Support packed remote struct without destructuring

serde/src/lib.rs

@@ -157,7 +157,7 @@ mod lib {
         pub use std::*;
     }
 
-    pub use self::core::{cmp, iter, mem, num, slice, str};
+    pub use self::core::{cmp, iter, mem, num, ptr, slice, str};
     pub use self::core::{f32, f64};
     pub use self::core::{i16, i32, i64, i8, isize};
     pub use self::core::{u16, u32, u64, u8, usize};

serde/src/private/mod.rs

@@ -14,6 +14,7 @@ pub use lib::default::Default;
 pub use lib::fmt::{self, Formatter};
 pub use lib::marker::PhantomData;
 pub use lib::option::Option::{self, None, Some};
+pub use lib::ptr;
 pub use lib::result::Result::{self, Err, Ok};
 
 pub use self::string::from_utf8_lossy;

serde_derive/build.rs

@@ -16,6 +16,12 @@ fn main() {
     if minor >= 37 {
         println!("cargo:rustc-cfg=underscore_consts");
     }
+
+    // The ptr::addr_of! macro stabilized in Rust 1.51:
+    // https://blog.rust-lang.org/2021/03/25/Rust-1.51.0.html#stabilized-apis
+    if minor >= 51 {
+        println!("cargo:rustc-cfg=ptr_addr_of");
+    }
 }
 
 fn rustc_minor_version() -> Option<u32> {

serde_derive/src/pretend.rs

@@ -1,7 +1,7 @@
-use proc_macro2::{Span, TokenStream};
-use syn::Ident;
+use proc_macro2::TokenStream;
+use quote::format_ident;
 
-use internals::ast::{Container, Data, Field, Style};
+use internals::ast::{Container, Data, Field, Style, Variant};
 
 // Suppress dead_code warnings that would otherwise appear when using a remote
 // derive. Other than this pretend code, a struct annotated with remote derive
@@ -32,49 +32,115 @@ pub fn pretend_used(cont: &Container, is_packed: bool) -> TokenStream {
 
 // For structs with named fields, expands to:
 //
+//     match None::<&T> {
+//         Some(T { a: __v0, b: __v1 }) => {}
+//         _ => {}
+//     }
+//
+// For packed structs on sufficiently new rustc, expands to:
+//
+//     match None::<&T> {
+//         Some(__v @ T { a: _, b: _ }) => {
+//             let _ = addr_of!(__v.a);
+//             let _ = addr_of!(__v.b);
+//         }
+//         _ => {}
+//     }
+//
+// For packed structs on older rustc, we assume Sized and !Drop, and expand to:
+//
 //     match None::<T> {
-//         Some(T { a: ref __v0, b: ref __v1 }) => {}
+//         Some(T { a: __v0, b: __v1 }) => {}
 //         _ => {}
 //     }
 //
 // For enums, expands to the following but only including struct variants:
 //
-//     match None::<T> {
-//         Some(T::A { a: ref __v0 }) => {}
-//         Some(T::B { b: ref __v0 }) => {}
+//     match None::<&T> {
+//         Some(T::A { a: __v0 }) => {}
+//         Some(T::B { b: __v0 }) => {}
 //         _ => {}
 //     }
 //
-// The `ref` is important in case the user has written a Drop impl on their
-// type. Rust does not allow destructuring a struct or enum that has a Drop
-// impl.
 fn pretend_fields_used(cont: &Container, is_packed: bool) -> TokenStream {
+    match &cont.data {
+        Data::Enum(variants) => pretend_fields_used_enum(cont, variants),
+        Data::Struct(Style::Struct, fields) => if is_packed {
+            pretend_fields_used_struct_packed(cont, fields)
+        } else {
+            pretend_fields_used_struct(cont, fields)
+        },
+        Data::Struct(_, _) => quote!(),
+    }
+}
+
+fn pretend_fields_used_struct(cont: &Container, fields: &[Field]) -> TokenStream {
     let type_ident = &cont.ident;
     let (_, ty_generics, _) = cont.generics.split_for_impl();
 
