Use diagnostic::on_unimplemented to make compile errors more helpful and less intimidating

[lloydmeta]

Right now, compile-time errors on transmogrify can be truly terrifying and unhelpful.

The new #[diagnostic::on_unimplemented] attribute from 1.78.0 might be helpful here.

That said, much of the functionality around LabelledGeneric and friends are built on top of smaller, more basic traits, which are based on other traits.... so I'm not really sure how far we can really go. Worth a think though.

frunk/core/src/labelled.rs

Lines 829 to 911 in 1ca9c99

	/// Non-trivial implementation of `Transmogrifier` where similarly-shaped `Source` and `Target` types are
	/// both Labelled HLists, but do not immediately transform into one another due to mis-matched
	/// fields, possibly recursively so.
	impl<
	SourceHead,
	SourceTail,
	TargetHeadName,
	TargetHeadValue,
	TargetTail,
	PluckSourceHeadNameIndex,
	TransMogSourceHeadValueIndices,
	TransMogTailIndices,
	>
	Transmogrifier<
	HCons<Field<TargetHeadName, TargetHeadValue>, TargetTail>,
	HCons<
	DoTransmog<PluckSourceHeadNameIndex, TransMogSourceHeadValueIndices>,
	TransMogTailIndices,
	>,
	> for HCons<SourceHead, SourceTail>
	where
	// Pluck a value out of the Source by the Head Target Name
	HCons<SourceHead, SourceTail>: ByNameFieldPlucker<TargetHeadName, PluckSourceHeadNameIndex>,
	// The value we pluck out needs to be able to be transmogrified to the Head Target Value type
	Field<
	TargetHeadName,
	<HCons<SourceHead, SourceTail> as ByNameFieldPlucker<
	TargetHeadName,
	PluckSourceHeadNameIndex,
	>>::TargetValue,
	>: Transmogrifier<TargetHeadValue, TransMogSourceHeadValueIndices>,
	// The remainder from plucking out the Head Target Name must be able to be transmogrified to the
	// target tail, utilising the other remaining indices
	<HCons<SourceHead, SourceTail> as ByNameFieldPlucker<
	TargetHeadName,
	PluckSourceHeadNameIndex,
	>>::Remainder: Transmogrifier<TargetTail, TransMogTailIndices>,
	{
	#[inline(always)]
	fn transmogrify(self) -> HCons<Field<TargetHeadName, TargetHeadValue>, TargetTail> {
	let (source_field_for_head_target_name, remainder) = self.pluck_by_name();
	let name = source_field_for_head_target_name.name;
	let transmogrified_value: TargetHeadValue =
	source_field_for_head_target_name.transmogrify();
	let as_field: Field<TargetHeadName, TargetHeadValue> =
	field_with_name(name, transmogrified_value);
	HCons {
	head: as_field,
	tail: remainder.transmogrify(),
	}
	}
	}
	impl<Source, Target, TransmogIndices>
	Transmogrifier<Target, LabelledGenericTransmogIndicesWrapper<TransmogIndices>> for Source
	where
	Source: LabelledGeneric,
	Target: LabelledGeneric,
	<Source as LabelledGeneric>::Repr:
	Transmogrifier<<Target as LabelledGeneric>::Repr, TransmogIndices>,
	{
	#[inline(always)]
	fn transmogrify(self) -> Target {
	let source_as_repr = self.into();
	let source_transmogged = source_as_repr.transmogrify();
	<Target as LabelledGeneric>::from(source_transmogged)
	}
	}
	// Implementation for when the source value is plucked
	impl<Source, TargetName, TargetValue, TransmogIndices>
	Transmogrifier<TargetValue, PluckedLabelledGenericIndicesWrapper<TransmogIndices>>
	for Field<TargetName, Source>
	where
	Source: LabelledGeneric,
	TargetValue: LabelledGeneric,
	Source: Transmogrifier<TargetValue, TransmogIndices>,
	{
	#[inline(always)]
	fn transmogrify(self) -> TargetValue {
	self.value.transmogrify()
	}
	}