Produce error message for a few more unsupported signatures

Author: dtolnay

syntax/parse.rs

@@ -379,6 +379,18 @@ fn parse_extern_fn(
             "async function is not directly supported yet, but see https://cxx.rs/async.html for a working approach",
         ));
     }
+    if foreign_fn.sig.constness.is_some() {
+        return Err(Error::new_spanned(
+            foreign_fn,
+            "const extern function is not supported",
+        ));
+    }
+    if let Some(abi) = &foreign_fn.sig.abi {
+        return Err(Error::new_spanned(
+            abi,
+            "explicit ABI on extern function is not supported",
+        ));
+    }
 
     let mut doc = Doc::new();
     let mut cxx_name = None;

tests/ui/const_fn.rs

@@ -0,0 +1,10 @@
+#[cxx::bridge]
+mod ffi {
+    extern "Rust" {
+        const fn f();
+    }
+}
+
+const fn f() {}
+
+fn main() {}

tests/ui/const_fn.stderr

@@ -0,0 +1,5 @@
+error: const extern function is not supported
+ --> $DIR/const_fn.rs:4:9
+  |
+4 |         const fn f();
+  |         ^^^^^^^^^^^^^

tests/ui/extern_fn_abi.rs

@@ -0,0 +1,8 @@
+#[cxx::bridge]
+mod ffi {
+    extern "C++" {
+        extern "Java" fn f();
+    }
+}
+
+fn main() {}

tests/ui/extern_fn_abi.stderr

@@ -0,0 +1,5 @@
+error: explicit ABI on extern function is not supported
+ --> $DIR/extern_fn_abi.rs:4:9
+  |
+4 |         extern "Java" fn f();
+  |         ^^^^^^^^^^^^^