Balance to Asset Balance Conversion

[apopiak]

This PR adds the BalanceConversion trait and BalanceToAssetBalance type to pallet_assets to allow converting native balances to asset specific ones.
This is a "low resolution" conversion based on the existential deposit (of the native currency) and the min_balance (of the asset).

Note: The implementation actually expects a generic fungible, so the incoming balance could theoretically be an asset as well. (So the same implementation could work for chains that only have the assets pallet.)

PR also adds some derives for Imbalance that seemed useful but are unconnected to the conversion.

needed for: paritytech/cumulus#488

supersedes #8776

[kianenigma]

LGTM, but I am not really familiar with its usage on cumulus yet.

[apopiak]

I guess this needs an audit?
The particular changes are pretty innocuous, but will be relevant for transaction payments?

[apopiak]

Should the trait live in frame_support?

[shawntabrizi]

Probably yes, somewhere in here: https://github.com/paritytech/substrate/tree/master/frame/support/src/traits/tokens

[emostov]

Code itself lgtm. But what is the use case and the justification for using min_balance of assets to establish a conversion rate? From what I understand, in order to faithfully use this conversion one must assume the min_balance between two assets is equivalent in relative "value" - is this the correct way to think about it?

[gui1117]

code is good to me,

but test needs to fixed, CI doesn't compile them

[gui1117]

But what is the use case and the justification for using min_balance of assets to establish a conversion rate? From what I understand, in order to faithfully use this conversion one must assume the min_balance between two assets is equivalent in relative "value" - is this the correct way to think about it?

Yes I think this is the correct usecase, when min_balance hold a relatively same value.
Usually the minimum balance cover the cost to have the balance and other account information on chain.

[apopiak]

bot merge

[ghost]

Trying merge.