rust/src/rustc/middle/borrowck/gather_loans.rs

// ----------------------------------------------------------------------
// Gathering loans
//
// The borrow check proceeds in two phases. In phase one, we gather the full
// set of loans that are required at any point.  These are sorted according to
// their associated scopes.  In phase two, checking loans, we will then make
// sure that all of these loans are honored.

import categorization::{public_methods, opt_deref_kind};
import loan::public_methods;
import preserve::public_methods;

export gather_loans;

enum gather_loan_ctxt = @{bccx: borrowck_ctxt, req_maps: req_maps};

fn gather_loans(bccx: borrowck_ctxt, crate: @ast::crate) -> req_maps {
    let glcx = gather_loan_ctxt(@{bccx: bccx,
                                  req_maps: {req_loan_map: int_hash(),
                                             pure_map: int_hash()}});
    let v = visit::mk_vt(@{visit_expr: req_loans_in_expr
                           with *visit::default_visitor()});
    visit::visit_crate(*crate, glcx, v);
    ret glcx.req_maps;
}

fn req_loans_in_expr(ex: @ast::expr,
                     &&self: gather_loan_ctxt,
                     vt: visit::vt<gather_loan_ctxt>) {
    let bccx = self.bccx;
    let tcx = bccx.tcx;

    #debug["req_loans_in_expr(ex=%s)", pprust::expr_to_str(ex)];

    // If this expression is borrowed, have to ensure it remains valid:
    for tcx.borrowings.find(ex.id).each { |borrow|
        let cmt = self.bccx.cat_borrow_of_expr(ex);
        let scope_r = ty::re_scope(borrow.scope_id);
        self.guarantee_valid(cmt, borrow.mutbl, scope_r);
    }

    // Special checks for various kinds of expressions:
    alt ex.node {
      ast::expr_addr_of(mutbl, base) {
        let base_cmt = self.bccx.cat_expr(base);

        // make sure that the thing we are pointing out stays valid
        // for the lifetime `scope_r` of the resulting ptr:
        let scope_r =
            alt check ty::get(tcx.ty(ex)).struct {
              ty::ty_rptr(r, _) { r }
            };
        self.guarantee_valid(base_cmt, mutbl, scope_r);
      }

      ast::expr_call(f, args, _) {
        let arg_tys = ty::ty_fn_args(ty::expr_ty(self.tcx(), f));
        let scope_r = ty::re_scope(ex.id);
        vec::iter2(args, arg_tys) { |arg, arg_ty|
            alt ty::resolved_mode(self.tcx(), arg_ty.mode) {
              ast::by_mutbl_ref {
                let arg_cmt = self.bccx.cat_expr(arg);
                self.guarantee_valid(arg_cmt, m_mutbl, scope_r);
              }
              ast::by_ref {
                let arg_cmt = self.bccx.cat_expr(arg);
                self.guarantee_valid(arg_cmt, m_imm,  scope_r);
              }
              ast::by_val {
                // Rust's by-val does not actually give ownership to
                // the callee.  This means that if a pointer type is
                // passed, it is effectively a borrow, and so the
                // caller must guarantee that the data remains valid.
                //
                // Subtle: we only guarantee that the pointer is valid
                // and const.  Technically, we ought to pass in the
                // mutability that the caller expects (e.g., if the
                // formal argument has type @mut, we should guarantee
                // validity and mutability, not validity and const).
                // However, the type system already guarantees that
                // the caller's mutability is compatible with the
                // callee, so this is not necessary.  (Note that with
                // actual borrows, typeck is more liberal and allows
                // the pointer to be borrowed as immutable even if it
                // is mutable in the caller's frame, thus effectively
                // passing the buck onto us to enforce this)

                alt opt_deref_kind(arg_ty.ty) {
                  some(deref_ptr(region_ptr)) {
                    /* region pointers are (by induction) guaranteed */
                  }
                  none {
                    /* not a pointer, no worries */
                  }
                  some(_) {
                    let arg_cmt = self.bccx.cat_borrow_of_expr(arg);
                    self.guarantee_valid(arg_cmt, m_const, scope_r);
                  }
                }
              }
              ast::by_move | ast::by_copy {}
            }
        }
      }

      ast::expr_alt(ex_v, arms, _) {
        let cmt = self.bccx.cat_expr(ex_v);
        for arms.each { |arm|
            for arm.pats.each { |pat|
                self.gather_pat(cmt, pat, arm.body.node.id, ex.id);
            }
        }
      }

      ast::expr_field(rcvr, _, _) |
      ast::expr_binary(_, rcvr, _) |
      ast::expr_unary(_, rcvr) if self.bccx.method_map.contains_key(ex.id) {
        // Receivers in method calls are always passed by ref.
        //
        // FIXME--this scope is both too large and too small.  We make
        // the scope the enclosing block, which surely includes any
        // immediate call (a.b()) but which is too big.  OTOH, in the
        // case of a naked field `a.b`, the value is copied
        // anyhow. This is probably best fixed if we address the
        // syntactic ambiguity.

