# Crate syn

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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::File` which represents a full source file, but there are other entry points that may be useful to procedural macros including `syn::Item`, `syn::Expr` and `syn::Type`.

- Derives — Of particular interest to derive macros is `syn::DeriveInput` which 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 `Span` that 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.

## 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.

    [dependencies]
    syn = "1.0"
    quote = "1.0"

    [lib]
    proc-macro = true

    use proc_macro::TokenStream;
    use quote::quote;
    use syn::{parse_macro_input, DeriveInput};

    #[proc_macro_derive(MyMacro)]
    pub fn my_macro(input: TokenStream) -> TokenStream {
        // Parse the input tokens into a syntax tree
        let input = parse_macro_input!(input as DeriveInput);

        // Build the output, possibly using quasi-quotation
        let expanded = quote! {
            // ...
        };

        // Hand the output tokens back to the compiler
        TokenStream::from(expanded)
    }

The `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.

    pub trait HeapSize {
        /// Total number of bytes of heap memory owned by `self`.
        fn heap_size_of_children(&self) -> usize;
    }

The derive macro allows users to write `#[derive(HeapSize)]` on data structures in their program.

    #[derive(HeapSize)]
    struct Demo<'a, T: ?Sized> {
        a: Box<T>,
        b: u8,
        c: &'a str,
        d: String,
    }

## 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 `HeapSize`.

    #[derive(HeapSize)]
    struct Broken {
        ok: String,
        bad: std::thread::Thread,
    }

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 `Thread`

## Parsing a custom syntax

The `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 procedural macro.

    lazy_static! {
        static ref USERNAME: Regex = Regex::new("^[a-z0-9_-]{3,16}$").unwrap();
    }

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();
       |                ^^^

## Testing

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 `trybuild` crate 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.

## Debugging

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 `.rs` extension.

This write-up by Brandon W Maister discusses debugging in more detail: Debugging Rust’s new Custom Derive system.

## Optional features

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.

## Modules

buffer

A stably addressed token buffer supporting efficient traversal based on a cheaply copyable cursor.

ext

Extension traits to provide parsing methods on foreign types.

parse

Parsing interface for parsing a token stream into a syntax tree node.

punctuated

A punctuated sequence of syntax tree nodes separated by punctuation.

spanned

A trait that can provide the `Span` of the complete contents of a syntax tree node.

token

Tokens representing Rust punctuation, keywords, and delimiters.

visit_mut

Syntax tree traversal to mutate an exclusive borrow of a syntax tree in place.

## Macros

Token

A type-macro that expands to the name of the Rust type representation of a given token.

braced

Parse a set of curly braces and expose their content to subsequent parsers.

bracketed

Parse a set of square brackets and expose their content to subsequent parsers.

custom_keyword

Define a type that supports parsing and printing a given identifier as if it were a keyword.

custom_punctuation

Define a type that supports parsing and printing a multi-character symbol as if it were a punctuation token.

parenthesized

Parse a set of parentheses and expose their content to subsequent parsers.

parse_macro_input

Parse the input TokenStream of a macro, triggering a compile error if the tokens fail to parse.

parse_quote

Quasi-quotation macro that accepts input like the `quote!` macro but uses type inference to figure out a return type for those tokens.

parse_quote_spanned

This macro is `parse_quote!` + `quote_spanned!`.

## Structs

Abi

The binary interface of a function: `extern "C"`.

AngleBracketedGenericArguments

Angle bracketed arguments of a path segment: the `<K, V>` in `HashMap<K, V>`.

Arm

One arm of a `match` expression: `0...10 => { return true; }`.

Attribute

An attribute like `#[repr(transparent)]`.

BareFnArg

An argument in a function type: the `usize` in `fn(usize) -> bool`.

Binding

A binding (equality constraint) on an associated type: `Item = u8`.

Block

A braced block containing Rust statements.

BoundLifetimes

A set of bound lifetimes: `for<'a, 'b, 'c>`.

ConstParam

A const generic parameter: `const LENGTH: usize`.

Constraint

An associated type bound: `Iterator<Item: Display>`.

DataEnum

An enum input to a `proc_macro_derive` macro.

DataStruct

A struct input to a `proc_macro_derive` macro.

DataUnion

An untagged union input to a `proc_macro_derive` macro.

