Recognize Rust source files automatically

core/prelude-packages.el

@@ -144,6 +144,7 @@ PACKAGE is installed only if not already present.  The file is opened in MODE."
     ("\\.pp\\'" puppet-mode puppet-mode)
     ("\\.php\\'" php-mode php-mode)
     ("PKGBUILD\\'" pkgbuild-mode pkgbuild-mode)
+    ("\\.rs\\'" rust-mode rust-mode)
     ("\\.sass\\'" sass-mode sass-mode)
     ("\\.scala\\'" scala-mode2 scala-mode)
     ("\\.scss\\'" scss-mode scss-mode)