src/librustc_ast_borrowck/borrowck/check_loans.rs

// ----------------------------------------------------------------------
// Checking loans
//
// Phase 2 of check: we walk down the tree and check that:
// 1. assignments are always made to mutable locations;
// 2. loans made in overlapping scopes do not conflict
// 3. assignments do not affect things loaned out as immutable
// 4. moves do not affect things loaned out in any way

use crate::borrowck::*;
use crate::borrowck::InteriorKind::{InteriorElement, InteriorField};
use rustc::middle::expr_use_visitor as euv;
use rustc::middle::expr_use_visitor::MutateMode;
use rustc::middle::mem_categorization as mc;
use rustc::middle::mem_categorization::Categorization;
use rustc::middle::region;
use rustc::ty::{self, TyCtxt, RegionKind};
use syntax_pos::Span;
use rustc::hir;
use rustc::hir::Node;
use log::debug;

use std::rc::Rc;

// FIXME (#16118): These functions are intended to allow the borrow checker to
// be less precise in its handling of Box while still allowing moves out of a
// Box. They should be removed when Unique is removed from LoanPath.

fn owned_ptr_base_path<'a, 'tcx>(loan_path: &'a LoanPath<'tcx>) -> &'a LoanPath<'tcx> {
    //! Returns the base of the leftmost dereference of an Unique in
    //! `loan_path`. If there is no dereference of an Unique in `loan_path`,
    //! then it just returns `loan_path` itself.

    return match helper(loan_path) {
        Some(new_loan_path) => new_loan_path,
        None => loan_path,
    };

    fn helper<'a, 'tcx>(loan_path: &'a LoanPath<'tcx>) -> Option<&'a LoanPath<'tcx>> {
        match loan_path.kind {
            LpVar(_) | LpUpvar(_) => None,
            LpExtend(ref lp_base, _, LpDeref(mc::Unique)) => {
                match helper(&lp_base) {
                    v @ Some(_) => v,
                    None => Some(&lp_base)
                }
            }
            LpDowncast(ref lp_base, _) |
            LpExtend(ref lp_base, ..) => helper(&lp_base)
        }
    }
}

fn owned_ptr_base_path_rc<'tcx>(loan_path: &Rc<LoanPath<'tcx>>) -> Rc<LoanPath<'tcx>> {
    //! The equivalent of `owned_ptr_base_path` for an &Rc<LoanPath> rather than
    //! a &LoanPath.

    return match helper(loan_path) {
        Some(new_loan_path) => new_loan_path,
        None => loan_path.clone()
    };

    fn helper<'tcx>(loan_path: &Rc<LoanPath<'tcx>>) -> Option<Rc<LoanPath<'tcx>>> {
        match loan_path.kind {
            LpVar(_) | LpUpvar(_) => None,
            LpExtend(ref lp_base, _, LpDeref(mc::Unique)) => {
                match helper(lp_base) {
                    v @ Some(_) => v,
                    None => Some(lp_base.clone())
                }
            }
            LpDowncast(ref lp_base, _) |
            LpExtend(ref lp_base, ..) => helper(lp_base)
        }
    }
}

struct CheckLoanCtxt<'a, 'tcx> {
    bccx: &'a BorrowckCtxt<'a, 'tcx>,
    dfcx_loans: &'a LoanDataFlow<'tcx>,
    move_data: &'a move_data::FlowedMoveData<'tcx>,
    all_loans: &'a [Loan<'tcx>],
    movable_generator: bool,
}

impl<'a, 'tcx> euv::Delegate<'tcx> for CheckLoanCtxt<'a, 'tcx> {
    fn consume(&mut self,
               consume_id: hir::HirId,
               _: Span,
               cmt: &mc::cmt_<'tcx>,
               mode: euv::ConsumeMode) {
        debug!("consume(consume_id={}, cmt={:?})", consume_id, cmt);

