[PATCH v2 09/20] rust: pin-init: internal: pin_data: project self-referential fields

[Gary Guo <gary@garyguo.net>]

This adds the projection for fields that are shared borrowed or that
borrows other fields but is covariant. Both cases allow a shared reference
to be accessed.

No mutable references can be created for these cases for different reasons:
* For fields that are shared borrowed, aliasing restriction prevents
  creation of mutable reference
* For fields that borrow other fields, their proper type contains field
  lifetimes. These lifetimes cannot be made available in the returned
  `project` struct (because there is no way to represent existential
  lifetime in return position). For covariant types, it is possible to
  shorten these lifetimes to that of `&self`; but doing so requires the
  reference to also be covariant over the pointee type, so we cannot give
  out `&mut` as it is invariant over the pointee.

Due to field-referencing fields being wrapped inside `Erase`, the normal
accessor syntax stop working; create accessor methods for these fields
instead.

Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/pin-init/internal/src/pin_data.rs | 109 ++++++++++++++++++++++++++++-----
 rust/pin-init/src/__internal.rs        |  12 ++++
 2 files changed, 107 insertions(+), 14 deletions(-)

diff --git a/rust/pin-init/internal/src/pin_data.rs b/rust/pin-init/internal/src/pin_data.rs
index 307d9b36078c..aa9a848c6670 100644
--- a/rust/pin-init/internal/src/pin_data.rs
+++ b/rust/pin-init/internal/src/pin_data.rs
@@ -12,7 +12,7 @@
     visit::Visit,
     visit_mut::VisitMut,
     Field, Fields, GenericParam, Generics, Ident, Index, Item, ItemStruct, Lifetime, LifetimeParam,
-    Member, PathSegment, Type, TypePath,
+    Member, PathSegment, Token, Type, TypePath,
 };
 
 use crate::{
@@ -849,7 +849,8 @@ fn generate_projections(info: &StructInfo) -> TokenStream {
         generics,
         ..
     } = &info.struct_;
-    let this_lt_generics: Generics = parse_quote!(<'__this>);
+    let this_lt = Lifetime::new("'__this", Span::mixed_site());
+    let this_lt_generics: Generics = parse_quote!(<#this_lt>);
     let generics_with_this_lt = CombinedGenerics(vec![&this_lt_generics, generics]);
 
     let (impl_generics, ty_generics, whr) = generics.split_for_impl();
@@ -860,33 +861,68 @@ fn generate_projections(info: &StructInfo) -> TokenStream {
     let (fields_decl, fields_proj): (Vec<_>, Vec<_>) = info
         .fields
         .iter()
-        .map(|field| {
-            let Field { vis, ty, .. } = &field.field;
-            let member = &field.member;
+        .map(|f| {
+            let vis = &f.field.vis;
+            let ident = f.member.as_ident();
+            let member = &f.member;
             // The projection of a tuple struct is a tuple struct itself, so its fields are
             // positional and must not be named.
-            let name = (!info.is_tuple_struct).then(|| {
-                let ident = field.member.as_ident();
-                quote!(#ident:)
-            });
+            let name = (!info.is_tuple_struct).then(|| quote!(#ident:));
+
+            // if `f.ty` contains field lifetimes, which we need to replace them with shorter
+            // `'__this` lifetime as field lifetimes are not available in this context.
+            let all_lifetimes: Vec<_> = f.captures.iter().map(|b| &b.lifetime).collect();
+            let ty = f
+                .field
+                .ty
+                .replace_lifetimes(&all_lifetimes, &vec![&this_lt; all_lifetimes.len()]);
+
+            // Fields sharedly borrowed by other fields can only be shared accessed. Fields that
+            // references other field and are covariant can also only be given shared reference
+            // as mutable reference is invariant.
+            let mut_token: Option<Token![mut]> = if f.borrowed.is_none() && f.captures.is_empty() {
+                Some(Default::default())
+            } else {
+                None
+            };
+
+            let mut accessor = quote!(&#mut_token #this.#member);
+            if !f.captures.is_empty() || f.borrowed.is_some() {
+                accessor = quote!(
+                    // SAFETY: we have `Erase<..>` which we know is layout compatible with `f.ty`.
+                    // Field lifetimes in `f.ty` can be shortened to `#ty` due to covariance.
+                    unsafe { ::core::mem::transmute::<_, &#mut_token #ty>(#accessor) }
+                )
+            }
 
