src/tools/clippy/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     ///
  29     /// // Bad
  30     /// let foo = "foo";
  31     /// format!("{}", foo);
  32     ///
  33     /// // Good
  34     /// foo.to_owned();
  35     /// ```
  36     #[clippy::version = "pre 1.29.0"]
  37     pub USELESS_FORMAT,
  38     complexity,
  39     "useless use of `format!`"
  40 }
  41
  42 declare_lint_pass!(UselessFormat => [USELESS_FORMAT]);
  43
  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);
  50             then {
  51                 (format_args, macro_call.span)
  52             } else {
  53                 return
  54             }
  55         };
  56
  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),
  61                 [_] => {
  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);
  67                     }
  68                 },
  69                 [..] => {},
  70             }
  71         } else if let [value] = *format_args.value_args {
  72             if_chain! {
  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),
  76                     ty::Str => true,
  77                     _ => false,
  78                 };
  79                 if let Some(args) = format_args.args();
  80                 if args.iter().all(|arg| arg.format_trait == sym::Display && !arg.has_string_formatting());
  81                 then {
  82                     let is_new_string = match value.kind {
  83                         ExprKind::Binary(..) => true,
  84                         ExprKind::MethodCall(path, ..) => path.ident.name.as_str() == "to_string",
  85                         _ => false,
  86                     };
  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();
  90                         let span = Span::new(
  91                             spdata.lo + BytePos(1),
  92                             spdata.hi - BytePos(1),
  93                             spdata.ctxt,
  94                             spdata.parent
  95                         );
  96                         let snip = snippet_with_applicability(cx, span, "..", &mut applicability);
  97                         if is_new_string {
  98                             snip.into()
  99                         } else {
 100                             format!("{snip}.to_string()")
 101                         }
 102                     } else if is_new_string {
 103                         snippet_with_applicability(cx, value.span, "..", &mut applicability).into_owned()
 104                     } else {
 105                         let sugg = Sugg::hir_with_applicability(cx, value, "<arg>", &mut applicability);
 106                         format!("{}.to_string()", sugg.maybe_par())
 107                     };
 108                     span_useless_format(cx, call_site, sugg, applicability);
 109                 }
 110             }
 111         };
 112     }
 113 }
 114
 115 fn span_useless_format_empty(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
 116     span_lint_and_sugg(
 117         cx,
 118         USELESS_FORMAT,
 119         span,
 120         "useless use of `format!`",
 121         "consider using `String::new()`",
 122         sugg,
 123         applicability,
 124     );
 125 }
 126
 127 fn span_useless_format(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
 128     span_lint_and_sugg(
 129         cx,
 130         USELESS_FORMAT,
 131         span,
 132         "useless use of `format!`",
 133         "consider using `.to_string()`",
 134         sugg,
 135         applicability,
 136     );
 137 }