rust/src/comp/middle/tstate/pre_post_conditions.rs

import core::{vec, option};
import option::{none, some};

import tstate::ann::*;
import aux::*;
import bitvectors::{bit_num, seq_preconds, seq_postconds,
                    intersect_states,
                    relax_precond_block, gen};
import tritv::*;
import syntax::ast::*;
import syntax::ast_util::*;
import syntax::visit;
import util::common::{new_def_hash, log_expr, field_exprs,
                      has_nonlocal_exits, log_stmt};
import syntax::codemap::span;
import util::ppaux::fn_ident_to_string;

fn find_pre_post_mod(_m: _mod) -> _mod {
    log "implement find_pre_post_mod!";
    fail;
}

fn find_pre_post_native_mod(_m: native_mod) -> native_mod {
    log "implement find_pre_post_native_mod";
    fail;
}

fn find_pre_post_method(ccx: crate_ctxt, m: @method) {
    assert (ccx.fm.contains_key(m.node.id));
    let fcx: fn_ctxt =
        {enclosing: ccx.fm.get(m.node.id),
         id: m.node.id,
         name: m.node.ident,
         ccx: ccx};
    find_pre_post_fn(fcx, m.node.meth.body);
}

fn find_pre_post_item(ccx: crate_ctxt, i: item) {
    alt i.node {
      item_const(_, e) {
        // make a fake fcx
        let v: @mutable [node_id] = @mutable [];
        let fake_fcx =
            {
             // just bogus
             enclosing:
                 {constrs: @new_def_hash::<constraint>(),
                  num_constraints: 0u,
                  cf: return_val,
                  i_return: ninit(0, ""),
                  i_diverge: ninit(0, ""),
                  used_vars: v},
             id: 0,
             name: "",
             ccx: ccx};
        find_pre_post_expr(fake_fcx, e);
      }
      item_fn(f, _) {
        assert (ccx.fm.contains_key(i.id));
        let fcx =
            {enclosing: ccx.fm.get(i.id), id: i.id, name: i.ident, ccx: ccx};
        find_pre_post_fn(fcx, f.body);
      }
      item_mod(m) { find_pre_post_mod(m); }
      item_native_mod(nm) { find_pre_post_native_mod(nm); }
      item_ty(_, _) { ret; }
      item_tag(_, _) { ret; }
      item_res(dtor, dtor_id, _, _) {
        let fcx =
            {enclosing: ccx.fm.get(dtor_id),
             id: dtor_id,
             name: i.ident,
             ccx: ccx};
        find_pre_post_fn(fcx, dtor.body);
      }
      item_obj(o, _, _) {for m in o.methods { find_pre_post_method(ccx, m); }}
      item_impl(_, _, ms) { for m in ms { find_pre_post_method(ccx, m); } }
    }
}


/* Finds the pre and postcondition for each expr in <args>;
   sets the precondition in a to be the result of combining
   the preconditions for <args>, and the postcondition in a to
   be the union of all postconditions for <args> */
fn find_pre_post_exprs(fcx: fn_ctxt, args: [@expr], id: node_id) {
    if vec::len::<@expr>(args) > 0u {
        log "find_pre_post_exprs: oper =";
        log_expr(*args[0]);
    }
    fn do_one(fcx: fn_ctxt, e: @expr) { find_pre_post_expr(fcx, e); }
    for e: @expr in args { do_one(fcx, e); }

    fn get_pp(ccx: crate_ctxt, &&e: @expr) -> pre_and_post {
        ret expr_pp(ccx, e);
    }
    let pps = vec::map(args, bind get_pp(fcx.ccx, _));

    set_pre_and_post(fcx.ccx, id, seq_preconds(fcx, pps),
                     seq_postconds(fcx, vec::map(pps, get_post)));
}

fn find_pre_post_loop(fcx: fn_ctxt, l: @local, index: @expr, body: blk,
                      id: node_id) {
    find_pre_post_expr(fcx, index);
    find_pre_post_block(fcx, body);
    pat_bindings(l.node.pat) {|p|
        let ident = alt p.node { pat_bind(id, _) { id } };
        let v_init = ninit(p.id, ident);
        relax_precond_block(fcx, bit_num(fcx, v_init) as node_id, body);
        // Hack: for-loop index variables are frequently ignored,
        // so we pretend they're used
        use_var(fcx, p.id);
    };

