1 use clippy_utils::diagnostics::span_lint_and_then;
2 use clippy_utils::source::snippet_with_applicability;
3 use clippy_utils::ty::{is_type_lang_item, walk_ptrs_ty_depth};
4 use clippy_utils::{match_def_path, paths};
5 use if_chain::if_chain;
6 use rustc_errors::Applicability;
8 use rustc_lint::LateContext;
9 use rustc_middle::ty::{self, Ty};
10 use rustc_span::symbol::{Symbol, sym};
12 use super::INEFFICIENT_TO_STRING;
14 /// Checks for the `INEFFICIENT_TO_STRING` lint
19 receiver: &hir::Expr<'_>,
20 args: &[hir::Expr<'_>],
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);
31 if specializes_tostring(cx, deref_self_ty);
35 INEFFICIENT_TO_STRING,
37 &format!("calling `to_string` on `{arg_ty}`"),
40 "`{self_ty}` implements `ToString` through a slower blanket impl, but `{deref_self_ty}` has a fast specialization of `ToString`"
42 let mut applicability = Applicability::MachineApplicable;
43 let arg_snippet = snippet_with_applicability(cx, receiver.span, "..", &mut applicability);
46 "try dereferencing the receiver",
47 format!("({}{arg_snippet}).to_string()", "*".repeat(deref_count)),
56 /// Returns whether `ty` specializes `ToString`.
57 /// Currently, these are `str`, `String`, and `Cow<'_, str>`.
58 fn specializes_tostring(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
59 if let ty::Str = ty.kind() {
63 if is_type_lang_item(cx, ty, hir::LangItem::String) {
67 if let ty::Adt(adt, substs) = ty.kind() {
68 cx.tcx.is_diagnostic_item(sym::Cow, adt.did()) && substs.type_at(1).is_str()