DeriveInput

Data structure sent to a `proc_macro_derive` macro.

Error

Error returned when a Syn parser cannot parse the input tokens.

ExprArray

A slice literal expression: `[a, b, c, d]`.

ExprAssign

An assignment expression: `a = compute()`.

ExprAssignOp

A compound assignment expression: `counter += 1`.

ExprAsync

An async block: `async { ... }`.

ExprAwait

An await expression: `fut.await`.

ExprBinary

A binary operation: `a + b`, `a * b`.

ExprBlock

A blocked scope: `{ ... }`.

ExprBox

A box expression: `box f`.

ExprBreak

A `break`, with an optional label to break and an optional expression.

ExprCall

A function call expression: `invoke(a, b)`.

ExprCast

A cast expression: `foo as f64`.

ExprClosure

A closure expression: `|a, b| a + b`.

ExprContinue

A `continue`, with an optional label.

ExprField

Access of a named struct field (`obj.k`) or unnamed tuple struct field (`obj.0`).

ExprForLoop

A for loop: `for pat in expr { ... }`.

ExprGroup

An expression contained within invisible delimiters.

ExprIf

An `if` expression with an optional `else` block: `if expr { ... } else { ... }`.

ExprIndex

A square bracketed indexing expression: `vector[2]`.

ExprLet

A `let` guard: `let Some(x) = opt`.

ExprLit

A literal in place of an expression: `1`, `"foo"`.

ExprLoop

Conditionless loop: `loop { ... }`.

ExprMacro

A macro invocation expression: `format!("{}", q)`.

ExprMatch

A `match` expression: `match n { Some(n) => {}, None => {} }`.

ExprMethodCall

A method call expression: `x.foo::<T>(a, b)`.

ExprParen

A parenthesized expression: `(a + b)`.

ExprPath

A path like `std::mem::replace` possibly containing generic parameters and a qualified self-type.

ExprRange

A range expression: `1..2`, `1..`, `..2`, `1..=2`, `..=2`.

ExprReference

A referencing operation: `&a` or `&mut a`.

ExprRepeat

An array literal constructed from one repeated element: `[0u8; N]`.

ExprReturn

A `return`, with an optional value to be returned.

ExprStruct

A struct literal expression: `Point { x: 1, y: 1 }`.

ExprTry

A try-expression: `expr?`.

ExprTryBlock

A try block: `try { ... }`.

ExprTuple

A tuple expression: `(a, b, c, d)`.

ExprType

A type ascription expression: `foo: f64`.

ExprUnary

A unary operation: `!x`, `*x`.

ExprUnsafe

An unsafe block: `unsafe { ... }`.

ExprWhile

A while loop: `while expr { ... }`.

ExprYield

A yield expression: `yield expr`.

Field

A field of a struct or enum variant.

FieldPat

A single field in a struct pattern.

FieldValue

A field-value pair in a struct literal.

FieldsNamed

Named fields of a struct or struct variant such as `Point { x: f64, y: f64 }`.

FieldsUnnamed

Unnamed fields of a tuple struct or tuple variant such as `Some(T)`.

File

A complete file of Rust source code.

ForeignItemFn

A foreign function in an `extern` block.

ForeignItemMacro

A macro invocation within an extern block.

ForeignItemStatic

A foreign static item in an `extern` block: `static ext: u8`.

ForeignItemType

A foreign type in an `extern` block: `type void`.

Generics

Lifetimes and type parameters attached to a declaration of a function, enum, trait, etc.

Ident

A word of Rust code, which may be a keyword or legal variable name.

ImplGenerics

Returned by `Generics::split_for_impl`.

ImplItemConst

An associated constant within an impl block.

ImplItemMacro

A macro invocation within an impl block.

ImplItemMethod

A method within an impl block.

ImplItemType

An associated type within an impl block.

Index

The index of an unnamed tuple struct field.

ItemConst

A constant item: `const MAX: u16 = 65535`.

ItemEnum

An enum definition: `enum Foo<A, B> { A(A), B(B) }`.

ItemExternCrate

An `extern crate` item: `extern crate serde`.

ItemFn

A free-standing function: `fn process(n: usize) -> Result<()> { ... }`.

ItemForeignMod

A block of foreign items: `extern "C" { ... }`.