        self.consume_common(consume_id.local_id, cmt, mode);
    }

    fn matched_pat(&mut self,
                   _matched_pat: &hir::Pat,
                   _cmt: &mc::cmt_<'_>,
                   _mode: euv::MatchMode) { }

    fn consume_pat(&mut self,
                   consume_pat: &hir::Pat,
                   cmt: &mc::cmt_<'tcx>,
                   mode: euv::ConsumeMode) {
        debug!("consume_pat(consume_pat={:?}, cmt={:?})", consume_pat, cmt);

        self.consume_common(consume_pat.hir_id.local_id, cmt, mode);
    }

    fn borrow(&mut self,
              borrow_id: hir::HirId,
              borrow_span: Span,
              cmt: &mc::cmt_<'tcx>,
              loan_region: ty::Region<'tcx>,
              bk: ty::BorrowKind,
              loan_cause: euv::LoanCause)
    {
        debug!("borrow(borrow_id={}, cmt={:?}, loan_region={:?}, \
               bk={:?}, loan_cause={:?})",
               borrow_id, cmt, loan_region,
               bk, loan_cause);

        if let Some(lp) = opt_loan_path(cmt) {
            self.check_if_path_is_moved(borrow_id.local_id, &lp);
        }

        self.check_for_conflicting_loans(borrow_id.local_id);

        self.check_for_loans_across_yields(cmt, loan_region, borrow_span);
    }

    fn mutate(&mut self,
              assignment_id: hir::HirId,
              _: Span,
              assignee_cmt: &mc::cmt_<'tcx>,
              mode: euv::MutateMode)
    {
        debug!("mutate(assignment_id={}, assignee_cmt={:?})",
               assignment_id, assignee_cmt);

        if let Some(lp) = opt_loan_path(assignee_cmt) {
            match mode {
                MutateMode::Init | MutateMode::JustWrite => {
                    // In a case like `path = 1`, then path does not
                    // have to be *FULLY* initialized, but we still
                    // must be careful lest it contains derefs of
                    // pointers.
                    self.check_if_assigned_path_is_moved(assignee_cmt.hir_id.local_id, &lp);
                }
                MutateMode::WriteAndRead => {
                    // In a case like `path += 1`, then path must be
                    // fully initialized, since we will read it before
                    // we write it.
                    self.check_if_path_is_moved(assignee_cmt.hir_id.local_id,
                                                &lp);
                }
            }
        }
        self.check_assignment(assignment_id.local_id, assignee_cmt);
    }

    fn decl_without_init(&mut self, _id: hir::HirId, _span: Span) { }
}

pub fn check_loans<'a, 'tcx>(
    bccx: &BorrowckCtxt<'a, 'tcx>,
    dfcx_loans: &LoanDataFlow<'tcx>,
    move_data: &move_data::FlowedMoveData<'tcx>,
    all_loans: &[Loan<'tcx>],
    body: &hir::Body,
) {
    debug!("check_loans(body id={})", body.value.hir_id);

    let def_id = bccx.tcx.hir().body_owner_def_id(body.id());

    let hir_id = bccx.tcx.hir().as_local_hir_id(def_id).unwrap();
    let movable_generator = !match bccx.tcx.hir().get(hir_id) {
        Node::Expr(&hir::Expr {
            node: hir::ExprKind::Closure(.., Some(hir::GeneratorMovability::Static)),
            ..
        }) => true,
        _ => false,
    };

    let param_env = bccx.tcx.param_env(def_id);
    let mut clcx = CheckLoanCtxt {
        bccx,
        dfcx_loans,
        move_data,
        all_loans,
        movable_generator,
    };
    let rvalue_promotable_map = bccx.tcx.rvalue_promotable_map(def_id);
    euv::ExprUseVisitor::new(&mut clcx,
                             bccx.tcx,
                             def_id,
                             param_env,
                             &bccx.region_scope_tree,
                             bccx.tables,
                             Some(rvalue_promotable_map))
        .consume_body(body);
}