    let loop_precond =
        seq_preconds(fcx, [expr_pp(fcx.ccx, index), block_pp(fcx.ccx, body)]);
    let loop_postcond =
        intersect_states(expr_postcond(fcx.ccx, index),
                         block_postcond(fcx.ccx, body));
    copy_pre_post_(fcx.ccx, id, loop_precond, loop_postcond);
}

// Generates a pre/post assuming that a is the
// annotation for an if-expression with consequent conseq
// and alternative maybe_alt
fn join_then_else(fcx: fn_ctxt, antec: @expr, conseq: blk,
                  maybe_alt: option::t<@expr>, id: node_id, chck: if_ty) {
    find_pre_post_expr(fcx, antec);
    find_pre_post_block(fcx, conseq);
    alt maybe_alt {
      none. {
        alt chck {
          if_check. {
            let c: sp_constr = expr_to_constr(fcx.ccx.tcx, antec);
            gen(fcx, antec.id, c.node);
          }
          _ { }
        }

        let precond_res =
            seq_preconds(fcx,
                         [expr_pp(fcx.ccx, antec),
                          block_pp(fcx.ccx, conseq)]);
        set_pre_and_post(fcx.ccx, id, precond_res,
                         expr_poststate(fcx.ccx, antec));
      }
      some(altern) {
        /*
          if check = if_check, then
          be sure that the predicate implied by antec
          is *not* true in the alternative
         */
        find_pre_post_expr(fcx, altern);
        let precond_false_case =
            seq_preconds(fcx,
                         [expr_pp(fcx.ccx, antec), expr_pp(fcx.ccx, altern)]);
        let postcond_false_case =
            seq_postconds(fcx,
                          [expr_postcond(fcx.ccx, antec),
                           expr_postcond(fcx.ccx, altern)]);

        /* Be sure to set the bit for the check condition here,
         so that it's *not* set in the alternative. */
        alt chck {
          if_check. {
            let c: sp_constr = expr_to_constr(fcx.ccx.tcx, antec);
            gen(fcx, antec.id, c.node);
          }
          _ { }
        }
        let precond_true_case =
            seq_preconds(fcx,
                         [expr_pp(fcx.ccx, antec),
                          block_pp(fcx.ccx, conseq)]);
        let postcond_true_case =
            seq_postconds(fcx,
                          [expr_postcond(fcx.ccx, antec),
                           block_postcond(fcx.ccx, conseq)]);

        let precond_res =
            seq_postconds(fcx, [precond_true_case, precond_false_case]);
        let postcond_res =
            intersect_states(postcond_true_case, postcond_false_case);
        set_pre_and_post(fcx.ccx, id, precond_res, postcond_res);
      }
    }
}

fn gen_if_local(fcx: fn_ctxt, lhs: @expr, rhs: @expr, larger_id: node_id,
                new_var: node_id, pth: @path) {
    alt node_id_to_def(fcx.ccx, new_var) {
      some(d) {
        alt d {
          def_local(d_id, _) {
            find_pre_post_expr(fcx, rhs);
            let p = expr_pp(fcx.ccx, rhs);
            set_pre_and_post(fcx.ccx, larger_id, p.precondition,
                             p.postcondition);
            gen(fcx, larger_id,
                ninit(d_id.node, path_to_ident(fcx.ccx.tcx, pth)));
          }
          _ { find_pre_post_exprs(fcx, [lhs, rhs], larger_id); }
        }
      }
      _ { find_pre_post_exprs(fcx, [lhs, rhs], larger_id); }
    }
}

fn handle_update(fcx: fn_ctxt, parent: @expr, lhs: @expr, rhs: @expr,
                 ty: oper_type) {
    find_pre_post_expr(fcx, rhs);
    alt lhs.node {
      expr_path(p) {
        let post = expr_postcond(fcx.ccx, parent);
        let tmp = tritv_clone(post);

