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use proc_macro2::TokenStream;
use quote::quote;
use syn::{
parse::{Parse, ParseStream},
Attribute, Result, Token, Visibility,
};
use super::PIN;
use crate::utils::SliceExt;
// To generate the correct `Unpin` implementation and the projection methods,
// we need to collect the types of the pinned fields.
// However, since proc-macro-attribute is applied before `cfg` and `cfg_attr`
// on fields, we cannot be collecting field types properly at this timing.
// So instead of generating the `Unpin` implementation and the projection
// methods here, delegate their processing to proc-macro-derive.
//
// At this stage, only attributes are parsed and the following attributes are
// added to the attributes of the item.
// - `#[derive(InternalDerive)]` - An internal helper macro that does the above
// processing.
// - `#[pin(__private(#args))]` - Pass the argument of `#[pin_project]` to
// proc-macro-derive (`InternalDerive`).
pub(super) fn parse_attribute(args: &TokenStream, input: TokenStream) -> Result<TokenStream> {
let Input { attrs, body } = syn::parse2(input)?;
Ok(quote! {
#(#attrs)*
#[derive(::pin_project::__private::__PinProjectInternalDerive)]
// Use `__private` to prevent users from trying to control `InternalDerive`
// manually. `__private` does not guarantee compatibility between patch
// versions, so it should be sufficient for this purpose in most cases.
#[pin(__private(#args))]
#body
})
}
#[allow(dead_code)] // false positive that fixed in Rust 1.39
struct Input {
attrs: Vec<Attribute>,
body: TokenStream,
}
impl Parse for Input {
fn parse(input: ParseStream<'_>) -> Result<Self> {
let attrs = input.call(Attribute::parse_outer)?;
let ahead = input.fork();
let _vis: Visibility = ahead.parse()?;
if !ahead.peek(Token![struct]) && !ahead.peek(Token![enum]) {
// If we check this only on proc-macro-derive, it may generate unhelpful error
// messages. So it is preferable to be able to detect it here.
bail!(
input.parse::<TokenStream>()?,
"#[pin_project] attribute may only be used on structs or enums"
);
} else if let Some(attr) = attrs.find(PIN) {
bail!(attr, "#[pin] attribute may only be used on fields of structs or variants");
} else if let Some(attr) = attrs.find("pin_project") {
bail!(attr, "duplicate #[pin_project] attribute");
}
Ok(Self { attrs, body: input.parse()? })
}
}