1 use clippy_utils::diagnostics::span_lint_and_sugg;
2 use clippy_utils::higher::FormatExpn;
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, 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 FormatExpn { call_site, format_args } = match FormatExpn::parse(expr) {
47 Some(e) if !e.call_site.from_expansion() => e,
51 let mut applicability = Applicability::MachineApplicable;
52 if format_args.value_args.is_empty() {
53 if format_args.format_string_parts.is_empty() {
54 span_useless_format_empty(cx, call_site, "String::new()".to_owned(), applicability);
57 if let [e] = &*format_args.format_string_parts;
58 if let ExprKind::Lit(lit) = &e.kind;
59 if let Some(s_src) = snippet_opt(cx, lit.span);
61 // Simulate macro expansion, converting {{ and }} to { and }.
62 let s_expand = s_src.replace("{{", "{").replace("}}", "}");
63 let sugg = format!("{}.to_string()", s_expand);
64 span_useless_format(cx, call_site, sugg, applicability);
68 } else if let [value] = *format_args.value_args {
70 if format_args.format_string_symbols == [kw::Empty];
71 if match cx.typeck_results().expr_ty(value).peel_refs().kind() {
72 ty::Adt(adt, _) => cx.tcx.is_diagnostic_item(sym::String, adt.did),
76 if let Some(args) = format_args.args();
77 if args.iter().all(|arg| arg.is_display() && !arg.has_string_formatting());
79 let is_new_string = match value.kind {
80 ExprKind::Binary(..) => true,
81 ExprKind::MethodCall(path, ..) => path.ident.name.as_str() == "to_string",
84 let sugg = if is_new_string {
85 snippet_with_applicability(cx, value.span, "..", &mut applicability).into_owned()
87 let sugg = Sugg::hir_with_applicability(cx, value, "<arg>", &mut applicability);
88 format!("{}.to_string()", sugg.maybe_par())
90 span_useless_format(cx, call_site, sugg, applicability);
97 fn span_useless_format_empty(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
102 "useless use of `format!`",
103 "consider using `String::new()`",
109 fn span_useless_format(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
114 "useless use of `format!`",
115 "consider using `.to_string()`",