rust: qemu_api_macros: add Wrapper derive macro

Add a derive macro that makes it easy to peel off all the layers of
specialness (UnsafeCell, MaybeUninit, etc.) and just get a pointer
to the wrapped type; and likewise add them back starting from a
*mut.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/devel/rust.rst

@@ -315,11 +315,13 @@ a raw pointer, for use in calls to C functions.  It can be used for
 example as follows::
 
     #[repr(transparent)]
-    #[derive(Debug)]
+    #[derive(Debug, qemu_api_macros::Wrapper)]
     pub struct Object(Opaque<bindings::Object>);
 
-The bindings will then manually check for the big QEMU lock with
-assertions, which allows the wrapper to be declared thread-safe::
+where the special ``derive`` macro provides useful methods such as
+``from_raw``, ``as_ptr`, ``as_mut_ptr`` and ``raw_get``.  The bindings will
+then manually check for the big QEMU lock with assertions, which allows
+the wrapper to be declared thread-safe::
 
     unsafe impl Send for Object {}
     unsafe impl Sync for Object {}

rust/qemu-api-macros/src/lib.rs

@@ -6,7 +6,7 @@
 use quote::quote;
 use syn::{
     parse_macro_input, parse_quote, punctuated::Punctuated, spanned::Spanned, token::Comma, Data,
-    DeriveInput, Field, Fields, Ident, Meta, Path, Token, Type, Variant, Visibility,
+    DeriveInput, Field, Fields, FieldsUnnamed, Ident, Meta, Path, Token, Type, Variant, Visibility,
 };
 
 mod utils;
@@ -33,6 +33,35 @@ fn get_fields<'a>(
     }
 }
 
+fn get_unnamed_field<'a>(input: &'a DeriveInput, msg: &str) -> Result<&'a Field, MacroError> {
+    if let Data::Struct(s) = &input.data {
+        let unnamed = match &s.fields {
+            Fields::Unnamed(FieldsUnnamed {
+                unnamed: ref fields,
+                ..
+            }) => fields,
+            _ => {
+                return Err(MacroError::Message(
+                    format!("Tuple struct required for {}", msg),
+                    s.fields.span(),
+                ))
+            }
+        };
+        if unnamed.len() != 1 {
+            return Err(MacroError::Message(
+                format!("A single field is required for {}", msg),
+                s.fields.span(),
+            ));
+        }
+        Ok(&unnamed[0])
+    } else {
+        Err(MacroError::Message(
+            format!("Struct required for {}", msg),
+            input.ident.span(),
+        ))
+    }
+}
+
 fn is_c_repr(input: &DeriveInput, msg: &str) -> Result<(), MacroError> {
     let expected = parse_quote! { #[repr(C)] };
 
@@ -46,6 +75,19 @@ fn is_c_repr(input: &DeriveInput, msg: &str) -> Result<(), MacroError> {
     }
 }
 
+fn is_transparent_repr(input: &DeriveInput, msg: &str) -> Result<(), MacroError> {
+    let expected = parse_quote! { #[repr(transparent)] };
+
+    if input.attrs.iter().any(|attr| attr == &expected) {
+        Ok(())
+    } else {
+        Err(MacroError::Message(
+            format!("#[repr(transparent)] required for {}", msg),
+            input.ident.span(),
+        ))
+    }
+}
+
 fn derive_object_or_error(input: DeriveInput) -> Result<proc_macro2::TokenStream, MacroError> {
     is_c_repr(&input, "#[derive(Object)]")?;
 
@@ -72,6 +114,52 @@ pub fn derive_object(input: TokenStream) -> TokenStream {
     TokenStream::from(expanded)
 }
 
