pin_project_internal/pin_project/
attribute.rs

1// SPDX-License-Identifier: Apache-2.0 OR MIT
2
3use proc_macro2::TokenStream;
4use quote::quote;
5use syn::{
6    parse::{Parse, ParseStream},
7    Attribute, Result, Token, Visibility,
8};
9
10use super::PIN;
11use crate::utils::SliceExt;
12
13// To generate the correct `Unpin` implementation and the projection methods,
14// we need to collect the types of the pinned fields.
15// However, since proc-macro-attribute is applied before `cfg` and `cfg_attr`
16// on fields, we cannot be collecting field types properly at this timing.
17// So instead of generating the `Unpin` implementation and the projection
18// methods here, delegate their processing to proc-macro-derive.
19//
20// At this stage, only attributes are parsed and the following attributes are
21// added to the attributes of the item.
22// - `#[derive(InternalDerive)]` - An internal helper macro that does the above
23//   processing.
24// - `#[pin(__private(#args))]` - Pass the argument of `#[pin_project]` to
25//   proc-macro-derive (`InternalDerive`).
26
27pub(super) fn parse_attribute(args: &TokenStream, input: TokenStream) -> Result<TokenStream> {
28    let Input { attrs, body } = syn::parse2(input)?;
29
30    Ok(quote! {
31        #(#attrs)*
32        #[derive(::pin_project::__private::__PinProjectInternalDerive)]
33        // Use `__private` to prevent users from trying to control `InternalDerive`
34        // manually. `__private` does not guarantee compatibility between patch
35        // versions, so it should be sufficient for this purpose in most cases.
36        #[pin(__private(#args))]
37        #body
38    })
39}
40
41struct Input {
42    attrs: Vec<Attribute>,
43    body: TokenStream,
44}
45
46impl Parse for Input {
47    fn parse(input: ParseStream<'_>) -> Result<Self> {
48        let attrs = input.call(Attribute::parse_outer)?;
49
50        let ahead = input.fork();
51        let _vis: Visibility = ahead.parse()?;
52        if !ahead.peek(Token![struct]) && !ahead.peek(Token![enum]) {
53            // If we check this only on proc-macro-derive, it may generate unhelpful error
54            // messages. So it is preferable to be able to detect it here.
55            bail!(
56                input.parse::<TokenStream>()?,
57                "#[pin_project] attribute may only be used on structs or enums"
58            );
59        } else if let Some(attr) = attrs.find(PIN) {
60            bail!(attr, "#[pin] attribute may only be used on fields of structs or variants");
61        } else if let Some(attr) = attrs.find("pin_project") {
62            bail!(attr, "duplicate #[pin_project] attribute");
63        }
64        Ok(Self { attrs, body: input.parse()? })
65    }
66}
pin_project_internal/pin_project/attribute.rs

pin_project_internal/pin_project/
attribute.rs
