[SR-13670] Sema/SILGen: lower `@derivative` attribute directly to SIL differentiability witnesses

[dan-zheng]

Previous ID	SR-13670
Radar	None
Original Reporter	@dan-zheng
Type	Sub-task
Status	Closed
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s
Labels	Sub-task
Assignee	@dan-zheng
Priority	Medium

md5: e5ce47fd74f7fc29e2c2f821747e676e

Parent-Task:

• SR-13659 Retroactive derivative registration

is blocked by:

• SR-13669 Differentiation: revamp transform to directly canonicalize differentiability witnesses

relates to:

• TF-1037 [AD] Crash for @differentiable + @derivative attributes with different derivative generic signatures
• TF-1042 [AD] RemoveASTContext::{Differentiable,Derivative}Attrs

Issue Description:

Do not generate implicit @differentiable attributes during @derivative attribute type-checking. This was a hack and recently caused problems in ASTScopeLookup logic (TF-815).

Instead, directly lower @derivative attributes to SIL differentiability witnesses during SILGen.

[dan-zheng]

I investigated this as part of SIL differentiability witnesses (TF-866). Changing attribute type-checking to not generate implicit @differentiable attributes was fine.
It seems blocked by SILGen: it is important that SIL [differentiable] attributes are generated in SILFunctionBuilder::addFunctionAttributes, and it's not clear how to do this if we remove implicit @differentiable attributes. This will likely be blocked until SIL differentiability witnesses are done.

[dan-zheng]

WIP at https://github.com/dan-zheng/swift/commits/derivative-attr-silgen.
The change is straightforward but is blocked by TF-1024: changing the differentiation transform to directly canonicalize differentiability witnesses instead of SIL [differentiable] attributes.

[dan-zheng]

Done in #28621
@derivative attribute is SILGen'd directly to differentiability witnesses and no longer generates implicit @differentiable attribute.