maintain force_explicit_abi = false behavior

Author: fee1-dead

src/utils.rs

@@ -136,6 +136,10 @@ pub(crate) fn format_extern(ext: ast::Extern, force_explicit_abi: bool) -> Cow<'
         ast::Extern::None => Cow::from(""),
         ast::Extern::Implicit(_) if force_explicit_abi => Cow::from("extern \"C\" "),
         ast::Extern::Implicit(_) => Cow::from("extern "),
+        // turn `extern "C"` into `extern` when `force_explicit_abi` is set to false
+        ast::Extern::Explicit(abi, _) if abi.symbol_unescaped == sym::C && !force_explicit_abi => {
+            Cow::from("extern ")
+        }
         ast::Extern::Explicit(abi, _) => {
             Cow::from(format!(r#"extern "{}" "#, abi.symbol_unescaped))
         }

tests/target/extern_not_explicit.rs

@@ -1,12 +1,12 @@
 // rustfmt-force_explicit_abi: false
 
-extern "C" {
+extern {
     fn some_fn() -> ();
 }
 
-extern "C" fn sup() {}
+extern fn sup() {}
 
-type funky_func = extern "C" fn(
+type funky_func = extern fn(
     unsafe extern "rust-call" fn(
         *const JSJitInfo,
         *mut JSContext,