clippy_lints/src/format.rs

   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};
   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, BytePos, 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     /// let foo = "foo";
  29     /// format!("{}", foo);
  30     /// ```
  31     ///
  32     /// Use instead:
  33     /// ```rust
  34     /// let foo = "foo";
  35     /// foo.to_owned();
  36     /// ```
  37     #[clippy::version = "pre 1.29.0"]
  38     pub USELESS_FORMAT,
  39     complexity,
  40     "useless use of `format!`"
  41 }
  42
  43 declare_lint_pass!(UselessFormat => [USELESS_FORMAT]);
  44
  45 impl<'tcx> LateLintPass<'tcx> for UselessFormat {
  46     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  47         let (format_args, call_site) = if_chain! {
  48             if let Some(macro_call) = root_macro_call_first_node(cx, expr);
  49             if cx.tcx.is_diagnostic_item(sym::format_macro, macro_call.def_id);
  50             if let Some(format_args) = FormatArgsExpn::find_nested(cx, expr, macro_call.expn);
  51             then {
  52                 (format_args, macro_call.span)
  53             } else {
  54                 return
  55             }
  56         };
  57
  58         let mut applicability = Applicability::MachineApplicable;
  59         if format_args.value_args.is_empty() {
  60             match *format_args.format_string_parts {
  61                 [] => span_useless_format_empty(cx, call_site, "String::new()".to_owned(), applicability),
  62                 [_] => {
  63                     if let Some(s_src) = snippet_opt(cx, format_args.format_string_span) {
  64                         // Simulate macro expansion, converting {{ and }} to { and }.
  65                         let s_expand = s_src.replace("{{", "{").replace("}}", "}");
  66                         let sugg = format!("{}.to_string()", s_expand);
  67                         span_useless_format(cx, call_site, sugg, applicability);
  68                     }
  69                 },
  70                 [..] => {},
  71             }
  72         } else if let [value] = *format_args.value_args {
  73             if_chain! {
  74                 if format_args.format_string_parts == [kw::Empty];
  75                 if match cx.typeck_results().expr_ty(value).peel_refs().kind() {
  76                     ty::Adt(adt, _) => cx.tcx.is_diagnostic_item(sym::String, adt.did()),
  77                     ty::Str => true,
  78                     _ => false,
  79                 };
  80                 if let Some(args) = format_args.args();
  81                 if args.iter().all(|arg| arg.format_trait == sym::Display && !arg.has_string_formatting());
  82                 then {
  83                     let is_new_string = match value.kind {
  84                         ExprKind::Binary(..) => true,
  85                         ExprKind::MethodCall(path, ..) => path.ident.name == sym::to_string,
  86                         _ => false,
  87                     };
  88                     let sugg = if format_args.format_string_span.contains(value.span) {
  89                         // Implicit argument. e.g. `format!("{x}")` span points to `{x}`
  90                         let spdata = value.span.data();
  91                         let span = Span::new(
  92                             spdata.lo + BytePos(1),
  93                             spdata.hi - BytePos(1),
  94                             spdata.ctxt,
  95                             spdata.parent
  96                         );
  97                         let snip = snippet_with_applicability(cx, span, "..", &mut applicability);
  98                         if is_new_string {
  99                             snip.into()
 100                         } else {
 101                             format!("{snip}.to_string()")
 102                         }
 103                     } else if is_new_string {
 104                         snippet_with_applicability(cx, value.span, "..", &mut applicability).into_owned()
 105                     } else {
 106                         let sugg = Sugg::hir_with_applicability(cx, value, "<arg>", &mut applicability);
 107                         format!("{}.to_string()", sugg.maybe_par())
 108                     };
 109                     span_useless_format(cx, call_site, sugg, applicability);
 110                 }
 111             }
 112         };
 113     }
 114 }
 115
 116 fn span_useless_format_empty(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
 117     span_lint_and_sugg(
 118         cx,
 119         USELESS_FORMAT,
 120         span,
 121         "useless use of `format!`",
 122         "consider using `String::new()`",
 123         sugg,
 124         applicability,
 125     );
 126 }
 127
 128 fn span_useless_format(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
 129     span_lint_and_sugg(
 130         cx,
 131         USELESS_FORMAT,
 132         span,
 133         "useless use of `format!`",
 134         "consider using `.to_string()`",
 135         sugg,
 136         applicability,
 137     );
 138 }