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/librustdoc/clean/mod.rs

@@ -1721,7 +1721,7 @@ impl<'tcx> Clean<Type> for ty::Ty<'tcx> {
                     where_predicates: Vec::new()
                 },
                 decl: (cx.map.local_def_id(0), &fty.sig).clean(cx),
-                abi: fty.abi.to_string(),
+                abi: fty.abi,
             }),
             ty::TyStruct(def, substs) |
             ty::TyEnum(def, substs) => {
@@ -2143,7 +2143,7 @@ pub struct BareFunctionDecl {
     pub unsafety: hir::Unsafety,
     pub generics: Generics,
     pub decl: FnDecl,
-    pub abi: String,
+    pub abi: Abi,
 }
 
 impl Clean<BareFunctionDecl> for hir::BareFnTy {
@@ -2156,7 +2156,7 @@ impl Clean<BareFunctionDecl> for hir::BareFnTy {
                 where_predicates: Vec::new()
             },
             decl: self.decl.clean(cx),
-            abi: self.abi.to_string(),
+            abi: self.abi,
         }
     }
 }

src/librustdoc/html/format.rs

@@ -451,11 +451,7 @@ impl fmt::Display for clean::Type {
             clean::BareFunction(ref decl) => {
                 write!(f, "{}{}fn{}{}",
                        UnsafetySpace(decl.unsafety),
-                       match &*decl.abi {
-                           "" => " extern ".to_string(),
-                           "\"Rust\"" => "".to_string(),
-                           s => format!(" extern {} ", s)
-                       },
+                       AbiSpace(decl.abi),
                        decl.generics,
                        decl.decl)
             }
@@ -770,8 +766,7 @@ impl fmt::Display for AbiSpace {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self.0 {
             Abi::Rust => Ok(()),
-            Abi::C => write!(f, "extern "),
-            abi => write!(f, "extern {} ", abi),
+            abi => write!(f, "extern "{}" ", abi.name()),
         }
     }
 }

src/librustdoc/html/render.rs

@@ -2129,8 +2129,6 @@ fn render_assoc_item(w: &mut fmt::Formatter,
               d: &clean::FnDecl,
               link: AssocItemLink)
               -> fmt::Result {
-        use syntax::abi::Abi;
-
         let name = meth.name.as_ref().unwrap();
         let anchor = format!("#{}.{}", shortty(meth), name);
         let href = match link {
@@ -2157,10 +2155,7 @@ fn render_assoc_item(w: &mut fmt::Formatter,
                    {generics}{decl}{where_clause}",
                ConstnessSpace(vis_constness),
                UnsafetySpace(unsafety),
-               match abi {
-                   Abi::Rust => String::new(),
-                   a => format!("extern {} ", a.to_string())
-               },
+               AbiSpace(abi),
                href = href,
                name = name,
                generics = *g,