Derive Macro StructMeta

#[derive(StructMeta)]
{
    // Attributes available to this derive:
    #[struct_meta]
}

Derive syn::parse::Parse for parsing attribute arguments.

• Example
• Named parameter
  • Supported field types for named parameter
  • Flag style
  • NameValue style
  • NameArgs style
  • NameArgs or Flag style
  • NameArgList style
  • NameArgList or Flag style
  • Optional named parameter
  • Rest named parameter
• Unnamed parameter
  • Required unnamed parameter
  • Optional unnamed parameter
  • Variadic unnamed parameter
• Parameter order
• Helper attribute #[struct_meta(...)]
• Uses with #[proc_macro_derive]
• Uses with #[proc_macro_attribute]
• Parsing ambiguous arguments
  • #[struct_meta(name_filter = "...")]

Example

use structmeta::StructMeta;
use syn::{parse_quote, Attribute};
use syn::{LitInt, LitStr};

#[derive(StructMeta)]
struct Args {
    #[struct_meta(unnamed)]
    a: LitStr,
    b: LitInt,
    c: Option<LitInt>,
}

let attr: Attribute = parse_quote!(#[attr("xyz", b = 10)]);
let args: Args = attr.parse_args()?;
assert_eq!(args.a.value(), "xyz");
assert_eq!(args.b.base10_parse::<u32>()?, 10);
assert!(args.c.is_none());

Named parameter

The following field will be “Named parameter”.

• field in record struct.
• field with #[struct_meta(name = "...")] in tuple struct.
• However, fields that meet the following conditions are excluded
  • field with #[struct_meta(unnamed)]
  • field with type HashMap<String, _>

“Named parameter” is a parameter that specifies with a name, such as #[attr(flag, x = 10, y(1, 2, 3))].

Supported field types for named parameter

“Named parameter” has the following four styles, and the style is determined by the type of the field.

• Flag style : name
• NameValue style : name = value
• NameArgs style : name(args)
• NameArgList style : name(arg, arg, ...)

field type	field type (with span)	style	example
bool	Flag	Flag	name
T	NameValue<T>	NameValue	name = value
	NameArgs<T>	NameArgs	name(args)
	NameArgs<Option<T>>	NameArgs or Flag	name(args) or name
Vec<T>	NameArgs<Vec<T>>	NameArgList	name(arg, arg, ...)
	NameArgs<Option<Vec<T>>>	NameArgList or Flag	name(arg, arg, ...) or name

Note: the type T in the table above needs to implement syn::parse::Parse.

With the above type as P (bool and Flag are excluded), you can also use the following types.

field type	effect
Option<P>	optional parameter
HashMap<String, P>	rest parameter

Flag style

A field with the type bool will be a parameter that specifies only its name.

use structmeta::StructMeta;
use syn::{parse_quote, Attribute};

#[derive(StructMeta)]
struct Args {
    a: bool,
    b: bool,
}

let attr: Attribute = parse_quote!(#[attr(a)]);
let args: Args = attr.parse_args()?;
assert_eq!(args.a, true);
assert_eq!(args.b, false);

let attr: Attribute = parse_quote!(#[attr(a, b)]);
let args: Args = attr.parse_args()?;
assert_eq!(args.a, true);
assert_eq!(args.b, true);

If you use Flag instead of bool, you will get its Span when the argument is specified.

use structmeta::{Flag, StructMeta};
use syn::{parse_quote, Attribute};

#[derive(StructMeta)]
struct Args {
    a: Flag,
}

let attr: Attribute = parse_quote!(#[attr(a)]);
let args: Args = attr.parse_args()?;
if let Some(_span) = args.a.span {
    // Use span.
}

NameValue style

A field with type T or NameValue<T> will be name = value style parameter.

use structmeta::{NameValue, StructMeta};
use syn::{parse_quote, Attribute, LitInt, LitStr};

#[derive(StructMeta)]
struct Args {
    a: LitStr,
    b: NameValue<LitInt>,
}

let attr: Attribute = parse_quote!(#[attr(a = "abc", b = 10)]);
let args: Args = attr.parse_args()?;
assert_eq!(args.a.value(), "abc");
assert_eq!(args.b.value.base10_parse::<u32>()?, 10);

