rust-clippy/clippy_lints/src/loops/mut_range_bound.rs

use super::MUT_RANGE_BOUND;
use clippy_utils::diagnostics::span_lint_and_note;
use clippy_utils::{get_enclosing_block, higher, path_to_local};
use if_chain::if_chain;
use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
use rustc_hir::{BindingAnnotation, Expr, ExprKind, HirId, Node, PatKind};
use rustc_infer::infer::TyCtxtInferExt;
use rustc_lint::LateContext;
use rustc_middle::hir::map::Map;
use rustc_middle::{mir::FakeReadCause, ty};
use rustc_span::source_map::Span;
use rustc_typeck::expr_use_visitor::{Delegate, ExprUseVisitor, PlaceBase, PlaceWithHirId};

pub(super) fn check(cx: &LateContext<'_>, arg: &Expr<'_>, body: &Expr<'_>) {
    if_chain! {
        if let Some(higher::Range {
            start: Some(start),
            end: Some(end),
            ..
        }) = higher::Range::hir(arg);
        let (mut_id_start, mut_id_end) = (check_for_mutability(cx, start), check_for_mutability(cx, end));
        if mut_id_start.is_some() || mut_id_end.is_some();
        then {
            let (span_low, span_high) = check_for_mutation(cx, body, mut_id_start, mut_id_end);
            mut_warn_with_span(cx, span_low);
            mut_warn_with_span(cx, span_high);
        }
    }
}

fn mut_warn_with_span(cx: &LateContext<'_>, span: Option<Span>) {
    if let Some(sp) = span {
        span_lint_and_note(
            cx,
            MUT_RANGE_BOUND,
            sp,
            "attempt to mutate range bound within loop",
            None,
            "the range of the loop is unchanged",
        );
    }
}

fn check_for_mutability(cx: &LateContext<'_>, bound: &Expr<'_>) -> Option<HirId> {
    if_chain! {
        if let Some(hir_id) = path_to_local(bound);
        if let Node::Binding(pat) = cx.tcx.hir().get(hir_id);
        if let PatKind::Binding(BindingAnnotation::Mutable, ..) = pat.kind;
        then {
            return Some(hir_id);
        }
    }
    None
}

fn check_for_mutation<'tcx>(
    cx: &LateContext<'tcx>,
    body: &Expr<'_>,
    bound_id_start: Option<HirId>,
    bound_id_end: Option<HirId>,
) -> (Option<Span>, Option<Span>) {
    let mut delegate = MutatePairDelegate {
        cx,
        hir_id_low: bound_id_start,
        hir_id_high: bound_id_end,
        span_low: None,
        span_high: None,
    };
    cx.tcx.infer_ctxt().enter(|infcx| {
        ExprUseVisitor::new(
            &mut delegate,
            &infcx,
            body.hir_id.owner,
            cx.param_env,
            cx.typeck_results(),
        )
        .walk_expr(body);
    });

    delegate.mutation_span()
}

struct MutatePairDelegate<'a, 'tcx> {
    cx: &'a LateContext<'tcx>,
    hir_id_low: Option<HirId>,
    hir_id_high: Option<HirId>,
    span_low: Option<Span>,
    span_high: Option<Span>,
}

impl<'tcx> Delegate<'tcx> for MutatePairDelegate<'_, 'tcx> {
    fn consume(&mut self, _: &PlaceWithHirId<'tcx>, _: HirId) {}

    fn borrow(&mut self, cmt: &PlaceWithHirId<'tcx>, diag_expr_id: HirId, bk: ty::BorrowKind) {
        if bk == ty::BorrowKind::MutBorrow {
            if let PlaceBase::Local(id) = cmt.place.base {
                if Some(id) == self.hir_id_low && !BreakAfterExprVisitor::is_found(self.cx, diag_expr_id) {
                    self.span_low = Some(self.cx.tcx.hir().span(diag_expr_id));
                }
                if Some(id) == self.hir_id_high && !BreakAfterExprVisitor::is_found(self.cx, diag_expr_id) {
                    self.span_high = Some(self.cx.tcx.hir().span(diag_expr_id));
                }
            }
        }
    }

    fn mutate(&mut self, cmt: &PlaceWithHirId<'tcx>, diag_expr_id: HirId) {
        if let PlaceBase::Local(id) = cmt.place.base {
            if Some(id) == self.hir_id_low && !BreakAfterExprVisitor::is_found(self.cx, diag_expr_id) {
                self.span_low = Some(self.cx.tcx.hir().span(diag_expr_id));
            }
            if Some(id) == self.hir_id_high && !BreakAfterExprVisitor::is_found(self.cx, diag_expr_id) {
                self.span_high = Some(self.cx.tcx.hir().span(diag_expr_id));
            }
        }
    }

    fn fake_read(&mut self, _: rustc_typeck::expr_use_visitor::Place<'tcx>, _: FakeReadCause, _: HirId) {}
}

impl MutatePairDelegate<'_, '_> {
    fn mutation_span(&self) -> (Option<Span>, Option<Span>) {
        (self.span_low, self.span_high)
    }
}

struct BreakAfterExprVisitor {
    hir_id: HirId,
    past_expr: bool,
    past_candidate: bool,
    break_after_expr: bool,
}

impl BreakAfterExprVisitor {
    pub fn is_found(cx: &LateContext<'_>, hir_id: HirId) -> bool {
        let mut visitor = BreakAfterExprVisitor {
            hir_id,
            past_expr: false,
            past_candidate: false,
            break_after_expr: false,
        };

        get_enclosing_block(cx, hir_id).map_or(false, |block| {
            visitor.visit_block(block);
            visitor.break_after_expr
        })
    }
}

impl intravisit::Visitor<'tcx> for BreakAfterExprVisitor {
    type Map = Map<'tcx>;

    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
        NestedVisitorMap::None
    }

    fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
        if self.past_candidate {
            return;
        }

        if expr.hir_id == self.hir_id {
            self.past_expr = true;
        } else if self.past_expr {
            if matches!(&expr.kind, ExprKind::Break(..)) {
                self.break_after_expr = true;
            }

            self.past_candidate = true;
        } else {
            intravisit::walk_expr(self, expr);
        }
    }
}