ItemImpl

An impl block providing trait or associated items: `impl<A> Trait for Data<A> { ... }`.

ItemMacro

A macro invocation, which includes `macro_rules!` definitions.

ItemMacro2

A 2.0-style declarative macro introduced by the `macro` keyword.

ItemMod

A module or module declaration: `mod m` or `mod m { ... }`.

ItemStatic

A static item: `static BIKE: Shed = Shed(42)`.

ItemStruct

A struct definition: `struct Foo<A> { x: A }`.

ItemTrait

A trait definition: `pub trait Iterator { ... }`.

ItemTraitAlias

A trait alias: `pub trait SharableIterator = Iterator + Sync`.

ItemType

A type alias: `type Result<T> = std::result::Result<T, MyError>`.

ItemUnion

A union definition: `union Foo<A, B> { x: A, y: B }`.

ItemUse

A use declaration: `use std::collections::HashMap`.

Label

A lifetime labeling a `for`, `while`, or `loop`.

Lifetime

A Rust lifetime: `'a`.

LifetimeDef

A lifetime definition: `'a: 'b + 'c + 'd`.

LitBool

A boolean literal: `true` or `false`.

LitByte

A byte literal: `b'f'`.

LitByteStr

A byte string literal: `b"foo"`.

LitChar

A character literal: `'a'`.

LitFloat

A floating point literal: `1f64` or `1.0e10f64`.

LitInt

An integer literal: `1` or `1u16`.

LitStr

A UTF-8 string literal: `"foo"`.

Local

A local `let` binding: `let x: u64 = s.parse()?`.

Macro

A macro invocation: `println!("{}", mac)`.

MetaList

A structured list within an attribute, like `derive(Copy, Clone)`.

MetaNameValue

A name-value pair within an attribute, like `feature = "nightly"`.

MethodTurbofish

The `::<>` explicit type parameters passed to a method call: `parse::<u64>()`.

ParenthesizedGenericArguments

Arguments of a function path segment: the `(A, B) -> C` in `Fn(A,B) -> C`.

PatBox

A box pattern: `box v`.

PatIdent

A pattern that binds a new variable: `ref mut binding @ SUBPATTERN`.

PatLit

A literal pattern: `0`.

PatMacro

A macro in pattern position.

PatOr

A pattern that matches any one of a set of cases.

PatPath

A path pattern like `Color::Red`, optionally qualified with a self-type.

PatRange

A range pattern: `1..=2`.

PatReference

A reference pattern: `&mut var`.

PatRest

The dots in a tuple or slice pattern: `[0, 1, ..]`

PatSlice

A dynamically sized slice pattern: `[a, b, ref i @ .., y, z]`.

PatStruct

A struct or struct variant pattern: `Variant { x, y, .. }`.

PatTuple

A tuple pattern: `(a, b)`.

PatTupleStruct

A tuple struct or tuple variant pattern: `Variant(x, y, .., z)`.

PatType

A type ascription pattern: `foo: f64`.

PatWild

A pattern that matches any value: `_`.

Path

A path at which a named item is exported (e.g. `std::collections::HashMap`).

PathSegment

A segment of a path together with any path arguments on that segment.

PredicateEq

An equality predicate in a `where` clause (unsupported).

PredicateLifetime

A lifetime predicate in a `where` clause: `'a: 'b + 'c`.

PredicateType

A type predicate in a `where` clause: `for<'c> Foo<'c>: Trait<'c>`.

QSelf

The explicit Self type in a qualified path: the `T` in `<T as Display>::fmt`.

Receiver

The `self` argument of an associated method, whether taken by value or by reference.

Signature

A function signature in a trait or implementation: `unsafe fn initialize(&self)`.

TraitBound

A trait used as a bound on a type parameter.

TraitItemConst

An associated constant within the definition of a trait.

TraitItemMacro

A macro invocation within the definition of a trait.

TraitItemMethod

A trait method within the definition of a trait.

TraitItemType

An associated type within the definition of a trait.

Turbofish

Returned by `TypeGenerics::as_turbofish`.

TypeArray

A fixed size array type: `[T; n]`.

TypeBareFn

A bare function type: `fn(usize) -> bool`.

TypeGenerics

Returned by `Generics::split_for_impl`.

TypeGroup

A type contained within invisible delimiters.

