Nota : this packages requires pdfTeX.

The linegoal package provides a macro \linegoal
to be used with \setlength :
   \setlength<some dimen>\linegoal

will assign to <some dimen> the horizontal length
of the remainder of the line.

This is achieved using the \pdfsavepos primitive
and the zref-savepos package.

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Typical usage :

Some text: \begin{tabularx}\linegoal{|l|X|} \hline
            one & two                       \\
            three & four                    \\\hline
           \end{tabularx}