src/tools/clippy/clippy_lints/src/format.rs

   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};
   9 use rustc_middle::ty;
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11 use rustc_span::symbol::kw;
  12 use rustc_span::{sym, Span};
  13
  14 declare_clippy_lint! {
  15     /// ### What it does
  16     /// Checks for the use of `format!("string literal with no
  17     /// argument")` and `format!("{}", foo)` where `foo` is a string.
  18     ///
  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()`
  24     /// if `foo: &str`.
  25     ///
  26     /// ### Examples
  27     /// ```rust
  28     ///
  29     /// // Bad
  30     /// let foo = "foo";
  31     /// format!("{}", foo);
  32     ///
  33     /// // Good
  34     /// foo.to_owned();
  35     /// ```
  36     pub USELESS_FORMAT,
  37     complexity,
  38     "useless use of `format!`"
  39 }
  40
  41 declare_lint_pass!(UselessFormat => [USELESS_FORMAT]);
  42
  43 impl<'tcx> LateLintPass<'tcx> for UselessFormat {
  44     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  45         let FormatExpn { call_site, format_args } = match FormatExpn::parse(expr) {
  46             Some(e) if !e.call_site.from_expansion() => e,
  47             _ => return,
  48         };
  49
  50         let mut applicability = Applicability::MachineApplicable;
  51         if format_args.value_args.is_empty() {
  52             if_chain! {
  53                 if let [e] = &*format_args.format_string_parts;
  54                 if let ExprKind::Lit(lit) = &e.kind;
  55                 if let Some(s_src) = snippet_opt(cx, lit.span);
  56                 then {
  57                     // Simulate macro expansion, converting {{ and }} to { and }.
  58                     let s_expand = s_src.replace("{{", "{").replace("}}", "}");
  59                     let sugg = format!("{}.to_string()", s_expand);
  60                     span_useless_format(cx, call_site, sugg, applicability);
  61                 }
  62             }
  63         } else if let [value] = *format_args.value_args {
  64             if_chain! {
  65                 if format_args.format_string_symbols == [kw::Empty];
  66                 if match cx.typeck_results().expr_ty(value).peel_refs().kind() {
  67                     ty::Adt(adt, _) => cx.tcx.is_diagnostic_item(sym::String, adt.did),
  68                     ty::Str => true,
  69                     _ => false,
  70                 };
  71                 if let Some(args) = format_args.args();
  72                 if args.iter().all(|arg| arg.is_display() && !arg.has_string_formatting());
  73                 then {
  74                     let is_new_string = match value.kind {
  75                         ExprKind::Binary(..) => true,
  76                         ExprKind::MethodCall(path, ..) => path.ident.name.as_str() == "to_string",
  77                         _ => false,
  78                     };
  79                     let sugg = if is_new_string {
  80                         snippet_with_applicability(cx, value.span, "..", &mut applicability).into_owned()
  81                     } else {
  82                         let sugg = Sugg::hir_with_applicability(cx, value, "<arg>", &mut applicability);
  83                         format!("{}.to_string()", sugg.maybe_par())
  84                     };
  85                     span_useless_format(cx, call_site, sugg, applicability);
  86                 }
  87             }
  88         };
  89     }
  90 }
  91
  92 fn span_useless_format(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
  93     span_lint_and_sugg(
  94         cx,
  95         USELESS_FORMAT,
  96         span,
  97         "useless use of `format!`",
  98         "consider using `.to_string()`",
  99         sugg,
 100         applicability,
 101     );
 102 }