Add a test for a shadow corner case

clippy_lints/src/shadow.rs

@@ -262,7 +262,7 @@ fn lint_shadow<T>(cx: &LateContext, name: Name, span: Span, pattern_span: Span,
         span_lint_and_then(cx,
                            SHADOW_UNRELATED,
                            span,
-                           &format!("{} shadows a previous declaration", snippet(cx, pattern_span, "_")),
+                           &format!("`{}` shadows a previous declaration", snippet(cx, pattern_span, "_")),
                            |db| { db.span_note(prev_span, "previous binding is here"); });
     }
 }

tests/compile-fail/shadow.rs

@@ -20,6 +20,9 @@ fn main() {
     let y = 1;
     let x = y; //~ERROR `x` is shadowed by `y`
 
+    let x; //~ERROR `x` shadows a previous declaration
+    x = 42;
+
     let o = Some(1_u8);
 
     if let Some(p) = o { assert_eq!(1, p); }