NameArgs style

A field with type NameArgs<T> will be name(args) style parameter.

use structmeta::{NameArgs, StructMeta};
use syn::{parse_quote, Attribute, LitInt, LitStr};

#[derive(StructMeta)]
struct Args {
    a: NameArgs<LitStr>,
    b: NameArgs<LitInt>,
}

let attr: Attribute = parse_quote!(#[attr(a("abc"), b(10))]);
let args: Args = attr.parse_args()?;
assert_eq!(args.a.args.value(), "abc");
assert_eq!(args.b.args.base10_parse::<u32>()?, 10);

NameArgs or Flag style

A field with type NameArgs<T> will be name(args) or name style parameter.

use structmeta::{NameArgs, StructMeta};
use syn::{parse_quote, Attribute, LitInt, LitStr};

#[derive(StructMeta)]
struct Args {
    a: NameArgs<Option<LitStr>>,
    b: NameArgs<Option<LitInt>>,
}

let attr: Attribute = parse_quote!(#[attr(a, b(10))]);
let args: Args = attr.parse_args()?;
assert!(args.a.args.is_none());
assert_eq!(args.b.args.unwrap().base10_parse::<u32>()?, 10);

NameArgList style

A field with type NameArgs<Vec<T>> will be name(arg, arg, ...) style parameter.

use structmeta::{NameArgs, StructMeta};
use syn::{parse_quote, Attribute, LitStr};

#[derive(StructMeta)]
struct Args {
    a: NameArgs<Vec<LitStr>>,
}

let attr: Attribute = parse_quote!(#[attr(a())]);
let args: Args = attr.parse_args()?;
assert_eq!(args.a.args.len(), 0);

let attr: Attribute = parse_quote!(#[attr(a("x"))]);
let args: Args = attr.parse_args()?;
assert_eq!(args.a.args.len(), 1);

let attr: Attribute = parse_quote!(#[attr(a("x", "y"))]);
let args: Args = attr.parse_args()?;
assert_eq!(args.a.args.len(), 2);

NameArgList or Flag style

A field with type NameArgs<Option<Vec<T>>> will be name(arg, arg, ...) or name style parameter.

use structmeta::{NameArgs, StructMeta};
use syn::{parse_quote, Attribute, LitStr};

#[derive(StructMeta)]
struct Args {
    abc: NameArgs<Option<Vec<LitStr>>>,
}

let attr: Attribute = parse_quote!(#[attr(abc)]);
let args: Args = attr.parse_args()?;
assert_eq!(args.abc.args.is_none(), true);

let attr: Attribute = parse_quote!(#[attr(abc())]);
let args: Args = attr.parse_args()?;
assert_eq!(args.abc.args.unwrap().len(), 0);

let attr: Attribute = parse_quote!(#[attr(abc("x"))]);
let args: Args = attr.parse_args()?;
assert_eq!(args.abc.args.unwrap().len(), 1);

let attr: Attribute = parse_quote!(#[attr(abc("x", "y"))]);
let args: Args = attr.parse_args()?;
assert_eq!(args.abc.args.unwrap().len(), 2);

Optional named parameter

If you use Option for the field type, it becomes an optional parameter.

use structmeta::{NameValue, StructMeta};
use syn::{parse_quote, Attribute, LitInt, LitStr};

#[derive(StructMeta)]
struct Args {
    a: Option<LitStr>,
    b: Option<NameValue<LitInt>>,
}

let attr: Attribute = parse_quote!(#[attr(a = "abc")]);
let args: Args = attr.parse_args()?;
assert!(args.a.is_some());
assert!(args.b.is_none());

let attr: Attribute = parse_quote!(#[attr(b = 10)]);
let args: Args = attr.parse_args()?;
assert!(args.a.is_none());
assert!(args.b.is_some());

Rest named parameter

If HashMap<String, _> is used for the field type, the field will contain named arguments that are not associated with the field.

use std::collections::HashMap;
use structmeta::StructMeta;
use syn::{parse_quote, Attribute, LitInt};

#[derive(StructMeta)]
struct Args {
    a: Option<LitInt>,
    rest: HashMap<String, LitInt>,
}

let attr: Attribute = parse_quote!(#[attr(a = 10, b = 20, c = 30)]);
let args: Args = attr.parse_args()?;
assert!(args.a.is_some());
let mut keys: Vec<_> = args.rest.keys().collect();
keys.sort();
assert_eq!(keys, vec!["b", "c"]);

Unnamed parameter

The following field will be “Unnamed parameter”.

