pin_project_internal/
pinned_drop.rs

1// SPDX-License-Identifier: Apache-2.0 OR MIT
2
3use proc_macro2::TokenStream;
4use quote::{format_ident, quote, ToTokens};
5use syn::{
6    parse_quote, spanned::Spanned, token::Colon, visit_mut::VisitMut, Error, FnArg,
7    GenericArgument, Ident, ImplItem, ItemImpl, Pat, PatIdent, PatType, Path, PathArguments,
8    Result, ReturnType, Signature, Token, Type, TypePath, TypeReference,
9};
10
11use crate::utils::{ReplaceReceiver, SliceExt};
12
13pub(crate) fn attribute(args: &TokenStream, mut input: ItemImpl) -> TokenStream {
14    let res = (|| -> Result<()> {
15        if !args.is_empty() {
16            bail!(args, "unexpected argument: `{}`", args)
17        }
18        validate_impl(&input)?;
19        expand_impl(&mut input);
20        Ok(())
21    })();
22
23    if let Err(e) = res {
24        let mut tokens = e.to_compile_error();
25        if let Type::Path(self_ty) = &*input.self_ty {
26            let (impl_generics, _, where_clause) = input.generics.split_for_impl();
27
28            // Generate a dummy impl of `PinnedDrop`.
29            // In many cases, `#[pinned_drop] impl` is declared after `#[pin_project]`.
30            // Therefore, if `pinned_drop` compile fails, you will also get an error
31            // about `PinnedDrop` not being implemented.
32            // This can be prevented to some extent by generating a dummy
33            // `PinnedDrop` implementation.
34            // We already know that we will get a compile error, so this won't
35            // accidentally compile successfully.
36            //
37            // However, if `input.self_ty` is not Type::Path, there is a high possibility that
38            // the type does not exist (since #[pin_project] can only be used on struct/enum
39            // definitions), so do not generate a dummy impl.
40            tokens.extend(quote! {
41                impl #impl_generics ::pin_project::__private::PinnedDrop for #self_ty
42                #where_clause
43                {
44                    unsafe fn drop(self: ::pin_project::__private::Pin<&mut Self>) {}
45                }
46            });
47        }
48        tokens
49    } else {
50        input.attrs.push(parse_quote!(#[allow(unused_qualifications)])); // https://github.com/rust-lang/rust/issues/122519
51        input.into_token_stream()
52    }
53}
54
55/// Validates the signature of given `PinnedDrop` impl.
56fn validate_impl(item: &ItemImpl) -> Result<()> {
57    const INVALID_ITEM: &str =
58        "#[pinned_drop] may only be used on implementation for the `PinnedDrop` trait";
59
60    if let Some(attr) = item.attrs.find("pinned_drop") {
61        bail!(attr, "duplicate #[pinned_drop] attribute");
62    }
63
64    if let Some((_, path, _)) = &item.trait_ {
65        if !path.is_ident("PinnedDrop") {
66            bail!(path, INVALID_ITEM);
67        }
68    } else {
69        bail!(item.self_ty, INVALID_ITEM);
70    }
71
72    if item.unsafety.is_some() {
73        bail!(item.unsafety, "implementing the trait `PinnedDrop` is not unsafe");
74    }
75    if item.items.is_empty() {
76        bail!(item, "not all trait items implemented, missing: `drop`");
77    }
78
79    match &*item.self_ty {
80        Type::Path(_) => {}
81        ty => {
82            bail!(ty, "implementing the trait `PinnedDrop` on this type is unsupported");
83        }
84    }
85
86    item.items.iter().enumerate().try_for_each(|(i, item)| match item {
87        ImplItem::Const(item) => {
88            bail!(item, "const `{}` is not a member of trait `PinnedDrop`", item.ident)
89        }
90        ImplItem::Type(item) => {
91            bail!(item, "type `{}` is not a member of trait `PinnedDrop`", item.ident)
92        }
93        ImplItem::Fn(method) => {
94            validate_sig(&method.sig)?;
95            if i == 0 {
96                Ok(())
97            } else {
98                bail!(method, "duplicate definitions with name `drop`")
99            }
100        }
101        _ => unreachable!("unexpected ImplItem"),
102    })
103}
104
105/// Validates the signature of given `PinnedDrop::drop` method.