        // let scope_r = ty::re_scope(ex.id);
        let scope_r = ty::re_scope(self.tcx().region_map.get(ex.id));
        let rcvr_cmt = self.bccx.cat_expr(rcvr);
        self.guarantee_valid(rcvr_cmt, m_imm, scope_r);
      }

      _ { /*ok*/ }
    }

    // Check any contained expressions:
    visit::visit_expr(ex, self, vt);
}

impl methods for gather_loan_ctxt {
    fn tcx() -> ty::ctxt { self.bccx.tcx }

    // guarantees that addr_of(cmt) will be valid for the duration of
    // `static_scope_r`, or reports an error.  This may entail taking
    // out loans, which will be added to the `req_loan_map`.  This can
    // also entail "rooting" GC'd pointers, which means ensuring
    // dynamically that they are not freed.
    fn guarantee_valid(cmt: cmt,
                       req_mutbl: ast::mutability,
                       scope_r: ty::region) {

        #debug["guarantee_valid(cmt=%s, req_mutbl=%s, scope_r=%s)",
               self.bccx.cmt_to_repr(cmt),
               self.bccx.mut_to_str(req_mutbl),
               region_to_str(self.tcx(), scope_r)];
        let _i = indenter();

        alt cmt.lp {
          // If this expression is a loanable path, we MUST take out a loan.
          // This is somewhat non-obvious.  You might think, for example, that
          // if we have an immutable local variable `x` whose value is being
          // borrowed, we could rely on `x` not to change.  This is not so,
          // however, because even immutable locals can be moved.  So we take
          // out a loan on `x`, guaranteeing that it remains immutable for the
          // duration of the reference: if there is an attempt to move it
          // within that scope, the loan will be detected and an error will be
          // reported.
          some(_) {
            alt scope_r {
              ty::re_scope(scope_id) {
                let loans = self.bccx.loan(cmt, req_mutbl);
                self.add_loans(scope_id, loans);
              }
              _ {
                self.bccx.span_err(
                    cmt.span,
                    #fmt["cannot guarantee the stability \
                          of this expression for the entirety of \
                          its lifetime, %s",
                         region_to_str(self.tcx(), scope_r)]);
              }
            }
          }

          // The path is not loanable: in that case, we must try and preserve
          // it dynamically (or see that it is preserved by virtue of being
          // rooted in some immutable path)
          none {
            let opt_scope_id = alt scope_r {
              ty::re_scope(scope_id) { some(scope_id) }
              _ { none }
            };

            let result = {
                self.check_mutbl(req_mutbl, cmt).chain { |_ok|
                    self.bccx.preserve(cmt, opt_scope_id)
                }
            };

            alt result {
              ok(()) {
                // we were able guarantee the validity of the ptr,
                // perhaps by rooting or because it is immutably
                // rooted.  good.
              }
              err(e) {
                // not able to guarantee the validity of the ptr.
                // rather than report an error, presuming that the
                // borrow is for a limited scope, we'll make one last
                // ditch effort and require that the scope where the
                // borrow occurs be pure.
                alt opt_scope_id {
                  some(scope_id) {
                    self.req_maps.pure_map.insert(scope_id, e);
                  }
                  none {
                    // otherwise, fine, I give up.
                    self.bccx.report(e);
                  }
                }
              }
            }
          }
        }
    }

    // Check that the pat `cmt` is compatible with the required
    // mutability, presuming that it can be preserved to stay alive
    // long enough.
    //
    // For example, if you have an expression like `&x.f` where `x`
    // has type `@mut{f:int}`, this check might fail because `&x.f`
    // reqires an immutable pointer, but `f` lives in (aliased)
    // mutable memory.
    fn check_mutbl(req_mutbl: ast::mutability,
                   cmt: cmt) -> bckres<()> {
        alt (req_mutbl, cmt.mutbl) {
          (m_const, _) |
          (m_imm, m_imm) |
          (m_mutbl, m_mutbl) {
            ok(())
          }

