Parser for Rust source code
Syn is a parsing library for parsing a stream of Rust tokens into a syntax tree of Rust source code.
Currently this library is geared toward use in Rust procedural macros, but contains some APIs that may be useful more generally.
Data structures — Syn provides a complete syntax tree that can represent any valid Rust source code. The syntax tree is rooted at
syn::Filewhich represents a full source file, but there are other entry points that may be useful to procedural macros including
Derives — Of particular interest to derive macros is
syn::DeriveInputwhich is any of the three legal input items to a derive macro. An example below shows using this type in a library that can derive implementations of a user-defined trait.
Parsing — Parsing in Syn is built around parser functions with the signature
fn(ParseStream) -> Result<T>. Every syntax tree node defined by Syn is individually parsable and may be used as a building block for custom syntaxes, or you may dream up your own brand new syntax without involving any of our syntax tree types.
Location information — Every token parsed by Syn is associated with a
Spanthat tracks line and column information back to the source of that token. These spans allow a procedural macro to display detailed error messages pointing to all the right places in the user's code. There is an example of this below.
Feature flags — Functionality is aggressively feature gated so your procedural macros enable only what they need, and do not pay in compile time for all the rest.
Version requirement: Syn supports rustc 1.31 and up.
The best way to learn about procedural macros is by writing some. Consider working through this procedural macro workshop to get familiar with the different types of procedural macros. The workshop contains relevant links into the Syn documentation as you work through each project.
Example of a derive macro
The canonical derive macro using Syn looks like this. We write an ordinary Rust
function tagged with a
proc_macro_derive attribute and the name of the trait
we are deriving. Any time that derive appears in the user's code, the Rust
compiler passes their data structure as tokens into our macro. We get to execute
arbitrary Rust code to figure out what to do with those tokens, then hand some
tokens back to the compiler to compile into the user's crate.
 = "1.0" = "1.0"  = true
use TokenStream; use quote; use ;
heapsize example directory shows a complete working implementation of a
derive macro. It works on any Rust compiler 1.31+. The example derives a
HeapSize trait which computes an estimate of the amount of heap memory owned
by a value.
The derive macro allows users to write
#[derive(HeapSize)] on data structures
in their program.
Spans and error reporting
The token-based procedural macro API provides great control over where the
compiler's error messages are displayed in user code. Consider the error the
user sees if one of their field types does not implement
By tracking span information all the way through the expansion of a procedural
macro as shown in the
heapsize example, token-based macros in Syn are able to
trigger errors that directly pinpoint the source of the problem.
error[E0277]: the trait bound `std::thread::Thread: HeapSize` is not satisfied --> src/main.rs:7:5 | 7 | bad: std::thread::Thread, | ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `HeapSize` is not implemented for `std::thread::Thread`
Parsing a custom syntax
lazy-static example directory shows the implementation of a
functionlike!(...) procedural macro in which the input tokens are parsed using
Syn's parsing API.
The example reimplements the popular
lazy_static crate from crates.io as a
The implementation shows how to trigger custom warnings and error messages on the macro input.
warning: come on, pick a more creative name --> src/main.rs:10:16 | 10 | static ref FOO: String = "lazy_static".to_owned(); | ^^^
When testing macros, we often care not just that the macro can be used
successfully but also that when the macro is provided with invalid input it
produces maximally helpful error messages. Consider using the
to write tests for errors that are emitted by your macro or errors detected by
the Rust compiler in the expanded code following misuse of the macro. Such tests
help avoid regressions from later refactors that mistakenly make an error no
longer trigger or be less helpful than it used to be.
When developing a procedural macro it can be helpful to look at what the
generated code looks like. Use
cargo rustc -- -Zunstable-options --pretty=expanded or the
cargo expand subcommand.
To show the expanded code for some crate that uses your procedural macro, run
cargo expand from that crate. To show the expanded code for one of your own
test cases, run
cargo expand --test the_test_case where the last argument is
the name of the test file without the
This write-up by Brandon W Maister discusses debugging in more detail: Debugging Rust's new Custom Derive system.
Syn puts a lot of functionality behind optional features in order to optimize compile time for the most common use cases. The following features are available.
derive(enabled by default) — Data structures for representing the possible input to a derive macro, including structs and enums and types.
full— Data structures for representing the syntax tree of all valid Rust source code, including items and expressions.
parsing(enabled by default) — Ability to parse input tokens into a syntax tree node of a chosen type.
printing(enabled by default) — Ability to print a syntax tree node as tokens of Rust source code.
visit— Trait for traversing a syntax tree.
visit-mut— Trait for traversing and mutating in place a syntax tree.
fold— Trait for transforming an owned syntax tree.
clone-impls(enabled by default) — Clone impls for all syntax tree types.
extra-traits— Debug, Eq, PartialEq, Hash impls for all syntax tree types.
proc-macro(enabled by default) — Runtime dependency on the dynamic library libproc_macro from rustc toolchain.
Proc macro shim
Syn operates on the token representation provided by the proc-macro2 crate from crates.io rather than using the compiler's built in proc-macro crate directly. This enables code using Syn to execute outside of the context of a procedural macro, such as in unit tests or build.rs, and we avoid needing incompatible ecosystems for proc macros vs non-macro use cases.
In general all of your code should be written against proc-macro2 rather than
proc-macro. The one exception is in the signatures of procedural macro entry
points, which are required by the language to use
The proc-macro2 crate will automatically detect and use the compiler's data structures when a procedural macro is active.