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     #[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 FormatExpn { call_site, format_args } = match FormatExpn::parse(expr) {
  47             Some(e) if !e.call_site.from_expansion() => e,
  48             _ => return,
  49         };
  50
  51         let mut applicability = Applicability::MachineApplicable;
  52         if format_args.value_args.is_empty() {
  53             if format_args.format_string_parts.is_empty() {
  54                 span_useless_format_empty(cx, call_site, "String::new()".to_owned(), applicability);
  55             } else {
  56                 if_chain! {
  57                     if let [e] = &*format_args.format_string_parts;
  58                     if let ExprKind::Lit(lit) = &e.kind;
  59                     if let Some(s_src) = snippet_opt(cx, lit.span);
  60                     then {
  61                         // Simulate macro expansion, converting {{ and }} to { and }.
  62                         let s_expand = s_src.replace("{{", "{").replace("}}", "}");
  63                         let sugg = format!("{}.to_string()", s_expand);
  64                         span_useless_format(cx, call_site, sugg, applicability);
  65                     }
  66                 }
  67             }
  68         } else if let [value] = *format_args.value_args {
  69             if_chain! {
  70                 if format_args.format_string_symbols == [kw::Empty];
  71                 if match cx.typeck_results().expr_ty(value).peel_refs().kind() {
  72                     ty::Adt(adt, _) => cx.tcx.is_diagnostic_item(sym::String, adt.did),
  73                     ty::Str => true,
  74                     _ => false,
  75                 };
  76                 if let Some(args) = format_args.args();
  77                 if args.iter().all(|arg| arg.is_display() && !arg.has_string_formatting());
  78                 then {
  79                     let is_new_string = match value.kind {
  80                         ExprKind::Binary(..) => true,
  81                         ExprKind::MethodCall(path, ..) => path.ident.name.as_str() == "to_string",
  82                         _ => false,
  83                     };
  84                     let sugg = if is_new_string {
  85                         snippet_with_applicability(cx, value.span, "..", &mut applicability).into_owned()
  86                     } else {
  87                         let sugg = Sugg::hir_with_applicability(cx, value, "<arg>", &mut applicability);
  88                         format!("{}.to_string()", sugg.maybe_par())
  89                     };
  90                     span_useless_format(cx, call_site, sugg, applicability);
  91                 }
  92             }
  93         };
  94     }
  95 }
  96
  97 fn span_useless_format_empty(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
  98     span_lint_and_sugg(
  99         cx,
 100         USELESS_FORMAT,
 101         span,
 102         "useless use of `format!`",
 103         "consider using `String::new()`",
 104         sugg,
 105         applicability,
 106     );
 107 }
 108
 109 fn span_useless_format(cx: &LateContext<'_>, span: Span, sugg: String, applicability: Applicability) {
 110     span_lint_and_sugg(
 111         cx,
 112         USELESS_FORMAT,
 113         span,
 114         "useless use of `format!`",
 115         "consider using `.to_string()`",
 116         sugg,
 117         applicability,
 118     );
 119 }