https://doc.rust-lang.org/nightly/reference.html#match-expressions

A pattern that's just an identifier, like Nil in the previous example, could either refer to an enum variant that's in scope, or bind a new variable. The compiler resolves this ambiguity by forbidding variable bindings that occur in match patterns from shadowing names of variants that are in scope. For example, wherever List is in scope, a match pattern would not be able to bind Nil as a new name.

Nil and List are not mentioned in the previous example