break long equation into lines (combine {align} and {multline})

Question

Is it possible to have aligned equations which one can split up if they are too long, but such that they are still aligned? I mean a single environment that combines align (equations remain aligned) and multline (if they are too long they may be split in a nice way)

If there is some math counterpart of the \hfill it will be great!

In the following the two vertical bars denote the default width of the math mode, and the minus signs denote empty space

the following is bad

|------------------------------------|

|aaa-=-bbbbbbbbbbbbbbbbbbbbbbbbbbbbbb|

|---=-cccccccccccccccc-+-dddddddddddddddd

It must be somehow turned into

|------------------------------------|

|aaa-=-bbbbbbbbbbbbbbbbbbbbbbbbbbbbbb|

|----=-cccccccccccccccc--------------|

|------------------+-dddddddddddddddd|

that is the second line when broken its other part must be

1. set in the third line but pushed to the right end and

2. that end is the farthest end of all lines (on the left) and

3. the equal signs are still aligned,

Answer 1

\documentclass{article}
\usepackage{mathtools}

\begin{document}
\begin{align*}
aaa & = bbbbbbbb           \\
    & = \begin{multlined}[t]
        cccccccccccccccc    \\
        + dddddddddddddddd
        \end{multlined}
\end{align*}
\end{document}

and one more example

\documentclass{article}
\usepackage{mathtools}
%---------------- show page layout. don't use in a real document!
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.15pt}
\renewcommand*\ShowFrameColor{\color{red}}
%---------------------------------------------------------------%

\begin{document}
\begin{align}
aaa & = bbbbbbbb           \\
    & \begin{multlined}[b][0.35\linewidth]
        = cccccccccccccccc    \\
        + dddddddddddddddd
        \end{multlined}
\end{align}
\end{document}

(red lines show borders of text)

Answer 2

I think the first form, using a fixed indent such as \qquad is most common, but the second form, using multlined is closest to what you ask for.

\documentclass{article}

\usepackage{mathtools}

\begin{document}

with qquad
\begin{align*}
  aaa&=bbb\\
     &=cccc\\
     &\qquad + ddddd
\end{align*}

with multlined
\begin{align*}
  aaa&=bbb\\
   &=\begin{multlined}[t]
      ccc\\
      {}+ ddd
     \end{multlined}
\end{align*}
\end{document}