I would say
\newcommand{\x}[1]{%
{}$% get out of math
\kern-2\mathsurround % in case it's non zero
$% reenter math
\binoppenalty10000 \relpenalty10000 #1% typeset the subformula
{}$% get out of math
\kern-2\mathsurround % in case it's non zero
$% reenter math for the rest of the formula
}
TeX breaks formulas only after binary operators or relation symbols, the desirability of such breaks is measured by the two mentioned parameters. However the values used the penalties are those valid at the end of the formula, so simply enclosing #1 in \begingroup...\endgroup and setting the values wouldn't do anything.
Of course this can work only if used in suitable places of the formula, for example $a+\x{b+c}$ would have the right spacing after the first + (because of the empty subformula); the last empty subformula does nothing.
My opinion is still that bad breaks must be solved with suitably placed \nobreak commands.
Some examples:
\documentclass[a4paper,draft]{book}
\newcommand{\x}[1]{{}$\kern-2\mathsurround${}
\binoppenalty10000 \relpenalty10000 #1{}$\kern-2\mathsurround${}}
\begin{document}
\parbox{5cm}{
A formula \(a+\x{c+d}\)\break showing that spaces are right
A new formula \(a+\x{c+d}\) showing that spaces are right
A brand new formula x \(a+\x{c+d}\) showing that spaces are right
A brand new formula xx \(a+\x{c+d}\) showing that spaces are right
A brand new formula xxx \(a+\x{c+d}\) showing that spaces are right
A brand new formula xxxx \(a+\x{c+d}\) showing that spaces are right
Another brand new formula \(a+\x{c+d}\) showing that spaces are right
Right: $\sin(\x{a+b})$
Wrong: $\sin\x{(a+b)}$
\mathsurround=30pt
A formula xxxxxxx \(a+\x{c+d}\) showing mathsurround
A formula xxxxxxx \(a+c+d\) showing mathsurround
}
\end{document}
Addition about usage
The \x macro (possibly with a more descriptive name) should be used in specific places. Its contents must
(1) start with an ordinary symbol or be preceded by an ordinary symbol;
(2) end with an ordinary symbol or be followed by one.
It doesn't support the style declarations \displaystyle, \textstyle, \scriptstyle, or \scriptscriptstyle; it may make sense to carry a \displaystyle declaration, this might be done with a *-variant.
It doesn't support \left or \right: it's not allowed something like
$...\left(\x{a+b}\right)...$
but this is not a problem, as no formula can be split at relation or operation symbols between \left and \right and the spaces around these symbols never participates to stretching or shrinking.
If you insert a formula as
$\smash{<formula>}$
its vertical size will not be considered during typesetting. However, spaces inside the formula won't stretch or shrink together with the other spaces in the line, which could change the typesetting. So, in the case of a formula with an equality you should type
$x=\smash{<big subformula>}$
I'd not do in this way, though: the enlarged line spacing is the only clue you have for manually fixing the output. So I'd leave the typesetting as is, and only then apply cautiously adjustments. In the showcase example, you could use
$\smash[b]{\binom{\xi}{\xi}}$
(requires amsmath) to remove the bottom vertical space.
Best Answer
TeX inserts some vertical space to ensure text elements don't overlap. For these special cases, you could consider using
\smash- this removes all vertical box lengths (height and depth) from its argument. So, you would use\smash{$\psi_\text{pseudo}$}, say.Left shows the original, right shows the output when using
\smash(Click to enlarge):Note though that any changes in the paragraph layout might cause problems in the paragraph flows differently.