refactor ABI formatting (rust-lang#5845)

fixes 5701

Whenever we see an `extern "Rust"` on a function, we don't strip it from the function.

If there's any future desire to have rustfmt remove an explicit "Rust" ABI, as it historically did prior to this change, then we can consider updating the rustfmt config surface to support that scenario

src/items.rs

@@ -248,7 +248,6 @@ impl<'a> Item<'a> {
             abi: format_extern(
                 ast::Extern::from_abi(fm.abi, DUMMY_SP),
                 config.force_explicit_abi(),
-                true,
             ),
             vis: None,
             body: fm
@@ -336,7 +335,6 @@ impl<'a> FnSig<'a> {
         result.push_str(&format_extern(
             self.ext,
             context.config.force_explicit_abi(),
-            false,
         ));
         result
     }

src/types.rs

@@ -892,7 +892,6 @@ fn rewrite_bare_fn(
     result.push_str(&format_extern(
         bare_fn.ext,
         context.config.force_explicit_abi(),
-        false,
     ));
 
     result.push_str("fn");

src/utils.rs

@@ -131,23 +131,18 @@ pub(crate) fn format_mutability(mutability: ast::Mutability) -> &'static str {
 }
 
 #[inline]
-pub(crate) fn format_extern(
-    ext: ast::Extern,
-    explicit_abi: bool,
-    is_mod: bool,
-) -> Cow<'static, str> {
-    let abi = match ext {
-        ast::Extern::None => "Rust".to_owned(),
-        ast::Extern::Implicit(_) => "C".to_owned(),
-        ast::Extern::Explicit(abi, _) => abi.symbol_unescaped.to_string(),
-    };
-
-    if abi == "Rust" && !is_mod {
-        Cow::from("")
-    } else if abi == "C" && !explicit_abi {
-        Cow::from("extern ")
-    } else {
-        Cow::from(format!(r#"extern "{abi}" "#))
+pub(crate) fn format_extern(ext: ast::Extern, explicit_abi: bool) -> Cow<'static, str> {
+    match ext {
+        ast::Extern::None => Cow::from(""),
+        ast::Extern::Implicit(_) if explicit_abi => Cow::from("extern \"C\" "),
+        ast::Extern::Implicit(_) => Cow::from("extern "),
+        // turn `extern "C"` into `extern` when `explicit_abi` is set to false
+        ast::Extern::Explicit(abi, _) if abi.symbol_unescaped == sym::C && !explicit_abi => {
+            Cow::from("extern ")
+        }
+        ast::Extern::Explicit(abi, _) => {
+            Cow::from(format!(r#"extern "{}" "#, abi.symbol_unescaped))
+        }
     }
 }
 

tests/target/extern-rust.rs

@@ -0,0 +1 @@
+extern "Rust" fn uwu() {}