rustc: Warn about dead constants

A few catch-all blocks ended up not having this case for constants.

Closes #17925

src/librustc/middle/dead.rs

@@ -441,6 +441,7 @@ impl<'a, 'tcx> DeadVisitor<'a, 'tcx> {
     fn should_warn_about_item(&mut self, item: &ast::Item) -> bool {
         let should_warn = match item.node {
             ast::ItemStatic(..)
+            | ast::ItemConst(..)
             | ast::ItemFn(..)
             | ast::ItemEnum(..)
             | ast::ItemStruct(..) => true,

src/libsyntax/ast.rs

@@ -1334,14 +1334,16 @@ impl Item_ {
     pub fn descriptive_variant(&self) -> &str {
         match *self {
             ItemStatic(..) => "static item",
+            ItemConst(..) => "constant item",
             ItemFn(..) => "function",
             ItemMod(..) => "module",
             ItemForeignMod(..) => "foreign module",
             ItemTy(..) => "type alias",
             ItemEnum(..) => "enum",
             ItemStruct(..) => "struct",
             ItemTrait(..) => "trait",
-            _ => "item"
+            ItemMac(..) |
+            ItemImpl(..) => "item"
         }
     }
 }

src/test/compile-fail/lint-dead-code-1.rs

@@ -35,6 +35,13 @@ pub static used_static2: int = used_static;
 const USED_STATIC: int = 0;
 const STATIC_USED_IN_ENUM_DISCRIMINANT: int = 10;
 
+pub const pub_const: int = 0;
+const priv_const: int = 0; //~ ERROR: constant item is never used
+const used_const: int = 0;
+pub const used_const2: int = used_const;
+const USED_CONST: int = 0;
+const CONST_USED_IN_ENUM_DISCRIMINANT: int = 10;
+
 pub type typ = *const UsedStruct4;
 pub struct PubStruct;
 struct PrivStruct; //~ ERROR: struct is never used