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::last_path_segment;
   4 use clippy_utils::source::{snippet_opt, snippet_with_applicability};
   5 use clippy_utils::sugg::Sugg;
   6 use if_chain::if_chain;
   7 use rustc_errors::Applicability;
   8 use rustc_hir::{BorrowKind, Expr, ExprKind, QPath};
   9 use rustc_lint::{LateContext, LateLintPass};
  10 use rustc_middle::ty;
  11 use rustc_session::{declare_lint_pass, declare_tool_lint};
  12 use rustc_span::symbol::kw;
  13 use rustc_span::{sym, Span};
  14
  15 declare_clippy_lint! {
  16     /// ### What it does
  17     /// Checks for the use of `format!("string literal with no
  18     /// argument")` and `format!("{}", foo)` where `foo` is a string.
  19     ///
  20     /// ### Why is this bad?
  21     /// There is no point of doing that. `format!("foo")` can
  22     /// be replaced by `"foo".to_owned()` if you really need a `String`. The even
  23     /// worse `&format!("foo")` is often encountered in the wild. `format!("{}",
  24     /// foo)` can be replaced by `foo.clone()` if `foo: String` or `foo.to_owned()`
  25     /// if `foo: &str`.
  26     ///
  27     /// ### Examples
  28     /// ```rust
  29     ///
  30     /// // Bad
  31     /// let foo = "foo";
  32     /// format!("{}", foo);
  33     ///
  34     /// // Good
  35     /// foo.to_owned();
  36     /// ```
  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_chain! {
  54                 if let [e] = &*format_args.format_string_parts;
  55                 if let ExprKind::Lit(lit) = &e.kind;
  56                 if let Some(s_src) = snippet_opt(cx, lit.span);
  57                 then {
  58                     // Simulate macro expansion, converting {{ and }} to { and }.
  59                     let s_expand = s_src.replace("{{", "{").replace("}}", "}");
  60                     let sugg = format!("{}.to_string()", s_expand);
  61                     span_useless_format(cx, call_site, sugg, applicability);
  62                 }
  63             }
  64         } else if let [value] = *format_args.value_args {
  65             if_chain! {
  66                 if format_args.format_string_symbols == [kw::Empty];
  67                 if match cx.typeck_results().expr_ty(value).peel_refs().kind() {
  68                     ty::Adt(adt, _) => cx.tcx.is_diagnostic_item(sym::string_type, adt.did),
  69                     ty::Str => true,
  70                     _ => false,
  71                 };
  72                 if format_args.args.iter().all(|e| is_display_arg(e));
  73                 if format_args.fmt_expr.map_or(true, |e| check_unformatted(e));
  74                 then {
  75                     let is_new_string = match value.kind {
  76                         ExprKind::Binary(..) => true,
  77                         ExprKind::MethodCall(path, ..) => path.ident.name.as_str() == "to_string",
  78                         _ => false,
  79                     };
  80                     let sugg = if is_new_string {
  81                         snippet_with_applicability(cx, value.span, "..", &mut applicability).into_owned()
  82                     } else {
  83                         let sugg = Sugg::hir_with_applicability(cx, value, "<arg>", &mut applicability);
  84                         format!("{}.to_string()", sugg.maybe_par())
  85                     };
  86                     span_useless_format(cx, call_site, sugg, applicability);
  87                 }
  88             }
  89         };
  90     }
  91 }
  92
  93 fn span_useless_format(cx: &LateContext<'_>, span: Span, mut sugg: String, mut applicability: Applicability) {
  94     // The callsite span contains the statement semicolon for some reason.
  95     if snippet_with_applicability(cx, span, "..", &mut applicability).ends_with(';') {
  96         sugg.push(';');
  97     }
  98
  99     span_lint_and_sugg(
 100         cx,
 101         USELESS_FORMAT,
 102         span,
 103         "useless use of `format!`",
 104         "consider using `.to_string()`",
 105         sugg,
 106         applicability,
 107     );
 108 }
 109
 110 fn is_display_arg(expr: &Expr<'_>) -> bool {
 111     if_chain! {
 112         if let ExprKind::Call(_, [_, fmt]) = expr.kind;
 113         if let ExprKind::Path(QPath::Resolved(_, path)) = fmt.kind;
 114         if let [.., t, _] = path.segments;
 115         if t.ident.name.as_str() == "Display";
 116         then { true } else { false }
 117     }
 118 }
 119
 120 /// Checks if the expression matches
 121 /// ```rust,ignore
 122 /// &[_ {
 123 ///    format: _ {
 124 ///         width: _::Implied,
 125 ///         precision: _::Implied,
 126 ///         ...
 127 ///    },
 128 ///    ...,
 129 /// }]
 130 /// ```
 131 fn check_unformatted(expr: &Expr<'_>) -> bool {
 132     if_chain! {
 133         if let ExprKind::AddrOf(BorrowKind::Ref, _, expr) = expr.kind;
 134         if let ExprKind::Array([expr]) = expr.kind;
 135         // struct `core::fmt::rt::v1::Argument`
 136         if let ExprKind::Struct(_, fields, _) = expr.kind;
 137         if let Some(format_field) = fields.iter().find(|f| f.ident.name == sym::format);
 138         // struct `core::fmt::rt::v1::FormatSpec`
 139         if let ExprKind::Struct(_, fields, _) = format_field.expr.kind;
 140         if let Some(precision_field) = fields.iter().find(|f| f.ident.name == sym::precision);
 141         if let ExprKind::Path(ref precision_path) = precision_field.expr.kind;
 142         if last_path_segment(precision_path).ident.name == sym::Implied;
 143         if let Some(width_field) = fields.iter().find(|f| f.ident.name == sym::width);
 144         if let ExprKind::Path(ref width_qpath) = width_field.expr.kind;
 145         if last_path_segment(width_qpath).ident.name == sym::Implied;
 146         then {
 147             return true;
 148         }
 149     }
 150
 151     false
 152 }