-            if field.pinned {
+            if !f.captures.iter().all(|b| b.variance == Variance::Covariant) {
+                // If the type is not covariant, it must omitted, as projection shortens the
+                // lifetime to `'__this`.
                 (
                     quote!(
-                        #vis #name ::core::pin::Pin<&'__this mut #ty>,
+                        #vis #name ::pin_init::__internal::NotVisible<&'__this #mut_token #ty>,
+                    ),
+                    quote!(
+                        #name ::pin_init::__internal::NotVisible::new(),
+                    ),
+                )
+            } else if f.pinned {
+                (
+                    quote!(
+                        #vis #name ::core::pin::Pin<&'__this #mut_token #ty>,
                     ),
                     quote!(
                         // SAFETY: this field is structurally pinned.
-                        #name unsafe { ::core::pin::Pin::new_unchecked(&mut #this.#member) },
+                        #name unsafe { ::core::pin::Pin::new_unchecked(#accessor) },
                     ),
                 )
             } else {
                 (
                     quote!(
-                        #vis #name &'__this mut #ty,
+                        #vis #name &'__this #mut_token #ty,
                     ),
                     quote!(
-                        #name &mut #this.#member,
+                        #name #accessor,
                     ),
                 )
             }
@@ -936,6 +972,49 @@ fn generate_projections(info: &StructInfo) -> TokenStream {
             },
         )
     };
+
+    // For fields that references other fields, field access syntax stops working as they're wrapped
+    // behind `Erase` because their actual lifetime is not on the struct.
+    //
+    // Generate an accessor method for them.
+    let mut accessors = Vec::new();
+    for f in info.fields.iter() {
+        let ident = f.member.as_ident();
+        let member = &f.member;
+
+        if f.captures.is_empty() {
+            // They can be accessed normally, no accessor to be generated.
+            continue;
+        }
+
+        if f.captures.iter().all(|b| b.variance == Variance::Covariant) {
+            let f_doc = format!("Access the `{ident}` field on a shared reference of `Self`.");
+            let vis = &f.field.vis;
+
+            // Use the span of type for better error message.
+            let span = f.field.ty.span().resolved_at(Span::mixed_site());
+
+            let all_lifetimes: Vec<_> = f.captures.iter().map(|b| &b.lifetime).collect();
+            let ty = f
+                .field
+                .ty
+                .replace_lifetimes(&all_lifetimes, &vec![&this_lt; all_lifetimes.len()]);
+
+            accessors.push(quote_spanned!(span =>
+                #[doc = #f_doc]
+                #[inline]
+                #[allow(clippy::mut_from_ref)] // false positive when `&&mut` is returned.
+                #vis fn #ident<#this_lt>(&#this_lt self) -> &#this_lt #ty {
+                    // SAFETY: we have `Erased<..>` which we know is layout compatible with `f.ty`.
+                    // Field lifetimes in `f.ty` can be shortened to `#ty` due to covariance.
+                    unsafe { ::core::mem::transmute(&self.#member) }
+                }
+            ))
+        } else {
+            continue;
+        }
+    }
+
     quote! {
         #[doc = #docs]
         // Allow `non_snake_case` since the same warning will be emitted on
@@ -962,6 +1041,8 @@ impl #impl_generics #ident #ty_generics
                 let #this = unsafe { ::core::pin::Pin::get_unchecked_mut(self) };
                 #projection_init
             }
+
+            #(#accessors)*
         }
     }
 }
diff --git a/rust/pin-init/src/__internal.rs b/rust/pin-init/src/__internal.rs
index df079e68ec1e..ddd99e705c93 100644
--- a/rust/pin-init/src/__internal.rs
+++ b/rust/pin-init/src/__internal.rs
@@ -5,6 +5,8 @@
 //! These items must not be used outside of this crate and the pin-init-internal crate located at
 //! `../internal`.
 
+#![expect(clippy::new_without_default, reason = "private API")]
+
 use core::marker::PhantomPinned;
 use core::ops::Deref;
 
@@ -677,3 +679,13 @@ fn deref(&self) -> &T {
         &self.1
     }
 }
+
+/// An alias of `PhantomData` but with a name to aid user in case of misuse.
+pub struct NotVisible<T: ?Sized>(PhantomData<T>);
+
+impl<T: ?Sized> NotVisible<T> {
+    #[inline(always)]
+    pub fn new() -> Self {
+        Self(PhantomData)
+    }
+}

-- 
2.54.0