vim: set Rust tab conventions

src/etc/vim/ftplugin/rust.vim

@@ -31,6 +31,12 @@ setlocal formatoptions-=t formatoptions+=croqnl
 " j was only added in 7.3.541, so stop complaints about its nonexistence
 silent! setlocal formatoptions+=j
 
+" smartindent will be overridden by indentexpr if filetype indent is on, but
+" otherwise it's better than nothing.
+setlocal smartindent nocindent
+
+setlocal tabstop=4 shiftwidth=4 expandtab
+
 " This includeexpr isn't perfect, but it's a good start
 setlocal includeexpr=substitute(v:fname,'::','/','g')