+fn derive_opaque_or_error(input: DeriveInput) -> Result<proc_macro2::TokenStream, MacroError> {
+    is_transparent_repr(&input, "#[derive(Wrapper)]")?;
+
+    let name = &input.ident;
+    let field = &get_unnamed_field(&input, "#[derive(Wrapper)]")?;
+    let typ = &field.ty;
+
+    // TODO: how to add "::qemu_api"?  For now, this is only used in the
+    // qemu_api crate so it's not a problem.
+    Ok(quote! {
+        unsafe impl crate::cell::Wrapper for #name {
+            type Wrapped = <#typ as crate::cell::Wrapper>::Wrapped;
+        }
+        impl #name {
+            pub unsafe fn from_raw<'a>(ptr: *mut <Self as crate::cell::Wrapper>::Wrapped) -> &'a Self {
+                let ptr = ::std::ptr::NonNull::new(ptr).unwrap().cast::<Self>();
+                unsafe { ptr.as_ref() }
+            }
+
+            pub const fn as_mut_ptr(&self) -> *mut <Self as crate::cell::Wrapper>::Wrapped {
+                self.0.as_mut_ptr()
+            }
+
+            pub const fn as_ptr(&self) -> *const <Self as crate::cell::Wrapper>::Wrapped {
+                self.0.as_ptr()
+            }
+
+            pub const fn as_void_ptr(&self) -> *mut ::core::ffi::c_void {
+                self.0.as_void_ptr()
+            }
+
+            pub const fn raw_get(slot: *mut Self) -> *mut <Self as crate::cell::Wrapper>::Wrapped {
+                slot.cast()
+            }
+        }
+    })
+}
+
+#[proc_macro_derive(Wrapper)]
+pub fn derive_opaque(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+    let expanded = derive_opaque_or_error(input).unwrap_or_else(Into::into);
+
+    TokenStream::from(expanded)
+}
+
 #[rustfmt::skip::macros(quote)]
 fn derive_offsets_or_error(input: DeriveInput) -> Result<proc_macro2::TokenStream, MacroError> {
     is_c_repr(&input, "#[derive(offsets)]")?;

rust/qemu-api/meson.build

@@ -42,16 +42,13 @@ _qemu_api_rs = static_library(
   override_options: ['rust_std=2021', 'build.rust_std=2021'],
   rust_abi: 'rust',
   rust_args: _qemu_api_cfg,
-  dependencies: libc_dep,
+  dependencies: [libc_dep, qemu_api_macros],
 )
 
 rust.test('rust-qemu-api-tests', _qemu_api_rs,
           suite: ['unit', 'rust'])
 
-qemu_api = declare_dependency(
-  link_with: _qemu_api_rs,
-  dependencies: qemu_api_macros,
-)
+qemu_api = declare_dependency(link_with: _qemu_api_rs)
 
 # Rust executables do not support objects, so add an intermediate step.
 rust_qemu_api_objs = static_library(

rust/qemu-api/src/cell.rs

@@ -1030,3 +1030,48 @@ pub unsafe fn new() -> Self {
         }
     }
 }
+
+/// Annotates [`Self`] as a transparent wrapper for another type.
+///
+/// Usually defined via the [`qemu_api_macros::Wrapper`] derive macro.
+///
+/// # Examples
+///
+/// ```
+/// # use std::mem::ManuallyDrop;
+/// # use qemu_api::cell::Wrapper;
+/// #[repr(transparent)]
+/// pub struct Example {
+///     inner: ManuallyDrop<String>,
+/// }
+///
+/// unsafe impl Wrapper for Example {
+///     type Wrapped = String;
+/// }
+/// ```
+///
+/// # Safety
+///
+/// `Self` must be a `#[repr(transparent)]` wrapper for the `Wrapped` type,
+/// whether directly or indirectly.
+///
+/// # Methods
+///
+/// By convention, types that implement Wrapper also implement the following
+/// methods:
+///
+/// ```ignore
+/// pub const unsafe fn from_raw<'a>(value: *mut Self::Wrapped) -> &'a Self;
+/// pub const unsafe fn as_mut_ptr(&self) -> *mut Self::Wrapped;
+/// pub const unsafe fn as_ptr(&self) -> *const Self::Wrapped;
+/// pub const unsafe fn raw_get(slot: *mut Self) -> *const Self::Wrapped;
+/// ```
+///
+/// They are not defined here to allow them to be `const`.
+pub unsafe trait Wrapper {
+    type Wrapped;
+}
+
+unsafe impl<T> Wrapper for Opaque<T> {
+    type Wrapped = T;
+}