[PATCH 2/4] rust: pin-init: internal: pin_data: support tuple struct projections

[Gary Guo <gary@garyguo.net>]

From: Mohamad Alsadhan <mo@sdhn.cc>

`#[pin_data]` rejects tuple structs because it assumes every field has a
name, which it uses for the projection field, the `__Unpin` field and the
pin-data accessor.

Identify fields by `syn::Member` instead, so that tuple fields are referred
to by their index in generated field accesses. The names that generated
items still need are derived from the index as `_0`, `_1`, etc.

The projection of a tuple struct is a tuple struct itself, so projected
fields are accessed with the same `.0`, `.1` syntax as on the input
type rather than through synthesised names.

Signed-off-by: Mohamad Alsadhan <mo@sdhn.cc>
[ Moved utility code to util.rs as extension trait - Gary ]
Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/pin-init/internal/src/pin_data.rs | 112 +++++++++++++++++++++++----------
 rust/pin-init/internal/src/util.rs     |  31 ++++++++-
 rust/pin-init/src/lib.rs               |  23 +++++++
 3 files changed, 130 insertions(+), 36 deletions(-)

diff --git a/rust/pin-init/internal/src/pin_data.rs b/rust/pin-init/internal/src/pin_data.rs
index 074bc6b3091a..8cd9bf139567 100644
--- a/rust/pin-init/internal/src/pin_data.rs
+++ b/rust/pin-init/internal/src/pin_data.rs
@@ -7,7 +7,8 @@
     parse_quote, parse_quote_spanned,
     spanned::Spanned,
     visit_mut::VisitMut,
-    Field, Fields, Generics, Ident, Item, PathSegment, Type, TypePath, Visibility, WhereClause,
+    Field, Fields, Generics, Ident, Index, Item, Member, PathSegment, Type, TypePath, Visibility,
+    WhereClause,
 };
 
 use crate::{
@@ -49,6 +50,7 @@ fn to_tokens(&self, tokens: &mut TokenStream) {
 
 struct FieldInfo<'a> {
     field: &'a Field,
+    member: Member,
     pinned: bool,
 }
 
@@ -129,10 +131,12 @@ pub(crate) fn pin_data(
     replacer.visit_generics_mut(&mut struct_.generics);
     replacer.visit_fields_mut(&mut struct_.fields);
 
+    let is_tuple_struct = matches!(struct_.fields, Fields::Unnamed(_));
     let fields: Vec<FieldInfo<'_>> = struct_
         .fields
         .iter_mut()
-        .map(|field| {
+        .enumerate()
+        .map(|(index, field)| {
             let len = field.attrs.len();
             field.attrs.retain(|a| !a.path().is_ident("pin"));
             let pinned_count = len - field.attrs.len();
@@ -144,23 +148,30 @@ pub(crate) fn pin_data(
                 !field.attrs.iter().any(|a| a.path().is_ident("cfg")),
                 "cfgs should be all resolved at this point"
             );
+            let member = match &field.ident {
+                Some(ident) => Member::Named(ident.clone()),
+                None => Member::Unnamed(Index {
+                    index: index as u32,
+                    span: field.span(),
+                }),
+            };
 
             FieldInfo {
                 field: &*field,
+                member,
                 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 \
+                    "The field {} of type `PhantomPinned` only has an effect \
                     if it has the `#[pin]` attribute",
+                    field.member.display_name(),
                 ),
             );
         }
@@ -168,8 +179,13 @@ pub(crate) fn pin_data(
 
     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 projections = generate_projections(
+        &struct_.vis,
+        &struct_.ident,
+        &struct_.generics,
+        is_tuple_struct,
+        &fields,
+    );
     let the_pin_data =
         generate_the_pin_data(&struct_.vis, &struct_.ident, &struct_.generics, &fields);
 
@@ -231,7 +247,7 @@ fn generate_unpin_impl(
         unreachable!()
     };
     let pinned_fields = fields.iter().filter(|f| f.pinned).map(|f| {
-        let ident = f.field.ident.as_ref().unwrap();
+        let ident = f.member.as_ident();
         let ty = &f.field.ty;
         quote!(
             #ident: #ty
@@ -313,6 +329,7 @@ fn generate_projections(
     vis: &Visibility,
     ident: &Ident,
     generics: &Generics,
+    is_tuple_struct: bool,
     fields: &[FieldInfo<'_>],
 ) -> TokenStream {
     let (impl_generics, ty_generics, _) = generics.split_for_impl();
@@ -325,28 +342,32 @@ fn generate_projections(
     let (fields_decl, fields_proj): (Vec<_>, Vec<_>) = fields
         .iter()
         .map(|field| {
-            let Field { vis, ident, ty, .. } = &field.field;
+            let Field { vis, ty, .. } = &field.field;
+            let member = &field.member;
+            // The projection of a tuple struct is a tuple struct itself, so its fields are
+            // positional and must not be named.
+            let name = (!is_tuple_struct).then(|| {
+                let ident = field.member.as_ident();
+                quote!(#ident:)
+            });
 
