Fix false-positive in panic_params

It might still have false positives, but it’s even less likely.

src/panic.rs

@@ -37,7 +37,8 @@ impl LateLintPass for PanicPass {
             match_path(path, &BEGIN_UNWIND),
             let ExprLit(ref lit) = params[0].node,
             let LitKind::Str(ref string, _) = lit.node,
-            string.contains('{'),
+            let Some(par) = string.find('{'),
+            string[par..].contains('}'),
             let Some(sp) = cx.sess().codemap()
                              .with_expn_info(expr.span.expn_id,
                                              |info| info.map(|i| i.call_site))

tests/compile-fail/panic.rs

@@ -4,10 +4,14 @@
 #[deny(panic_params)]
 
 fn missing() {
+    if true {
         panic!("{}"); //~ERROR: You probably are missing some parameter
+    } else {
+        panic!("{:?}"); //~ERROR: You probably are missing some parameter
+    }
 }
 
-fn ok_sigle() {
+fn ok_single() {
     panic!("foo bar");
 }
 
@@ -15,8 +19,18 @@ fn ok_multiple() {
     panic!("{}", "This is {ok}");
 }
 
+fn ok_bracket() {
+    // the match is just here because of #759, it serves no other purpose for the lint
+    match 42 {
+        1337 => panic!("{so is this"),
+        666 => panic!("so is this}"),
+        _ => panic!("}so is that{"),
+    }
+}
+
 fn main() {
     missing();
-    ok_sigle();
+    ok_single();
     ok_multiple();
+    ok_bracket();
 }