Crate syn 

  

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 = "2.0"
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. 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 -Zunpretty=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

bufferparsing

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

ext

Extension traits to provide parsing methods on foreign types.

foldfold

Syntax tree traversal to transform the nodes of an owned syntax tree.

metaparsing and (derive or full)

Facility for interpreting structured content inside of an Attribute.

parseparsing

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

punctuated

A punctuated sequence of syntax tree nodes separated by punctuation.

spannedparsing and printing

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

token

Tokens representing Rust punctuation, keywords, and delimiters.

visitvisit

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

visit_mutvisit-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.

bracedparsing

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

bracketedparsing

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.

parenthesizedparsing

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

parse_macro_inputparsing and proc-macro

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

parse_quoteparsing and printing

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_spannedparsing and printing

This macro is parse_quote! + quote_spanned!.

Structs

Abiderive or full

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

AngleBracketedGenericArgumentsderive or full

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

Armfull

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

AssocConstderive or full

An equality constraint on an associated constant: the PANIC = false in Trait<PANIC = false>.

AssocTypederive or full

A binding (equality constraint) on an associated type: the Item = u8 in Iterator<Item = u8>.

Attributederive or full

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

BareFnArgderive or full

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

BareVariadicderive or full

The variadic argument of a function pointer like fn(usize, ...).

Blockfull

A braced block containing Rust statements.

BoundLifetimesderive or full

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

ConstParamderive or full

A const generic parameter: const LENGTH: usize.

Constraintderive or full

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

DataEnumderive

An enum input to a proc_macro_derive macro.

DataStructderive

A struct input to a proc_macro_derive macro.

DataUnionderive

An untagged union input to a proc_macro_derive macro.

DeriveInputderive

Data structure sent to a proc_macro_derive macro.

Error

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

ExprArrayfull

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

ExprAssignfull

An assignment expression: a = compute().

ExprAsyncfull

An async block: async { ... }.

ExprAwaitfull

An await expression: fut.await.

ExprBinaryderive or full

A binary operation: a + b, a += b.

ExprBlockfull

A blocked scope: { ... }.

ExprBreakfull

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

ExprCallderive or full

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

ExprCastderive or full

A cast expression: foo as f64.

ExprClosurefull

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

ExprConstfull

A const block: const { ... }.

ExprContinuefull

A continue, with an optional label.

ExprFieldderive or full

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

ExprForLoopfull

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

ExprGroupfull

An expression contained within invisible delimiters.

ExprIffull

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

ExprIndexderive or full

A square bracketed indexing expression: vector[2].

ExprInferfull

The inferred value of a const generic argument, denoted _.

ExprLetfull

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

ExprLitderive or full

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

ExprLoopfull

Conditionless loop: loop { ... }.

ExprMacroderive or full

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

ExprMatchfull

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

ExprMethodCallderive or full

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

ExprParenderive or full

A parenthesized expression: (a + b).

ExprPathderive or full

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

ExprRangefull

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

ExprRawAddrfull

Address-of operation: &raw const place or &raw mut place.

ExprReferencederive or full

A referencing operation: &a or &mut a.

ExprRepeatfull

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

ExprReturnfull

A return, with an optional value to be returned.

ExprStructderive or full

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

ExprTryfull

A try-expression: expr?.

ExprTryBlockfull

A try block: try { ... }.

ExprTuplefull

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

ExprUnaryderive or full

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

ExprUnsafefull

An unsafe block: unsafe { ... }.

ExprWhilefull

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

ExprYieldfull

A yield expression: yield expr.

Fieldderive or full

A field of a struct or enum variant.

FieldPatfull

A single field in a struct pattern.

FieldValuederive or full

A field-value pair in a struct literal.

FieldsNamedderive or full

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

FieldsUnnamedderive or full

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

Filefull

A complete file of Rust source code.

ForeignItemFnfull

A foreign function in an extern block.

ForeignItemMacrofull

A macro invocation within an extern block.

ForeignItemStaticfull

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

ForeignItemTypefull

A foreign type in an extern block: type void.

Genericsderive or full

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.

ImplGenericsprinting and (derive or full)

Returned by Generics::split_for_impl.

ImplItemConstfull

An associated constant within an impl block.

ImplItemFnfull

An associated function within an impl block.

ImplItemMacrofull

A macro invocation within an impl block.

ImplItemTypefull

An associated type within an impl block.

Indexderive or full

The index of an unnamed tuple struct field.

ItemConstfull

A constant item: const MAX: u16 = 65535.

ItemEnumfull

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

ItemExternCratefull

An extern crate item: extern crate serde.

ItemFnfull

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

ItemForeignModfull

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

ItemImplfull

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

ItemMacrofull

A macro invocation, which includes macro_rules! definitions.

