Major optimization refactor, across the board

+ enable lexical-scope everywhere (lexical-binding = t): ~5-10% faster
  startup; ~5-20% general boost
+ reduce consing, function calls & garbage collection by preferring
  cl-loop & dolist over lambda closures (for mapc[ar], add-hook, and
  various cl-lib filter/map/reduce functions) -- where possible
+ prefer functions with dedicated opcodes, like assq (see byte-defop's
  in bytecomp.el for more)
+ prefer pcase & cond (faster) over cl-case
+ general refactor for code readability
+ ensure naming & style conventions are adhered to
+ appease byte-compiler by marking unused variables with underscore
+ defer minor mode activation to after-init, emacs-startup or
  window-setup hooks; a customization opportunity for users + ensures
  custom functionality won't interfere with startup.

modules/lang/rust/autoload.el

@@ -1,4 +1,4 @@
-;;; lang/rust/autoload.el
+;;; lang/rust/autoload.el -*- lexical-binding: t; -*-
 
 ;; TODO (defun +rust/run-cargo () (interactive))
 

modules/lang/rust/config.el

@@ -1,4 +1,4 @@
-;;; module-rust.el
+;;; lang/rust/config.el -*- lexical-binding: t; -*-
 
 (defvar +rust-ext-dir (concat doom-etc-dir "rust/")
   "TODO")
@@ -16,11 +16,14 @@
   :after rust-mode
   :preface
   :init
-  (add-hook! rust-mode '(racer-mode eldoc-mode flycheck-rust-setup))
+  (add-hook! 'rust-mode-hook #'(racer-mode eldoc-mode flycheck-rust-setup))
   :config
   (setq racer-cmd (expand-file-name "racer/target/release/racer" +rust-ext-dir)
         racer-rust-src-path (expand-file-name "rust/src/" +rust-ext-dir))
 
+  (unless (file-exists-p racer-cmd)
+    (warn "rust-mode: racer binary can't be found; auto-completion is disabled"))
+
   ;; TODO Unit test keybinds
   (map! :map rust-mode-map :m "gd" #'racer-find-definition))