From 59fdfaff7850dd92d2b150d198cd88eea2d43d6b Mon Sep 17 00:00:00 2001
From: Deadbeef <ent3rm4n@gmail.com>
Date: Tue, 18 Jul 2023 18:07:54 +0000
Subject: [PATCH 1/2] refactor ABI formatting

Here are two paths of behavior this changes:

1. whenever we see an `extern "Rust"` on a function, we don't strip it
from the function (which fixes #5701)
2. if `force_explicit_abi` is disabled, we preserve what the user has
written. Previously, rustfmt would change `extern "C"` to `extern `,
which is not something a code formatter should do.
---
 src/items.rs                        |  2 --
 src/types.rs                        |  1 -
 src/utils.rs                        | 25 ++++++++-----------------
 tests/target/extern-rust.rs         |  1 +
 tests/target/extern_not_explicit.rs |  6 +++---
 5 files changed, 12 insertions(+), 23 deletions(-)
 create mode 100644 tests/target/extern-rust.rs

diff --git a/src/items.rs b/src/items.rs
index a72646ef897..1f4a14211b5 100644
--- a/src/items.rs
+++ b/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
     }
diff --git a/src/types.rs b/src/types.rs
index 8be474d5bca..f2e229d7477 100644
--- a/src/types.rs
+++ b/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");
diff --git a/src/utils.rs b/src/utils.rs
index d1cb197cb51..940dffc6da6 100644
--- a/src/utils.rs
+++ b/src/utils.rs
@@ -131,23 +131,14 @@ 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 "),
+        ast::Extern::Explicit(abi, _) => {
+            Cow::from(format!(r#"extern "{}" "#, abi.symbol_unescaped))
+        }
     }
 }
 
diff --git a/tests/target/extern-rust.rs b/tests/target/extern-rust.rs
new file mode 100644
index 00000000000..32824c91203
--- /dev/null
+++ b/tests/target/extern-rust.rs
@@ -0,0 +1 @@
+extern "Rust" fn uwu() {}
diff --git a/tests/target/extern_not_explicit.rs b/tests/target/extern_not_explicit.rs
index b55b64d05b3..1f800cf1460 100644
--- a/tests/target/extern_not_explicit.rs
+++ b/tests/target/extern_not_explicit.rs
@@ -1,12 +1,12 @@
 // rustfmt-force_explicit_abi: false
 
-extern {
+extern "C" {
     fn some_fn() -> ();
 }
 
-extern fn sup() {}
+extern "C" fn sup() {}
 
-type funky_func = extern fn(
+type funky_func = extern "C" fn(
     unsafe extern "rust-call" fn(
         *const JSJitInfo,
         *mut JSContext,

From 267e8c21fdde9d22cbfc5101489cd58d87a48a34 Mon Sep 17 00:00:00 2001
From: Deadbeef <ent3rm4n@gmail.com>
Date: Wed, 19 Jul 2023 16:12:07 +0000
Subject: [PATCH 2/2] maintain `force_explicit_abi = false` behavior

---
 src/utils.rs                        | 4 ++++
 tests/target/extern_not_explicit.rs | 6 +++---
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/utils.rs b/src/utils.rs
index 940dffc6da6..ae7c50b4ca2 100644
--- a/src/utils.rs
+++ b/src/utils.rs
@@ -136,6 +136,10 @@ pub(crate) fn format_extern(ext: ast::Extern, explicit_abi: bool) -> Cow<'static
         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))
         }
diff --git a/tests/target/extern_not_explicit.rs b/tests/target/extern_not_explicit.rs
index 1f800cf1460..b55b64d05b3 100644
--- a/tests/target/extern_not_explicit.rs
+++ b/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,