Fix slice const generic length display

src/librustdoc/clean/mod.rs

@@ -2768,7 +2768,10 @@ impl Clean<Type> for hir::Ty {
                 };
                 let length = match cx.tcx.const_eval(param_env.and(cid)) {
                     Ok(length) => print_const(cx, length),
-                    Err(_) => "_".to_string(),
+                    Err(_) => cx.sess()
+                                .source_map()
+                                .span_to_snippet(cx.tcx.def_span(def_id))
+                                .unwrap_or_else(|_| "_".to_string()),
                 };
                 Array(box ty.clean(cx), length)
             },

src/test/rustdoc/const-generics/const-generic-slice.rs

@@ -0,0 +1,12 @@
+#![crate_name = "foo"]
+#![feature(const_generics)]
+
+pub trait Array {
+    type Item;
+}
+
+// @has foo/trait.Array.html
+// @has - '//h3[@class="impl"]' 'impl<T, const N: usize> Array for [T; N]'
+impl <T, const N: usize> Array for [T; N] {
+    type Item = T;
+}