fn compatible_borrow_kinds(borrow_kind1: ty::BorrowKind,
                           borrow_kind2: ty::BorrowKind)
                           -> bool {
    borrow_kind1 == ty::ImmBorrow && borrow_kind2 == ty::ImmBorrow
}

impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
    pub fn tcx(&self) -> TyCtxt<'tcx> { self.bccx.tcx }

    pub fn each_issued_loan<F>(&self, node: hir::ItemLocalId, mut op: F) -> bool where
        F: FnMut(&Loan<'tcx>) -> bool,
    {
        //! Iterates over each loan that has been issued
        //! on entrance to `node`, regardless of whether it is
        //! actually *in scope* at that point. Sometimes loans
        //! are issued for future scopes and thus they may have been
        //! *issued* but not yet be in effect.

        self.dfcx_loans.each_bit_on_entry(node, |loan_index| {
            let loan = &self.all_loans[loan_index];
            op(loan)
        })
    }

    pub fn each_in_scope_loan<F>(&self, scope: region::Scope, mut op: F) -> bool where
        F: FnMut(&Loan<'tcx>) -> bool,
    {
        //! Like `each_issued_loan()`, but only considers loans that are
        //! currently in scope.

        self.each_issued_loan(scope.item_local_id(), |loan| {
            if self.bccx.region_scope_tree.is_subscope_of(scope, loan.kill_scope) {
                op(loan)
            } else {
                true
            }
        })
    }

    fn each_in_scope_loan_affecting_path<F>(&self,
                                            scope: region::Scope,
                                            loan_path: &LoanPath<'tcx>,
                                            mut op: F)
                                            -> bool where
        F: FnMut(&Loan<'tcx>) -> bool,
    {
        //! Iterates through all of the in-scope loans affecting `loan_path`,
        //! calling `op`, and ceasing iteration if `false` is returned.

        // First, we check for a loan restricting the path P being used. This
        // accounts for borrows of P but also borrows of subpaths, like P.a.b.
        // Consider the following example:
        //
        //     let x = &mut a.b.c; // Restricts a, a.b, and a.b.c
        //     let y = a;          // Conflicts with restriction

        let loan_path = owned_ptr_base_path(loan_path);
        let cont = self.each_in_scope_loan(scope, |loan| {
            let mut ret = true;
            for restr_path in &loan.restricted_paths {
                if **restr_path == *loan_path {
                    if !op(loan) {
                        ret = false;
                        break;
                    }
                }
            }
            ret
        });

        if !cont {
            return false;
        }

        // Next, we must check for *loans* (not restrictions) on the path P or
        // any base path. This rejects examples like the following:
        //
        //     let x = &mut a.b;
        //     let y = a.b.c;
        //
        // Limiting this search to *loans* and not *restrictions* means that
        // examples like the following continue to work:
        //
        //     let x = &mut a.b;
        //     let y = a.c;

        let mut loan_path = loan_path;
        loop {
            match loan_path.kind {
                LpVar(_) | LpUpvar(_) => {
                    break;
                }
                LpDowncast(ref lp_base, _) |
                LpExtend(ref lp_base, ..) => {
                    loan_path = &lp_base;
                }
            }

            let cont = self.each_in_scope_loan(scope, |loan| {
                if *loan.loan_path == *loan_path {
                    op(loan)
                } else {
                    true
                }
            });

            if !cont {
                return false;
            }
        }

        return true;
    }

    pub fn loans_generated_by(&self, node: hir::ItemLocalId) -> Vec<usize> {
        //! Returns a vector of the loans that are generated as
        //! we enter `node`.

