Fix warning for @formname without @onsubmit

[jjonescz]

Related to #9077.
Discovered when updating SDK in ASP.NET Core repo: dotnet/aspnetcore#50734

This removes warning RZ10021: Attribute '@formname' can only be used when '@onsubmit' event handler is also present. which was reported in wrong cases (even if user had valid @onsubmit) plus it became clear that it's not needed at all (@formname should be allowed without @onsubmit).

(The warning condition was wrong because the integration tests didn't register/process the event like real ASP.NET Core setting would - I fixed that in this PR for the @formname tests by adding @using Microsoft.AspNetCore.Components.Web.)

[SteveSandersonMS]

Could you clarify why there even is a warning/error for Attribute '@formname' can only be used when '@onsubmit' event handler is also present? When would it occur?

If interpreted with no other context, it's not true - you can have either @formname or @onsubmit without the other. They are both useful on their own, so we shouldn't be blocking that.

In all cases, @formname should compile as a call to AddNamedEvent and not as a call to AddAttribute. If it did compile as AddAttribute, then it would not work at runtime as soon as we finish removing the temporary API support.

[jjonescz]

you can have either @formname or @onsubmit without the other

Ah, OK, I thought it's not allowed, probably looking at the runtime implementation:

https://github.com/dotnet/aspnetcore/blob/d3122c7d16b85812891aced9c23e975b46f58f12/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs#L117

I will remove the warning completely then.

In all cases, @formname should compile as a call to AddNamedEvent and not as a call to AddAttribute. If it did compile as AddAttribute, then it would not work at runtime as soon as we finish removing the temporary API support.

Even if allowing @formname without @onsubmit, there's still one case where it won't compile to AddNamedEvent - when used on a non-<form> element (as there's also a warning for that). That is fine?

[SteveSandersonMS]

Ah, OK, I thought it's not allowed, probably looking at the runtime implementation

Understood. There is a misleading comment left behind there. Fortunately we do have E2E test cases that exercise @formname without @onsubmit.

Even if allowing @formname without @onsubmit, there's still one case where it won't compile to AddNamedEvent - when used on a non-

element (as there's also a warning for that). That is fine?

Yes, that's fine. Thanks for checking! There is currently no use case for @formname on elements other than <form>, so the warning and non-support is good. If there's ever a reason to change that we'll coordinate with you.

Removed the warning completely

[jaredpar]

Think we can delet this comment

[jaredpar]

For this test it appears no code is really generated and no diagnostics are emitted. Is that correct? If so it seems a bit odd to me that we do nothing and don't alert the user.

[jjonescz]

The @formname is translated into AddNamedEvent normally - https://github.com/dotnet/razor/blob/f808c223fdbdbc2a05069005c9693022d4cc693e/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/TestFiles/IntegrationTests/ComponentRuntimeCodeGenerationTest/FormName_MissingUsing/TestComponent.codegen.cs. The @onsubmit is emitted as-is but that's a pre-existing behavior - all events behave like this when the event handler is not registered.

Could perhaps implement a warning for this specific scenario - like "you seem to be using @formname with broken @onsubmit, did you mean to have event handler for @onsubmit registered?" - but that's something that shouldn't happen to users normally and if we want to do that, I guess it could be a follow up PR.