Crate syn[][src]

githubcrates-iodocs-rs


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.


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.

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

Syntax tree traversal to walk a shared borrow of a syntax tree.

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.

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 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.