Institutes of the Christian Religion (1559) - John Calvin
Book 2 - God the Redeemer
Chapter 8 - Explanation of the Moral Law:
The 10 Commandments
Sections 49-50 - Tenth Commandment - Don't Covet
Section 49 - Meaning: forbids all desire contrary to love.
This is in addition to the intent to commit murder or adultery, as covered there; coveting is playing with the desire without committing to sin with your will.
"Wouldn't THAT be nice" = coveting.
"If I could, I would" = intent.
Section 50 - Innermost rightesouness!
When we entertain the desire, we have become greedy - focused on self rather than neighbor.
God requires a tempered heart that always loves the other.