Allow impl UniquePtrTarget for T;

cxx allows UniquePtr<T> only if T implements a trait,
UniquePtrTarget. This trait implementation is automatically generated
for type T only if:

* UniquePtr<T> is used in a struct or a function parameter/return
  type; AND
* the type T is a C++ type.

Until now, it was not possible to generate UniquePtrTarget
implementations for type aliases where a cxx type refers to an
externally defined type (e.g. from bindgen).

There were also legitimate uses for UniquePtr<T> even if none
of the structs/functions within the cxx::bridge actually used it;
for example storing a UniquePtr<T> in some other struct outside
the cxx::bridge mod.

This change allows any cxx::bridge block to contain
  impl UniquePtrTarget for T {}
which will force cxx to generate the UniquePtrTarget trait (and
associated C++ code) for that type.

It's not desirable to do this for _every_ type since it does
generate additional C++ code as well as Rust code, and thus in some
toolchains may cause binary bloat.

Future changes could allow similar syntax for types which should
be containable in a C++ vector, e.g.:
  impl VectorElement for T {}
but that's not yet done here.

Author: adetaylor

gen/src/write.rs

@@ -154,6 +154,9 @@ fn write_includes(out: &mut OutFile, types: &Types) {
             _ => {}
         }
     }
+    if !types.unique_ptr_impls.is_empty() {
+        out.include.memory = true;
+    }
 }
 
 fn write_include_cxxbridge(out: &mut OutFile, apis: &[Api], types: &Types) {
@@ -1054,13 +1057,6 @@ fn write_generic_instantiations(out: &mut OutFile, types: &Types) {
                     write_rust_vec_extern(out, inner);
                 }
             }
-        } else if let Type::UniquePtr(ptr) = ty {
-            if let Type::Ident(inner) = &ptr.inner {
-                if Atom::from(inner).is_none() && !types.aliases.contains_key(inner) {
-                    out.next_section();
-                    write_unique_ptr(out, inner, types);
-                }
-            }
         } else if let Type::CxxVector(ptr) = ty {
             if let Type::Ident(inner) = &ptr.inner {
                 if Atom::from(inner).is_none() && !types.aliases.contains_key(inner) {
@@ -1070,6 +1066,12 @@ fn write_generic_instantiations(out: &mut OutFile, types: &Types) {
             }
         }
     }
+
+    for inner in types.unique_ptr_impls.into_iter() {
+        out.next_section();
+        write_unique_ptr(out, inner, types);
+    }
+
     out.end_block("extern \"C\"");
 
     out.begin_block("namespace rust");

macro/src/expand.rs

@@ -39,7 +39,7 @@ fn expand(ffi: Module, apis: &[Api], types: &Types) -> TokenStream {
 
     for api in apis {
         match api {
-            Api::Include(_) | Api::RustType(_) => {}
+            Api::Include(_) | Api::RustType(_) | Api::Impl(_) => {}
             Api::Struct(strct) => expanded.extend(expand_struct(strct)),
             Api::Enum(enm) => expanded.extend(expand_enum(enm)),
             Api::CxxType(ety) => {
@@ -74,12 +74,6 @@ fn expand(ffi: Module, apis: &[Api], types: &Types) -> TokenStream {
                     hidden.extend(expand_rust_vec(namespace, ident));
                 }
             }
-        } else if let Type::UniquePtr(ptr) = ty {
-            if let Type::Ident(ident) = &ptr.inner {
-                if Atom::from(ident).is_none() && !types.aliases.contains_key(ident) {
-                    expanded.extend(expand_unique_ptr(namespace, ident, types));
-                }
-            }
         } else if let Type::CxxVector(ptr) = ty {
             if let Type::Ident(ident) = &ptr.inner {
                 if Atom::from(ident).is_none() && !types.aliases.contains_key(ident) {
@@ -92,6 +86,10 @@ fn expand(ffi: Module, apis: &[Api], types: &Types) -> TokenStream {
         }
     }
 
