Somewhat related:
There are definitely people that think +(String, &str) was a mistake (and along with it, +=(&mut String, &str), as it doesn’t fit the mathematical meaning of +. Historically, operator overloading, and especially when “nonstandard” operators are used outside the set of {+,-,/,*}, has led to some very hard-to-read code. Any time you bring up using operators, you’re going to face an uphill battle convincing people that the use of operators is valid for your proposed use case. This is even if they’re an addendum to the main point; the easier a point is to argue, the more it will be argued, whether it’s a main point or not.
On top of this, Rust has a focus on being explicit (or rather, locally apparent) about behavior, and towards that point doesn’t allow integer conversions silently (instead requiring as or .[try_]into(). As you’ve described it, your proposed operator does “conversion” of a sort behind the scenes and therefor obscures the behavior from the reader.
That being said, what you want has definite parallels with “placement in”, in that what it wants to do is not conversion (otherwise place = value.into() would be fine), but rather it wants to put a value into the old allocation.
let x: BigNum = default();
x <- 5;
let x: String = default();
x <- "wow";
(note: just applying the idea, these are not necessarily versions of placement in that existed before it was unaccepted.)
The key point to push here is that you are trying to reuse the allocation. Framing it as clear(); += may be correct for numerical types (and coincidentally for the “abuse” of summation for String), but isn’t the operation you’re going for, so only serves to muddy your point.
clone_from is what you want, but you want one that’s more generic such that it doesn’t just operate over a borrowed version of Self, but can accept the “stack” version as well. So String.clone_from(&str), which doesn’t work either.
I can see how a lack of some way to set the value of a bignum like resource other than it.set(5) would be useful; it could also be used for Cell and maybe many other container types. (But since I learned in Java, I come from the world where there was no operator overloading, so bignum was just always done with methods.)
TL;DR: presenting this as an operator and a muddiness around “convert then assign” are not helping your push.
