Compiler:
- Add `Forward` and `Reverse` to `DifferentiabilityKind`.
- Expand `DifferentiabilityMask` in `ExtInfo` to 3 bits so that it now holds all 4 cases of `DifferentiabilityKind`.
- Parse `@differentiable(reverse)` and `@differentiable(_forward)` declaration attributes and type attributes.
- Emit a warning for `@differentiable` without `reverse`.
- Emit an error for `@differentiable(_forward)`.
- Rename `@differentiable(linear)` to `@differentiable(_linear)`.
- Make `@differentiable(reverse)` type lowering go through today's `@differentiable` code path. We will specialize it to reverse-mode in a follow-up patch.
ABI:
- Add `Forward` and `Reverse` to `FunctionMetadataDifferentiabilityKind`.
- Extend `TargetFunctionTypeFlags` by 1 bit to store the highest bit of differentiability kind (linear). Note that there is a 2-bit gap in `DifferentiabilityMask` which is reserved for `AsyncMask` and `ConcurrentMask`; `AsyncMask` is ABI-stable so we cannot change that.
_Differentiation module:
- Replace all occurrences of `@differentiable` with `@differentiable(reverse)`.
- Delete `_transpose(of:)`.
Resolves rdar://69980056.
Enable `@differentiable` attribute on setters of properties and
subscripts in `Differentiable`-conforming types.
Add automatically-differentiated `@differentiable` setter test.
Resolves TF-1166.
Fix SIL differential function type calculation to handle non-wrt `inout`
parameters.
Patch `SILFunctionType::getDifferentiabilityResultIndices` to prevent returning
empty result indices for `@differentiable` function types with no formal results
where all `inout` parameters are `@noDerivative`. TF-1305 tracks a robust fix.
Resolves SR-13305.
Exposes TF-1305: parameter/result differentiability hole for `inout` parameters.