        let mut result = Vec::new();
        self.dfcx_loans.each_gen_bit(node, |loan_index| {
            result.push(loan_index);
            true
        });
        return result;
    }

    pub fn check_for_loans_across_yields(&self,
                                         cmt: &mc::cmt_<'tcx>,
                                         loan_region: ty::Region<'tcx>,
                                         borrow_span: Span) {
        pub fn borrow_of_local_data(cmt: &mc::cmt_<'_>) -> bool {
            match cmt.cat {
                // Borrows of static items is allowed
                Categorization::StaticItem => false,
                // Reborrow of already borrowed data is ignored
                // Any errors will be caught on the initial borrow
                Categorization::Deref(..) => false,

                // By-ref upvars has Derefs so they will get ignored.
                // Generators counts as FnOnce so this leaves only
                // by-move upvars, which is local data for generators
                Categorization::Upvar(..) => true,

                Categorization::ThreadLocal(region) |
                Categorization::Rvalue(region) => {
                    // Rvalues promoted to 'static are no longer local
                    if let RegionKind::ReStatic = *region {
                        false
                    } else {
                        true
                    }
                }

                // Borrow of local data must be checked
                Categorization::Local(..) => true,

                // For interior references and downcasts, find out if the base is local
                Categorization::Downcast(ref cmt_base, _) |
                Categorization::Interior(ref cmt_base, _) => borrow_of_local_data(&cmt_base),
            }
        }

        if !self.movable_generator {
            return;
        }

        if !borrow_of_local_data(cmt) {
            return;
        }

        let scope = match *loan_region {
            // A concrete region in which we will look for a yield expression
            RegionKind::ReScope(scope) => scope,

            // There cannot be yields inside an empty region
            RegionKind::ReEmpty => return,

            // Local data cannot have these lifetimes
            RegionKind::ReEarlyBound(..) |
            RegionKind::ReLateBound(..) |
            RegionKind::ReFree(..) |
            RegionKind::ReStatic => {
                self.bccx
                    .tcx
                    .sess.delay_span_bug(borrow_span,
                                         &format!("unexpected region for local data {:?}",
                                                  loan_region));
                return
            }

            // These cannot exist in borrowck
            RegionKind::ReVar(..) |
            RegionKind::RePlaceholder(..) |
            RegionKind::ReClosureBound(..) |
            RegionKind::ReErased => span_bug!(borrow_span,
                                              "unexpected region in borrowck {:?}",
                                              loan_region),
        };

        let body_id = self.bccx.body.value.hir_id.local_id;

        if self.bccx.region_scope_tree.containing_body(scope) != Some(body_id) {
            // We are borrowing local data longer than its storage.
            // This should result in other borrowck errors.
            self.bccx.tcx.sess.delay_span_bug(borrow_span,
                                              "borrowing local data longer than its storage");
            return;
        }

        if let Some(_) = self.bccx.region_scope_tree
            .yield_in_scope_for_expr(scope, cmt.hir_id, self.bccx.body)
        {
            self.bccx.signal_error();
        }
    }

    pub fn check_for_conflicting_loans(&self, node: hir::ItemLocalId) {
        //! Checks to see whether any of the loans that are issued
        //! on entrance to `node` conflict with loans that have already been
        //! issued when we enter `node` (for example, we do not
        //! permit two `&mut` borrows of the same variable).
        //!
        //! (Note that some loans can be *issued* without necessarily
        //! taking effect yet.)

        debug!("check_for_conflicting_loans(node={:?})", node);

        let new_loan_indices = self.loans_generated_by(node);
        debug!("new_loan_indices = {:?}", new_loan_indices);

        for &new_loan_index in &new_loan_indices {
            self.each_issued_loan(node, |issued_loan| {
                let new_loan = &self.all_loans[new_loan_index];
                // Only report an error for the first issued loan that conflicts
                // to avoid O(n^2) errors.
                self.report_error_if_loans_conflict(issued_loan, new_loan)
            });
        }

        for (i, &x) in new_loan_indices.iter().enumerate() {
            let old_loan = &self.all_loans[x];
            for &y in &new_loan_indices[(i+1) ..] {
                let new_loan = &self.all_loans[y];
                self.report_error_if_loans_conflict(old_loan, new_loan);
            }
        }
    }

    pub fn report_error_if_loans_conflict(
        &self,
        old_loan: &Loan<'tcx>,
        new_loan: &Loan<'tcx>,
    ) -> bool {
        //! Checks whether `old_loan` and `new_loan` can safely be issued
        //! simultaneously.

