clippy_lints/src/methods/inefficient_to_string.rs

use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::msrvs::{self, Msrv};
use clippy_utils::res::MaybeDef;
use clippy_utils::source::snippet_with_applicability;
use clippy_utils::sym;
use clippy_utils::ty::peel_and_count_ty_refs;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::LateContext;
use rustc_middle::ty::{self, Ty};

use super::INEFFICIENT_TO_STRING;

pub fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, receiver: &hir::Expr<'_>, msrv: Msrv) {
    if let Some(to_string_meth_did) = cx.typeck_results().type_dependent_def_id(expr.hir_id)
        && cx.tcx.is_diagnostic_item(sym::to_string_method, to_string_meth_did)
        && let Some(args) = cx.typeck_results().node_args_opt(expr.hir_id)
        && let arg_ty = cx.typeck_results().expr_ty_adjusted(receiver)
        && let self_ty = args.type_at(0)
        && let (deref_self_ty, deref_count, _) = peel_and_count_ty_refs(self_ty)
        && deref_count >= 1
        // Since Rust 1.82, the specialized `ToString` is properly called
        && !msrv.meets(cx, msrvs::SPECIALIZED_TO_STRING_FOR_REFS)
        && specializes_tostring(cx, deref_self_ty)
    {
        span_lint_and_then(
            cx,
            INEFFICIENT_TO_STRING,
            expr.span,
            format!("calling `to_string` on `{arg_ty}`"),
            |diag| {
                diag.help(format!(
                    "`{self_ty}` implements `ToString` through a slower blanket impl, but `{deref_self_ty}` has a fast specialization of `ToString`"
                ));
                let mut applicability = Applicability::MachineApplicable;
                let arg_snippet = snippet_with_applicability(cx, receiver.span, "..", &mut applicability);
                diag.span_suggestion(
                    expr.span,
                    "try dereferencing the receiver",
                    format!("({}{arg_snippet}).to_string()", "*".repeat(deref_count)),
                    applicability,
                );
            },
        );
    }
}

/// Returns whether `ty` specializes `ToString`.
/// Currently, these are `str`, `String`, and `Cow<'_, str>`.
fn specializes_tostring(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
    if let ty::Str = ty.kind() {
        return true;
    }

    if ty.is_lang_item(cx, hir::LangItem::String) {
        return true;
    }

    if let ty::Adt(adt, args) = ty.kind() {
        cx.tcx.is_diagnostic_item(sym::Cow, adt.did()) && args.type_at(1).is_str()
    } else {
        false
    }
}