rust/pin-init/internal/src/pin_data.rs

// SPDX-License-Identifier: Apache-2.0 OR MIT

use proc_macro2::TokenStream;
use quote::{format_ident, quote, ToTokens};
use syn::{
    parse::{End, Nothing, Parse},
    parse_quote, parse_quote_spanned,
    spanned::Spanned,
    visit_mut::VisitMut,
    Field, Fields, Generics, Ident, Item, PathSegment, Type, TypePath, Visibility, WhereClause,
};

use crate::diagnostics::{DiagCtxt, ErrorGuaranteed};

pub(crate) mod kw {
    syn::custom_keyword!(PinnedDrop);
}

pub(crate) enum Args {
    Nothing(Nothing),
    #[allow(dead_code)]
    PinnedDrop(kw::PinnedDrop),
}

impl Parse for Args {
    fn parse(input: syn::parse::ParseStream<'_>) -> syn::Result<Self> {
        let lh = input.lookahead1();
        if lh.peek(End) {
            input.parse().map(Self::Nothing)
        } else if lh.peek(kw::PinnedDrop) {
            input.parse().map(Self::PinnedDrop)
        } else {
            Err(lh.error())
        }
    }
}

impl ToTokens for Args {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        match self {
            Self::Nothing(_) => (),
            Self::PinnedDrop(kw) => kw.to_tokens(tokens),
        }
    }
}

struct FieldInfo<'a> {
    field: &'a Field,
    pinned: bool,
}

pub(crate) fn pin_data(
    args: Args,
    input: Item,
    dcx: &mut DiagCtxt,
) -> Result<TokenStream, ErrorGuaranteed> {
    let mut struct_ = match input {
        Item::Struct(struct_) => struct_,
        Item::Enum(enum_) => {
            return Err(dcx.error(
                enum_.enum_token,
                "`#[pin_data]` only supports structs for now",
            ));
        }
        Item::Union(union) => {
            return Err(dcx.error(
                union.union_token,
                "`#[pin_data]` only supports structs for now",
            ));
        }
        rest => {
            return Err(dcx.error(
                rest,
                "`#[pin_data]` can only be applied to struct, enum and union definitions",
            ));
        }
    };

    // Handling cfg can gets very complicated, especially for tuple structs. Therefore, resolve all
    // field cfgs first before continuing.
    //
    // We need to perform this after parsing so we can reliably detect field cfgs.
    for (field_idx, field) in struct_.fields.iter_mut().enumerate() {
        let cfg: Vec<_> = field
            .attrs
            .iter()
            .filter(|a| a.path().is_ident("cfg"))
            .map(|a| {
                a.parse_args::<TokenStream>()
                    .expect("parse as token stream cannot fail")
            })
            .collect();

        if cfg.is_empty() {
            continue;
        }

        field.attrs.retain(|a| !a.path().is_ident("cfg"));
        let cfg_true_struct = quote!(#struct_);

        let punctuated = match &mut struct_.fields {
            Fields::Named(fields) => &mut fields.named,
            Fields::Unnamed(fields) => &mut fields.unnamed,
            Fields::Unit => unreachable!(),
        };
        *punctuated = std::mem::take(punctuated)
            .into_pairs()
            .enumerate()
            .filter(|&(i, _)| i != field_idx)
            .map(|(_, p)| p)
            .collect();
        let cfg_false_struct = quote!(#struct_);

        // Resolve one field at a time until we've got no more field cfgs.
        //
        // This is linear time because macro invocations with false cfg will not be expanded.
        return Ok(quote!(
            #[cfg(all(#(#cfg,)*))]
            #[::pin_init::pin_data(#args)]
            #cfg_true_struct

            #[cfg(not(all(#(#cfg,)*)))]
            #[::pin_init::pin_data(#args)]
            #cfg_false_struct
        ));
    }

    // The generics might contain the `Self` type. Since this macro will define a new type with the
    // same generics and bounds, this poses a problem: `Self` will refer to the new type as opposed
    // to this struct definition. Therefore we have to replace `Self` with the concrete name.
    let mut replacer = {
        let name = &struct_.ident;
        let (_, ty_generics, _) = struct_.generics.split_for_impl();
        SelfReplacer(parse_quote!(#name #ty_generics))
    };
    replacer.visit_generics_mut(&mut struct_.generics);
    replacer.visit_fields_mut(&mut struct_.fields);

    let fields: Vec<FieldInfo<'_>> = struct_
        .fields
        .iter_mut()
        .map(|field| {
            let len = field.attrs.len();
            field.attrs.retain(|a| !a.path().is_ident("pin"));
            let pinned_count = len - field.attrs.len();
            if pinned_count > 1 {
                dcx.error(&field, "#[pin] attribute specified more than once");
            }

            assert!(
                !field.attrs.iter().any(|a| a.path().is_ident("cfg")),
                "cfgs should be all resolved at this point"
            );

