Function is_valid_identifier_or_path 

pub fn is_valid_identifier_or_path(s: &str) -> bool

Identifier & path validation.

A path may optionally start with :: and is otherwise a ::‑separated list of identifiers. Identifiers follow these rules:

• Must not be empty.
• Must not be just _.
• Must not start with two underscores (__).
• First char: Unicode XID_Start or _.
• Remaining chars: Unicode XID_Continue.

Any colon that is not part of a :: token is rejected, and empty path segments such as foo:: or foo:::bar are invalid.