+    for ident in types.unique_ptr_impls.into_iter() {
+        expanded.extend(expand_unique_ptr(namespace, ident, types));
+    }
+
     // Work around https://github.com/rust-lang/rust/issues/67851.
     if !hidden.is_empty() {
         expanded.extend(quote! {

syntax/file.rs

@@ -2,8 +2,8 @@ use crate::syntax::namespace::Namespace;
 use quote::quote;
 use syn::parse::{Error, Parse, ParseStream, Result};
 use syn::{
-    braced, token, Abi, Attribute, ForeignItem, Ident, Item as RustItem, ItemEnum, ItemStruct,
-    ItemUse, LitStr, Token, Visibility,
+    braced, token, Abi, Attribute, ForeignItem, Ident, Item as RustItem, ItemEnum, ItemImpl,
+    ItemStruct, ItemUse, LitStr, Token, Visibility,
 };
 
 pub struct Module {
@@ -22,6 +22,7 @@ pub enum Item {
     Enum(ItemEnum),
     ForeignMod(ItemForeignMod),
     Use(ItemUse),
+    Impl(ItemImpl),
     Other(RustItem),
 }
 
@@ -99,6 +100,7 @@ impl Parse for Item {
                 brace_token: item.brace_token,
                 items: item.items,
             })),
+            RustItem::Impl(item) => Ok(Item::Impl(ItemImpl { attrs, ..item })),
             RustItem::Use(item) => Ok(Item::Use(ItemUse { attrs, ..item })),
             other => Ok(Item::Other(other)),
         }

syntax/ident.rs

@@ -20,7 +20,7 @@ pub(crate) fn check_all(cx: &mut Check, namespace: &Namespace, apis: &[Api]) {
 
     for api in apis {
         match api {
-            Api::Include(_) => {}
+            Api::Include(_) | Api::Impl(_) => {}
             Api::Struct(strct) => {
                 check(cx, &strct.ident);
                 for field in &strct.fields {

syntax/mod.rs

@@ -42,6 +42,7 @@ pub enum Api {
     RustType(ExternType),
     RustFunction(ExternFn),
     TypeAlias(TypeAlias),
+    Impl(Impl),
 }
 
 pub struct ExternType {
@@ -87,6 +88,13 @@ pub struct TypeAlias {
     pub semi_token: Token![;],
 }
 
+pub struct Impl {
+    pub impl_token: Token![impl],
+    pub trait_ident: Ident,
+    pub for_token: Token![for],
+    pub ident: Ident,
+}
+
 pub struct Signature {
     pub unsafety: Option<Token![unsafe]>,
     pub fn_token: Token![fn],

syntax/parse.rs

@@ -3,17 +3,17 @@ use crate::syntax::file::{Item, ItemForeignMod};
 use crate::syntax::report::Errors;
 use crate::syntax::Atom::*;
 use crate::syntax::{
-    attrs, error, Api, Doc, Enum, ExternFn, ExternType, Lang, Receiver, Ref, Signature, Slice,
-    Struct, Ty1, Type, TypeAlias, Var, Variant,
+    attrs, error, Api, Doc, Enum, ExternFn, ExternType, Impl, Lang, Receiver, Ref, Signature,
+    Slice, Struct, Ty1, Type, TypeAlias, Var, Variant,
 };
 use proc_macro2::{TokenStream, TokenTree};
 use quote::{format_ident, quote, quote_spanned};
 use syn::parse::{ParseStream, Parser};
 use syn::punctuated::Punctuated;
 use syn::{
     Abi, Attribute, Error, Fields, FnArg, ForeignItem, ForeignItemFn, ForeignItemType,
-    GenericArgument, Ident, ItemEnum, ItemStruct, LitStr, Pat, PathArguments, Result, ReturnType,
-    Token, Type as RustType, TypeBareFn, TypePath, TypeReference, TypeSlice,
+    GenericArgument, Ident, ItemEnum, ItemImpl, ItemStruct, LitStr, Pat, PathArguments, Result,
+    ReturnType, Token, Type as RustType, TypeBareFn, TypePath, TypeReference, TypeSlice,
 };
 
