Add option to force explicit extern ABI's

src/config.rs

@@ -328,6 +328,7 @@ create_config! {
         "Maximum width of the args of a function call before falling back to vertical formatting";
     struct_lit_width: usize, 16,
         "Maximum width in the body of a struct lit before falling back to vertical formatting";
+    force_explicit_abi: bool, true, "Always print the abi for extern items";
     newline_style: NewlineStyle, NewlineStyle::Unix, "Unix or Windows line endings";
     fn_brace_style: BraceStyle, BraceStyle::SameLineWhere, "Brace style for functions";
     item_brace_style: BraceStyle, BraceStyle::SameLineWhere, "Brace style for structs and enums";

src/items.rs

@@ -80,7 +80,8 @@ impl Rewrite for ast::Local {
 
 impl<'a> FmtVisitor<'a> {
     pub fn format_foreign_mod(&mut self, fm: &ast::ForeignMod, span: Span) {
-        self.buffer.push_str(&::utils::format_abi(fm.abi));
+        let abi_str = ::utils::format_abi(fm.abi, self.config.force_explicit_abi);
+        self.buffer.push_str(&abi_str);
 
         let snippet = self.snippet(span);
         let brace_pos = snippet.find_uncommented("{").unwrap();
@@ -1265,7 +1266,7 @@ fn rewrite_fn_base(context: &RewriteContext,
     result.push_str(::utils::format_unsafety(unsafety));
 
     if abi != abi::Abi::Rust {
-        result.push_str(&::utils::format_abi(abi));
+        result.push_str(&::utils::format_abi(abi, context.config.force_explicit_abi));
     }
 
     // fn foo

src/types.rs

@@ -608,7 +608,7 @@ fn rewrite_bare_fn(bare_fn: &ast::BareFnTy,
     result.push_str(&::utils::format_unsafety(bare_fn.unsafety));
 
     if bare_fn.abi != abi::Abi::Rust {
-        result.push_str(&::utils::format_abi(bare_fn.abi));
+        result.push_str(&::utils::format_abi(bare_fn.abi, context.config.force_explicit_abi));
     }
 
     result.push_str("fn");

src/utils.rs

@@ -91,9 +91,12 @@ pub fn format_mutability(mutability: ast::Mutability) -> &'static str {
 }
 
 #[inline]
-// FIXME(#451): include "C"?
-pub fn format_abi(abi: abi::Abi) -> String {
-    format!("extern {} ", abi)
+pub fn format_abi(abi: abi::Abi, explicit_abi: bool) -> String {
+    if abi == abi::Abi::C && !explicit_abi {
+        "extern ".into()
+    } else {
+        format!("extern {} ", abi)
+    }
 }
 
 // The width of the first line in s.

tests/source/extern_not_explicit.rs

@@ -0,0 +1,14 @@
+// rustfmt-force_explicit_abi: false
+
+ extern  "C" {
+   fn some_fn() -> ();
+ }
+
+ extern "C" fn sup() {
+
+ }
+
+type funky_func = extern "C" fn (unsafe extern "rust-call" fn(*const JSJitInfo, *mut JSContext,
+                                              HandleObject, *mut libc::c_void, u32,
+                                              *mut JSVal)
+                                              -> u8);

tests/target/extern_not_explicit.rs

@@ -0,0 +1,15 @@
+// rustfmt-force_explicit_abi: false
+
+extern {
+    fn some_fn() -> ();
+}
+
+extern fn sup() {}
+
+type funky_func = extern fn(unsafe extern "rust-call" fn(*const JSJitInfo,
+                                                         *mut JSContext,
+                                                         HandleObject,
+                                                         *mut libc::c_void,
+                                                         u32,
+                                                         *mut JSVal)
+                                                         -> u8);