[LLVM][Casting.h] Add ForwardToPointerCast trait

[bzcheeseman]

Summary

Addresses use cases in Clang/MLIR that need pointer-to-pointer, reference-to-reference, and value-to-value casts from/to the same types. This should reduce boilerplate by allowing the user to simply specify the pointer cast and forward the reference cast directly to the pointer cast.

This cast trait DOES NOT implement castFailed and doCastIfPossible because in the general case doing so could result in a nullptr dereference. Users can use NullableValueCastFailed and DefaultDoCastIfPossible as desired for those cases where nullptr is acceptable.

[rriddle]

Nice.

Was there somewhere in MLIR that could take advantage of this? Trying to recall from another patch.

[bzcheeseman]

I think Operation casting probably could have used it IIRC

llvm/include/llvm/Support/Casting.h

static inline decltype(auto) doCast(const From &f) {
     return ForwardTo::doCast(const_cast<NonConstFrom>(f));
   }
 
   static inline decltype(auto) doCastIfPossible(const From &f) {
     return ForwardTo::doCastIfPossible(const_cast<NonConstFrom>(f));
   }
 };
 
+/// Provides a cast trait that uses a defined pointer to pointer cast as a base
+/// for reference-to-reference casts. Note that it does not provide castFailed
+/// and doCastIfPossible because a pointer-to-pointer cast would likely just
+/// return `nullptr` which could cause nullptr dereference. You can use it like
+/// this:
+///
+///   template <> struct CastInfo<foo, bar *> { ... verbose implementation... };
+///
+///   template <>
+///   struct CastInfo<foo, bar>
+///       : public ForwardToPointerCast<foo, bar, CastInfo<foo, bar *>> {};
+///
+template <typename To, typename From, typename ForwardTo>
+struct ForwardToPointerCast {
+  static inline bool isPossible(const From &f) {
+    return ForwardTo::isPossible(&f);
+  }
+
+  static inline decltype(auto) doCast(const From &f) {
+    return *ForwardTo::doCast(&f);
+  }
+};
+
 //===----------------------------------------------------------------------===//
 // CastInfo
 //===----------------------------------------------------------------------===//
 
 /// This struct provides a method for customizing the way a cast is performed.
 /// It inherits from CastIsPossible, to support the case of declaring many
 /// CastIsPossible specializations without having to specialize the full
 /// CastInfo.