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

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::source::snippet_with_applicability;
use clippy_utils::ty::is_type_diagnostic_item;
use if_chain::if_chain;
use rustc_ast::LitKind;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::LateContext;
use rustc_span::source_map::Spanned;
use rustc_span::sym;

use super::GET_FIRST;

pub(super) fn check<'tcx>(
    cx: &LateContext<'tcx>,
    expr: &'tcx hir::Expr<'_>,
    recv: &'tcx hir::Expr<'_>,
    arg: &'tcx hir::Expr<'_>,
) {
    if_chain! {
        if let Some(method_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id);
        if let Some(impl_id) = cx.tcx.impl_of_method(method_id);
        let identity = cx.tcx.type_of(impl_id).instantiate_identity();
        if let hir::ExprKind::Lit(Spanned { node: LitKind::Int(0, _), .. }) = arg.kind;
        then {
            if identity.is_slice() {
                let mut app = Applicability::MachineApplicable;
                let slice_name = snippet_with_applicability(cx, recv.span, "..", &mut app);
                span_lint_and_sugg(
                    cx,
                    GET_FIRST,
                    expr.span,
                    &format!("accessing first element with `{slice_name}.get(0)`"),
                    "try",
                    format!("{slice_name}.first()"),
                    app,
                );
            } else if is_type_diagnostic_item(cx, identity, sym::VecDeque){
                    let mut app = Applicability::MachineApplicable;
                    let slice_name = snippet_with_applicability(cx, recv.span, "..", &mut app);
                    span_lint_and_sugg(
                        cx,
                        GET_FIRST,
                        expr.span,
                        &format!("accessing first element with `{slice_name}.get(0)`"),
                        "try",
                        format!("{slice_name}.front()"),
                        app,
                    );
            }
        }
    }
}