This PR introduces `@differentiable` attribute to mark functions as differentiable. This PR only contains changes related to parsing the attribute. Type checking and other changes will be added in subsequent patches.
See https://github.com/apple/swift/pull/27506/files#diff-f3216f4188fd5ed34e1007e5a9c2490f for examples and tests for the new attribute.
The `_Differentiation` module is the experimental support library for
differentiable programming. It is built when the build-script flag
`--enable-experimental-differentiable-programming` is enabled.
The `Differentiable` protocol generalizes all types that work with
differentiation. It is a core piece of the differentiable programming
project. Other parts depending on the `Differentiable` protocol will
be upstreamed piece by piece.
The `Differentiable` protocol is compiler-known and will be used during
type-checking, SILGen, and the SIL differentiation transform.
Adds parsing for a type attribute `@differentiable`, which is optionally allowed to have argument `@differentiable(linear)`.
The typechecker currently rejects all uses of `@differentiable` with "error: attribute does not apply to type". Future work (https://bugs.swift.org/browse/TF-871https://bugs.swift.org/browse/TF-873) will update the typechecker to allow this attribute in places where it is allowed.
Resolves https://bugs.swift.org/browse/TF-822.