 pub mod kw {
@@ -34,6 +34,7 @@ pub fn parse_items(cx: &mut Errors, items: Vec<Item>, trusted: bool) -> Vec<Api>
             },
             Item::ForeignMod(foreign_mod) => parse_foreign_mod(cx, foreign_mod, &mut apis, trusted),
             Item::Use(item) => cx.error(item, error::USE_NOT_ALLOWED),
+            Item::Impl(item) => parse_impl(cx, item, &mut apis),
             Item::Other(item) => cx.error(item, "unsupported item"),
         }
     }
@@ -621,3 +622,48 @@ fn parse_return_type(
         ty => Ok(Some(ty)),
     }
 }
+
+fn parse_impl(cx: &mut Errors, imp: ItemImpl, api: &mut Vec<Api>) {
+    // At present we only support impl UniquePtrTarget for <ty>;
+    if !imp.items.is_empty() {
+        cx.error(imp, "impl must be empty");
+        return;
+    }
+    match &imp.trait_ {
+        None => cx.error(imp, "impl must be for a trait"),
+        Some((_, ty_path, for_token)) => {
+            let ident = ty_path.get_ident();
+            match ident {
+                None => {
+                    cx.error(imp, "path incomplete");
+                    return;
+                }
+                Some(trait_ident) if trait_ident.to_string() == "UniquePtrTarget" => {
+                    let ty = parse_type(&imp.self_ty);
+                    match ty {
+                        Err(_) => {
+                            cx.error(imp, "unable to parse type");
+                            return;
+                        }
+                        Ok(ty) => match ty {
+                            Type::Ident(ty) => {
+                                api.push(Api::Impl(Impl {
+                                    for_token: for_token.clone(),
+                                    trait_ident: trait_ident.clone(),
+                                    impl_token: imp.impl_token,
+                                    ident: ty,
+                                }));
+                            }
+                            _ => cx
+                                .error(imp, "can only impl UniquePtrTarget for a plain identifier"),
+                        },
+                    }
+                }
+                Some(_) => cx.error(
+                    imp,
+                    "only supported trait which can be implemented is UniquePtrTarget",
+                ),
+            }
+        }
+    }
+}

syntax/set.rs

@@ -30,6 +30,10 @@ where
     pub fn contains(&self, value: &T) -> bool {
         self.set.contains(value)
     }
+
+    pub fn is_empty(&self) -> bool {
+        self.set.is_empty()
+    }
 }
 
 impl<'s, 'a, T> IntoIterator for &'s OrderedSet<&'a T> {

syntax/types.rs

@@ -15,6 +15,7 @@ pub struct Types<'a> {
     pub aliases: Map<&'a Ident, &'a TypeAlias>,
     pub untrusted: Map<&'a Ident, &'a ExternType>,
     pub required_trivial: Map<&'a Ident, TrivialReason<'a>>,
+    pub unique_ptr_impls: Set<&'a Ident>,
 }
 
 impl<'a> Types<'a> {
@@ -26,6 +27,7 @@ impl<'a> Types<'a> {
         let mut rust = Set::new();
         let mut aliases = Map::new();
         let mut untrusted = Map::new();
+        let mut unique_ptr_impls = Set::new();
 
         fn visit<'a>(all: &mut Set<&'a Type>, ty: &'a Type) {
             all.insert(ty);
@@ -133,6 +135,9 @@ impl<'a> Types<'a> {
                     cxx.insert(ident);
                     aliases.insert(ident, alias);
                 }
+                Api::Impl(imp) => {
+                    unique_ptr_impls.insert(&imp.ident);
+                }
             }
         }
 
@@ -170,6 +175,16 @@ impl<'a> Types<'a> {
             }
         }
 
+        for ty in &all {
+            if let Type::UniquePtr(ptr) = ty {
+                if let Type::Ident(ident) = &ptr.inner {
+                    if Atom::from(ident).is_none() && !aliases.contains_key(ident) {
+                        unique_ptr_impls.insert(ident);
+                    }
+                }
+            }
+        }
+
         Types {
             all,
             structs,
@@ -179,6 +194,7 @@ impl<'a> Types<'a> {
             aliases,
             untrusted,
             required_trivial,
+            unique_ptr_impls,
         }
     }