Mini-rust in Rust 020: Gaps in our type checker

mini-rust (21 part Series)
Mini-rust in Rust 001: Lexing an int
Mini-rust in Rust 002: Errors while lexing
Mini-rust in Rust 003: Much prettier errors
Mini-rust in Rust 004: Refactoring our lexer
Mini-rust in Rust 005: Making our lexer an iterator
Mini-rust in Rust 006: Parsing an operation
Mini-rust in Rust 007: Evaluating an operation
Mini-rust in Rust 008: Refactoring our parser
Mini-rust in Rust 009: Handling parsing errors
Mini-rust in Rust 010: Lexing an assignment
Mini-rust in Rust 011: Parsing an assignment plus expression
Mini-rust in Rust 012: Parsing with backtracking
Mini-rust in Rust 013: Fixing a bug in our lexer
Mini-rust in Rust 014: Evaluating an assignment
Mini-rust in Rust 015: Parsing a typed assignment
Mini-rust in Rust 016: Tidying our parser
Mini-rust in Rust 017: Handling float literals
Mini-rust in Rust 018: Type checking, the happy path
Mini-rust in Rust 019: Type checking an actual type error
Mini-rust in Rust 020: Gaps in our type checker
Mini-rust in Rust 021: Including type information in our syntax tree