src/tools/clippy/clippy_lints/src/matches/match_on_vec_items.rs

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::source::snippet;
use clippy_utils::ty::{is_type_diagnostic_item, is_type_lang_item};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::{Expr, ExprKind, LangItem};
use rustc_lint::LateContext;
use rustc_span::sym;

use super::MATCH_ON_VEC_ITEMS;

pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, scrutinee: &'tcx Expr<'_>) {
    if_chain! {
        if let Some(idx_expr) = is_vec_indexing(cx, scrutinee);
        if let ExprKind::Index(vec, idx, _) = idx_expr.kind;

        then {
            // FIXME: could be improved to suggest surrounding every pattern with Some(_),
            // but only when `or_patterns` are stabilized.
            span_lint_and_sugg(
                cx,
                MATCH_ON_VEC_ITEMS,
                scrutinee.span,
                "indexing into a vector may panic",
                "try",
                format!(
                    "{}.get({})",
                    snippet(cx, vec.span, ".."),
                    snippet(cx, idx.span, "..")
                ),
                Applicability::MaybeIncorrect
            );
        }
    }
}

fn is_vec_indexing<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> Option<&'tcx Expr<'tcx>> {
    if_chain! {
        if let ExprKind::Index(array, index, _) = expr.kind;
        if is_vector(cx, array);
        if !is_full_range(cx, index);

        then {
            return Some(expr);
        }
    }

    None
}

fn is_vector(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
    let ty = cx.typeck_results().expr_ty(expr);
    let ty = ty.peel_refs();
    is_type_diagnostic_item(cx, ty, sym::Vec)
}

fn is_full_range(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
    let ty = cx.typeck_results().expr_ty(expr);
    let ty = ty.peel_refs();
    is_type_lang_item(cx, ty, LangItem::RangeFull)
}