[Tex/LaTex] Use different math font in inline-style and display-style formulas

Question

I would like to use a different font in inline-style math mode ($...$) and displayed equations (e.g., \begin{equation} ... \end{equation}). Currently, I set my fonts with

\setmainfont{Frutiger 45 Light}
\setmathrm{Cambria Math}
\setmathfont(Digits,Latin){Cambria Math}

but I would like to use Frutiger in mathmode as well so there is no break in font style for in-line math. I tried defining a new command

\newcommandx{\textvar}[1]{
{
\setmathrm{Frutiger 45 Light}
\setmathfont(Digits){Frutiger 45 Light}
#1
}
\setmathrm{Cambria Math}
\setmathfont(Digits,Latin){Cambria Math}
}

but \setmath* seem to work only in the preamble and not in the document itself.

Is there any way to do this?

Answer 1

As we are fans of good typography, the discussion about best practises is always mandatory and important.

I want to second what Chris says above:

Please do not try that at home!

% arara: lualatex

\documentclass{article}
\usepackage{unicode-math}
\setmathfont[version=lm]{Latin Modern Math}
\setmathfont[version=asana]{Asana Math}

\begingroup
\catcode`\$=\active
\protected\gdef$#1${\mathversion{asana}\(#1\)\mathversion{lm}}
\endgroup
\AtBeginDocument{\catcode`\$=\active}

\begin{document}
    \mathversion{lm}
    \begin{equation}E=mc^2\end{equation} where $E$ stands for energy \begin{equation}E=mc^2\end{equation}
\end{document}

---

Edit:

In comment you mention that you want to keep the old behaviour. In this case you should (highly recommended) define a new command for such things:

\newcommand*{\myinlinemath}[1]{{\mathversion{asana}\ensuremath{#1}}}

---

Edit 2:

For your very example I would just recommend to use the siuntix package:

% arara: xelatex

\documentclass{article}
\usepackage{fontspec}
\setmainfont{linux libertine o}
\usepackage{siunitx}

\begin{document}
\begin{equation}E=mc^2\end{equation}

something like \SI{2.8 \pm 0.5}{\mu\metre} in the middle of a line

\sisetup{detect-all}
something like \SI{2.8 \pm 0.5}{\mu\metre} in the middle of a line
\end{document}