Do not allow subword-match when detecting built-in formatting macros

Before this change, a user-created macro like ‘cprintln!’ would see the
‘println’ subword highlighted as a built-in macro, and the leading ‘c’
left without color.

After this commit, only full words will be recognized as built-ins, and
‘cprintln!’ is highlighted as a regular macro in full, not as a built-in.

In addition, this commit fixes the same issue for the write! macro.

Author: dato

rust-mode.el

@@ -676,15 +676,15 @@ Returns nil if the point is not within a Rust string."
       1 font-lock-preprocessor-face keep)
 
      ;; Builtin formatting macros
-     (,(concat (rust-re-grab (concat (regexp-opt rust-builtin-formatting-macros) "!")) (concat rust-formatting-macro-opening-re "\\(?:" rust-start-of-string-re) "\\)?")
+     (,(concat (rust-re-grab (concat (rust-re-word (regexp-opt rust-builtin-formatting-macros)) "!")) (concat rust-formatting-macro-opening-re "\\(?:" rust-start-of-string-re) "\\)?")
       (1 'rust-builtin-formatting-macro-face)
       (rust-string-interpolation-matcher
        (rust-end-of-string)
        nil
        (0 'rust-string-interpolation-face t nil)))
 
      ;; write! macro
-     (,(concat (rust-re-grab "write\\(ln\\)?!") (concat rust-formatting-macro-opening-re "[[:space:]]*[^\"]+,[[:space:]]*" rust-start-of-string-re))
+     (,(concat (rust-re-grab (concat (rust-re-word "write\\(ln\\)?") "!")) (concat rust-formatting-macro-opening-re "[[:space:]]*[^\"]+,[[:space:]]*" rust-start-of-string-re))
       (1 'rust-builtin-formatting-macro-face)
       (rust-string-interpolation-matcher
        (rust-end-of-string)