Move ligatures for Fira Code font to private unicode area at U+e100

fira_code_patch.py

#!/bin/env python3

import os
import sys
import argparse
from glob import glob
from itertools import chain

import fontforge

ADDITIONAL_LIGATURES = [
    'x.multiply',
    'colon.uc',
    'plus.lc',
    'plus.tosf2',
]


def run(fontpath, outpath, starting_point=0xe100):
    try:
        font = fontforge.open(fontpath)
        ligatures = list(filter(lambda x: x.glyphname.endswith('.liga'),
                                font.glyphs()))
        for i, glyph in enumerate(ligatures):
            point = starting_point + i
            # You could also create a reference instead of defining an
            # encoding, but I don't recommend that, as it causes some
            # problems with character width
            # name = glyph.glyphname
            # newchar = font.createChar(point, name + '.private')
            # newchar.addReference(name)
            glyph.unicode = point
        font.generate(outpath)
    finally:
        font.close()


def main(argv):
    parser = argparse.ArgumentParser()
    parser.add_argument('--output-dir', '-o', type=str)
    parser.add_argument('fonts', nargs='+',
                        help='font files to process')
    argvals = parser.parse_args(argv)

    output = argvals.output_dir
    fonts = argvals.fonts

    os.makedirs(output, exist_ok=True)

    for f in chain.from_iterable(map(glob, fonts)):
        run(f, os.path.join(output, os.path.basename(f)))


if __name__ == '__main__':
    main(sys.argv[1:])

ligatures.el

(defun fira-code-mode--make-alist (list)
  "Generate prettify-symbols alist from LIST."
  (let ((idx -1))
    (mapcar
    (lambda (s)
      (setq idx (1+ idx))
      (let* ((code (+ #Xe100 idx))
          (width (string-width s))
          (prefix ())
          (suffix '(?\s (Br . Br)))
          (n 1))
    (while (< n width)
      (setq prefix (append prefix '(?\s (Br . Bl))))
      (setq n (1+ n)))
    (cons s (append prefix suffix (list (decode-char 'ucs code))))))
    list)))

(defconst fira-code-mode--ligatures
  '("www" "--" "---" "-->" "-|" "->" "->>" "-<" "-<<" "-~" "{|" "[|" "]#" ".-"
    ".." "..." "..=" "..<" ".?" ".=" "::" ":::" "::=" ":=" ":>" ":<" ";;" "!!"
    "!!." "!=" "!==" "?." "??" "?=" "**" "***" "*>" "*/" "#(" "#{" "#[" "#:"
    "#!" "#?" "##" "###" "####" "#=" "#_" "#_(" "/*" "/=" "/==" "/>" "//" "///"
    "/\\" "\\/" "_|_" "__" "&&" "|-" "|->" "|}" "|]" "||" "||-" "|||>" "||="
    "||>" "|=" "|=>" "|>" "$>" "++" "+++" "+>" "=:=" "=!=" "==" "===" "==>"
    "=>" "=>>" "=<<" "=/=" ">-" ">->" ">:" ">=" ">=>" ">>" ">>-" ">>=" ">>>"
    "<-" "<--" "<-|" "<->" "<-<" "<:" "<!--" "<*" "<*>" "<|" "<||" "<|||"
    "<|>" "<$" "<$>" "<+" "<+>" "<=" "<=|" "<==" "<==>" "<=>" "<=<" "<>" "<<"
    "<<-" "<<->>" "<<=" "<<<" "<~" "<~>" "<~~" "</" "</>" "~-" "~@" "~>" "~~"
    "~~>" "^=" "%%")) 

(defvar fira-code-mode--old-prettify-alist)

(defun fira-code-mode--enable ()
  "Enable Fira Code ligatures in current buffer."
  (setq-local fira-code-mode--old-prettify-alist prettify-symbols-alist)
  (setq-local prettify-symbols-alist (append (fira-code-mode--make-alist fira-code-mode--ligatures) fira-code-mode--old-prettify-alist))
  (prettify-symbols-mode t))

(defun fira-code-mode--disable ()
  "Disable Fira Code ligatures in current buffer."
  (setq-local prettify-symbols-alist fira-code-mode--old-prettify-alist)
  (prettify-symbols-mode -1))

(define-minor-mode fira-code-mode
  "Fira Code ligatures minor mode"
  :lighter " Fira Code"
  (setq-local prettify-symbols-unprettify-at-point 'right-edge)
  (if fira-code-mode
      (fira-code-mode--enable)
    (fira-code-mode--disable)))

(provide 'fira-code-mode)
  (define-minor-mode fira-code-mode
    "Fira Code ligatures minor mode"
    :lighter " Fira Code"
    (setq-local prettify-symbols-unprettify-at-point 'right-edge)
    (if fira-code-mode
        (fira-code-mode--enable)
      (fira-code-mode--disable)))

  (provide 'fira-code-mode)

Lines 1 to 41 of ligatures.el are a duplication of lines 42 to 82, you can delete them.

Also I had to change the ligatures list or some ligatures didn't display properly, I probably left some out though.
I'm leaving my list here just in case someone finds it useful:

(defconst fira-code-mode--ligatures
  '( "www" "--" "---" "-->" "-|" "->" "->>" "-<" "-<<" "-~" "{|"
    "[|" "]#" ".-" ".." "..." "..=" "..<" ".?" ".=" "::" ":::" "::=" ":=" ":>" ":<"
    ";;" "!!" "!!." "!=" "!==" "?." "??" "?=" "**" "***" "*>" "*/" "#("
    "#{" "#[" "#:" "#!" "#?" "##" "###" "####" "#=" "#_" "#_(" "/*" "/=" "/=="
    "/>" "//" "///" "/\\" "\\/" "_|_" "__" "&&" "|-" "->" "|}" "|]" "||" "||-"
    "|||>" "||=" "||>" "|=" "|=>" "|>" "$>" "++" "+++" "+>" "=:=" "=!=" "=="
    "===" "==>" "=>" "=>>" "=<<" "=/=" ">-" ">->" ">:" ">=" ">=>" ">>" ">>-"
    ">>=" ">>>" "<-" "<--" "<-|" "<->" "<-<" "<:" "<!--" "<*" "<*>" "<|" "<||"
    "<|||" "<|>" "<$" "<$>" "<+" "<+>" "<=" "<=|" "<==" "<==>" "<=>" "<=<"
    "<>" "<<" "<<-" "<<-->>" "<<=" "<<<" "<~" "<~>" "<~~" "</" "</>" "~-" "~@"
    "~>" "~~" "~~>" "^=" "%%"))

Lines 1 to 41 of ligatures.el are a duplication of lines 42 to 82, you can delete them.

Also I had to change the ligatures list or some ligatures didn't display properly, I probably left some out though.

Deleted the redundant lines (how'd they get there, anyway?!) and checked your updated list against FontForge. Good job, I found 2 or 3 errors in the less widely used ones, that was it! Thank you for your contribution, this should now be compatible with Fira Code 2.x.