misc/emacs: handle backslash in raw string in Emacs 23

Go-mode in Emacs 23 does not recognize a backslash followed
by a backquote as end of raw string literal, as it does not
support syntax-propertize-function which Go-mode uses to
remove special meaning from backslashes in ``.

This patch provides a fallback mechanism to do the same thing
using font-lock-syntactic-keywords, which is supported by
Emacs 23.

LGTM=dominik.honnef
R=golang-codereviews, dominik.honnef
CC=adonovan, golang-codereviews
https://golang.org/cl/78730048

misc/emacs/go-mode.el

@@ -272,6 +272,7 @@ For mode=set, all covered lines will have this weight."
      `((,go-func-meth-regexp 2 font-lock-function-name-face))) ;; method name
 
    `(
+     ("\\(`[^`]*`\\)" 1 font-lock-multiline) ;; raw string literal, needed for font-lock-syntactic-keywords
      (,(concat (go--regexp-enclose-in-symbol "type") "[[:space:]]+\\([^[:space:]]+\\)") 1 font-lock-type-face) ;; types
      (,(concat (go--regexp-enclose-in-symbol "type") "[[:space:]]+" go-identifier-regexp "[[:space:]]*" go-type-name-regexp) 1 font-lock-type-face) ;; types
      (,(concat "[^[:word:][:multibyte:]]\\[\\([[:digit:]]+\\|\\.\\.\\.\\)?\\]" go-type-name-regexp) 2 font-lock-type-face) ;; Arrays/slices
@@ -290,6 +291,14 @@ For mode=set, all covered lines will have this weight."
      (,(concat "^[[:space:]]*\\(" go-label-regexp "\\)[[:space:]]*:\\(\\S.\\|$\\)") 1 font-lock-constant-face) ;; Labels and compound literal fields
      (,(concat (go--regexp-enclose-in-symbol "\\(goto\\|break\\|continue\\)") "[[:space:]]*\\(" go-label-regexp "\\)") 2 font-lock-constant-face)))) ;; labels in goto/break/continue
 
+(defconst go--font-lock-syntactic-keywords
+  ;; Override syntax property of raw string literal contents, so that
+  ;; backslashes have no special meaning in ``. Used in Emacs 23 or older.
+  '(("\\(`\\)\\([^`]*\\)\\(`\\)"
+     (1 (7 . ?`))
+     (2 (15 . nil))  ;; 15 = "generic string"
+     (3 (7 . ?`)))))
+
 (defvar go-mode-map
   (let ((m (make-sparse-keymap)))
     (define-key m "}" #'go-mode-insert-and-indent)
@@ -564,7 +573,10 @@ recommended that you look at goflymake
 
   (set (make-local-variable 'parse-sexp-lookup-properties) t)
   (if (boundp 'syntax-propertize-function)
-      (set (make-local-variable 'syntax-propertize-function) #'go-propertize-syntax))
+      (set (make-local-variable 'syntax-propertize-function) #'go-propertize-syntax)
+    (set (make-local-variable 'font-lock-syntactic-keywords)
+         go--font-lock-syntactic-keywords)
+    (set (make-local-variable 'font-lock-multiline) t))
 
   (set (make-local-variable 'go-dangling-cache) (make-hash-table :test 'eql))
   (add-hook 'before-change-functions (lambda (x y) (setq go-dangling-cache (make-hash-table :test 'eql))) t t)