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 format_args.format_string_parts.is_empty() {
  53                 span_useless_format_empty(cx, call_site, "String::new()".to_owned(), applicability);
  54             } else {
  55                 if_chain! {
  56                     if let [e] = &*format_args.format_string_parts;
  57                     if let ExprKind::Lit(lit) = &e.kind;
  58                     if let Some(s_src) = snippet_opt(cx, lit.span);
  59                     then {
  60                         // Simulate macro expansion, converting {{ and }} to { and }.
  61                         let s_expand = s_src.replace("{{", "{").replace("}}", "}");
  62                         let sugg = format!("{}.to_string()", s_expand);
  63                         span_useless_format(cx, call_site, sugg, applicability);
  64                     }
  65                 }
  66             }
  67         } else if let [value] = *format_args.value_args {
  68             if_chain! {
  69                 if format_args.format_string_symbols == [kw::Empty];
  70                 if match cx.typeck_results().expr_ty(value).peel_refs().kind() {
  71                     ty::Adt(adt, _) => cx.tcx.is_diagnostic_item(sym::String, adt.did),
  72                     ty::Str => true,
  73                     _ => false,
  74                 };
  75                 if let Some(args) = format_args.args();
  76                 if args.iter().all(|arg| arg.is_display() && !arg.has_string_formatting());
  77                 then {
  78                     let is_new_string = match value.kind {
  79                         ExprKind::Binary(..) => true,
  80                         ExprKind::MethodCall(path, ..) => path.ident.name.as_str() == "to_string",
  81                         _ => false,
  82                     };
  83                     let sugg = if is_new_string {
  84                         snippet_with_applicability(cx, value.span, "..", &mut applicability).into_owned()
  85                     } else {
  86                         let sugg = Sugg::hir_with_applicability(cx, value, "<arg>", &mut applicability);
  87                         format!("{}.to_string()", sugg.maybe_par())
  88                     };
  89                     span_useless_format(cx, call_site, sugg, applicability);
  90                 }
  91             }
  92         };
  93     }
  94 }
  95
  96 fn span_useless_format_empty(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
  97     span_lint_and_sugg(
  98         cx,
  99         USELESS_FORMAT,
 100         span,
 101         "useless use of `format!`",
 102         "consider using `String::new()`",
 103         sugg,
 104         applicability,
 105     );
 106 }
 107
 108 fn span_useless_format(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
 109     span_lint_and_sugg(
 110         cx,
 111         USELESS_FORMAT,
 112         span,
 113         "useless use of `format!`",
 114         "consider using `.to_string()`",
 115         sugg,
 116         applicability,
 117     );
 118 }