        debug!("report_error_if_loans_conflict(old_loan={:?}, new_loan={:?})",
               old_loan,
               new_loan);

        // Should only be called for loans that are in scope at the same time.
        assert!(self.bccx.region_scope_tree.scopes_intersect(old_loan.kill_scope,
                                                       new_loan.kill_scope));

        self.report_error_if_loan_conflicts_with_restriction(
            old_loan, new_loan)
        && self.report_error_if_loan_conflicts_with_restriction(
                new_loan, old_loan)
    }

    pub fn report_error_if_loan_conflicts_with_restriction(
        &self,
        loan1: &Loan<'tcx>,
        loan2: &Loan<'tcx>,
    ) -> bool {
        //! Checks whether the restrictions introduced by `loan1` would
        //! prohibit `loan2`.
        debug!("report_error_if_loan_conflicts_with_restriction(\
                loan1={:?}, loan2={:?})",
               loan1,
               loan2);

        if compatible_borrow_kinds(loan1.kind, loan2.kind) {
            return true;
        }

        let loan2_base_path = owned_ptr_base_path_rc(&loan2.loan_path);
        for restr_path in &loan1.restricted_paths {
            if *restr_path != loan2_base_path { continue; }

            self.bccx.signal_error();
            return false;
        }

        true
    }

    fn consume_common(
        &self,
        id: hir::ItemLocalId,
        cmt: &mc::cmt_<'tcx>,
        mode: euv::ConsumeMode,
    ) {
        if let Some(lp) = opt_loan_path(cmt) {
            match mode {
                euv::Copy => {
                    self.check_for_copy_of_frozen_path(id, &lp);
                }
                euv::Move(_) => {
                    // Sometimes moves aren't from a move path;
                    // this either means that the original move
                    // was from something illegal to move,
                    // or was moved from referent of an unsafe
                    // pointer or something like that.
                    if self.move_data.is_move_path(id, &lp) {
                        self.check_for_move_of_borrowed_path(id, &lp);
                    }
                }
            }
            self.check_if_path_is_moved(id, &lp);
        }
    }

    fn check_for_copy_of_frozen_path(&self,
                                     id: hir::ItemLocalId,
                                     copy_path: &LoanPath<'tcx>) {
        self.analyze_restrictions_on_use(id, copy_path, ty::ImmBorrow);
    }

    fn check_for_move_of_borrowed_path(&self,
                                       id: hir::ItemLocalId,
                                       move_path: &LoanPath<'tcx>) {
        // We want to detect if there are any loans at all, so we search for
        // any loans incompatible with MutBorrrow, since all other kinds of
        // loans are incompatible with that.
        self.analyze_restrictions_on_use(id, move_path, ty::MutBorrow);
    }

    fn analyze_restrictions_on_use(&self,
                                       expr_id: hir::ItemLocalId,
                                       use_path: &LoanPath<'tcx>,
                                       borrow_kind: ty::BorrowKind) {
        debug!("analyze_restrictions_on_use(expr_id={:?}, use_path={:?})",
               expr_id, use_path);

        let scope = region::Scope {
            id: expr_id,
            data: region::ScopeData::Node
        };
        self.each_in_scope_loan_affecting_path(
            scope, use_path, |loan| {
            if !compatible_borrow_kinds(loan.kind, borrow_kind) {
                self.bccx.signal_error();
                false
            } else {
                true
            }
        });
    }

    /// Reports an error if `expr` (which should be a path)
    /// is using a moved/uninitialized value
    fn check_if_path_is_moved(&self,
                              id: hir::ItemLocalId,
                              lp: &Rc<LoanPath<'tcx>>) {
        debug!("check_if_path_is_moved(id={:?}, lp={:?})", id, lp);

