Updated E0423 to new format

Author: Knight

src/librustc_resolve/lib.rs

@@ -336,12 +336,14 @@ fn resolve_struct_error<'b, 'a: 'b, 'c>(resolver: &'b Resolver<'a>,
             err
         }
         ResolutionError::StructVariantUsedAsFunction(path_name) => {
-            struct_span_err!(resolver.session,
+            let mut err = struct_span_err!(resolver.session,
                              span,
                              E0423,
                              "`{}` is the name of a struct or struct variant, but this expression \
                              uses it like a function name",
-                             path_name)
+                             path_name);
+            err.span_label(span, &format!("struct called like a function"));
+            err
         }
         ResolutionError::SelfNotAvailableInStaticMethod => {
             struct_span_err!(resolver.session,

src/test/compile-fail/E0423.rs

@@ -12,4 +12,5 @@ fn main () {
     struct Foo { a: bool };
 
     let f = Foo(); //~ ERROR E0423
+                   //~^ struct called like a function
 }

src/test/compile-fail/issue-18252.rs

@@ -13,5 +13,7 @@ enum Foo {
 }
 
 fn main() {
-    let f = Foo::Variant(42); //~ ERROR uses it like a function
+    let f = Foo::Variant(42);
+    //~^ ERROR uses it like a function
+    //~| struct called like a function
 }

src/test/compile-fail/issue-19452.rs

@@ -13,5 +13,7 @@ enum Homura {
 }
 
 fn main() {
-    let homura = Homura::Madoka; //~ ERROR uses it like a function
+    let homura = Homura::Madoka;
+    //~^ ERROR uses it like a function
+    //~| struct called like a function
 }