            FieldInfo {
                field: &*field,
                pinned: pinned_count != 0,
            }
        })
        .collect();

    for field in &fields {
        let ident = field.field.ident.as_ref().unwrap();

        if !field.pinned && is_phantom_pinned(&field.field.ty) {
            dcx.warn(
                field.field,
                format!(
                    "The field `{ident}` of type `PhantomPinned` only has an effect \
                    if it has the `#[pin]` attribute",
                ),
            );
        }
    }

    let unpin_impl = generate_unpin_impl(&struct_.ident, &struct_.generics, &fields);
    let drop_impl = generate_drop_impl(&struct_.ident, &struct_.generics, args);
    let projections =
        generate_projections(&struct_.vis, &struct_.ident, &struct_.generics, &fields);
    let the_pin_data =
        generate_the_pin_data(&struct_.vis, &struct_.ident, &struct_.generics, &fields);

    Ok(quote! {
        #struct_
        #projections
        // We put the rest into this const item, because it then will not be accessible to anything
        // outside.
        const _: () = {
            #the_pin_data
            #unpin_impl
            #drop_impl
        };
    })
}

fn is_phantom_pinned(ty: &Type) -> bool {
    match ty {
        Type::Path(TypePath { qself: None, path }) => {
            // Cannot possibly refer to `PhantomPinned` (except alias, but that's on the user).
            if path.segments.len() > 3 {
                return false;
            }
            // If there is a `::`, then the path needs to be `::core::marker::PhantomPinned` or
            // `::std::marker::PhantomPinned`.
            if path.leading_colon.is_some() && path.segments.len() != 3 {
                return false;
            }
            let expected: Vec<&[&str]> = vec![&["PhantomPinned"], &["marker"], &["core", "std"]];
            for (actual, expected) in path.segments.iter().rev().zip(expected) {
                if !actual.arguments.is_empty() || expected.iter().all(|e| actual.ident != e) {
                    return false;
                }
            }
            true
        }
        _ => false,
    }
}

fn generate_unpin_impl(
    ident: &Ident,
    generics: &Generics,
    fields: &[FieldInfo<'_>],
) -> TokenStream {
    let (_, ty_generics, _) = generics.split_for_impl();
    let mut generics_with_pin_lt = generics.clone();
    generics_with_pin_lt.params.insert(0, parse_quote!('__pin));
    generics_with_pin_lt.make_where_clause();
    let (
        impl_generics_with_pin_lt,
        ty_generics_with_pin_lt,
        Some(WhereClause {
            where_token,
            predicates,
        }),
    ) = generics_with_pin_lt.split_for_impl()
    else {
        unreachable!()
    };
    let pinned_fields = fields.iter().filter(|f| f.pinned).map(|f| {
        let ident = f.field.ident.as_ref().unwrap();
        let ty = &f.field.ty;
        quote!(
            #ident: #ty
        )
    });
    quote! {
        // This struct will be used for the unpin analysis. It is needed, because only structurally
        // pinned fields are relevant whether the struct should implement `Unpin`.
        #[allow(
            dead_code, // The fields below are never used.
            non_snake_case // The warning will be emitted on the struct definition.
        )]
        struct __Unpin #generics_with_pin_lt
        #where_token
            #predicates
        {
            __phantom_pin: ::pin_init::__internal::PhantomInvariantLifetime<'__pin>,
            __phantom: ::pin_init::__internal::PhantomInvariant<#ident #ty_generics>,
            #(#pinned_fields),*
        }

        #[doc(hidden)]
        impl #impl_generics_with_pin_lt ::core::marker::Unpin for #ident #ty_generics
        #where_token
            __Unpin #ty_generics_with_pin_lt: ::core::marker::Unpin,
            #predicates
        {}
    }
}

