Don't suggest to_string().to_string in USELESS_FORMAT

clippy_lints/src/format.rs

@@ -57,12 +57,24 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
                         if check_single_piece(&args[0]);
                         if let Some(format_arg) = get_single_string_arg(cx, &args[1]);
                         if check_unformatted(&args[2]);
+                        if let ExprKind::AddrOf(_, ref format_arg) = format_arg.node;
                         then {
-                            let sugg = format!("{}.to_string()", snippet(cx, format_arg, "<arg>").into_owned());
+                            let (message, sugg) = if_chain! {
+                                if let ExprKind::MethodCall(ref path, ref span, ref expr) = format_arg.node;
+                                if path.ident.as_interned_str() == "to_string";
+                                then {
+                                    ("`to_string()` is enough",
+                                    snippet(cx, format_arg.span, "<arg>").to_string())
+                                } else {
+                                    ("consider using .to_string()",
+                                    format!("{}.to_string()", snippet(cx, format_arg.span, "<arg>")))
+                                }
+                            };
+
                             span_lint_and_then(cx, USELESS_FORMAT, span, "useless use of `format!`", |db| {
                                 db.span_suggestion_with_applicability(
                                     expr.span,
-                                    "consider using .to_string()",
+                                    message,
                                     sugg,
                                     Applicability::MachineApplicable,
                                 );
@@ -113,9 +125,9 @@ fn check_single_piece(expr: &Expr) -> bool {
 /// ::std::fmt::Display::fmt)],
 /// }
 /// ```
-/// and that type of `__arg0` is `&str` or `String`
+/// and that the type of `__arg0` is `&str` or `String`,
-/// then returns the span of first element of the matched tuple
+/// then returns the span of first element of the matched tuple.
-fn get_single_string_arg(cx: &LateContext<'_, '_>, expr: &Expr) -> Option<Span> {
+fn get_single_string_arg<'a>(cx: &LateContext<'_, '_>, expr: &'a Expr) -> Option<&'a Expr> {
     if_chain! {
         if let ExprKind::AddrOf(_, ref expr) = expr.node;
         if let ExprKind::Match(ref match_expr, ref arms, _) = expr.node;
@@ -134,7 +146,7 @@ fn get_single_string_arg(cx: &LateContext<'_, '_>, expr: &Expr) -> Option<Span>
             let ty = walk_ptrs_ty(cx.tables.pat_ty(&pat[0]));
             if ty.sty == ty::Str || match_type(cx, ty, &paths::STRING) {
                 if let ExprKind::Tup(ref values) = match_expr.node {
-                    return Some(values[0].span);
+                    return Some(&values[0]);
                 }
             }
         }

tests/ui/format.rs

@@ -52,4 +52,8 @@ fn main() {
     format!("{:.10}", "foo"); // could not be "foo"[..10]
     format!("{:.prec$}", "foo", prec = 1);
     format!("{:.prec$}", "foo", prec = 10);
+
+    format!("{}", 42.to_string());
+    let x = std::path::PathBuf::from("/bar/foo/qux");
+    format!("{}", x.display().to_string());
 }

tests/ui/format.stderr

@@ -54,5 +54,21 @@ error: useless use of `format!`
    |
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
-error: aborting due to 7 previous errors
+error: useless use of `format!`
+  --> $DIR/format.rs:56:5
+   |
+56 |     format!("{}", 42.to_string());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: `to_string()` is enough: `42.to_string()`
+   |
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+error: useless use of `format!`
+  --> $DIR/format.rs:58:5
+   |
+58 |     format!("{}", x.display().to_string());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: `to_string()` is enough: `x.display().to_string()`
+   |
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+error: aborting due to 9 previous errors