          (_, m_const) |
          (m_imm, m_mutbl) |
          (m_mutbl, m_imm) {
            err({cmt: cmt,
                 code: err_mutbl(req_mutbl, cmt.mutbl)})
          }
        }
    }

    fn add_loans(scope_id: ast::node_id, loans: @const [loan]) {
        alt self.req_maps.req_loan_map.find(scope_id) {
          some(l) {
            *l += [loans];
          }
          none {
            self.req_maps.req_loan_map.insert(scope_id, @mut [loans]);
          }
        }
    }

    fn gather_pat(cmt: cmt, pat: @ast::pat,
                  arm_id: ast::node_id, alt_id: ast::node_id) {

        // Here, `cmt` is the categorization for the value being
        // matched and pat is the pattern it is being matched against.
        //
        // In general, the way that this works is that we walk down
        // the pattern, constructing a cmt that represents the path
        // that will be taken to reach the value being matched.
        //
        // When we encounter named bindings, we take the cmt that has
        // been built up and pass it off to guarantee_valid() so that
        // we can be sure that the binding will remain valid for the
        // duration of the arm.
        //
        // The correspondence between the id in the cmt and which
        // pattern is being referred to is somewhat...subtle.  In
        // general, the id of the cmt is the id of the node that
        // produces the value.  For patterns, that's actually the
        // *subpattern*, generally speaking.
        //
        // To see what I mean about ids etc, consider:
        //
        //     let x = @@3;
        //     alt x {
        //       @@y { ... }
        //     }
        //
        // Here the cmt for `y` would be something like
        //
        //     local(x)->@->@
        //
        // where the id of `local(x)` is the id of the `x` that appears
        // in the alt, the id of `local(x)->@` is the `@y` pattern,
        // and the id of `local(x)->@->@` is the id of the `y` pattern.

        #debug["gather_pat: id=%d pat=%s cmt=%s arm_id=%d alt_id=%d",
               pat.id, pprust::pat_to_str(pat),
               self.bccx.cmt_to_repr(cmt), arm_id, alt_id];
        let _i = indenter();

        let tcx = self.tcx();
        alt pat.node {
          ast::pat_wild {
            // _
          }

          ast::pat_enum(_, none) {
            // variant(*)
          }
          ast::pat_enum(_, some(subpats)) {
            // variant(x, y, z)
            for subpats.each { |subpat|
                let subcmt = self.bccx.cat_variant(subpat, cmt);
                self.gather_pat(subcmt, subpat, arm_id, alt_id);
            }
          }

          ast::pat_ident(_, none) if self.pat_is_variant(pat) {
            // nullary variant
            #debug["nullary variant"];
          }
          ast::pat_ident(id, o_pat) {
            // x or x @ p --- `x` must remain valid for the scope of the alt
            #debug["defines identifier %s", pprust::path_to_str(id)];

            // Note: there is a discussion of the function of
            // cat_discr in the method preserve():
            let cmt1 = self.bccx.cat_discr(cmt, alt_id);
            let arm_scope = ty::re_scope(arm_id);
            self.guarantee_valid(cmt1, m_const, arm_scope);

            for o_pat.each { |p|
                self.gather_pat(cmt, p, arm_id, alt_id);
            }
          }

          ast::pat_rec(field_pats, _) {
            // {f1: p1, ..., fN: pN}
            for field_pats.each { |fp|
                let cmt_field = self.bccx.cat_field(fp.pat, cmt, fp.ident);
                self.gather_pat(cmt_field, fp.pat, arm_id, alt_id);
            }
          }

          ast::pat_tup(subpats) {
            // (p1, ..., pN)
            for subpats.each { |subpat|
                let subcmt = self.bccx.cat_tuple_elt(subpat, cmt);
                self.gather_pat(subcmt, subpat, arm_id, alt_id);
            }
          }

          ast::pat_box(subpat) | ast::pat_uniq(subpat) {
            // @p1, ~p1
            alt self.bccx.cat_deref(subpat, cmt, 0u, true) {
              some(subcmt) {
                self.gather_pat(subcmt, subpat, arm_id, alt_id);
              }
              none {
                tcx.sess.span_bug(pat.span, "Non derefable type");
              }
            }
          }

          ast::pat_lit(_) | ast::pat_range(_, _) { /*always ok*/ }
        }
    }

    fn pat_is_variant(pat: @ast::pat) -> bool {
        pat_util::pat_is_variant(self.bccx.tcx.def_map, pat)
    }
}