1 use clippy_utils::diagnostics::span_lint_and_sugg;
2 use clippy_utils::macros::{root_macro_call_first_node, FormatArgsExpn};
3 use clippy_utils::source::{snippet_opt, snippet_with_applicability};
4 use clippy_utils::sugg::Sugg;
5 use if_chain::if_chain;
6 use rustc_errors::Applicability;
7 use rustc_hir::{Expr, ExprKind};
8 use rustc_lint::{LateContext, LateLintPass};
10 use rustc_session::{declare_lint_pass, declare_tool_lint};
11 use rustc_span::symbol::kw;
12 use rustc_span::{sym, BytePos, Span};
14 declare_clippy_lint! {
16 /// Checks for the use of `format!("string literal with no
17 /// argument")` and `format!("{}", foo)` where `foo` is a string.
19 /// ### Why is this bad?
20 /// There is no point of doing that. `format!("foo")` can
21 /// be replaced by `"foo".to_owned()` if you really need a `String`. The even
22 /// worse `&format!("foo")` is often encountered in the wild. `format!("{}",
23 /// foo)` can be replaced by `foo.clone()` if `foo: String` or `foo.to_owned()`
31 /// format!("{}", foo);
36 #[clippy::version = "pre 1.29.0"]
39 "useless use of `format!`"
42 declare_lint_pass!(UselessFormat => [USELESS_FORMAT]);
44 impl<'tcx> LateLintPass<'tcx> for UselessFormat {
45 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
46 let (format_args, call_site) = if_chain! {
47 if let Some(macro_call) = root_macro_call_first_node(cx, expr);
48 if cx.tcx.is_diagnostic_item(sym::format_macro, macro_call.def_id);
49 if let Some(format_args) = FormatArgsExpn::find_nested(cx, expr, macro_call.expn);
51 (format_args, macro_call.span)
57 let mut applicability = Applicability::MachineApplicable;
58 if format_args.value_args.is_empty() {
59 match *format_args.format_string_parts {
60 [] => span_useless_format_empty(cx, call_site, "String::new()".to_owned(), applicability),
62 if let Some(s_src) = snippet_opt(cx, format_args.format_string_span) {
63 // Simulate macro expansion, converting {{ and }} to { and }.
64 let s_expand = s_src.replace("{{", "{").replace("}}", "}");
65 let sugg = format!("{}.to_string()", s_expand);
66 span_useless_format(cx, call_site, sugg, applicability);
71 } else if let [value] = *format_args.value_args {
73 if format_args.format_string_parts == [kw::Empty];
74 if match cx.typeck_results().expr_ty(value).peel_refs().kind() {
75 ty::Adt(adt, _) => cx.tcx.is_diagnostic_item(sym::String, adt.did()),
79 if let Some(args) = format_args.args();
80 if args.iter().all(|arg| arg.format_trait == sym::Display && !arg.has_string_formatting());
82 let is_new_string = match value.kind {
83 ExprKind::Binary(..) => true,
84 ExprKind::MethodCall(path, ..) => path.ident.name.as_str() == "to_string",
87 let sugg = if format_args.format_string_span.contains(value.span) {
88 // Implicit argument. e.g. `format!("{x}")` span points to `{x}`
89 let spdata = value.span.data();
91 spdata.lo + BytePos(1),
92 spdata.hi - BytePos(1),
96 let snip = snippet_with_applicability(cx, span, "..", &mut applicability);
100 format!("{snip}.to_string()")
102 } else if is_new_string {
103 snippet_with_applicability(cx, value.span, "..", &mut applicability).into_owned()
105 let sugg = Sugg::hir_with_applicability(cx, value, "<arg>", &mut applicability);
106 format!("{}.to_string()", sugg.maybe_par())
108 span_useless_format(cx, call_site, sugg, applicability);
115 fn span_useless_format_empty(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
120 "useless use of `format!`",
121 "consider using `String::new()`",
127 fn span_useless_format(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
132 "useless use of `format!`",
133 "consider using `.to_string()`",