Begin development on async fn support

gen/build/Cargo.toml

@@ -14,6 +14,8 @@ categories = ["development-tools::ffi"]
 
 [features]
 parallel = ["cc/parallel"]
+# incomplete features that are not covered by a compatibility guarantee:
+experimental-async-fn = []
 
 [dependencies]
 cc = "1.0.49"

gen/cmd/Cargo.toml

@@ -16,6 +16,10 @@ categories = ["development-tools::ffi"]
 name = "cxxbridge"
 path = "src/main.rs"
 
+[features]
+# incomplete features that are not covered by a compatibility guarantee:
+experimental-async-fn = []
+
 [dependencies]
 clap = { version = "3.0", default-features = false, features = ["std", "suggestions"] }
 codespan-reporting = "0.11"

macro/Cargo.toml

@@ -16,6 +16,8 @@ categories = ["development-tools::ffi"]
 proc-macro = true
 
 [features]
+# incomplete features that are not covered by a compatibility guarantee:
+experimental-async-fn = []
 experimental-enum-variants-from-header = ["clang-ast", "flate2", "memmap", "serde", "serde_json"]
 
 [dependencies]
@@ -23,7 +25,7 @@ proc-macro2 = "1.0"
 quote = "1.0.4"
 syn = { version = "1.0.70", features = ["full"] }
 
-# optional dependencies
+# optional dependencies:
 clang-ast = { version = "0.1", optional = true }
 flate2 = { version = "1.0", optional = true }
 memmap = { version = "0.7", optional = true }

syntax/impls.rs

@@ -309,6 +309,7 @@ impl Eq for Signature {}
 impl PartialEq for Signature {
     fn eq(&self, other: &Self) -> bool {
         let Signature {
+            asyncness,
             unsafety,
             fn_token: _,
             generics: _,
@@ -320,6 +321,7 @@ impl PartialEq for Signature {
             throws_tokens: _,
         } = self;
         let Signature {
+            asyncness: asyncness2,
             unsafety: unsafety2,
             fn_token: _,
             generics: _,
@@ -330,7 +332,8 @@ impl PartialEq for Signature {
             paren_token: _,
             throws_tokens: _,
         } = other;
-        unsafety.is_some() == unsafety2.is_some()
+        asyncness.is_some() == asyncness2.is_some()
+            && unsafety.is_some() == unsafety2.is_some()
             && receiver == receiver2
             && ret == ret2
             && throws == throws2
@@ -362,6 +365,7 @@ impl PartialEq for Signature {
 impl Hash for Signature {
     fn hash<H: Hasher>(&self, state: &mut H) {
         let Signature {
+            asyncness,
             unsafety,
             fn_token: _,
             generics: _,
@@ -372,6 +376,7 @@ impl Hash for Signature {
             paren_token: _,
             throws_tokens: _,
         } = self;
+        asyncness.is_some().hash(state);
         unsafety.is_some().hash(state);
         receiver.hash(state);
         for arg in args {

syntax/mod.rs

@@ -179,6 +179,7 @@ pub struct Lifetimes {
 }
 
 pub struct Signature {
+    pub asyncness: Option<Token![async]>,
     pub unsafety: Option<Token![unsafe]>,
     pub fn_token: Token![fn],
     pub generics: Generics,

syntax/parse.rs

@@ -562,7 +562,7 @@ fn parse_extern_fn(
         ));
     }
 
-    if foreign_fn.sig.asyncness.is_some() {
+    if foreign_fn.sig.asyncness.is_some() && !cfg!(feature = "experimental-async-fn") {
         return Err(Error::new_spanned(
             foreign_fn,
             "async function is not directly supported yet, but see https://cxx.rs/async.html \
@@ -664,6 +664,7 @@ fn parse_extern_fn(
     let mut throws_tokens = None;
     let ret = parse_return_type(&foreign_fn.sig.output, &mut throws_tokens)?;
     let throws = throws_tokens.is_some();
+    let asyncness = foreign_fn.sig.asyncness;
     let unsafety = foreign_fn.sig.unsafety;
     let fn_token = foreign_fn.sig.fn_token;
     let inherited_span = unsafety.map_or(fn_token.span, |unsafety| unsafety.span);
@@ -684,6 +685,7 @@ fn parse_extern_fn(
         visibility,
         name,
         sig: Signature {
+            asyncness,
             unsafety,
             fn_token,
             generics,
@@ -1400,6 +1402,7 @@ fn parse_type_fn(ty: &TypeBareFn) -> Result<Type> {
     let ret = parse_return_type(&ty.output, &mut throws_tokens)?;
     let throws = throws_tokens.is_some();
 
+    let asyncness = None;
     let unsafety = ty.unsafety;
     let fn_token = ty.fn_token;
     let generics = Generics::default();
@@ -1407,6 +1410,7 @@ fn parse_type_fn(ty: &TypeBareFn) -> Result<Type> {
     let paren_token = ty.paren_token;
 
     Ok(Type::Fn(Box::new(Signature {
+        asyncness,
         unsafety,
         fn_token,
         generics,

syntax/tokens.rs

@@ -248,6 +248,7 @@ impl ToTokens for Lifetimes {
 impl ToTokens for Signature {
     fn to_tokens(&self, tokens: &mut TokenStream) {
         let Signature {
+            asyncness: _,
             unsafety: _,
             fn_token,
             generics: _,