Once isn't a keyword.

Stebalien · Stebalien · commit fdce8edf81b2 · 2015-05-23T14:39:49.000-04:00
diff --git a/syntax/rust.vim b/syntax/rust.vim
@@ -24,7 +24,7 @@ syn keyword   rustKeyword     continue
 syn keyword   rustKeyword     extern nextgroup=rustExternCrate,rustObsoleteExternMod skipwhite skipempty
 syn keyword   rustKeyword     fn nextgroup=rustFuncName skipwhite skipempty
 syn keyword   rustKeyword     for in if impl let
-syn keyword   rustKeyword     loop once pub
+syn keyword   rustKeyword     loop pub
 syn keyword   rustKeyword     return super
 syn keyword   rustKeyword     unsafe where while
 syn keyword   rustKeyword     use nextgroup=rustModPath skipwhite skipempty
