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

fira_code_patch.py

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
            name = glyph.glyphname
            newchar = font.createChar(point, name + '.private')
            newchar.addReference(name)
            # Other option is to use alternative unicode encoding
            # fontforge recommends to use references
            # glygh.altuni = (point, -1, 0)
        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

(defconst ligatures-fira-code-start #Xe100)

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

(defun ligatures-correct-symbol-bounds (len char)
  "Prepend up to LEN non-breaking spaces with reference points to CHAR.
This way `compose-region' called by function `prettify-symbols-mode'
will use the correct width of the symbols instead of the width
measured by `char-width'."
  (let ((acc (list char)))
    (while (> len 1)
      (setq acc (cons #X00a0 (cons '(Br . Bl) acc)))
      (setq len (1- len)))
    acc))


(defun ligatures-make-alist (ligatures starting-code)
  "Construct text to ligature character.
For each string in LIGATURES list add replacement from STARTING-CODE
sequentially."
  (mapcar (lambda (l)
            (let ((n starting-code))
              (setq starting-code (1+ starting-code))
              (when l
                (cons l (ligatures-correct-symbol-bounds
                         (length l) n)))))
          ligatures))

(defun ligatures-fira-code-setup ()
  "Add Fira Code ligatures to `prettify-symbols-alist'."
  (setq prettify-symbols-alist (append (ligatures-make-alist
                                        ligatures-fira-code-list
                                        ligatures-fira-code-start)
                                       prettify-symbols-alist)))

@reiver-dev: Ah, I need the fontforge complete software package. Please ignore my previous comment.