Left-aligned display math mode

Question

How could one achieve, as primitively as possible, left-aligned display math so that \abovedisplayshortskip would still work as intended?

It is my understanding that one can achieve left-alignment via an immediate \halign inside display math mode, but this makes (again, in my understanding) it occupy the whole width of the display, and hence makes it impossible for there to be \abovedisplayshortskip (or \belowditto).

Here is what I have now:

Some text $$\displaylines{\kern2\parindent % i.e. \halign
  a + b = c
\hfill}$$ and so continues the paragraph. Until
\bye

Answer 1

My first thought (you didn't wait long enough to get my answer) was to set the math left aligned in an \hbox that was shorter than the \displaywidth by twice the indentation value:

\newdimen\mydisplayboxwidth
\newdimen\mydisplayindent
\mydisplayindent 2\parindent % 
\noindent OK:
$$\mydisplayboxwidth\displaywidth
  \advance\mydisplayboxwidth-2\mydisplayindent
  \hbox to \mydisplayboxwidth
    {$\mathsurround0pt \displaystyle a + b = c$\hss}
\eqno(1)$$
and so continues the paragraph.
\bye

Answer 2

No takers? I'm sorry I'm doing the unthinkable and answering my own question.

Thanks to @Thruston for finding the relevant part from The TeXbook (p. 190):

TeX also allows “alignment displays”, which are not processed in math mode because they contain no formulas at the outer level. An alignment display is created by commands of the general form

$$<assignments>\halign{<alignment>}<assignments>$$

where the <assignments> are optional things like parameter changes that do not produce any math lists. In such displays, the \halign is processed exactly as if it had appeared in vertical mode, and it will construct a vertical list v as usual, except that each row of the alignment will be shifted by the \displayindent. After the alignment and the closing assignments have been processed, TeX will put a \predisplaypenalty item and some \abovedisplayskip glue on the main vertical list, followed by v, followed by \postdisplaypenalty item and \belowdisplayskip glue.

(Emphasis mine.)

So building on this knowledge, one could do something like the following for \displaylines-like command with indenting (I've left out all the other things it's doing):

\everydisplay={\displayindent=3\parindent}

\def\indentlines#1{
  \halign{
    \mathsurround=0pt
    $\displaystyle ##\hfil$\crcr
    \noalign{
      \ifdim\predisplaysize<\displayindent
        \kern-\abovedisplayskip\vskip\abovedisplayshortskip
      \fi}
    #1\crcr}}

\hsize=38em
Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie
consequat, vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan
et iusto odio dignissim qui blandit praesent luptatum zzril delenit augue duis
dolore te feugait nulla facilisi.$$
  \indentlines{ a + b = c. }
$$ Consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut
labore et dolore magna aliquyam erat, sed diam voluptua.
\bye

For some reason the \displayindent needs to be set inside display math mode and not outside.