What is the right way to input prime with Unicode-math (LuaTeX backend)

In traditional TeX fonts, \prime is a big glyph like the one in Latin Modern Math or Asana Math. However, there isn't such a character in Unicode. Symbol U+2032 is used as prime, but it looks like $^\prime$ i.e. $'$ in TeX. That makes the problem.

As a Unicode math font, Cambria/Minion/XITS is correct. And \prime command should be unavailable then. It seems a bug. I don't think the maintainers of unicode-math can do much. Asana Math and Latin Modern Math fonts may be modified to coincide with Unicode standard; then unicode-math can produce consistent result.

Definition in Unicode: http://www.unicode.org/charts/PDF/U2000.pdf

---

A temporary fix for Cambria Math:

\documentclass{article}
\usepackage{unicode-math}
\setmathfont{Cambria Math}
\ExplSyntaxOn
\group_begin:
  \char_set_catcode_active:N \'
  \char_set_catcode_active:N \`
  \char_set_catcode_active:n {"2032}
  \char_set_catcode_active:n {"2033}
  \char_set_catcode_active:n {"2034}
  \char_set_catcode_active:n {"2057}
  \char_set_catcode_active:n {"2035}
  \char_set_catcode_active:n {"2036}
  \char_set_catcode_active:n {"2037}
  \cs_gset:Nn \um_define_prime_chars: {
    \cs_set_eq:NN '        \um_scan_prime:
    \cs_set_eq:NN ^^^^2032 \um_scan_prime:
    \cs_set_eq:NN ^^^^2033 \um_scan_dprime:
    \cs_set_eq:NN ^^^^2034 \um_scan_trprime:
    \cs_set_eq:NN ^^^^2057 \um_scan_qprime:
    \cs_set_eq:NN `        \um_scan_backprime:
    \cs_set_eq:NN ^^^^2035 \um_scan_backprime:
    \cs_set_eq:NN ^^^^2036 \um_scan_backdprime:
    \cs_set_eq:NN ^^^^2037 \um_scan_backtrprime:
  }
\group_end:
\ExplSyntaxOff

\begin{document}

$A' A'' A''' A''''$

\end{document}

You can either use f' or f^\prime, which is the proper way for inputting prime even if not using unicode-math. The fact that some fonts give superscripted prime with \prime is not something you should rely on. See Why isn't \prime automatically set to be a superscript? for the reason why old TeX fonts have such a big prime glyph (In OpenType math the issue is dealt with a bit differently, but some fonts still following the old practices).