Add diagnostic::on_unimplemented attributes to Transmogrifier and ByNameFieldPlucker traits

Co-authored-by: lloydmeta <914805+lloydmeta@users.noreply.github.com>

Author: Copilot

core/src/labelled.rs

@@ -586,6 +586,12 @@ where
 }
 
 /// Trait for plucking out a `Field` from a type by type-level `TargetKey`.
+#[diagnostic::on_unimplemented(
+    message = "Cannot find field with key `{TargetKey}` in `{Self}`",
+    label = "Field not found",
+    note = "The source type does not contain a field with the target key.",
+    note = "Make sure the field name exists in the source struct and matches exactly."
+)]
 pub trait ByNameFieldPlucker<TargetKey, Index> {
     type TargetValue;
     type Remainder;
@@ -752,6 +758,13 @@ where
 ///
 /// Credit:
 /// 1. Haskell "transmogrify" Github repo: <https://github.com/ivan-m/transmogrify>
+#[diagnostic::on_unimplemented(
+    message = "Cannot transmogrify `{Self}` into `{Target}`",
+    label = "Cannot convert this type into the target type",
+    note = "Transmogrify requires that the source and target types have compatible structures.",
+    note = "The source type must have all the fields needed for the target type, possibly in a different order or nested structure.",
+    note = "Check that field names match and types are compatible between the source and target."
+)]
 pub trait Transmogrifier<Target, TransmogrifyIndexIndices> {
     /// Consume this current object and return an object of the Target type.
     ///