Removal of all unstable placement features

[arielb1]

So from my POV, the problem is that we promised RVO/NRVO, but never quite followed on that promise.

@kennytm

place <- [$expr; $n]

would expand to

for i in 0..$n {
    place[i] <- $expr;
}

Isn't this a misoptimization for the current semantics of ExprRepeat, as it evaluates $expr multiple times? The correct version would be this:

if $n > 0 {
    place[0] = $expr;
    place[1..] = [place[0]; $n-1];
}

Maybe we should also have a variant of ExprRepeat that works with non-Copy but constexpr $expr.

@kennytm

I was thinking if it is possible to do a syntax-based transformation of the RHS at HIR→HAIR level to perform the proper lowering, e.g.

For years there was supposed to be a guaranteed MIR optimization that does exactly this, except also in a few more cases (e.g., NRVO). It just never happened to be implemented because nobody found the time for it.