Use correct article in help message for conversion or cast

Before it always used `an`; now it uses the correct article for the type.

compiler/rustc_middle/src/ty/sty.rs

@@ -210,6 +210,18 @@ impl TyKind<'tcx> {
             _ => false,
         }
     }
+
+    /// Get the article ("a" or "an") to use with this type.
+    ///
+    /// **Panics if `self` is [`TyKind::Error`].**
+    pub fn article(&self) -> &'static str {
+        match self {
+            Int(_) | Float(_) | Array(_, _) => "an",
+            Adt(def, _) if def.is_enum() => "an",
+            Error(_) => panic!(),
+            _ => "a",
+        }
+    }
 }
 
 // `TyKind` is used a lot. Make sure it doesn't unintentionally get bigger.