        alt ty {
          oper_move. {
            if is_path(rhs) { forget_in_postcond(fcx, parent.id, rhs.id); }
          }
          oper_swap. {
            forget_in_postcond_still_init(fcx, parent.id, lhs.id);
            forget_in_postcond_still_init(fcx, parent.id, rhs.id);
          }
          oper_assign. {
            forget_in_postcond_still_init(fcx, parent.id, lhs.id);
          }
          _ {
            // pure and assign_op require the lhs to be init'd
            let df = node_id_to_def_strict(fcx.ccx.tcx, lhs.id);
            alt df {
              def_local(d_id, _) {
                let i =
                    bit_num(fcx,
                            ninit(d_id.node, path_to_ident(fcx.ccx.tcx, p)));
                require_and_preserve(i, expr_pp(fcx.ccx, lhs));
              }
              _ { }
            }
          }
        }

        gen_if_local(fcx, lhs, rhs, parent.id, lhs.id, p);
        alt rhs.node {
          expr_path(p1) {
            let d = local_node_id_to_local_def_id(fcx, lhs.id);
            let d1 = local_node_id_to_local_def_id(fcx, rhs.id);
            alt d {
              some(id) {
                alt d1 {
                  some(id1) {
                    let instlhs =
                        {ident: path_to_ident(fcx.ccx.tcx, p), node: id};
                    let instrhs =
                        {ident: path_to_ident(fcx.ccx.tcx, p1), node: id1};
                    copy_in_poststate_two(fcx, tmp, post, instlhs, instrhs,
                                          ty);
                  }
                  _ { }
                }
              }
              _ { }
            }
          }
          _ {/* do nothing */ }
        }
      }
      _ { find_pre_post_expr(fcx, lhs); }
    }
}

fn handle_var(fcx: fn_ctxt, rslt: pre_and_post, id: node_id, name: ident) {
    handle_var_def(fcx, rslt, node_id_to_def_strict(fcx.ccx.tcx, id), name);
}

fn handle_var_def(fcx: fn_ctxt, rslt: pre_and_post, def: def, name: ident) {
    log ("handle_var_def: ", def, name);
    alt def {
      def_local(d_id, _) | def_arg(d_id, _) {
        use_var(fcx, d_id.node);
        let i = bit_num(fcx, ninit(d_id.node, name));
        require_and_preserve(i, rslt);
      }
      _ {/* nothing to check */ }
    }
}

fn forget_args_moved_in(fcx: fn_ctxt, parent: @expr, modes: [ty::mode],
                        operands: [@expr]) {
    let i = 0u;
    for mode: ty::mode in modes {
        if mode == by_move {
            forget_in_postcond(fcx, parent.id, operands[i].id);
        }
        i += 1u;
    }
}

fn find_pre_post_expr_fn_upvars(fcx: fn_ctxt, e: @expr) {
    let rslt = expr_pp(fcx.ccx, e);
    clear_pp(rslt);
    for def in *freevars::get_freevars(fcx.ccx.tcx, e.id) {
        log ("handle_var_def: def=", def);
        handle_var_def(fcx, rslt, def.def, "upvar");
    }
}

/* Fills in annotations as a side effect. Does not rebuild the expr */
fn find_pre_post_expr(fcx: fn_ctxt, e: @expr) {
    let enclosing = fcx.enclosing;
    let num_local_vars = num_constraints(enclosing);
    fn do_rand_(fcx: fn_ctxt, e: @expr) { find_pre_post_expr(fcx, e); }


    alt e.node {
      expr_call(operator, operands, _) {
        /* copy */

        let args = operands;
        args += [operator];

        find_pre_post_exprs(fcx, args, e.id);
        /* see if the call has any constraints on its type */
        for c: @ty::constr in constraints_expr(fcx.ccx.tcx, operator) {
            let i =
                bit_num(fcx, substitute_constr_args(fcx.ccx.tcx, args, c));
            require(i, expr_pp(fcx.ccx, e));
        }

        forget_args_moved_in(fcx, e, callee_modes(fcx, operator.id),
                             operands);

