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_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     /// 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.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                     // Simulate macro expansion, converting {{ and }} to { and }.
  64                     let s_expand = format_args.format_string.snippet.replace("{{", "{").replace("}}", "}");
  65                     let sugg = format!("{s_expand}.to_string()");
  66                     span_useless_format(cx, call_site, sugg, applicability);
  67                 },
  68                 [..] => {},
  69             }
  70         } else if let [arg] = &*format_args.args {
  71             let value = arg.param.value;
  72             if_chain! {
  73                 if format_args.format_string.parts == [kw::Empty];
  74                 if arg.format.is_default();
  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                 then {
  81                     let is_new_string = match value.kind {
  82                         ExprKind::Binary(..) => true,
  83                         ExprKind::MethodCall(path, ..) => path.ident.name == sym::to_string,
  84                         _ => false,
  85                     };
  86                     let sugg = if is_new_string {
  87                         snippet_with_applicability(cx, value.span, "..", &mut applicability).into_owned()
  88                     } else {
  89                         let sugg = Sugg::hir_with_applicability(cx, value, "<arg>", &mut applicability);
  90                         format!("{}.to_string()", sugg.maybe_par())
  91                     };
  92                     span_useless_format(cx, call_site, sugg, applicability);
  93                 }
  94             }
  95         };
  96     }
  97 }
  98
  99 fn span_useless_format_empty(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
 100     span_lint_and_sugg(
 101         cx,
 102         USELESS_FORMAT,
 103         span,
 104         "useless use of `format!`",
 105         "consider using `String::new()`",
 106         sugg,
 107         applicability,
 108     );
 109 }
 110
 111 fn span_useless_format(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
 112     span_lint_and_sugg(
 113         cx,
 114         USELESS_FORMAT,
 115         span,
 116         "useless use of `format!`",
 117         "consider using `.to_string()`",
 118         sugg,
 119         applicability,
 120     );
 121 }