where clause is permitted to be empty

WhereClause is permitted to be empty (i.e. a bare where) in baseline Rust. The following statements all compile:

struct Struct where {
}
enum Enum where {
}
type TypeAlias where = Struct;
trait Trait where {
}
fn function() where {
}
impl Struct where {
}
impl Trait for Struct where {
}

In the Ferrocene spec, 12.2 "Where Clauses" specifies that a WhereClause contains a WhereClausePredicateList which must be a non-empty list. However, Rust permits an empty list (which has the same effect as if where was not specified at all). The specification should probably be amended to reflect the actual grammar.

Read section 12.2, “Where Clauses,” and compare its requirement for a non-empty WhereClausePredicateList with the Rust examples in the issue. Amend the specification grammar to permit an empty predicate list, while preserving the existing behavior for non-empty clauses; the examples should then be consistent with the specification.