auto merge of #13579 : hirschenberger/rust/lint_unsigned_negate, r=alexcrichton

See #11273 and #13318

src/test/compile-fail/lint-type-limits.rs

@@ -36,3 +36,14 @@ fn qux() {
         i += 1;
     }
 }
+
+fn quy() {
+    let i = -23u; //~ WARNING negation of unsigned int literal may be unintentional
+                  //~^ WARNING unused variable
+}
+
+fn quz() {
+    let i = 23u;
+    let j = -i;   //~ WARNING negation of unsigned int variable may be unintentional
+                  //~^ WARNING unused variable
+}