Crate derive_syn_parse[−][src]

Derive macro for syn::parse::Parse

A common pattern when writing custom syn parsers is repeating <name>: input.parse()? for each field in the output. #[derive(Parse)] handles that for you, with some extra helpful customization.

Usage

Using this crate is as simple as adding it to your ‘Cargo.toml’ and importing the derive macro:

# Cargo.toml

[dependencies]
derive-syn-parse = "0.1"

// your_file.rs
use derive_syn_parse::Parse;

#[derive(Parse)]
struct CustomParsable {
    // ...
}

The derived implementation of Parse always parses in the order that the fields are given. Note that deriving Parse is also available on enums. For more information, see the dedicated section.

This crate is intended for users who are already making heavy use of syn.

Motivation

When writing rust code that makes heavy use of syn’s parsing functionality, we often end up writing things like:

use syn::parse::{Parse, ParseStream};
use syn::{Ident, Token, Type};

// A simplified struct field
//
//     x: i32
struct MyField {
    ident: Ident,
    colon_token: Token![:],
    ty: Type,
}

impl Parse for MyField {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        Ok(MyField {
            ident: input.parse()?,
            colon_token: input.parse()?,
            ty: input.parse()?,
        })
    }
}

This is really repetitive! Ideally, we’d like to just #[derive(Parse)] and have it work. And so we can! (for the most part) Adding #[derive(Parse)] to the previous struct produces an equivalent implementation of Parse:

use syn::{Ident, Token, Type};
use derive_syn_parse::Parse;

#[derive(Parse)]
struct MyField {
    ident: Ident,
    colon_token: Token![:],
    ty: Type,
}

Of course, there are more complicated cases. This is mainly covered below in the ‘Advanced usage’ section.

Enum parsing

Parsing enums is a complex feature. When writing manual implementations of Parse, it doesn’t come up as often, but there are also typically many ways to do it: syn provides both forking the ParseBuffer and peeking in order to handle this, with peeking to be preferred if possible.

This library does not support forking; it tends to suffer from poor error messages and general inefficiency. That being said, manual implementations of Parse can and should be written when this library is insufficient.

We do support peeking - in two different ways. These are handled by the #[peek] and #[peek_with] attributes, which are required on- and only available for enum variants. The general syntax can be thought of as:

#[peek($TYPE, name = $NAME)]

and

#[peek_with($EXPR, name = $NAME)]

where $TYPE, $EXPR, and $NAME are meta-variables that correspond to the particular input given to the attribute.

These can be thought of as translating literally to:

if input.peek($TYPE) {
    // parse variant
} else {
    // parse other variants
}

and

if ($EXPR)(input) {
    // parse variant
} else {
    // parse other variants
}

respectively. If no variant matches, we produce an error message, using the names that were provided for each type.

Advanced usage

There’s a moderate collection of helper attributes that can be applied to fields to customize the generated implementation of Parse. Each of these are demonstrated with the implementation that they produce. Please note that the produced implementation is typically not identical to what’s shown here.

All of the examples are fairly contrived, I know. The reality of the matter is that - if you would find this useful - it’s probably true that your use-case is much more complicated than would make sense for a short example. (If it isn’t, let me know! It would be great to include it here!)

`#[paren]` / `#[bracket]` / `#[brace]`

Because the derive macro has no fool-proof method for determining by itself whether a field type is any of syn::token::{Paren, Bracket, Brace}, these three serve to provide that information instead.

These are typically used in conjunction with #[inside].


