Rust Macro Expansion Visualizer
See what Rust macros expand to. Our Rust Macro Expansion Visualizer detects and visualizes over 20 macro types including println!, vec!, format!, macro_rules! definitions, #[derive()] attribute macros, and custom procedural macros. Each detection includes the expanded form, detailed explanation, and source line number. Free, private, and no signup required.
Detect and visualize Rust macro expansions. Automatically identifies built-in macros (println!, vec!, format!, assert!), macro_rules! definitions, #[derive(...)] attribute macros, and custom proc macros. Shows expanded forms, explanations, and recursion detection. All processing is local - no server upload required.
Paste Rust code above and click Analyze Macros to detect and visualize macro expansions. The tool identifies built-in macros (println!, vec!, format!), macro_rules! definitions, #[derive] attributes, and custom procedural macros. Try loading an example to see how it works!
Why Use Our Rust Macro Expansion Visualizer?
Comprehensive Macro Detection
Automatically detects 20+ Rust macro types: built-in macros (println!, vec!, format!, concat!, stringify!), declarations (macro_rules!), attribute macros (#[derive(...)]), and custom procedural macros. Each detection includes the macro name, arguments, and source line number for complete visibility into your Rust code's macro usage.
Simulated Expansion & Recursion Detection
Visualizes what each macro expands to with simulated expanded forms. Built-in macros show their equivalent code, macro_rules! definitions show their match arms, and recursive macros are flagged with depth indicators. Understand exactly how template-heavy Rust code works at compile time.
Educational Explanations & Context
Every detected macro includes a detailed explanation of its purpose, how it works, and what it produces at compile time. Perfect for learning Rust's macro system - understand the difference between declarative macro_rules!, procedural macros, and built-in compiler macros with real examples.
100% Browser-Local & Private
All Rust macro analysis runs entirely in your browser. Your Rust code, all detected macros, expansion visualizations, and analysis results never leave your device. No server uploads, no API calls, no data storage, and no tracking. Perfect for analyzing proprietary code or competitive analysis.
Common Use Cases for Our Rust Macro Expansion Visualizer
Understanding Complex Macro-Heavy Rust Codebases
When working on Rust projects that extensively use macros (like serde, actix-web, diesel, or rocket), use this tool to expand macro invocations inline. Understanding what serde::Serialize, #[tokio::main], or sqlx::query! actually generates is critical for debugging compilation errors and unexpected behavior.
Debugging Macro Expansion Errors
Rust's macro expansion errors can be cryptic. Paste failing macro code to see what it actually expands to, making it easier to identify mismatched types, missing trait implementations, or incorrect pattern matching in macro_rules! macros. Spot common pitfalls like hygiene issues and variable capture.
Learning Rust's Macro System
New to Rust macros? Load the built-in examples and experiment with different macro patterns. Visualize how println!, vec!, format!, and matches! expand to regular Rust code. Understand the distinction between declarative macro_rules!, procedural macros, and built-in compiler intrinsics with side-by-side comparisons.
Code Review & Obfuscation Analysis
During security code reviews of Rust crates, scan for suspicious macro usage that might hide malicious behavior. Malicious Rust code can use macro_rules! or proc macros to conceal payloads, obfuscate network calls, or hide data exfiltration logic. This tool reveals what those macros actually produce.
Analyzing Recursive & Complex Macro Patterns
Rust macros can be recursive, making them hard to reason about. The tool detects recursive macro patterns and shows recursion depth. Use it to understand macro-based DSLs (domain-specific languages), builder patterns, state machines, and other complex compile-time code generation patterns.
Documentation & Teaching Tool
Generate expansion visualizations for documentation, blog posts, or classroom materials. Show students exactly how Rust's macro system works with concrete examples. The clear invocation-to-expansion mapping makes it easy to explain the relationship between macro calls and their generated code.
Understanding Rust Macro Expansion
What is Rust Macro Expansion?
Rust macro expansion is the compile-time process of transforming macro invocations into actual Rust code. When the Rust compiler encounters a macro call likeprintln!("Hello, ", name), it replaces it with generated code before type checking and compilation proceed. There are three kinds of macros in Rust:declarative macros (macro_rules!) that use pattern matching,procedural macros that run arbitrary Rust code at compile time (like #[derive(Serialize)]), and built-in compiler macros (like println!, vec!, and format!) that are built into the compiler itself. Understanding macro expansion is essential for debugging, code review, and learning advanced Rust patterns.
How Our Rust Macro Expansion Visualizer Works
The visualizer scans Rust source code using language-aware pattern matching to identify different macro types. Built-in macros like println!, vec!, format!, and concat! are detected by their known names and argument structures - each has a tailored expansion simulator that shows the equivalent generated code. macro_rules!definitions are detected and their match arms are parsed to show pattern-transcriber pairs.#[derive(...)] attributes are parsed to list the trait implementations being generated. Custom macrosare identified as any identifier followed by ! that isn't a known built-in. Recursive macros are detected by searching for self-referential macro invocations within a definition. All processing runs locally in your browser with no server interaction.
Rust Macros vs Other Languages
Rust's macro system is fundamentally different from C/C++ preprocessor macros. Unlike C's #define which is a simple text substitution, Rust macros operate at the token leveland respect syntax boundaries. This means Rust macros are hygienic (they won't accidentally capture variables from the calling scope), they support pattern matching on syntax structures, and they integrate with Rust's type system. Procedural macros in Rust are even more powerful - they receive a token stream at compile time and produce a new token stream, enabling advanced code generation for serialization (serde), web frameworks (actix-web, rocket), and database ORMs (diesel, sqlx). This makes Rust macros safe, composable, and far more capable than traditional preprocessor macros or even C++ templates.
Privacy, Security & Limitations
This tool runs entirely in your browser using client-side JavaScript. The Rust code you paste, all detected macros, expansion visualizations, and analysis results are never uploaded to any server. However, there are important limitations to be aware of. The tool provides simulated expansions for built-in macros and structural analysis for macro_rules! definitions, but it does not execute procedural macros or fully expand recursive macros with runtime state. The expanded form is an educational approximation, not the exact compiler output. For precise macro expansion, use cargo expand or rustc -Z macro-backtrace. The tool may not detect macros from external crates unless their call patterns are included in the source being analyzed.