fix: formatting arbitrary extern abi

Author: calebcartwright

Cargo.lock

@@ -1,7 +1,7 @@
 [package]
 
 name = "rustfmt-nightly"
-version = "1.4.12"
+version = "1.4.13"
 authors = ["Nicholas Cameron <[email protected]>", "The Rustfmt developers"]
 description = "Tool to find and fix Rust formatting issues"
 repository = "https://github.com/rust-lang/rustfmt"
@@ -86,10 +86,6 @@ version = "642.0.0"
 package = "rustc-ap-rustc_span"
 version = "642.0.0"
 
-[dependencies.rustc_target]
-package = "rustc-ap-rustc_target"
-version = "642.0.0"
-
 [dependencies.syntax]
 package = "rustc-ap-syntax"
 version = "642.0.0"

Cargo.toml

@@ -2,7 +2,6 @@ use std::borrow::Cow;
 
 use rustc_ast_pretty::pprust;
 use rustc_span::{sym, BytePos, ExpnId, Span, Symbol, SyntaxContext};
-use rustc_target::spec::abi;
 use syntax::ast::{
     self, Attribute, CrateSugar, MetaItem, MetaItemKind, NestedMetaItem, NodeId, Path, Visibility,
     VisibilityKind,
@@ -140,19 +139,17 @@ pub(crate) fn format_extern(
     is_mod: bool,
 ) -> Cow<'static, str> {
     let abi = match ext {
-        ast::Extern::None => abi::Abi::Rust,
-        ast::Extern::Implicit => abi::Abi::C,
-        ast::Extern::Explicit(abi) => {
-            abi::lookup(&abi.symbol_unescaped.as_str()).unwrap_or(abi::Abi::Rust)
-        }
+        ast::Extern::None => "Rust".to_owned(),
+        ast::Extern::Implicit => "C".to_owned(),
+        ast::Extern::Explicit(abi) => abi.symbol_unescaped.to_string(),
     };
 
-    if abi == abi::Abi::Rust && !is_mod {
+    if abi == "Rust" && !is_mod {
         Cow::from("")
-    } else if abi == abi::Abi::C && !explicit_abi {
+    } else if abi == "C" && !explicit_abi {
         Cow::from("extern ")
     } else {
-        Cow::from(format!("extern {} ", abi))
+        Cow::from(format!(r#"extern "{}" "#, abi))
     }
 }
 

src/utils.rs

@@ -0,0 +1,2 @@
+#[cfg(any())]
+extern  "C++"     {}

tests/source/issue_4086.rs

@@ -0,0 +1,2 @@
+#[cfg(any())]
+extern "C++" {}