        /* if this is a failing call, its postcondition sets everything */
        alt controlflow_expr(fcx.ccx, operator) {
          noreturn. { set_postcond_false(fcx.ccx, e.id); }
          _ { }
        }
      }
      expr_vec(args, _) { find_pre_post_exprs(fcx, args, e.id); }
      expr_path(p) {
        let rslt = expr_pp(fcx.ccx, e);
        clear_pp(rslt);
        handle_var(fcx, rslt, e.id, path_to_ident(fcx.ccx.tcx, p));
      }
      expr_log(_, arg) {
        find_pre_post_expr(fcx, arg);
        copy_pre_post(fcx.ccx, e.id, arg);
      }
      expr_fn(f, cap_clause) {
        find_pre_post_expr_fn_upvars(fcx, e);

        let use_cap_item = lambda(&&cap_item: @capture_item) {
            let d = local_node_id_to_local_def_id(fcx, cap_item.id);
            option::may(d, { |id| use_var(fcx, id) });
        };
        vec::iter(cap_clause.copies, use_cap_item);
        vec::iter(cap_clause.moves, use_cap_item);

        vec::iter(cap_clause.moves) { |cap_item|
            log ("forget_in_postcond: ", cap_item);
            forget_in_postcond(fcx, e.id, cap_item.id);
        }
      }
      expr_fn_block(_, _) {
        find_pre_post_expr_fn_upvars(fcx, e);
      }
      expr_block(b) {
        find_pre_post_block(fcx, b);
        let p = block_pp(fcx.ccx, b);
        set_pre_and_post(fcx.ccx, e.id, p.precondition, p.postcondition);
      }
      expr_rec(fields, maybe_base) {
        let es = field_exprs(fields);
        alt maybe_base { none. {/* no-op */ } some(b) { es += [b]; } }
        find_pre_post_exprs(fcx, es, e.id);
      }
      expr_tup(elts) { find_pre_post_exprs(fcx, elts, e.id); }
      expr_copy(a) {
        find_pre_post_expr(fcx, a);
        copy_pre_post(fcx.ccx, e.id, a);
      }
      expr_move(lhs, rhs) { handle_update(fcx, e, lhs, rhs, oper_move); }
      expr_swap(lhs, rhs) { handle_update(fcx, e, lhs, rhs, oper_swap); }
      expr_assign(lhs, rhs) { handle_update(fcx, e, lhs, rhs, oper_assign); }
      expr_assign_op(_, lhs, rhs) {
        /* Different from expr_assign in that the lhs *must*
           already be initialized */

        find_pre_post_exprs(fcx, [lhs, rhs], e.id);
        forget_in_postcond_still_init(fcx, e.id, lhs.id);
      }
      expr_lit(_) { clear_pp(expr_pp(fcx.ccx, e)); }
      expr_ret(maybe_val) {
        alt maybe_val {
          none. {
            clear_precond(fcx.ccx, e.id);
            set_postcond_false(fcx.ccx, e.id);
          }
          some(ret_val) {
            find_pre_post_expr(fcx, ret_val);
            set_precondition(node_id_to_ts_ann(fcx.ccx, e.id),
                             expr_precond(fcx.ccx, ret_val));
            set_postcond_false(fcx.ccx, e.id);
          }
        }
      }
      expr_be(val) {
        find_pre_post_expr(fcx, val);
        set_pre_and_post(fcx.ccx, e.id, expr_prestate(fcx.ccx, val),
                         false_postcond(num_local_vars));
      }
      expr_if(antec, conseq, maybe_alt) {
        join_then_else(fcx, antec, conseq, maybe_alt, e.id, plain_if);
      }
      expr_ternary(_, _, _) { find_pre_post_expr(fcx, ternary_to_if(e)); }
      expr_binary(bop, l, r) {
        if lazy_binop(bop) {
            find_pre_post_expr(fcx, l);
            find_pre_post_expr(fcx, r);
            let overall_pre =
                seq_preconds(fcx, [expr_pp(fcx.ccx, l), expr_pp(fcx.ccx, r)]);
            set_precondition(node_id_to_ts_ann(fcx.ccx, e.id), overall_pre);
            set_postcondition(node_id_to_ts_ann(fcx.ccx, e.id),
                              expr_postcond(fcx.ccx, l));
        } else { find_pre_post_exprs(fcx, [l, r], e.id); }
      }
      expr_unary(_, operand) {
        find_pre_post_expr(fcx, operand);
        copy_pre_post(fcx.ccx, e.id, operand);
      }
      expr_cast(operand, _) {
        find_pre_post_expr(fcx, operand);
        copy_pre_post(fcx.ccx, e.id, operand);
      }
      expr_while(test, body) {
        find_pre_post_expr(fcx, test);
        find_pre_post_block(fcx, body);
        set_pre_and_post(fcx.ccx, e.id,
                         seq_preconds(fcx,
                                      [expr_pp(fcx.ccx, test),
                                       block_pp(fcx.ccx, body)]),
                         intersect_states(expr_postcond(fcx.ccx, test),
                                          block_postcond(fcx.ccx, body)));
      }
      expr_do_while(body, test) {
        find_pre_post_block(fcx, body);
        find_pre_post_expr(fcx, test);
        let loop_postcond =
            seq_postconds(fcx,
                          [block_postcond(fcx.ccx, body),
                           expr_postcond(fcx.ccx, test)]);
        /* conservative approximation: if the body
           could break or cont, the test may never be executed */