-            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>,
+                        #vis #name ::core::pin::Pin<&'__pin mut #ty>,
                     ),
                     quote!(
                         // SAFETY: this field is structurally pinned.
-                        #ident: unsafe { ::core::pin::Pin::new_unchecked(&mut #this.#ident) },
+                        #name unsafe { ::core::pin::Pin::new_unchecked(&mut #this.#member) },
                     ),
                 )
             } else {
                 (
                     quote!(
-                        #vis #ident: &'__pin mut #ty,
+                        #vis #name &'__pin mut #ty,
                     ),
                     quote!(
-                        #ident: &mut #this.#ident,
+                        #name &mut #this.#member,
                     ),
                 )
             }
@@ -355,24 +376,52 @@ fn generate_projections(
     let structurally_pinned_fields_docs = fields
         .iter()
         .filter(|f| f.pinned)
-        .map(|f| format!(" - `{}`", f.field.ident.as_ref().unwrap()));
+        .map(|f| format!(" - {}", f.member.display_name()));
     let not_structurally_pinned_fields_docs = fields
         .iter()
         .filter(|f| !f.pinned)
-        .map(|f| format!(" - `{}`", f.field.ident.as_ref().unwrap()));
+        .map(|f| format!(" - {}", f.member.display_name()));
     let docs = format!(" Pin-projections of [`{ident}`]");
+    let (projection_def, projection_init) = if is_tuple_struct {
+        (
+            quote! {
+                #vis struct #projection #generics_with_pin_lt (
+                    #(#fields_decl)*
+                    ::core::marker::PhantomData<&'__pin mut ()>,
+                ) #whr;
+            },
+            quote! {
+                #projection(
+                    #(#fields_proj)*
+                    ::core::marker::PhantomData,
+                )
+            },
+        )
+    } else {
+        (
+            quote! {
+                #vis struct #projection #generics_with_pin_lt
+                    #whr
+                {
+                    #(#fields_decl)*
+                    ___pin_phantom_data: ::core::marker::PhantomData<&'__pin mut ()>,
+                }
+            },
+            quote! {
+                #projection {
+                    #(#fields_proj)*
+                    ___pin_phantom_data: ::core::marker::PhantomData,
+                }
+            },
+        )
+    };
     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 ()>,
-        }
+        #projection_def
 
         impl #impl_generics #ident #ty_generics
             #whr
@@ -390,10 +439,7 @@ impl #impl_generics #ident #ty_generics
             ) -> #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,
-                }
+                #projection_init
             }
         }
     }
@@ -414,11 +460,9 @@ fn generate_the_pin_data(
     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 Field { vis, ty, .. } = f.field;
+            let field_name = f.member.as_ident();
+            let member = &f.member;
             let pin_marker = if f.pinned {
                 quote!(Pinned)
             } else {
@@ -443,7 +487,7 @@ fn generate_the_pin_data(
                     // - 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) }
+                    unsafe { ::pin_init::__internal::Slot::new(&raw mut (*slot).#member) }
                 }
             }
         })
diff --git a/rust/pin-init/internal/src/util.rs b/rust/pin-init/internal/src/util.rs
index ed18ab7d45e6..ed2c78f0658f 100644
--- a/rust/pin-init/internal/src/util.rs
+++ b/rust/pin-init/internal/src/util.rs
@@ -1,7 +1,8 @@
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
-use proc_macro2::TokenStream;
-use syn::Attribute;
+use proc_macro2::{Ident, TokenStream};
+use quote::format_ident;
+use syn::{Attribute, Index, Member};
 
 pub(crate) trait AttrListExt {
     fn extract_cfg_attrs(&mut self) -> Vec<TokenStream>;
@@ -25,3 +26,29 @@ fn extract_cfg_attrs(&mut self) -> Vec<TokenStream> {
         cfg
     }
 }
+
+pub(crate) trait MemberExt {
+    /// Returns an identifier for the member.
+    ///
+    /// Tuple fields have no name of their own, so they are named `_0`, `_1`, ... instead.
+    fn as_ident(&self) -> Ident;
+
+    /// Obtain a display name for the member in diagnostics.
+    fn display_name(&self) -> String;
+}
+
+impl MemberExt for Member {
+    fn as_ident(&self) -> Ident {
+        match self {
+            Member::Named(ident) => ident.clone(),
+            Member::Unnamed(Index { index, .. }) => format_ident!("_{index}"),
+        }
+    }
+
+    fn display_name(&self) -> String {
+        match self {
+            Member::Named(ident) => format!("`{ident}`"),
+            Member::Unnamed(Index { index, .. }) => format!("index `{index}`"),
+        }
+    }
+}
diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
index 7600cdbbbf98..bf77b76c43c8 100644
--- a/rust/pin-init/src/lib.rs
+++ b/rust/pin-init/src/lib.rs
@@ -304,6 +304,9 @@
 /// This macro enables the use of the [`pin_init!`] macro. When pin-initializing a `struct`,
 /// then `#[pin]` directs the type of initializer that is required.
 ///
+/// Tuple structs are supported as well. Their fields have no names, so the generated projection
+/// is a tuple struct too and its fields are accessed by index.
+///
 /// If your `struct` implements `Drop`, then you need to add `PinnedDrop` as arguments to this
 /// macro, and change your `Drop` implementation to `PinnedDrop` annotated with
 /// `#[`[`macro@pinned_drop`]`]`, since dropping pinned values requires extra care.
@@ -327,6 +330,26 @@
 /// }
 /// ```
 ///
+/// The same as a tuple struct, projected by index:
+///
+/// ```
+/// # #![feature(allocator_api)]
+/// # #[path = "../examples/mutex.rs"] mod mutex; use mutex::*;
+/// use core::pin::Pin;
+/// use pin_init::pin_data;
+///
+/// enum Command {
+///     /* ... */
+/// }
+///
+/// #[pin_data]
+/// struct DriverData(#[pin] CMutex<Vec<Command>>, Box<[u8; 1024 * 1024]>);
+///
+/// fn queue(data: Pin<&mut DriverData>) -> Pin<&mut CMutex<Vec<Command>>> {
+///     data.project().0
+/// }
+/// ```
+///
 /// ```
 /// # #![feature(allocator_api)]
 /// # #[path = "../examples/mutex.rs"] mod mutex; use mutex::*;

-- 
2.54.0