rustdoc: Cleanup ABI rendering

Use a common method for rendering `extern "<abi>"`.

This now consistently shows `extern "C" fn` rather than just `extern fn`.

src/test/rustdoc/extern-impl.rs

@@ -0,0 +1,37 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![crate_name = "foo"]
+
+// @has foo/struct.Foo.html
+pub struct Foo;
+
+impl Foo {
+    // @has - '//code' 'fn rust0()'
+    pub fn rust0() {}
+    // @has - '//code' 'fn rust1()'
+    pub extern "Rust" fn rust1() {}
+    // @has - '//code' 'extern "C" fn c0()'
+    pub extern fn c0() {}
+    // @has - '//code' 'extern "C" fn c1()'
+    pub extern "C" fn c1() {}
+    // @has - '//code' 'extern "system" fn system0()'
+    pub extern "system" fn system0() {}
+}
+
+// @has foo/trait.Bar.html
+pub trait Bar {}
+
+// @has - '//code' 'impl Bar for fn()'
+impl Bar for fn() {}
+// @has - '//code' 'impl Bar for extern "C" fn()'
+impl Bar for extern fn() {}
+// @has - '//code' 'impl Bar for extern "system" fn()'
+impl Bar for extern "system" fn() {}

src/test/rustdoc/ffi.rs

@@ -13,10 +13,10 @@
 
 extern crate rustdoc_ffi as lib;
 
-// @has ffi/fn.foreigner.html //pre 'pub unsafe extern fn foreigner(cold_as_ice: u32)'
+// @has ffi/fn.foreigner.html //pre 'pub unsafe extern "C" fn foreigner(cold_as_ice: u32)'
 pub use lib::foreigner;
 
 extern "C" {
-    // @has ffi/fn.another.html //pre 'pub unsafe extern fn another(cold_as_ice: u32)'
+    // @has ffi/fn.another.html //pre 'pub unsafe extern "C" fn another(cold_as_ice: u32)'
     pub fn another(cold_as_ice: u32);
 }

src/test/rustdoc/issue-22038.rs

@@ -10,7 +10,7 @@
 
 extern {
     // @has issue_22038/fn.foo1.html \
-    //      '//*[@class="rust fn"]' 'pub unsafe extern fn foo1()'
+    //      '//*[@class="rust fn"]' 'pub unsafe extern "C" fn foo1()'
     pub fn foo1();
 }
 
@@ -21,7 +21,7 @@ extern "system" {
 }
 
 // @has issue_22038/fn.bar.html \
-//      '//*[@class="rust fn"]' 'pub extern fn bar()'
+//      '//*[@class="rust fn"]' 'pub extern "C" fn bar()'
 pub extern fn bar() {}
 
 // @has issue_22038/fn.baz.html \

src/test/rustdoc/variadic.rs

@@ -9,6 +9,6 @@
 // except according to those terms.
 
 extern "C" {
-    // @has variadic/fn.foo.html //pre 'pub unsafe extern fn foo(x: i32, ...)'
+    // @has variadic/fn.foo.html //pre 'pub unsafe extern "C" fn foo(x: i32, ...)'
     pub fn foo(x: i32, ...);
 }