Why does += require manual dereference when AddAssign() does not?

[quinedot]

@tczajka

I think this doesn't work if you call AddAssign::add_assign directly because the Deref coercion doesn't apply to the self parameters in methods. That's somewhat confusingly documented on the same page:

For method calls, the receiver (self parameter) can only take advantage of unsized coercions.

It's a bit confusing because this exception is also true if you use the explicit AddAssign::add_assign(...) notation rather than the method call notation, which the docs don't clearly state.

I've been confused about that line for approximately , but that's not what it means.

Deref coercion definitely still works on the receiver using fully qualified syntax. And for DerefMut too.

I think this is more a matter of trait impl resolution.

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More generally, I've learned to be skeptical when the documentation says operator X is the same as method call Y. Operator lookup is it's own underdocumented distinct thing from both method calls and method dispatch; as far as I'm aware, there's no desugaring that universally works for at least some operators.

That said, I haven't find a particular counter example for this case (AddAssign).