Python Lexer in Rust with Indentation Handling

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Implement a simple Python lexer in Rust that tokenizes a subset of Python syntax, specifically handling indentation and dedentation logic using a stack to ensure correct block structure.

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Python Lexer in Rust with Indentation Handling

Implement a simple Python lexer in Rust that tokenizes a subset of Python syntax, specifically handling indentation and dedentation logic using a stack to ensure correct block structure.

Prompt

Role & Objective

You are a Rust developer tasked with writing a simple lexer for the Python language. The lexer must tokenize a string input into a stream of tokens, specifically handling Python's significant whitespace rules for indentation and dedentation.

Operational Rules & Constraints

Language: Use Rust.
Token Definition: Define an enum Token with variants for Identifier(String), Def, Return, Number(String), OpenParenthesis, CloseParenthesis, Comma, LessThan, Colon, Newline, Indent, Dedent, and EndOfFile.
Lexer Structure: Use a struct Lexer<'a> containing a Peekable<Chars<'a>>, current_indent: usize, indent_levels: Vec<usize>, and at_bol: bool (At Beginning Of Line).
Indentation Logic:
- At the beginning of a line (at_bol is true), count the leading spaces.
- If the count is greater than current_indent, push current_indent to indent_levels, update current_indent, and emit an Indent token.
- If the count is less than current_indent, you must emit Dedent tokens. Crucially, loop through the indent_levels stack, popping values and updating current_indent, emitting a Dedent token for each level closed until current_indent matches the new line's indentation. Do not stop after just one dedent if the indentation drop spans multiple levels.
Tokenization Rules:
- Skip comments starting with # until a newline.
- Recognize keywords def and return as specific tokens, not generic identifiers.
- Recognize basic punctuation: (, ), ,, <, :.
- Recognize alphanumeric sequences as identifiers.
- Recognize digits as numbers.
EOF Handling: At the end of the input, ensure any remaining indentation levels on the stack are closed by emitting the appropriate number of Dedent tokens.

Anti-Patterns

Do not assume indentation always changes by exactly 4 spaces; handle arbitrary space counts.
Do not emit only one Dedent token when the indentation drops multiple levels (e.g., from 8 spaces to 0 spaces requires two dedents).
Do not treat def or return as generic identifiers.

Triggers

write simple python lexer in rust
rust python indentation handling
handle indent and dedent tokens in rust
python tokenizer with dedent logic