Automatic conversion from WinRT class to implemented interface

[kaivol]

Suggestion

Use case

Currently, the following code does not compile:

let vector: Collections::IVector<IStringable> = ...;
let uri = Uri::CreateUri(h!("https://github.com"))?;
vector.Append(&uri)?;
       ------  ^^^ the trait `CanInto<IStringable>` is not implemented for `Uri`
       |
       required by a bound introduced by this call
= note: required for `&windows::Foundation::Uri` to implement `IntoParam<windows::Foundation::IStringable, ReferenceType>`

Instead, one has to manually cast the Uri to a IStringable. This should not be necessary, because it is guaranteed that Uri implements IStringable.

I understand that Uri implements CanTryInto<IStringable>, which makes it compatible with TryIntoParam<IStringable> arguments, but TryIntoParam arguments only seem to be used for a few methods.

The same problem also exists for WinRT interfaces which require another interface.

Suggestion

I'm not sure if I'm missing something, but I would suggest getting rid of the distinction between CanInto/IntoParam and CanTryInto/TryIntoParam, replacing the latter with the former.
This would require changes to CanInto to support conversions using QueryInterface, but that should not be a problem, as the conversions are guaranteed to succeed.

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I would be happy to hear any opinion on this; whether the proposal makes sense or if there are reasons for not doing this and keeping the current solution.

[kennykerr]

It may be possible to combine TryIntoParam and IntoParam so long as we can do it without a run time cost to existing uses of IntoParam.

[kennykerr]

This should fix it: #2775