E0118 probably shouldn't use the terminology of "base type"

[CAD97]

[playground]

struct Animal;

impl &Animal {}

error[E0118]: no base type found for inherent implementation
 --> src/lib.rs:3:6
  |
3 | impl &Animal {}
  |      ^^^^^^^ impl requires a base type
  |
  = note: either implement a trait on it or create a newtype to wrap it instead

To quote GIGAOTSOS:

impl requeres a base type error screams about presence of sybtyping

This should probably use the covered/local terminology from re-rebalancing coherence instead of "base type" to avoid the problematic overlap with subtyping/inheritance terminology.

[Havvy]

In the reference, we use the term "nominal type". The error code explanation should also mention unions and trait objects (impl dyn Send {} is valid). If the person use &NominalType or &mut NominalType, it should also hint at removing the reference and putting them on Self in the associated items. Not sure if I should open a new issue for that one or not though.