Constant UNREPRESENTABLE_KEYWORDS 

pub const UNREPRESENTABLE_KEYWORDS: &'static [&'static str];

Keywords that cannot be represented as a Rust identifier at all, not even as a raw identifier. Per the Rust reference, r#self, r#Self, r#super, and r#crate are explicitly disallowed by the compiler — there is no valid spelling, so a field with one of these names must be rejected at schema-parse time rather than escaped.