106///
107/// The correct signature is: `(mut) self: (<path>::)Pin<&mut Self>`
108fn validate_sig(sig: &Signature) -> Result<()> {
109    fn get_ty_path(ty: &Type) -> Option<&Path> {
110        if let Type::Path(TypePath { qself: None, path }) = ty {
111            Some(path)
112        } else {
113            None
114        }
115    }
116
117    const INVALID_ARGUMENT: &str = "method `drop` must take an argument `self: Pin<&mut Self>`";
118
119    if sig.ident != "drop" {
120        bail!(sig.ident, "method `{}` is not a member of trait `PinnedDrop", sig.ident);
121    }
122
123    if let ReturnType::Type(_, ty) = &sig.output {
124        match &**ty {
125            Type::Tuple(ty) if ty.elems.is_empty() => {}
126            _ => bail!(ty, "method `drop` must return the unit type"),
127        }
128    }
129
130    match sig.inputs.len() {
131        1 => {}
132        0 => return Err(Error::new(sig.paren_token.span.join(), INVALID_ARGUMENT)),
133        _ => bail!(sig.inputs, INVALID_ARGUMENT),
134    }
135
136    if let Some(arg) = sig.receiver() {
137        // (mut) self: <path>
138        if let Some(path) = get_ty_path(&arg.ty) {
139            let ty =
140                path.segments.last().expect("Type paths should always have at least one segment");
141            if let PathArguments::AngleBracketed(args) = &ty.arguments {
142                // (mut) self: (<path>::)<ty><&mut <elem>..>
143                if let Some(GenericArgument::Type(Type::Reference(TypeReference {
144                    mutability: Some(_),
145                    elem,
146                    ..
147                }))) = args.args.first()
148                {
149                    // (mut) self: (<path>::)Pin<&mut Self>
150                    if args.args.len() == 1
151                        && ty.ident == "Pin"
152                        && get_ty_path(elem).map_or(false, |path| path.is_ident("Self"))
153                    {
154                        if sig.unsafety.is_some() {
155                            bail!(sig.unsafety, "implementing the method `drop` is not unsafe");
156                        }
157                        return Ok(());
158                    }
159                }
160            }
161        }
162    }
163
164    bail!(sig.inputs[0], INVALID_ARGUMENT)
165}
166
167// from:
168//
169// fn drop(self: Pin<&mut Self>) {
170//     // ...
171// }
172//
173// into:
174//
175// unsafe fn drop(self: Pin<&mut Self>) {
176//     fn __drop_inner<T>(__self: Pin<&mut Foo<'_, T>>) {
177//         fn __drop_inner() {}
178//         // ...
179//     }
180//     __drop_inner(self);
181// }
182//
183fn expand_impl(item: &mut ItemImpl) {
184    // `PinnedDrop` is a private trait and should not appear in docs.
185    item.attrs.push(parse_quote!(#[doc(hidden)]));
186
187    let path = &mut item.trait_.as_mut().expect("unexpected inherent impl").1;
188    *path = parse_quote_spanned! { path.span() =>
189        ::pin_project::__private::PinnedDrop
190    };
191
192    let method =
193        if let ImplItem::Fn(method) = &mut item.items[0] { method } else { unreachable!() };
194
195    // `fn drop(mut self: Pin<&mut Self>)` -> `fn __drop_inner<T>(mut __self: Pin<&mut Receiver>)`
196    let drop_inner = {
197        let mut drop_inner = method.clone();
198        let ident = format_ident!("__drop_inner");
199        // Add a dummy `__drop_inner` function to prevent users call outer `__drop_inner`.
200        drop_inner.block.stmts.insert(0, parse_quote!(fn #ident() {}));
201        drop_inner.sig.ident = ident;
202        drop_inner.sig.generics = item.generics.clone();
203        let receiver = drop_inner.sig.receiver().expect("drop() should have a receiver").clone();
204        let pat = Box::new(Pat::Ident(PatIdent {
205            attrs: vec![],
206            by_ref: None,
207            mutability: receiver.mutability,
208            ident: Ident::new("__self", receiver.self_token.span()),
209            subpat: None,
210        }));
211        drop_inner.sig.inputs[0] = FnArg::Typed(PatType {
212            attrs: receiver.attrs,
213            pat,
214            colon_token: Colon::default(),
215            ty: receiver.ty,
216        });
217        let self_ty = if let Type::Path(ty) = &*item.self_ty { ty } else { unreachable!() };
218        let mut visitor = ReplaceReceiver(self_ty);
219        visitor.visit_signature_mut(&mut drop_inner.sig);
220        visitor.visit_block_mut(&mut drop_inner.block);
221        drop_inner
222    };
223
224    // `fn drop(mut self: Pin<&mut Self>)` -> `unsafe fn drop(self: Pin<&mut Self>)`
225    method.sig.unsafety = Some(<Token![unsafe]>::default());
226    let self_token = if let FnArg::Receiver(ref mut rec) = method.sig.inputs[0] {
227        rec.mutability = None;
228        &rec.self_token
229    } else {
230        panic!("drop() should have a receiver")
231    };
232
233    method.block.stmts = parse_quote! {
234        #[allow(clippy::needless_pass_by_value)] // This lint does not warn the receiver.
235        #drop_inner
236        __drop_inner(#self_token);
237    };
238}
pin_project_internal/pinned_drop.rs

pin_project_internal/
pinned_drop.rs