        // FIXME: if you find yourself tempted to cut and paste
        // the body below and then specializing the error reporting,
        // consider refactoring this instead!

        let base_lp = owned_ptr_base_path_rc(lp);
        self.move_data.each_move_of(id, &base_lp, |_, _| {
            self.bccx.signal_error();
            false
        });
    }

    /// Reports an error if assigning to `lp` will use a
    /// moved/uninitialized value. Mainly this is concerned with
    /// detecting derefs of uninitialized pointers.
    ///
    /// For example:
    ///
    /// ```
    /// let a: i32;
    /// a = 10; // ok, even though a is uninitialized
    /// ```
    ///
    /// ```
    /// struct Point { x: u32, y: u32 }
    /// let mut p: Point;
    /// p.x = 22; // ok, even though `p` is uninitialized
    /// ```
    ///
    /// ```compile_fail,E0381
    /// # struct Point { x: u32, y: u32 }
    /// let mut p: Box<Point>;
    /// (*p).x = 22; // not ok, p is uninitialized, can't deref
    /// ```
    fn check_if_assigned_path_is_moved(&self,
                                       id: hir::ItemLocalId,
                                       lp: &Rc<LoanPath<'tcx>>)
    {
        match lp.kind {
            LpVar(_) | LpUpvar(_) => {
                // assigning to `x` does not require that `x` is initialized
            }
            LpDowncast(ref lp_base, _) => {
                // assigning to `(P->Variant).f` is ok if assigning to `P` is ok
                self.check_if_assigned_path_is_moved(id, lp_base);
            }
            LpExtend(ref lp_base, _, LpInterior(_, InteriorField(_))) => {
                match lp_base.to_type().sty {
                    ty::Adt(def, _) if def.has_dtor(self.tcx()) => {
                        // In the case where the owner implements drop, then
                        // the path must be initialized to prevent a case of
                        // partial reinitialization
                        //
                        // FIXME: could refactor via hypothetical
                        // generalized check_if_path_is_moved
                        let loan_path = owned_ptr_base_path_rc(lp_base);
                        self.move_data.each_move_of(id, &loan_path, |_, _| {
                            self.bccx
                                .signal_error();
                            false
                        });
                        return;
                    },
                    _ => {},
                }

                // assigning to `P.f` is ok if assigning to `P` is ok
                self.check_if_assigned_path_is_moved(id, lp_base);
            }
            LpExtend(ref lp_base, _, LpInterior(_, InteriorElement)) |
            LpExtend(ref lp_base, _, LpDeref(_)) => {
                // assigning to `P[i]` requires `P` is initialized
                // assigning to `(*P)` requires `P` is initialized
                self.check_if_path_is_moved(id, lp_base);
            }
        }
    }

    fn check_assignment(&self,
                        assignment_id: hir::ItemLocalId,
                        assignee_cmt: &mc::cmt_<'tcx>) {
        debug!("check_assignment(assignee_cmt={:?})", assignee_cmt);

        // Check that we don't invalidate any outstanding loans
        if let Some(loan_path) = opt_loan_path(assignee_cmt) {
            let scope = region::Scope {
                id: assignment_id,
                data: region::ScopeData::Node
            };
            self.each_in_scope_loan_affecting_path(scope, &loan_path, |_| {
                self.bccx.signal_error();
                false
            });
        }

        // Check for reassignments to (immutable) local variables. This
        // needs to be done here instead of in check_loans because we
        // depend on move data.
        if let Categorization::Local(_) = assignee_cmt.cat {
            let lp = opt_loan_path(assignee_cmt).unwrap();
            self.move_data.each_assignment_of(assignment_id, &lp, |_| {
                if !assignee_cmt.mutbl.is_mutable() {
                    self.bccx.signal_error();
                }
                false
            });
            return
        }
    }
}