Make diagnostic more user-friendly.

Author: dan-zheng

include/swift/AST/DiagnosticsSIL.def

@@ -493,8 +493,10 @@ WARNING(autodiff_nonvaried_result_fixit,none,
         ())
 // TODO(TF-1149): Remove this diagnostic.
 NOTE(autodiff_loadable_value_addressonly_tangent_unsupported,none,
-     "cannot differentiate value with loadable type %0 but address-only "
-     "'TangentVector' type %1", (Type, Type))
+     "cannot yet differentiate value whose type %0 has a compile-time known "
+     "size, but whose 'TangentVector' contains stored properties of unknown "
+     "size; consider modifying %1 to use fewer generic parameters in stored "
+     "properties", (Type, Type))
 NOTE(autodiff_enums_unsupported,none,
      "differentiating enum values is not yet supported", ())
 NOTE(autodiff_global_let_closure_not_differentiable,none,

test/AutoDiff/downstream/differentiation_transform_diagnostics.swift

@@ -310,7 +310,7 @@ class C<T: Differentiable>: Differentiable {
   // expected-error @+1 {{function is not differentiable}}
   @differentiable
   // expected-note @+2 {{when differentiating this function definition}}
-  // expected-note @+1 {{cannot differentiate value with loadable type 'C<T>' but address-only 'TangentVector' type 'C<τ_0_0>.TangentVector'}}
+  // expected-note @+1 {{cannot yet differentiate value whose type 'C<T>' has a compile-time known size, but whose 'TangentVector' contains stored properties of unknown size; consider modifying 'C<τ_0_0>.TangentVector' to use fewer generic parameters in stored properties}}
   var stored: T
 
   init(_ stored: T) {
@@ -320,7 +320,7 @@ class C<T: Differentiable>: Differentiable {
   // expected-error @+1 {{function is not differentiable}}
   @differentiable
   // expected-note @+2 {{when differentiating this function definition}}
-  // expected-note @+1 {{cannot differentiate value with loadable type 'C<T>' but address-only 'TangentVector' type 'C<τ_0_0>.TangentVector'}}
+  // expected-note @+1 {{cannot yet differentiate value whose type 'C<T>' has a compile-time known size, but whose 'TangentVector' contains stored properties of unknown size; consider modifying 'C<τ_0_0>.TangentVector' to use fewer generic parameters in stored properties}}
   func foo(_ x: T) -> T {
     stored
   }