        if has_nonlocal_exits(body) {
            loop_postcond = empty_poststate(num_local_vars);
        }
        set_pre_and_post(fcx.ccx, e.id,
                         seq_preconds(fcx,
                                      [block_pp(fcx.ccx, body),
                                       expr_pp(fcx.ccx, test)]),
                         loop_postcond);
      }
      expr_for(d, index, body) {
        find_pre_post_loop(fcx, d, index, body, e.id);
      }
      expr_index(val, sub) { find_pre_post_exprs(fcx, [val, sub], e.id); }
      expr_alt(ex, alts) {
        find_pre_post_expr(fcx, ex);
        fn do_an_alt(fcx: fn_ctxt, an_alt: arm) -> pre_and_post {
            alt an_alt.guard {
              some(e) { find_pre_post_expr(fcx, e); }
              _ {}
            }
            find_pre_post_block(fcx, an_alt.body);
            ret block_pp(fcx.ccx, an_alt.body);
        }
        let alt_pps = [];
        for a: arm in alts { alt_pps += [do_an_alt(fcx, a)]; }
        fn combine_pp(antec: pre_and_post, fcx: fn_ctxt, &&pp: pre_and_post,
                      &&next: pre_and_post) -> pre_and_post {
            union(pp.precondition, seq_preconds(fcx, [antec, next]));
            intersect(pp.postcondition, next.postcondition);
            ret pp;
        }
        let antec_pp = pp_clone(expr_pp(fcx.ccx, ex));
        let e_pp =
            @{precondition: empty_prestate(num_local_vars),
              postcondition: false_postcond(num_local_vars)};
        let g = bind combine_pp(antec_pp, fcx, _, _);
        let alts_overall_pp =
            vec::foldl(e_pp, alt_pps, g);
        set_pre_and_post(fcx.ccx, e.id, alts_overall_pp.precondition,
                         alts_overall_pp.postcondition);
      }
      expr_field(operator, _, _) {
        find_pre_post_expr(fcx, operator);
        copy_pre_post(fcx.ccx, e.id, operator);
      }
      expr_fail(maybe_val) {
        let prestate;
        alt maybe_val {
          none. { prestate = empty_prestate(num_local_vars); }
          some(fail_val) {
            find_pre_post_expr(fcx, fail_val);
            prestate = expr_precond(fcx.ccx, fail_val);
          }
        }
        set_pre_and_post(fcx.ccx, e.id,
                         /* if execution continues after fail,
                            then everything is true! */
                         prestate, false_postcond(num_local_vars));
      }
      expr_assert(p) {
        find_pre_post_expr(fcx, p);
        copy_pre_post(fcx.ccx, e.id, p);
      }
      expr_check(_, p) {
        find_pre_post_expr(fcx, p);
        copy_pre_post(fcx.ccx, e.id, p);
        /* predicate p holds after this expression executes */

