Warn when an `@` is optional in an attribute value

[davidwengier]

See dotnet/aspire#1449 for context, but in general there is code like:

<Component OnSomeEvent="DoSomething(context)" />

and the user changed it to:

<Component OnSomeEvent="@DoSomething(context)" />

This seemed to have no effect, but nobody is really sure what the rules are for whether the @ is necessary in this context or not. It would be great if the compiler could warn when it expects a C# expression, and encounters a C# implicit expression.

[chsienki]

@jjonescz Please take a look, thanks

[jjonescz]

We would need warning waves to avoid breaking users. (Another case where we need them: #9144)

Also feels like a question for language design whether we want this or not.

[jjonescz]

@chsienki I'm not sure if you want me to work on this given what I've written above

[chsienki]

@jjonescz Can you open an issue on the razor LDM for warning waves (if we don't already have one?) and start the email thread to discuss it? Seems like something we probably want to do.