• field in tuple struct.
• field with #[struct_meta(unnamed)] in record struct.
• However, fields that meet the following conditions are excluded
  • field with #[struct_meta(name = "...")]
  • field with type HashMap<String, _>

“Unnamed parameter” is a value-only parameter, such as #[attr("abc", 10, 20)].

field type	effect
T	required parameter
Option<T>	optional parameter
Vec<T>	variadic parameter

The type T in the table above needs to implement syn::parse::Parse.

Required unnamed parameter

Fields of the type that implement syn::parse::Parse will be required parameters.

use structmeta::StructMeta;
use syn::{parse_quote, Attribute, LitStr, Result};

#[derive(StructMeta)]
struct Args(LitStr);

let attr: Attribute = parse_quote!(#[attr()]);
let args: Result<Args> = attr.parse_args();
assert!(args.is_err());

let attr: Attribute = parse_quote!(#[attr("a")]);
let args: Args = attr.parse_args()?;
assert_eq!(args.0.value(), "a");

Optional unnamed parameter

Fields of type Option will be optional parameters.

use structmeta::StructMeta;
use syn::{parse_quote, Attribute, LitStr};

#[derive(StructMeta)]
struct Args(Option<LitStr>);

let attr: Attribute = parse_quote!(#[attr()]);
let args: Args = attr.parse_args()?;
assert!(args.0.is_none());

let attr: Attribute = parse_quote!(#[attr("a")]);
let args: Args = attr.parse_args()?;
assert_eq!(args.0.unwrap().value(), "a");

Variadic unnamed parameter

If you use Vec as the field type, multiple arguments can be stored in a single field.

use structmeta::StructMeta;
use syn::{parse_quote, Attribute, LitStr};

#[derive(StructMeta)]
struct Args(Vec<LitStr>);

let attr: Attribute = parse_quote!(#[attr()]);
let args: Args = attr.parse_args()?;
assert_eq!(args.0.len(), 0);

let attr: Attribute = parse_quote!(#[attr("a")]);
let args: Args = attr.parse_args()?;
assert_eq!(args.0.len(), 1);
assert_eq!(args.0[0].value(), "a");

let attr: Attribute = parse_quote!(#[attr("a", "b")]);
let args: Args = attr.parse_args()?;
assert_eq!(args.0.len(), 2);
assert_eq!(args.0[0].value(), "a");
assert_eq!(args.0[1].value(), "b");

Parameter order

The parameters must be in the following order.

• Unnamed
  • Required
  • Optional
  • Variadic
• Named

Helper attribute #[struct_meta(...)]

argument	struct	field	effect
dump	✔		Causes a compile error and outputs the automatically generated code as an error message.
name_filter = "..."	✔		Specify how to distinguish between a parameter name and a value of an unnamed parameter.
name = "..."		✔	Specify a parameter name.
unnamed		✔	Make the field be treated as an unnamed parameter.

Uses with #[proc_macro_derive]

A type with #[derive(StructMeta)] can be used with syn::Attribute::parse_args.

use proc_macro::TokenStream;
use quote::quote;
use structmeta::StructMeta;
use syn::{parse, parse_macro_input, DeriveInput, LitStr};

#[derive(StructMeta)]
struct MyAttr {
    msg: LitStr,
}

#[proc_macro_derive(MyMsg, attributes(my_msg))]
pub fn derive_my_msg(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);
    let mut msg = String::new();
    for attr in input.attrs {
        if attr.path.is_ident("my_msg") {
            let attr = attr.parse_args::<MyAttr>().unwrap();
            msg = attr.msg.value();
        }
    }
    quote!(const MSG: &str = #msg;).into()
}

