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

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::source::snippet_with_applicability;
use clippy_utils::ty::walk_ptrs_ty_depth;
use clippy_utils::{get_parent_expr, is_trait_method};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::LateContext;
use rustc_span::sym;

use super::USELESS_ASREF;

/// Checks for the `USELESS_ASREF` lint.
pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, call_name: &str, recvr: &hir::Expr<'_>) {
    // when we get here, we've already checked that the call name is "as_ref" or "as_mut"
    // check if the call is to the actual `AsRef` or `AsMut` trait
    if is_trait_method(cx, expr, sym::AsRef) || is_trait_method(cx, expr, sym::AsMut) {
        // check if the type after `as_ref` or `as_mut` is the same as before
        let rcv_ty = cx.typeck_results().expr_ty(recvr);
        let res_ty = cx.typeck_results().expr_ty(expr);
        let (base_res_ty, res_depth) = walk_ptrs_ty_depth(res_ty);
        let (base_rcv_ty, rcv_depth) = walk_ptrs_ty_depth(rcv_ty);
        if base_rcv_ty == base_res_ty && rcv_depth >= res_depth {
            // allow the `as_ref` or `as_mut` if it is followed by another method call
            if_chain! {
                if let Some(parent) = get_parent_expr(cx, expr);
                if let hir::ExprKind::MethodCall(segment, ..) = parent.kind;
                if segment.ident.span != expr.span;
                then {
                    return;
                }
            }

            let mut applicability = Applicability::MachineApplicable;
            span_lint_and_sugg(
                cx,
                USELESS_ASREF,
                expr.span,
                &format!("this call to `{call_name}` does nothing"),
                "try this",
                snippet_with_applicability(cx, recvr.span, "..", &mut applicability).to_string(),
                applicability,
            );
        }
    }
}