single_match should suggest plain `if` when possible

[Manishearth]

Currently, for things like:

match foo
  Enum::Variant => {...}
  _ => {...}
}

we suggest using if let Enum::Variant = foo {...}. However, in many cases, the enum implements PartialEq, so we can just suggest if Enum::Variant == foo.

This involves checking if the patterns in the match are simple PatIdents or PatEnums with empty pat-vecs (http://manishearth.github.io/rust-internals-docs/syntax/ast/enum.Pat_.html), and then checking if the type implements Eq (not sure how this can be done, probably needs some mucking about in middle::ty)

[llogiq]

Is there any downside in using if let as opposed to equality? Performance-wise?

Also equality may be implemented differently (e.g. broader than a match) – we should probably tread carefully here.

[birkenfeld]

I agree, we should not propose changes that could alter semantics.

[llogiq]

We could note the alternative but caution that semantics may be different depending on the PartialEq implementation.

I note that there is a case where this tripped me in eq_op.

[llogiq]

Also the same blacklist approach as in #181 could apply.