fn generate_drop_impl(ident: &Ident, generics: &Generics, args: Args) -> TokenStream {
    let (impl_generics, ty_generics, whr) = generics.split_for_impl();
    let has_pinned_drop = matches!(args, Args::PinnedDrop(_));
    // We need to disallow normal `Drop` implementation, the exact behavior depends on whether
    // `PinnedDrop` was specified in `args`.
    if has_pinned_drop {
        // When `PinnedDrop` was specified we just implement `Drop` and delegate.
        quote! {
            impl #impl_generics ::core::ops::Drop for #ident #ty_generics
                #whr
            {
                fn drop(&mut self) {
                    // SAFETY: Since this is a destructor, `self` will not move after this function
                    // terminates, since it is inaccessible.
                    let pinned = unsafe { ::core::pin::Pin::new_unchecked(self) };
                    // SAFETY: Since this is a drop function, we can create this token to call the
                    // pinned destructor of this type.
                    let token = unsafe { ::pin_init::__internal::OnlyCallFromDrop::new() };
                    ::pin_init::PinnedDrop::drop(pinned, token);
                }
            }
        }
    } else {
        // When no `PinnedDrop` was specified, then we have to prevent implementing drop.
        quote! {
            // We prevent this by creating a trait that will be implemented for all types implementing
            // `Drop`. Additionally we will implement this trait for the struct leading to a conflict,
            // if it also implements `Drop`
            trait MustNotImplDrop {}
            impl<T: ::core::ops::Drop + ?::core::marker::Sized> MustNotImplDrop for T {}
            impl #impl_generics MustNotImplDrop for #ident #ty_generics
                #whr
            {}
            // We also take care to prevent users from writing a useless `PinnedDrop` implementation.
            // They might implement `PinnedDrop` correctly for the struct, but forget to give
            // `PinnedDrop` as the parameter to `#[pin_data]`.
            trait UselessPinnedDropImpl_you_need_to_specify_PinnedDrop {}
            impl<T: ::pin_init::PinnedDrop + ?::core::marker::Sized>
                UselessPinnedDropImpl_you_need_to_specify_PinnedDrop for T {}
            impl #impl_generics
                UselessPinnedDropImpl_you_need_to_specify_PinnedDrop for #ident #ty_generics
                #whr
            {}
        }
    }
}

fn generate_projections(
    vis: &Visibility,
    ident: &Ident,
    generics: &Generics,
    fields: &[FieldInfo<'_>],
) -> TokenStream {
    let (impl_generics, ty_generics, _) = generics.split_for_impl();
    let mut generics_with_pin_lt = generics.clone();
    generics_with_pin_lt.params.insert(0, parse_quote!('__pin));
    let (_, ty_generics_with_pin_lt, whr) = generics_with_pin_lt.split_for_impl();
    let projection = format_ident!("{ident}Projection");
    let this = format_ident!("this");

    let (fields_decl, fields_proj): (Vec<_>, Vec<_>) = fields
        .iter()
        .map(|field| {
            let Field { vis, ident, ty, .. } = &field.field;

            let ident = ident
                .as_ref()
                .expect("only structs with named fields are supported");
            if field.pinned {
                (
                    quote!(
                        #vis #ident: ::core::pin::Pin<&'__pin mut #ty>,
                    ),
                    quote!(
                        // SAFETY: this field is structurally pinned.
                        #ident: unsafe { ::core::pin::Pin::new_unchecked(&mut #this.#ident) },
                    ),
                )
            } else {
                (
                    quote!(
                        #vis #ident: &'__pin mut #ty,
                    ),
                    quote!(
                        #ident: &mut #this.#ident,
                    ),
                )
            }
        })
        .collect();
    let structurally_pinned_fields_docs = fields
        .iter()
        .filter(|f| f.pinned)
        .map(|f| format!(" - `{}`", f.field.ident.as_ref().unwrap()));
    let not_structurally_pinned_fields_docs = fields
        .iter()
        .filter(|f| !f.pinned)
        .map(|f| format!(" - `{}`", f.field.ident.as_ref().unwrap()));
    let docs = format!(" Pin-projections of [`{ident}`]");
    quote! {
        #[doc = #docs]
        // Allow `non_snake_case` since the same warning will be emitted on
        // the struct definition.
        #[allow(dead_code, non_snake_case)]
        #[doc(hidden)]
        #vis struct #projection #generics_with_pin_lt
            #whr
        {
            #(#fields_decl)*
            ___pin_phantom_data: ::core::marker::PhantomData<&'__pin mut ()>,
        }

        impl #impl_generics #ident #ty_generics
            #whr
        {
            /// Pin-projects all fields of `Self`.
            ///
            /// These fields are structurally pinned:
            #(#[doc = #structurally_pinned_fields_docs])*
            ///
            /// These fields are **not** structurally pinned:
            #(#[doc = #not_structurally_pinned_fields_docs])*
            #[inline]
            #vis fn project<'__pin>(
                self: ::core::pin::Pin<&'__pin mut Self>,
            ) -> #projection #ty_generics_with_pin_lt {
                // SAFETY: we only give access to `&mut` for fields not structurally pinned.
                let #this = unsafe { ::core::pin::Pin::get_unchecked_mut(self) };
                #projection {
                    #(#fields_proj)*
                    ___pin_phantom_data: ::core::marker::PhantomData,
                }
            }
        }
    }
}

fn generate_the_pin_data(
    vis: &Visibility,
    struct_name: &Ident,
    generics: &Generics,
    fields: &[FieldInfo<'_>],
) -> TokenStream {
    let (impl_generics, ty_generics, whr) = generics.split_for_impl();

