Extend control-flow declarations to preserve memory management?

[vasslitvinov]

This is a fork-off from #13639 (comment)
"Control-flow declarations" were formerly called "if-var" and "while-var", introduced in #17047 and #17304.

Right now the control-flow variable borrows from the control-flow expression of an owned or shared type:

var myC = new shared C();
if const temp = myC {
  // here 'temp' is a borrowed C
}

This issue request allowing such temp to preserve the memory management of the control-flow expression, rather than borrowing from it:

var myC = new shared C();
if const temp = /* Please share the ownership with me. -User */ myC { 
  // here 'temp' is a shared C
}

One possible syntax for this is to have the user specify the type of the temp, with the semantics of a cast from myC to that type, for example:

var myC = new shared C();
if const temp: shared = myC { 
  // here 'temp' is a shared C
}

[bradcray]

I think this is a reasonable direction, but would probably opt to file it away until someone has a pressing use-case for it.