Tikz node with regular text AND text inside rectangle

Question

I want to draw with tikz the following:

That is,

abc, then def in green AND surrounded by a red rectangle, and then a ,.

What I have now is three different nodes. But they are not aligned to one another, and the distances between them are rather chosen for arbitrary distances by me, which is not systematic; If I change the font and font size, the distances may need to be amended.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
\node at (0,0) (A) {abc,};
\node[draw,color=red,right = 1ex of A] (B) {\textcolor{blue}{def}};
\node[right = 1ex of B] (C) {,};
\end{tikzpicture}
\end{document}

My output:

Answer 1

\documentclass[tikz, border=1cm]{standalone}
\usetikzlibrary{tikzmark, fit}
\begin{document}
\begin{tikzpicture}[remember picture]
\node {abc, \subnode{sub}{\textcolor{green}{def}} ,};
\node[fit=(sub), draw, inner sep=0pt, red] {};
\end{tikzpicture}
\end{document}

Answer 2

This version uses a savebox.

\documentclass{standalone}% so I don't have to crop it
\usepackage{tikz}

\begin{document}
\bgroup% use local registers
\sbox0{\begin{tikzpicture}[baseline=(B.base)]
  \node[draw=red, text=blue] (B) {def};
\end{tikzpicture}}%
\begin{tikzpicture}
\node at (0,0) (A) {abc, \usebox0,};
\end{tikzpicture}
\egroup
\end{document}

Answer 3

For a simple rectangle, the \fbox is enough.

In this example, I'm ignoring inner ysep and assume it has the same value as inner xsep.

Code

\documentclass[tikz, border=1cm]{standalone}
\DeclareDocumentCommand{\node}{O{.}O{.}m}{%
  \vphantom{#3}\smash{%
    \setlength\fboxrule{\pgflinewidth}%
    \pgfmathsetlength\fboxsep{\pgfkeysvalueof{/pgf/inner xsep}-.5\pgflinewidth}%
    \textcolor{#1}{\fbox{\textcolor{#2}{#3}}}%
  }%
}
\begin{document}
\tikz\node{abc,\node[red][green]{def},};
\end{document}

Output