Use bidirectional marshallers for elements.

[jkoritzinsky]

Fixes #89893

Also updates the analyzer to require the element shapes to be bidirectional in all cases.

Technically this is a source-breaking change in the analyzer, but since it can be suppressed like all analyzers, I'm not sure if we need to fill out a breaking-change form.

[ghost]

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Issue Details

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Fixes #89893

Also updates the analyzer to require the element shapes to be bidirectional in all cases.

Technically this is a source-breaking change in the analyzer, but since it can be suppressed like all analyzers, I'm not sure if we need to fill out a breaking-change form.

Author:	jkoritzinsky
Assignees:	-
Labels:	area-System.Runtime.InteropServices
Milestone:	8.0.0

[jkoritzinsky]

I've figured out a way to limit the breaking change to cases where the user explicitly specifies the ElementIn, ElementRef, or ElementOut marshal modes. I think those cases will be rare enough that merging this in this late in the release may be okay.

I found one user outside of dotnet/runtime and dotnet/samples that uses the Element marshaller modes specifically: https://grep.app/search?q=MarshalMode.Element.

If we're not sure, we can also merge this into .NET 9 as soon as we branch.

@AaronRobinsonMSFT @elinor-fung what do you think?

[AaronRobinsonMSFT]

I found one user outside of dotnet/runtime and dotnet/samples that uses the Element marshaller modes specifically: https://grep.app/search?q=MarshalMode.Element.

That looks like a rather popular library. How impactful will this be to them?

[jkoritzinsky]

They should be able to change the marshaller that they're using to the stateless marshaller that they already have for the ElementRef case.

I can also try to make the errors into warnings for the Element modes (like we do with the Default mode) to reduce the impact further.

[ghost]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[lambdageek]

@jkoritzinsky I'm seeing

##[error]src/libraries/System.Runtime.InteropServices/tests/TestAssets/SharedTypes/ComInterfaces/IStatelessPinnableCollectionBlittableElements.cs(53,92): error SYSLIB1057: (NETCORE_ENGINEERING_TELEMETRY=Build) The type 'SharedTypes.ComInterfaces.StatelessPinnableCollectionMarshaller<T, TUnmanagedElement>.ManagedToUnmanaged' specifies that it supports the 'ElementIn' marshal mode for 'SharedTypes.ComInterfaces.StatelessPinnableCollection<T>' but does not provide a two-parameter 'AllocateContainerForManagedElements' method that takes the unmanaged type as the first parameter and an 'int' as the second parameter

from Linux and Windows libraries builds on my PR (#90166)

[jtschuster]

@lambdageek I think I merged a PR with a new test type that this PR warns on between when this passed CI and when it was merged. I'll get a fix out, thanks for the head up!

edit: PR is here: #90176