Can a \hbox be made to attain all of its surrounding space?

Question

If I have two boxes like so:

\hbox to 2cm{\vrule\hfil\hbox{\vrule keke\vrule}\hfil\vrule}
\bye

, then is it possible somehow for the inner \hbox to take all of that 2cm? (the \hfils and \vrules are there only to illustrate the sizes)

Answer 1

This doesn't answer your question, but perhaps you want something like this?

\documentclass{article}
\makeatletter
\newcommand\labeleddownbracefill[1]{%
        $\m@th
        \setbox\z@\hbox{$\braceld$}%
        \braceld
        \leaders\vrule\@height\ht\z@\@depth\z@\hfill
        \braceru
        \raise1ex\hbox to\z@{\hss$\scriptstyle#1$\hss}%
        \bracelu
        \leaders\vrule\@height\ht\z@\@depth\z@\hfill
        \bracerd
        $%
}
\makeatother
\begin{document}
\begin{tabular}{cc}
foo&\labeleddownbracefill{\lambda}\\
bar&Here is some text to be braced\\
\end{tabular}
\end{document}

Answer 2

My idea was hairier than I thought, and falls a bit too far from a solution to count as an answer, but since I spent some time on it ... the basic idea, given code as follows (all code in Plain Tex):

\setbox0=\hbox to 3in 
    {\hfill a\hfill} % Box 0 is an hbox containing a skip, an hbox-like item, and a skip
\showbox0

is that (i) \showbox will tell you the information you need to reengineer the hbox by telling you how it "sets" glue:

\hbox(4.30554+0.0)x216.81, glue set 105.905fill
.\glue 0.0 plus 1.0fill
.\tenrm a
.\glue 0.0 plus 1.0fill

namely that 1.0fill is 105.905 of Tex's internal measure; and (ii), you can use this glue setting factor to go back and substitute the vrule for the last skip:

\skip0=105.95 % the glue setting factor
\setbox1=\hbox to 3in
   {\unhcopy0 % Make contents so far of this box be contents of box 0
    \unskip %remove last skip
    \hbox to \skip0{\vrule}} %put in hbox of same width as removed skip

The the two box registers will contain three items of the same width, but the last skip from box 0 will be replaced by a rule in box 1. You can make this step easier by defining a macro with a "hole" where the skip/hbox are to go.

We don't want to have to calculate the width based on looking up the results by hand: we want to get the "setting" width for the glue dimension automatically. This is where it gets hard. I think there is no way in Tex or Etex to internally extract the stretch/shrink factor of skips, although there is the possibility to spit out the information using \showbox and perform the calculation externally. Luatex does allow you to look inside the stretch/shrink factor, but it doesn't seem to directly allow you to inspect the setting factor.

So I can think of two problematic ways to get the value wanted here:

1. By parsing the output of \showbox externally;
2. In Luatex, by recursing over the structure of a box and computing the glue setting based on the information gathered (i.e., reconstructing what the Tex algorithm does already).