    // For every field, we create an initializing projection function according to its projection
    // type. If a field is structurally pinned, we create a `Slot` with `Pinned` which must be
    // initialized via `PinInit`; if it is not structurally pinned, then we create a `Slot` with
    // `Unpinned` which allows initialization via `Init`.
    let field_accessors = fields
        .iter()
        .map(|f| {
            let Field { vis, ident, ty, .. } = f.field;

            let field_name = ident
                .as_ref()
                .expect("only structs with named fields are supported");
            let pin_marker = if f.pinned {
                quote!(Pinned)
            } else {
                quote!(Unpinned)
            };
            quote! {
                /// # Safety
                ///
                /// - `slot` is valid and properly aligned.
                /// - `(*slot).#field_name` is properly aligned.
                /// - `(*slot).#field_name` points to uninitialized and exclusively accessed
                ///   memory.
                // Allow `non_snake_case` since the same warning will be emitted on
                // the struct definition.
                #[allow(non_snake_case)]
                #[inline(always)]
                #vis unsafe fn #field_name(
                    self,
                    slot: *mut #struct_name #ty_generics,
                ) -> ::pin_init::__internal::Slot<::pin_init::__internal::#pin_marker, #ty> {
                    // SAFETY:
                    // - If `#pin_marker` is `Pinned`, the corresponding field is structurally
                    //   pinned.
                    // - Other safety requirements follows the safety requirement.
                    unsafe { ::pin_init::__internal::Slot::new(&raw mut (*slot).#field_name) }
                }
            }
        })
        .collect::<TokenStream>();
    quote! {
        // We declare this struct which will host all of the projection function for our type. It
        // will be invariant over all generic parameters which are inherited from the struct.
        #[doc(hidden)]
        #vis struct __ThePinData #generics
            #whr
        {
            __phantom: ::pin_init::__internal::PhantomInvariant<#struct_name #ty_generics>,
        }

        impl #impl_generics ::core::clone::Clone for __ThePinData #ty_generics
            #whr
        {
            #[inline]
            fn clone(&self) -> Self { *self }
        }

        impl #impl_generics ::core::marker::Copy for __ThePinData #ty_generics
            #whr
        {}

        #[allow(dead_code)] // Some functions might never be used and private.
        impl #impl_generics __ThePinData #ty_generics
            #whr
        {
            /// Type inference helper function.
            #[inline(always)]
            #vis fn __make_closure<__F, __E>(self, f: __F) -> __F
            where
                __F: FnOnce(*mut #struct_name #ty_generics) ->
                    ::core::result::Result<::pin_init::__internal::InitOk, __E>,
            {
                f
            }

            #field_accessors
        }

        // SAFETY: We have added the correct projection functions above to `__ThePinData` and
        // we also use the least restrictive generics possible.
        unsafe impl #impl_generics ::pin_init::__internal::HasPinData for #struct_name #ty_generics
            #whr
        {
            type PinData = __ThePinData #ty_generics;

            #[inline]
            unsafe fn __pin_data() -> Self::PinData {
                __ThePinData { __phantom: ::pin_init::__internal::PhantomInvariant::new() }
            }
        }
    }
}

struct SelfReplacer(PathSegment);

impl VisitMut for SelfReplacer {
    fn visit_path_mut(&mut self, i: &mut syn::Path) {
        if i.is_ident("Self") {
            let span = i.span();
            let seg = &self.0;
            *i = parse_quote_spanned!(span=> #seg);
        } else {
            syn::visit_mut::visit_path_mut(self, i);
        }
    }

    fn visit_path_segment_mut(&mut self, seg: &mut PathSegment) {
        if seg.ident == "Self" {
            let span = seg.span();
            let this = &self.0;
            *seg = parse_quote_spanned!(span=> #this);
        } else {
            syn::visit_mut::visit_path_segment_mut(self, seg);
        }
    }

    fn visit_item_mut(&mut self, _: &mut Item) {
        // Do not descend into items, since items reset/change what `Self` refers to.
    }
}