// A single-argument function call
//
//     so_long(and_thanks + for_all * the_fish)
#[derive(Parse)]
struct SingleArgFn {
    ident: Ident,
    #[paren]
    paren_token: Paren,
    #[inside(paren_token)]
    arg: Expr,
}

produces


impl Parse for SingleArgFn {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let paren;
        Ok(SingleArgFn {
            ident: input.parse()?,
            paren_token: syn::parenthesized!(paren in input),
            arg: paren.parse()?,
        })
    }
}

`#[inside(..)]`

This is a companion to #[paren]/#[bracket]/#[brace] - given a field name to use, this attribute indicates that the field should be parsed using a previous field as the source.

use syn::{Type, Token, Expr};
use syn::token::Bracket;

// An array type required to have a length
//
//     [i32; 4]
#[derive(Parse)]
struct KnownLengthArrayType {
    #[bracket]
    bracket_token: Bracket,

    // Note that `#[inside(..)]` must be applied to all of the fields that
    // are in the brackets!
    #[inside(bracket_token)]
    ty: Type,
    #[inside(bracket_token)]
    semi_token: Token![;],
    #[inside(bracket_token)]
    expr: Expr,
}

produces


impl Parse for KnownLengthArrayType {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let bracket;
        Ok(KnownLengthArrayType {
            bracket_token: syn::bracketed!(bracket in input),
            ty: bracket.parse()?,
            semi_token: bracket.parse()?,
            expr: bracket.parse()?,
        })
    }
}

Conditional parsing

When implementing Parse for structs, it is occasionally the case that certain fields are optional - or should only be parsed under certain circumstances. There are attributes for that!

Say we want to represent enums with the following, different syntax:

enum Foo {
    Bar: Baz,
    Qux,
}

where the equivalent Rust code would be:

enum Foo {
    Bar(Baz),
    Qux,
}

There’s two ways we could parse the variants here – either with a colon (and following type), or without! To handle this, we can write:

#[derive(Parse)]
struct Variant {
    name: Ident,
    // `syn` already supports optional parsing of simple tokens
    colon: Option<Token![:]>,
    // We only want to parse the trailing type if there's a colon:
    #[parse_if(colon.is_some())]
    ty: Option<Type>,
}

Note that in this case, ty must be an Option. In addition to conditional parsing based on the values of what’s already been parsed, we can also peek - just as described above in the section on parsing enums. The only difference here is that we do not need to provide a name for the optional field. We could have equally implemented the above as:

#[derive(Parse)]
struct Variant {
    name: Ident,
    #[peek(Token![:])]
    ty: Option<VariantType>,
}

#[derive(Parse)]
struct VariantType {
    colon: Token![:],
    ty: Type,
}

`#[call(..)]`

Given a path to a function, this attribute specifies that the value of the field should be instead calculated by a call to input.parse(..) with a given path. The best example is taken straight from the syn documentation itself:

use syn::{Attribute, Ident, Token};

// Parses a unit struct with attributes.
//
//     #[path = "s.tmpl"]
//     struct S;
#[derive(Parse)]
struct UnitStruct {
    #[call(Attribute::parse_outer)]
    attrs: Vec<Attribute>,
    struct_token: Token![struct],
    name: Ident,
    semi_token: Token![;],
}

produces


impl Parse for UnitStruct {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        Ok(UnitStruct {
            attrs: input.call(Attribute::parse_outer)?,
            struct_token: input.parse()?,
            name: input.parse()?,
            semi_token: input.parse()?,
        })
    }
}

`#[parse_terminated(..)]`

Just as we have #[call(..)] for ParseStream::call, we have #[parse_terminated] for ParseStream::parse_terminated. Here’s the same example that the ParseStream method uses:


// Parse a simplified tuple struct syntax like:
//
//     struct S(A, B);
#[derive(Parse)]
struct TupleStruct {
    struct_token: Token![struct],
    ident: Ident,
    #[paren]
    paren_token: token::Paren,
    #[inside(paren_token)]
    #[parse_terminated(Type::parse)]
    fields: Punctuated<Type, Token![,]>,
    semi_token: Token![;],
}

produces


impl Parse for TupleStruct {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let content;
        Ok(TupleStruct {
            struct_token: input.parse()?,
            ident: input.parse()?,
            paren_token: parenthesized!(content in input),
            fields: content.parse_terminated(Type::parse)?,
            semi_token: input.parse()?,
        })
    }
}

Derive Macros

Parse