rust_fold

g:rust_fold allows folding to be enabled. This lets the user turn on
folding without having to define autocommands.

src/etc/vim/doc/rust.txt

@@ -53,6 +53,18 @@ g:rust_conceal_pub~
 	    let g:rust_conceal_pub = 1
 <
 
+                                                                 *g:rust_fold*
+g:rust_fold~
+	Set this option to turn on |folding|: >
+	    let g:rust_fold = 1
+<
+	Value		Effect ~
+	0		No folding
+	1		Braced blocks are folded. All folds are open by
+			default.
+	2		Braced blocks are folded. 'foldlevel' is left at the
+			global value (all folds are closed by default).
+
                                                   *g:rust_bang_comment_leader*
 g:rust_bang_comment_leader~
 	Set this option to 1 to preserve the leader on multi-line doc comments

src/etc/vim/syntax/rust.vim

@@ -3,7 +3,7 @@
 " Maintainer:   Patrick Walton <pcwalton@mozilla.com>
 " Maintainer:   Ben Blum <bblum@cs.cmu.edu>
 " Maintainer:   Chris Morgan <me@chrismorgan.info>
-" Last Change:  2014 Feb 27
+" Last Change:  July 06, 2014
 
 if version < 600
   syntax clear
@@ -11,6 +11,17 @@ elseif exists("b:current_syntax")
   finish
 endif
 
+" Fold settings {{{1
+
+if has("folding") && exists('g:rust_fold') && g:rust_fold != 0
+  setlocal foldmethod=syntax
+  if g:rust_fold == 2
+    setlocal foldlevel<
+  else
+    setlocal foldlevel=99
+  endif
+endif
+
 " Syntax definitions {{{1
 " Basic keywords {{{2
 syn keyword   rustConditional match if else
@@ -213,8 +224,6 @@ syn keyword rustTodo contained TODO FIXME XXX NB NOTE
 " Trivial folding rules to begin with.
 " TODO: use the AST to make really good folding
 syn region rustFoldBraces start="{" end="}" transparent fold
-" If you wish to enable this, setlocal foldmethod=syntax
-" It's not enabled by default as it would drive some people mad.
 
 " Default highlighting {{{1
 hi def link rustDecNumber       rustNumber