ItemModfull

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

ItemStaticfull

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

ItemStructfull

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

ItemTraitfull

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

ItemTraitAliasfull

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

ItemTypefull

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

ItemUnionfull

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

ItemUsefull

A use declaration: use alloc::collections::HashMap.

Labelfull

A lifetime labeling a for, while, or loop.

Lifetime

A Rust lifetime: 'a.

LifetimeParamderive or full

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".

LitCStr

A nul-terminated C-string literal: c"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".

Localfull

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

LocalInitfull

The expression assigned in a local let binding, including optional diverging else block.

Macroderive or full

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

MetaListderive or full

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

MetaNameValuederive or full

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

ParenthesizedGenericArgumentsderive or full

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

PatConstfull

A const block: const { ... }.

PatIdentfull

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

PatLitfull

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

PatMacrofull

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

PatOrfull

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

PatParenfull

A parenthesized pattern: (A | B).

PatPathfull

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

PatRangefull

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

PatReferencefull

A reference pattern: &mut var.

PatRestfull

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

PatSlicefull

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

PatStructfull

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

PatTuplefull

A tuple pattern: (a, b).

PatTupleStructfull

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

PatTypefull

A type ascription pattern: foo: f64.

PatWildfull

A pattern that matches any value: _.

Pathderive or full

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

PathSegmentderive or full

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

PreciseCapturefull

Precise capturing bound: the ‘use<…>’ in impl Trait + use<'a, T>.

PredicateLifetimederive or full

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

PredicateTypederive or full

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

QSelfderive or full

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

Receiverfull

The self argument of an associated method.

Signaturefull

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

StmtMacrofull

A macro invocation in statement position.

TraitBoundderive or full

A trait used as a bound on a type parameter.

TraitItemConstfull

An associated constant within the definition of a trait.

TraitItemFnfull

An associated function within the definition of a trait.

TraitItemMacrofull

A macro invocation within the definition of a trait.

TraitItemTypefull

An associated type within the definition of a trait.

Turbofishprinting and (derive or full)

Returned by TypeGenerics::as_turbofish.

TypeArrayderive or full

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

TypeBareFnderive or full

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

TypeGenericsprinting and (derive or full)

Returned by Generics::split_for_impl.

TypeGroupderive or full

A type contained within invisible delimiters.

TypeImplTraitderive or full

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

TypeInferderive or full

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

TypeMacroderive or full

A macro in the type position.

TypeNeverderive or full

The never type: !.

TypeParamderive or full

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

TypeParenderive or full

A parenthesized type equivalent to the inner type.

TypePathderive or full

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

TypePtrderive or full

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

TypeReferencederive or full

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

TypeSlicederive or full

A dynamically sized slice type: [T].

TypeTraitObjectderive or full

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

TypeTuplederive or full

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

UseGlobfull

A glob import in a use item: *.

UseGroupfull

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

UseNamefull

An identifier imported by a use item: HashMap.

UsePathfull

A path prefix of imports in a use item: core::....

UseRenamefull

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

Variadicfull

The variadic argument of a foreign function.

Variantderive or full

An enum variant.

VisRestrictedderive or full

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

WhereClausederive or full

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

Enums

AttrStylederive or full

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

BinOpderive or full

A binary operator: +, +=, &.

CapturedParamfull

Single parameter in a precise capturing bound.

Dataderive

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

Exprderive or full

A Rust expression.

FieldMutabilityderive or full

Unused, but reserved for RFC 3323 restrictions.

Fieldsderive or full

Data stored within an enum variant or struct.

FnArgfull

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

ForeignItemfull

An item within an extern block.

GenericArgumentderive or full

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

GenericParamderive or full

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

ImplItemfull

An item within an impl block.

ImplRestrictionfull

Unused, but reserved for RFC 3323 restrictions.

Itemfull

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

Lit

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

MacroDelimiterderive or full

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

Memberderive or full

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

Metaderive or full

Content of a compile-time structured attribute.

Patfull

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

PathArgumentsderive or full

Angle bracketed or parenthesized arguments of a path segment.

PointerMutabilityfull

Mutability of a raw pointer (*const T, *mut T), in which non-mutable isn’t the implicit default.

RangeLimitsfull

Limit types of a range, inclusive or exclusive.

ReturnTypederive or full

Return type of a function signature.

StaticMutabilityfull

The mutability of an Item::Static or ForeignItem::Static.

Stmtfull

A statement, usually ending in a semicolon.

TraitBoundModifierderive or full

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

TraitItemfull

An item declaration within the definition of a trait.

Typederive or full

The possible types that a Rust value could have.

TypeParamBoundderive or full

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

UnOpderive or full

A unary operator: *, !, -.

UseTreefull

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

Visibilityderive or full

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

WherePredicatederive or full

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

Functions

parseparsing and proc-macro

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

parse2parsing

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

parse_filefull and parsing

Parse the content of a file of Rust code.

parse_strparsing

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

Type Aliases

Result

The result of a Syn parser.