#[derive(MyMsg)]
#[my_msg(msg = "abc")]
struct TestType;

assert_eq!(MSG, "abc");

Uses with #[proc_macro_attribute]

A type with #[derive(StructMeta)] can be used with attr parameter in attribute proc macro.

use proc_macro::TokenStream;
use quote::quote;
use structmeta::StructMeta;
use syn::{parse, parse_macro_input, DeriveInput, LitStr};

#[derive(StructMeta)]
struct MyAttr {
    msg: LitStr,
}
#[proc_macro_attribute]
pub fn my_attr(attr: TokenStream, _item: TokenStream) -> TokenStream {
    let attr = parse::<MyAttr>(attr).unwrap();
    let msg = attr.msg.value();
    quote!(const MSG: &str = #msg;).into()
}

#[my_attr(msg = "xyz")]
struct TestType;

assert_eq!(MSG, "xyz");

Parsing ambiguous arguments

If one or more name = value style parameters are defined, arguments beginning with name = will be parsed as name = value style, even if the name is different from what it is defined as.

This specification is intended to prevent name = value from being treated as unnamed parameter due to typo.

use structmeta::StructMeta;
use syn::{parse_quote, Attribute, Expr, LitInt, Result};

#[derive(StructMeta)]
struct WithNamed {
    #[struct_meta(unnamed)]
    unnamed: Option<Expr>,
    x: Option<LitInt>,
}

let attr_x: Attribute = parse_quote!(#[attr(x = 10)]);
let result: WithNamed = attr_x.parse_args().unwrap();
assert_eq!(result.unnamed.is_some(), false);
assert_eq!(result.x.is_some(), true);

// `y = 10` is parsed as a wrong named parameter.
let attr_y: Attribute = parse_quote!(#[attr(y = 10)]);
let result: Result<WithNamed> = attr_y.parse_args();
assert!(result.is_err());

If name = value style parameter is not defined, it will be parsed as unnamed parameter.

use structmeta::StructMeta;
use syn::{parse_quote, Attribute, Expr, LitInt, Result};

#[derive(StructMeta)]
struct WithoutNamed {
    #[struct_meta(unnamed)]
    unnamed: Option<Expr>,
}

// `y = 10` is parsed as a unnamed parameter.
let attr_y: Attribute = parse_quote!(#[attr(y = 10)]);
let result: WithoutNamed = attr_y.parse_args().unwrap();
assert_eq!(result.unnamed, Some(parse_quote!(y = 10)));

Similarly, if one or more name(args) style parameters are defined, arguments with name(args) will be parsed as name(args) style. If name(args) style parameter is not defined, it will be parsed as unnamed parameter.

The same is true for name or name = value style parameter.

#[struct_meta(name_filter = "...")]

By attaching #[struct_meta(name_filter = "...")] to struct definition, you can restrict the names that can be used as parameter names and treat other identifiers as a value of unnamed parameter.

The following value can be used.

• "snake_case"

use structmeta::StructMeta;
use syn::{parse_quote, Attribute, Expr, LitInt, Result};

let attr_x: Attribute = parse_quote!(#[attr(X)]);
let attr_y: Attribute = parse_quote!(#[attr(Y)]);

#[derive(StructMeta)]
struct NoFilter {
    #[struct_meta(unnamed)]
    unnamed: Option<Expr>,
    #[struct_meta(name = "X")]
    x: bool,
}
let result: NoFilter = attr_x.parse_args().unwrap();
assert_eq!(result.unnamed, None);
assert_eq!(result.x, true); // `X` is parsed as a named parameter.

let result: Result<NoFilter> = attr_y.parse_args();
assert!(result.is_err()); // `Y` is parsed as a wrong named parameter.


#[derive(StructMeta)]
#[struct_meta(name_filter = "snake_case")]
struct WithFilter {
    #[struct_meta(unnamed)]
    unnamed: Option<Expr>,
    #[struct_meta(name = "X")]
    x: bool,
}
let result: WithFilter = attr_x.parse_args().unwrap();
assert_eq!(result.unnamed, Some(parse_quote!(X))); // `X` is parsed as a unnamed parameter.
assert_eq!(result.x, false);