        let c: sp_constr = expr_to_constr(fcx.ccx.tcx, p);
        gen(fcx, e.id, c.node);
      }
      expr_if_check(p, conseq, maybe_alt) {
        join_then_else(fcx, p, conseq, maybe_alt, e.id, if_check);
      }






      expr_bind(operator, maybe_args) {
        let args = [];
        let cmodes = callee_modes(fcx, operator.id);
        let modes = [];
        let i = 0;
        for expr_opt: option::t<@expr> in maybe_args {
            alt expr_opt {
              none. {/* no-op */ }
              some(expr) { modes += [cmodes[i]]; args += [expr]; }
            }
            i += 1;
        }
        args += [operator]; /* ??? order of eval? */
        forget_args_moved_in(fcx, e, modes, args);
        find_pre_post_exprs(fcx, args, e.id);
      }
      expr_break. { clear_pp(expr_pp(fcx.ccx, e)); }
      expr_cont. { clear_pp(expr_pp(fcx.ccx, e)); }
      expr_mac(_) { fcx.ccx.tcx.sess.bug("unexpanded macro"); }
      expr_anon_obj(anon_obj) {
        alt anon_obj.inner_obj {
          some(ex) {
            find_pre_post_expr(fcx, ex);
            copy_pre_post(fcx.ccx, e.id, ex);
          }
          none. { clear_pp(expr_pp(fcx.ccx, e)); }
        }
      }
    }
}

fn find_pre_post_stmt(fcx: fn_ctxt, s: stmt) {
    log "stmt =";
    log_stmt(s);
    alt s.node {
      stmt_decl(adecl, id) {
        alt adecl.node {
          decl_local(alocals) {
            let e_pp;
            let prev_pp = empty_pre_post(num_constraints(fcx.enclosing));
            for (_, alocal) in alocals {
                alt alocal.node.init {
                  some(an_init) {
                    /* LHS always becomes initialized,
                     whether or not this is a move */
                    find_pre_post_expr(fcx, an_init.expr);
                    pat_bindings(alocal.node.pat) {|p|
                        copy_pre_post(fcx.ccx, p.id, an_init.expr);
                    };
                    /* Inherit ann from initializer, and add var being
                       initialized to the postcondition */
                    copy_pre_post(fcx.ccx, id, an_init.expr);

                    let p = none;
                    alt an_init.expr.node {
                      expr_path(_p) { p = some(_p); }
                      _ { }
                    }

                    pat_bindings(alocal.node.pat) {|pat|
                        /* FIXME: This won't be necessary when typestate
                        works well enough for pat_bindings to return a
                        refinement-typed thing. */
                        let ident = alt pat.node { pat_bind(n, _) { n } };
                        alt p {
                          some(p) {
                            copy_in_postcond(fcx, id,
                                             {ident: ident, node: pat.id},
                                             {ident:
                                                  path_to_ident(fcx.ccx.tcx,
                                                                p),
                                              node: an_init.expr.id},
                                             op_to_oper_ty(an_init.op));
                          }
                          none. { }
                        }
                        gen(fcx, id, ninit(pat.id, ident));
                    };

                    if an_init.op == init_move && is_path(an_init.expr) {
                        forget_in_postcond(fcx, id, an_init.expr.id);
                    }

