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

use super::utils::make_iterator_snippet;
use super::MANUAL_FIND;
use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::source::snippet_with_applicability;
use clippy_utils::ty::implements_trait;
use clippy_utils::{higher, is_res_lang_ctor, path_res, peel_blocks_with_stmt};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::def::Res;
use rustc_hir::lang_items::LangItem;
use rustc_hir::{BindingAnnotation, Block, Expr, ExprKind, HirId, Node, Pat, PatKind, Stmt, StmtKind};
use rustc_lint::LateContext;
use rustc_span::Span;

pub(super) fn check<'tcx>(
    cx: &LateContext<'tcx>,
    pat: &'tcx Pat<'_>,
    arg: &'tcx Expr<'_>,
    body: &'tcx Expr<'_>,
    span: Span,
    expr: &'tcx Expr<'_>,
) {
    let inner_expr = peel_blocks_with_stmt(body);
    // Check for the specific case that the result is returned and optimize suggestion for that (more
    // cases can be added later)
    if_chain! {
        if let Some(higher::If { cond, then, r#else: None, }) = higher::If::hir(inner_expr);
        if let Some(binding_id) = get_binding(pat);
        if let ExprKind::Block(block, _) = then.kind;
        if let [stmt] = block.stmts;
        if let StmtKind::Semi(semi) = stmt.kind;
        if let ExprKind::Ret(Some(ret_value)) = semi.kind;
        if let ExprKind::Call(ctor, [inner_ret]) = ret_value.kind;
        if is_res_lang_ctor(cx, path_res(cx, ctor), LangItem::OptionSome);
        if path_res(cx, inner_ret) == Res::Local(binding_id);
        if let Some((last_stmt, last_ret)) = last_stmt_and_ret(cx, expr);
        then {
            let mut applicability = Applicability::MachineApplicable;
            let mut snippet = make_iterator_snippet(cx, arg, &mut applicability);
            // Checks if `pat` is a single reference to a binding (`&x`)
            let is_ref_to_binding =
                matches!(pat.kind, PatKind::Ref(inner, _) if matches!(inner.kind, PatKind::Binding(..)));
            // If `pat` is not a binding or a reference to a binding (`x` or `&x`)
            // we need to map it to the binding returned by the function (i.e. `.map(|(x, _)| x)`)
            if !(matches!(pat.kind, PatKind::Binding(..)) || is_ref_to_binding) {
                snippet.push_str(
                    &format!(
                        ".map(|{}| {})",
                        snippet_with_applicability(cx, pat.span, "..", &mut applicability),
                        snippet_with_applicability(cx, inner_ret.span, "..", &mut applicability),
                    )[..],
                );
            }
            let ty = cx.typeck_results().expr_ty(inner_ret);
            if cx.tcx.lang_items().copy_trait().map_or(false, |id| implements_trait(cx, ty, id, &[])) {
                snippet.push_str(
                    &format!(
                        ".find(|{}{}| {})",
                        "&".repeat(1 + usize::from(is_ref_to_binding)),
                        snippet_with_applicability(cx, inner_ret.span, "..", &mut applicability),
                        snippet_with_applicability(cx, cond.span, "..", &mut applicability),
                    )[..],
                );
                if is_ref_to_binding {
                    snippet.push_str(".copied()");
                }
            } else {
                applicability = Applicability::MaybeIncorrect;
                snippet.push_str(
                    &format!(
                        ".find(|{}| {})",
                        snippet_with_applicability(cx, inner_ret.span, "..", &mut applicability),
                        snippet_with_applicability(cx, cond.span, "..", &mut applicability),
                    )[..],
                );
            }
            // Extends to `last_stmt` to include semicolon in case of `return None;`
            let lint_span = span.to(last_stmt.span).to(last_ret.span);
            span_lint_and_then(
                cx,
                MANUAL_FIND,
                lint_span,
                "manual implementation of `Iterator::find`",
                |diag| {
                    if applicability == Applicability::MaybeIncorrect {
                        diag.note("you may need to dereference some variables");
                    }
                    diag.span_suggestion(
                        lint_span,
                        "replace with an iterator",
                        snippet,
                        applicability,
                    );
                },
            );
        }
    }
}

fn get_binding(pat: &Pat<'_>) -> Option<HirId> {
    let mut hir_id = None;
    let mut count = 0;
    pat.each_binding(|annotation, id, _, _| {
        count += 1;
        if count > 1 {
            hir_id = None;
            return;
        }
        if let BindingAnnotation::NONE = annotation {
            hir_id = Some(id);
        }
    });
    hir_id
}

// Returns the last statement and last return if function fits format for lint
fn last_stmt_and_ret<'tcx>(
    cx: &LateContext<'tcx>,
    expr: &'tcx Expr<'_>,
) -> Option<(&'tcx Stmt<'tcx>, &'tcx Expr<'tcx>)> {
    // Returns last non-return statement and the last return
    fn extract<'tcx>(block: &Block<'tcx>) -> Option<(&'tcx Stmt<'tcx>, &'tcx Expr<'tcx>)> {
        if let [.., last_stmt] = block.stmts {
            if let Some(ret) = block.expr {
                return Some((last_stmt, ret));
            }
            if_chain! {
                if let [.., snd_last, _] = block.stmts;
                if let StmtKind::Semi(last_expr) = last_stmt.kind;
                if let ExprKind::Ret(Some(ret)) = last_expr.kind;
                then {
                    return Some((snd_last, ret));
                }
            }
        }
        None
    }
    let mut parent_iter = cx.tcx.hir().parent_iter(expr.hir_id);
    if_chain! {
        // This should be the loop
        if let Some((node_hir, Node::Stmt(..))) = parent_iter.next();
        // This should be the function body
        if let Some((_, Node::Block(block))) = parent_iter.next();
        if let Some((last_stmt, last_ret)) = extract(block);
        if last_stmt.hir_id == node_hir;
        if is_res_lang_ctor(cx, path_res(cx, last_ret), LangItem::OptionNone);
        if let Some((_, Node::Expr(_block))) = parent_iter.next();
        // This includes the function header
        if let Some((_, func)) = parent_iter.next();
        if func.fn_kind().is_some();
        then {
            Some((block.stmts.last().unwrap(), last_ret))
        } else {
            None
        }
    }
}