warning about implicit copies should explain why type is not implicitly copyable

[nikomatsakis]

It would be nice if the warning "implicitly copying a non-implicitly-copyable value" varied depending on why the variable was "not-implicitly-copyable". For example:

• warning: implicitly copying a record with mutable fields
• warning: implicitly copying unique values, which entails a deep clone

etc.

[catamorphism]

I think some of the error messages have reasons now, but not all do.

[catamorphism]

@nikomatsakis - is this still relevant? I thought we got rid of the notion of impliciy copyability, but I still see code involving it in the compiler...

[nikomatsakis]

I think this is not still relevant? Certainly those messages no longer occur.