                    /* Clear out anything that the previous initializer
                    guaranteed */
                    e_pp = expr_pp(fcx.ccx, an_init.expr);
                    tritv_copy(prev_pp.precondition,
                               seq_preconds(fcx, [prev_pp, e_pp]));
                    /* Include the LHSs too, since those aren't in the
                     postconds of the RHSs themselves */
                    pat_bindings(alocal.node.pat) {|pat|
                        alt pat.node {
                          pat_bind(n, _) {
                            set_in_postcond(bit_num(fcx, ninit(pat.id, n)),
                                            prev_pp);
                          }
                        }
                    };
                    copy_pre_post_(fcx.ccx, id, prev_pp.precondition,
                                   prev_pp.postcondition);
                  }
                  none. {
                    pat_bindings(alocal.node.pat) {|p|
                        clear_pp(node_id_to_ts_ann(fcx.ccx, p.id).conditions);
                    };
                    clear_pp(node_id_to_ts_ann(fcx.ccx, id).conditions);
                  }
                }
            }
          }
          decl_item(anitem) {
            clear_pp(node_id_to_ts_ann(fcx.ccx, id).conditions);
            find_pre_post_item(fcx.ccx, *anitem);
          }
        }
      }
      stmt_expr(e, id) {
        find_pre_post_expr(fcx, e);
        copy_pre_post(fcx.ccx, id, e);
      }
    }
}

fn find_pre_post_block(fcx: fn_ctxt, b: blk) {
    /* Want to say that if there is a break or cont in this
     block, then that invalidates the poststate upheld by
    any of the stmts after it.
    Given that the typechecker has run, we know any break will be in
    a block that forms a loop body. So that's ok. There'll never be an
    expr_break outside a loop body, therefore, no expr_break outside a block.
    */

    /* Conservative approximation for now: This says that if a block contains
     *any* breaks or conts, then its postcondition doesn't promise anything.
     This will mean that:
     x = 0;
     break;

     won't have a postcondition that says x is initialized, but that's ok.
     */

    let nv = num_constraints(fcx.enclosing);
    fn do_one_(fcx: fn_ctxt, s: @stmt) {
        find_pre_post_stmt(fcx, *s);
        /*
                log_err "pre_post for stmt:";
                log_stmt_err(*s);
                log_err "is:";
                log_pp_err(stmt_pp(fcx.ccx, *s));
        */
    }
    for s: @stmt in b.node.stmts { do_one_(fcx, s); }
    fn do_inner_(fcx: fn_ctxt, &&e: @expr) { find_pre_post_expr(fcx, e); }
    let do_inner = bind do_inner_(fcx, _);
    option::map::<@expr, ()>(b.node.expr, do_inner);

    let pps: [pre_and_post] = [];
    for s: @stmt in b.node.stmts { pps += [stmt_pp(fcx.ccx, *s)]; }
    alt b.node.expr {
      none. {/* no-op */ }
      some(e) { pps += [expr_pp(fcx.ccx, e)]; }
    }

    let block_precond = seq_preconds(fcx, pps);

    let postconds = [];
    for pp: pre_and_post in pps { postconds += [get_post(pp)]; }

    /* A block may be empty, so this next line ensures that the postconds
       vector is non-empty. */
    postconds += [block_precond];

    let block_postcond = empty_poststate(nv);
    /* conservative approximation */

    if !has_nonlocal_exits(b) {
        block_postcond = seq_postconds(fcx, postconds);
    }
    set_pre_and_post(fcx.ccx, b.node.id, block_precond, block_postcond);
}

fn find_pre_post_fn(fcx: fn_ctxt, body: blk) {
    // hack
    use_var(fcx, tsconstr_to_node_id(fcx.enclosing.i_return));
    use_var(fcx, tsconstr_to_node_id(fcx.enclosing.i_diverge));

    find_pre_post_block(fcx, body);

    // Treat the tail expression as a return statement
    alt body.node.expr {
      some(tailexpr) { set_postcond_false(fcx.ccx, tailexpr.id); }
      none. {/* fallthrough */ }
    }
}

fn fn_pre_post(decl: fn_decl, body: blk, sp: span,
               i: fn_ident, id: node_id,
               ccx: crate_ctxt, v: visit::vt<crate_ctxt>) {
    visit::visit_fn_body(decl, body, sp, i, id, ccx, v);
    assert (ccx.fm.contains_key(id));
    let fcx =
        {enclosing: ccx.fm.get(id),
         id: id,
         name: fn_ident_to_string(id, i),
         ccx: ccx};
    find_pre_post_fn(fcx, body);
}

//
// Local Variables:
// mode: rust
// fill-column: 78;
// indent-tabs-mode: nil
// c-basic-offset: 4
// buffer-file-coding-system: utf-8-unix
// End:
//