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

use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::source::snippet_with_applicability;
use clippy_utils::ty::{is_type_lang_item, walk_ptrs_ty_depth};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::LateContext;
use rustc_middle::ty::{self, Ty};
use rustc_span::symbol::{sym, Symbol};

use super::INEFFICIENT_TO_STRING;

/// Checks for the `INEFFICIENT_TO_STRING` lint
pub fn check(
    cx: &LateContext<'_>,
    expr: &hir::Expr<'_>,
    method_name: Symbol,
    receiver: &hir::Expr<'_>,
    args: &[hir::Expr<'_>],
) {
    if_chain! {
        if args.is_empty() && method_name == sym::to_string;
        if let Some(to_string_meth_did) = cx.typeck_results().type_dependent_def_id(expr.hir_id);
        if cx.tcx.is_diagnostic_item(sym::to_string_method, to_string_meth_did);
        if 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) = walk_ptrs_ty_depth(self_ty);
        if deref_count >= 1;
        if specializes_tostring(cx, deref_self_ty);
        then {
            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 is_type_lang_item(cx, ty, 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
    }
}