Fix type annotations incorrectly highlighted as modules.

Previously, we were always treating :: as a module, but Rust
allows type annotations using :: e.g.

    parse::<i32>();

This also changes module highlighting so that only the module name is
highlighted, excluding the ::. This makes rust-mode consistent with
other Emacs modes, such as c++-mode and ruby-mode.

Author: Wilfred

rust-mode-tests.el

@@ -1403,6 +1403,27 @@ this_is_not_a_string();)"
    "\"/*! doc */\""
    '("\"/*! doc */\"" font-lock-string-face)))
 
+(ert-deftest font-lock-module ()
+  (rust-test-font-lock
+   "foo::bar"
+   '("foo" font-lock-type-face)))
+
+(ert-deftest font-lock-submodule ()
+  (rust-test-font-lock
+   "foo::bar::baz"
+   '("foo" font-lock-type-face
+     "bar" font-lock-type-face)))
+
+(ert-deftest font-lock-type-annotation ()
+  "Ensure type annotations are not confused with modules."
+  (rust-test-font-lock
+   "parse::<i32>();"
+   ;; Only the i32 should have been highlighted.
+   '("i32" font-lock-type-face))
+  (rust-test-font-lock
+   "foo:: <i32>"
+   ;; Only the i32 should have been highlighted.
+   '("i32" font-lock-type-face)))
 
 (ert-deftest indent-method-chains-no-align ()
   (let ((rust-indent-method-chain nil)) (test-indent

rust-mode.el

@@ -14,6 +14,7 @@
 ;;; Code:
 
 (eval-when-compile (require 'rx)
+                   (require 'cl)
                    (require 'compile)
                    (require 'url-vars))
 
@@ -525,6 +526,21 @@ function or trait.  When nil, where will be aligned with fn or trait."
   (concat "\\_<" (regexp-opt words t) "\\_>"))
 (defconst rust-re-special-types (regexp-opt-symbols rust-special-types))
 
+
+(defun rust-module-font-lock-matcher (limit)
+  "Matches module names \"foo::\" but does not match type annotations \"foo::<\"."
+  (block nil
+    (while t
+      (let* ((symbol-then-colons (rx-to-string `(seq (group (regexp ,rust-re-ident)) "::")))
+             (match (re-search-forward symbol-then-colons limit t)))
+        (cond
+         ;; If we didn't find a match, there are no more occurrences
+         ;; of foo::, so return.
+         ((null match) (return nil))
+         ;; If this isn't a type annotation foo::<, we've found a
+         ;; match, so a return it!
+         ((not (looking-at (rx (0+ space) "<"))) (return match)))))))
+
 (defvar rust-mode-font-lock-keywords
   (append
    `(
@@ -548,8 +564,8 @@ function or trait.  When nil, where will be aligned with fn or trait."
      ;; Field names like `foo:`, highlight excluding the :
      (,(concat (rust-re-grab rust-re-ident) ":[^:]") 1 font-lock-variable-name-face)
 
-     ;; Module names like `foo::`, highlight including the ::
-     (,(rust-re-grab (concat rust-re-ident "::")) 1 font-lock-type-face)
+     ;; Module names like `foo::`, highlight excluding the ::
+     (rust-module-font-lock-matcher 1 font-lock-type-face)
 
      ;; Lifetimes like `'foo`
      (,(concat "'" (rust-re-grab rust-re-ident) "[^']") 1 font-lock-variable-name-face)