src/tools/clippy/clippy_lints/src/loops/for_kv_map.rs

use super::FOR_KV_MAP;
use clippy_utils::diagnostics::{multispan_sugg, span_lint_and_then};
use clippy_utils::source::snippet;
use clippy_utils::sugg;
use clippy_utils::ty::is_type_diagnostic_item;
use clippy_utils::visitors::is_local_used;
use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability, Pat, PatKind};
use rustc_lint::LateContext;
use rustc_middle::ty;
use rustc_span::sym;

/// Checks for the `FOR_KV_MAP` lint.
pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>, arg: &'tcx Expr<'_>, body: &'tcx Expr<'_>) {
    let pat_span = pat.span;

    if let PatKind::Tuple(pat, _) = pat.kind {
        if pat.len() == 2 {
            let arg_span = arg.span;
            let (new_pat_span, kind, ty, mutbl) = match *cx.typeck_results().expr_ty(arg).kind() {
                ty::Ref(_, ty, mutbl) => match (&pat[0].kind, &pat[1].kind) {
                    (key, _) if pat_is_wild(cx, key, body) => (pat[1].span, "value", ty, mutbl),
                    (_, value) if pat_is_wild(cx, value, body) => (pat[0].span, "key", ty, Mutability::Not),
                    _ => return,
                },
                _ => return,
            };
            let mutbl = match mutbl {
                Mutability::Not => "",
                Mutability::Mut => "_mut",
            };
            let arg = match arg.kind {
                ExprKind::AddrOf(BorrowKind::Ref, _, expr) => expr,
                _ => arg,
            };

            if is_type_diagnostic_item(cx, ty, sym::HashMap) || is_type_diagnostic_item(cx, ty, sym::BTreeMap) {
                span_lint_and_then(
                    cx,
                    FOR_KV_MAP,
                    arg_span,
                    &format!("you seem to want to iterate on a map's {kind}s"),
                    |diag| {
                        let map = sugg::Sugg::hir(cx, arg, "map");
                        multispan_sugg(
                            diag,
                            "use the corresponding method",
                            vec![
                                (pat_span, snippet(cx, new_pat_span, kind).into_owned()),
                                (arg_span, format!("{}.{kind}s{mutbl}()", map.maybe_par())),
                            ],
                        );
                    },
                );
            }
        }
    }
}

/// Returns `true` if the pattern is a `PatWild` or an ident prefixed with `_`.
fn pat_is_wild<'tcx>(cx: &LateContext<'tcx>, pat: &'tcx PatKind<'_>, body: &'tcx Expr<'_>) -> bool {
    match *pat {
        PatKind::Wild => true,
        PatKind::Binding(_, id, ident, None) if ident.as_str().starts_with('_') => !is_local_used(cx, body, id),
        _ => false,
    }
}