"large enum variant" lint should report sizes of variants

[osa1]

clippy version: clippy 0.0.212 (d342cee 2020-04-07) (installed via rustup)

First of all, this lint seems useful, thanks for adding it.

I recently run this lint on my code base and it reported this warning:

warning: large size difference between variants
  --> libtiny_client/src/stream.rs:35:5
   |
35 |     TlsStream(TlsStream<TcpStream>),
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(clippy::large_enum_variant)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#large_enum_variant
help: consider boxing the large fields to reduce the total size of the enum
   |
35 |     TlsStream(Box<TlsStream<TcpStream>>),
   |               ^^^^^^^^^^^^^^^^^^^^^^^^^

The first question I asked when I see this is "OK, but how much is the difference, and what is the sizes of variants currently". I think this information is useful to decide whether to box the variant or not. For example, if one variant is one word and the other is 5, I probably wouldn't refactor this as 5 words is still not large enough for my purposes. But if one is one word and the other is 100 words then this is probably with refactoring.

I checked the link in the warning message, but couldn't see anything about when is a variant considered too different than others.