Re-add false positive check

clippy_lints/src/format.rs

@@ -90,6 +90,10 @@ fn on_argumentv1_new<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr, arm
         if let PatKind::Tuple(ref pats, None) = arms[0].pats[0].node;
         if pats.len() == 1;
         then {
+            let ty = walk_ptrs_ty(cx.tables.pat_ty(&pats[0]));
+            if ty.sty != rustc::ty::Str && !match_type(cx, ty, &paths::STRING) {
+                return None;
+            }
             if let ExprKind::Lit(ref lit) = format_args.node {
                 if let LitKind::Str(ref s, _) = lit.node {
                     return Some(format!("{:?}.to_string()", s.as_str()));
@@ -100,6 +104,8 @@ fn on_argumentv1_new<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr, arm
                     if path.ident.name == sym!(to_string) {
                         return Some(format!("{}", snip));
                     }
+                } else if let ExprKind::Binary(..) = format_args.node {
+                    return Some(format!("{}", snip));
                 }
                 return Some(format!("{}.to_string()", snip));
             }

tests/ui/format.fixed

@@ -60,4 +60,8 @@ fn main() {
     42.to_string();
     let x = std::path::PathBuf::from("/bar/foo/qux");
     x.display().to_string();
+
+    // False positive
+    let a = "foo".to_string();
+    let _ = Some(a + "bar");
 }

tests/ui/format.rs

@@ -63,4 +63,8 @@ fn main() {
     format!("{}", 42.to_string());
     let x = std::path::PathBuf::from("/bar/foo/qux");
     format!("{}", x.display().to_string());
+
+    // False positive
+    let a = "foo".to_string();
+    let _ = Some(format!("{}", a + "bar"));
 }

tests/ui/format.stderr

@@ -75,5 +75,11 @@ error: useless use of `format!`
 LL |     format!("{}", x.display().to_string());
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using .to_string(): `x.display().to_string();`
 
-error: aborting due to 12 previous errors
+error: useless use of `format!`
+  --> $DIR/format.rs:69:18
+   |
+LL |     let _ = Some(format!("{}", a + "bar"));
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using .to_string(): `a + "bar"`
+
+error: aborting due to 13 previous errors