TypeImplTrait

An `impl Bound1 + Bound2 + Bound3` type where `Bound` is a trait or a lifetime.

TypeInfer

Indication that a type should be inferred by the compiler: `_`.

TypeMacro

A macro in the type position.

TypeNever

The never type: `!`.

TypeParam

A generic type parameter: `T: Into<String>`.

TypeParen

A parenthesized type equivalent to the inner type.

TypePath

A path like `std::slice::Iter`, optionally qualified with a self-type as in `<Vec<T> as SomeTrait>::Associated`.

TypePtr

A raw pointer type: `*const T` or `*mut T`.

TypeReference

A reference type: `&'a T` or `&'a mut T`.

TypeSlice

A dynamically sized slice type: `[T]`.

TypeTraitObject

A trait object type `dyn Bound1 + Bound2 + Bound3` where `Bound` is a trait or a lifetime.

TypeTuple

A tuple type: `(A, B, C, String)`.

UseGlob

A glob import in a `use` item: `*`.

UseGroup

A braced group of imports in a `use` item: `{A, B, C}`.

UseName

An identifier imported by a `use` item: `HashMap`.

UsePath

A path prefix of imports in a `use` item: `std::...`.

UseRename

An renamed identifier imported by a `use` item: `HashMap as Map`.

Variadic

The variadic argument of a foreign function.

Variant

An enum variant.

VisCrate

A crate-level visibility: `crate`.

VisPublic

A public visibility level: `pub`.

VisRestricted

A visibility level restricted to some path: `pub(self)` or `pub(super)` or `pub(crate)` or `pub(in some::module)`.

WhereClause

A `where` clause in a definition: `where T: Deserialize<'de>, D: 'static`.

## Enums

AttrStyle

Distinguishes between attributes that decorate an item and attributes that are contained within an item.

BinOp

A binary operator: `+`, `+=`, `&`.

Data

The storage of a struct, enum or union data structure.

Expr

A Rust expression.

Fields

Data stored within an enum variant or struct.

FnArg

An argument in a function signature: the `n: usize` in `fn f(n: usize)`.

ForeignItem

An item within an `extern` block.

GenericArgument

An individual generic argument, like `'a`, `T`, or `Item = T`.

GenericMethodArgument

An individual generic argument to a method, like `T`.

GenericParam

A generic type parameter, lifetime, or const generic: `T: Into<String>`, `'a: 'b`, `const LEN: usize`.

ImplItem

An item within an impl block.

Item

Things that can appear directly inside of a module or scope.

Lit

A Rust literal such as a string or integer or boolean.

MacroDelimiter

A grouping token that surrounds a macro body: `m!(...)` or `m!{...}` or `m![...]`.

Member

A struct or tuple struct field accessed in a struct literal or field expression.

Meta

Content of a compile-time structured attribute.

NestedMeta

Element of a compile-time attribute list.

Pat

A pattern in a local binding, function signature, match expression, or various other places.

PathArguments

Angle bracketed or parenthesized arguments of a path segment.

RangeLimits

Limit types of a range, inclusive or exclusive.

ReturnType

Return type of a function signature.

Stmt

A statement, usually ending in a semicolon.

StrStyle

The style of a string literal, either plain quoted or a raw string like `r##"data"##`.

TraitBoundModifier

A modifier on a trait bound, currently only used for the `?` in `?Sized`.

TraitItem

An item declaration within the definition of a trait.

Type

The possible types that a Rust value could have.

TypeParamBound

A trait or lifetime used as a bound on a type parameter.

UnOp

A unary operator: `*`, `!`, `-`.

UseTree

A suffix of an import tree in a `use` item: `Type as Renamed` or `*`.

Visibility

The visibility level of an item: inherited or `pub` or `pub(restricted)`.

WherePredicate

A single predicate in a `where` clause: `T: Deserialize<'de>`.

## Functions

parse

Parse tokens of source code into the chosen syntax tree node.

parse2

Parse a proc-macro2 token stream into the chosen syntax tree node.

parse_file

Parse the content of a file of Rust code.

parse_str

Parse a string of Rust code into the chosen syntax tree node.

## Type Definitions

AttributeArgs

Conventional argument type associated with an invocation of an attribute macro.

Result

The result of a Syn parser.