Mono C# low-level struct feature support

[lambdageek]

See #46104. Basically there are two new C#1language features that we need to support. ref fields in ref structs, and fixed size buffers of any type. (See C# and CoreCLR issues in the linked issue).

• ref fields in ref structs. Consume Roslyn with ref fields support #71498
  For Mono due to conservative stack scanning, it is likely that the only additional work for ref fields is to verify that we don't reject ref fields during loading and class initialization.

• safe fixed size buffers (This is in flux pending the outcome of the FixSizedBufferAttribute proposal Expose an attribute to allow sizing a fixed sized buffer based on type and element count. #12320). [API Proposal]: InlineArrayAttribute #61135
  The approach with InlineArrayAttribute is to modify the struct layout algorithm to adjust the inferred size of a single-field struct to contain space for N extra copies of the field. Also the GC descriptor will need to be array-like.
  The rest is based on the proposed translation in https://github.com/dotnet/csharplang/blob/master/proposals/fixed-sized-buffers.md
  The proposed translation relies on creating a generic struct type with potentially thousands of fields.
  ~ - [ ] First cut at support: Just verify that class layout and initialization don't perform pathologically on such giant declarations.~
  ~ - [ ] Second cut at support: Look for the FixedBufferAttribute on the field declaration and synthesize the class layout for the defining class without initializing the underlying thousand-field struct.~
  - [ ] Stretch goal: Ideally also compile the indexer in the struct without doing class layout. (This is hard)

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Issue Details

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See #46014. Basically there are two new C#10 language features tht we need to support. ref fields in ref structs, and fixed size buffers of any type. (See C# and CoreCLR issues in the linked issue).

• ref fields in ref structs.
  For Mono due to conservative stack scanning, it is likely that the only additional work for ref fields is to verify that we don't reject ref fields during loading and class initialization.

• safe fixed size buffers
  The proposed translation relies on creating a generic struct type with potentially thousands of fields.

  • First cut at support: Just verify that class layout and initialization don't perform pathologically on such giant declarations.
  • Second cut at support: Look for the FixedBufferAttribute on the field declaration and synthesize the class layout for the defining class without initializing the underlying thousand-field struct.
  • Ideally also compile the indexer in the struct without doing class layout. (This is hard)

Author:	lambdageek
Assignees:	-
Labels:	area-VM-meta-mono, tracking
Milestone:	-

[lambdageek]

.NET 8 work continues in #80798