-    let patterns = match &cont.data {
-        Data::Enum(variants) => variants
-            .iter()
-            .filter_map(|variant| match variant.style {
-                Style::Struct => {
-                    let variant_ident = &variant.ident;
-                    let pat = struct_pattern(&variant.fields, is_packed);
-                    Some(quote!(#type_ident::#variant_ident #pat))
+    let members = fields.iter().map(|field| &field.member);
+    let placeholders = (0usize..).map(|i| format_ident!("__v{}", i));
+
+    quote! {
+        match _serde::__private::None::<&#type_ident #ty_generics> {
+            _serde::__private::Some(#type_ident { #(#members: #placeholders),* }) => {}
+            _ => {}
+        }
+    }
+}
+
+fn pretend_fields_used_struct_packed(cont: &Container, fields: &[Field]) -> TokenStream {
+    let type_ident = &cont.ident;
+    let (_, ty_generics, _) = cont.generics.split_for_impl();
+
+    let members = fields.iter().map(|field| &field.member).collect::<Vec<_>>();
+
+    #[cfg(ptr_addr_of)]
+    {
+        quote! {
+            match _serde::__private::None::<&#type_ident #ty_generics> {
+                _serde::__private::Some(__v @ #type_ident { #(#members: _),* }) => {
+                    #(
+                        let _ = _serde::__private::ptr::addr_of!(__v.#members);
+                    )*
                 }
-                _ => None,
-            })
-            .collect::<Vec<_>>(),
-        Data::Struct(Style::Struct, fields) => {
-            let pat = struct_pattern(fields, is_packed);
-            vec![quote!(#type_ident #pat)]
+                _ => {}
+            }
         }
-        Data::Struct(_, _) => {
-            return quote!();
+    }
+
+    #[cfg(not(ptr_addr_of))]
+    {
+        let placeholders = (0usize..).map(|i| format_ident!("__v{}", i));
+
+        quote! {
+            match _serde::__private::None::<#type_ident #ty_generics> {
+                _serde::__private::Some(#type_ident { #(#members: #placeholders),* }) => {}
+                _ => {}
+            }
         }
-    };
+    }
+}
+
+fn pretend_fields_used_enum(cont: &Container, variants: &[Variant]) -> TokenStream {
+    let type_ident = &cont.ident;
+    let (_, ty_generics, _) = cont.generics.split_for_impl();
+
+    let patterns = variants
+        .iter()
+        .filter_map(|variant| match variant.style {
+            Style::Struct => {
+                let variant_ident = &variant.ident;
+                let members = variant.fields.iter().map(|field| &field.member);
+                let placeholders = (0usize..).map(|i| format_ident!("__v{}", i));
+                Some(quote!(#type_ident::#variant_ident { #(#members: #placeholders),* }))
+            }
+            _ => None,
+        })
+        .collect::<Vec<_>>();
 
     quote! {
-        match _serde::__private::None::<#type_ident #ty_generics> {
+        match _serde::__private::None::<&#type_ident #ty_generics> {
             #(
                 _serde::__private::Some(#patterns) => {}
             )*
@@ -107,7 +173,7 @@ fn pretend_variants_used(cont: &Container) -> TokenStream {
     let cases = variants.iter().map(|variant| {
         let variant_ident = &variant.ident;
         let placeholders = &(0..variant.fields.len())
-            .map(|i| Ident::new(&format!("__v{}", i), Span::call_site()))
+            .map(|i| format_ident!("__v{}", i))
             .collect::<Vec<_>>();
 
         let pat = match variant.style {
@@ -131,15 +197,3 @@ fn pretend_variants_used(cont: &Container) -> TokenStream {
 
     quote!(#(#cases)*)
 }
-
-fn struct_pattern(fields: &[Field], is_packed: bool) -> TokenStream {
-    let members = fields.iter().map(|field| &field.member);
-    let placeholders =
-        (0..fields.len()).map(|i| Ident::new(&format!("__v{}", i), Span::call_site()));
-    let take_reference = if is_packed {
-        None
-    } else {
-        Some(quote!(ref))
-    };
-    quote!({ #(#members: #take_reference #placeholders),* })
-}

test_suite/tests/test_gen.rs

@@ -851,14 +851,36 @@ where
 
 #[repr(packed)]
 pub struct RemotePacked {
-    pub a: u8,
-    pub b: u16,
+    pub a: u16,
+    pub b: u32,
 }
 
-#[derive(Serialize, Deserialize)]
+#[derive(Serialize)]
 #[repr(packed)]
 #[serde(remote = "RemotePacked")]
 pub struct RemotePackedDef {
-    a: u8,
-    b: u16,
+    a: u16,
+    b: u32,
+}
+
+impl Drop for RemotePackedDef {
+    fn drop(&mut self) {}
+}
+
+#[repr(packed)]
+pub struct RemotePackedNonCopy {
+    pub a: u16,
+    pub b: String,
+}
+
+#[derive(Deserialize)]
+#[repr(packed)]
+#[serde(remote = "RemotePackedNonCopy")]
+pub struct RemotePackedNonCopyDef {
+    a: u16,
+    b: String,
+}
+
+impl Drop for RemotePackedNonCopyDef {
+    fn drop(&mut self) {}
 }