src/tools/clippy/clippy_lints/src/methods/inefficient_to_string.rs

   1 use clippy_utils::diagnostics::span_lint_and_then;
   2 use clippy_utils::source::snippet_with_applicability;
   3 use clippy_utils::ty::{is_type_diagnostic_item, walk_ptrs_ty_depth};
   4 use clippy_utils::{match_def_path, paths};
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir as hir;
   8 use rustc_lint::LateContext;
   9 use rustc_middle::ty::{self, Ty};
  10 use rustc_span::symbol::{sym, Symbol};
  11
  12 use super::INEFFICIENT_TO_STRING;
  13
  14 /// Checks for the `INEFFICIENT_TO_STRING` lint
  15 pub fn check<'tcx>(
  16     cx: &LateContext<'tcx>,
  17     expr: &hir::Expr<'_>,
  18     method_name: Symbol,
  19     receiver: &hir::Expr<'_>,
  20     args: &[hir::Expr<'_>],
  21 ) {
  22     if_chain! {
  23         if args.is_empty() && method_name == sym::to_string;
  24         if let Some(to_string_meth_did) = cx.typeck_results().type_dependent_def_id(expr.hir_id);
  25         if match_def_path(cx, to_string_meth_did, &paths::TO_STRING_METHOD);
  26         if let Some(substs) = cx.typeck_results().node_substs_opt(expr.hir_id);
  27         let arg_ty = cx.typeck_results().expr_ty_adjusted(receiver);
  28         let self_ty = substs.type_at(0);
  29         let (deref_self_ty, deref_count) = walk_ptrs_ty_depth(self_ty);
  30         if deref_count >= 1;
  31         if specializes_tostring(cx, deref_self_ty);
  32         then {
  33             span_lint_and_then(
  34                 cx,
  35                 INEFFICIENT_TO_STRING,
  36                 expr.span,
  37                 &format!("calling `to_string` on `{}`", arg_ty),
  38                 |diag| {
  39                     diag.help(&format!(
  40                         "`{}` implements `ToString` through a slower blanket impl, but `{}` has a fast specialization of `ToString`",
  41                         self_ty, deref_self_ty
  42                     ));
  43                     let mut applicability = Applicability::MachineApplicable;
  44                     let arg_snippet = snippet_with_applicability(cx, receiver.span, "..", &mut applicability);
  45                     diag.span_suggestion(
  46                         expr.span,
  47                         "try dereferencing the receiver",
  48                         format!("({}{}).to_string()", "*".repeat(deref_count), arg_snippet),
  49                         applicability,
  50                     );
  51                 },
  52             );
  53         }
  54     }
  55 }
  56
  57 /// Returns whether `ty` specializes `ToString`.
  58 /// Currently, these are `str`, `String`, and `Cow<'_, str>`.
  59 fn specializes_tostring(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
  60     if let ty::Str = ty.kind() {
  61         return true;
  62     }
  63
  64     if is_type_diagnostic_item(cx, ty, sym::String) {
  65         return true;
  66     }
  67
  68     if let ty::Adt(adt, substs) = ty.kind() {
  69         match_def_path(cx, adt.did(), &paths::COW) && substs.type_at(1).is_str()
  70     } else {
  71         false
  72     }
  73 }