clippy_lints/src/methods/inefficient_to_string.rs

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