Auto merge of #27185 - pnkfelix:test-cyclic-collections, r=alexcrichton

This test attempts to exercise cyclic structure in much of `std::collections`

(as much as is possible; e.g. things like `EnumSet` and `BitVec` do not really support carrying references at all, so trying to represent cyclic structure within them dooes not really make sense.)

This work should land before we make future revisions to `dropck`; that is, I am attempting to catch potential regressions to cases where we supported cyclic structure circa Rust 1.2.

src/test/run-pass/dropck_legal_cycles.rs

@@ -0,0 +1,759 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// This test exercises cases where cyclic structure is legal,
+// including when the cycles go through data-structures such
+// as `Vec` or `TypedArena`.
+//
+// The intent is to cover as many such cases as possible, ensuring
+// that if the compiler did not complain circa Rust 1.x (1.2 as of
+// this writing), then it will continue to not complain in the future.
+//
+// Note that while some of the tests are only exercising using the
+// given collection as a "backing store" for a set of nodes that hold
+// the actual cycle (and thus the cycle does not go through the
+// collection itself in such cases), in general we *do* want to make
+// sure to have at least one example exercising a cycle that goes
+// through the collection, for every collection type that supports
+// this.
+
+#![feature(vecmap)]
+
+use std::cell::Cell;
+use std::cmp::Ordering;
+use std::collections::BinaryHeap;
+use std::collections::HashMap;
+use std::collections::LinkedList;
+use std::collections::VecDeque;
+use std::collections::VecMap;
+use std::collections::btree_map::BTreeMap;
+use std::collections::btree_set::BTreeSet;
+use std::hash::{Hash, Hasher};
+
+const PRINT: bool = false;
+
+pub fn main() {
+    let c_orig = ContextData {
+        curr_depth: 0,
+        max_depth: 3,
+        visited: 0,
+        max_visits: 1000,
+        skipped: 0,
+        curr_mark: 0,
+        saw_prev_marked: false,
+    };
+
+    // Cycle 1: { v[0] -> v[1], v[1] -> v[0] };
+    // does not exercise `v` itself
+    let v: Vec<S> = vec![Named::new("s0"),
+                         Named::new("s1")];
+    v[0].next.set(Some(&v[1]));
+    v[1].next.set(Some(&v[0]));
+
+    let mut c = c_orig.clone();
+    c.curr_mark = 10;
+    assert!(!c.saw_prev_marked);
+    v[0].for_each_child(&mut c);
+    assert!(c.saw_prev_marked);
+
+    if PRINT { println!(""); }
+
+    // Cycle 2: { v[0] -> v, v[1] -> v }
+    let v: V = Named::new("v");
+    v.contents[0].set(Some(&v));
+    v.contents[1].set(Some(&v));
+
+    let mut c = c_orig.clone();
+    c.curr_mark = 20;
+    assert!(!c.saw_prev_marked);
+    v.for_each_child(&mut c);
+    assert!(c.saw_prev_marked);
+
+    if PRINT { println!(""); }
+
+    // Cycle 3: { hk0 -> hv0, hv0 -> hk0, hk1 -> hv1, hv1 -> hk1 };
+    // does not exercise `h` itself
+
+    let mut h: HashMap<H,H> = HashMap::new();
+    h.insert(Named::new("hk0"), Named::new("hv0"));
+    h.insert(Named::new("hk1"), Named::new("hv1"));
+    for (key, val) in h.iter() {
+        val.next.set(Some(key));
+        key.next.set(Some(val));
+    }
+
+    let mut c = c_orig.clone();
+    c.curr_mark = 30;
+    for (key, _) in h.iter() {
+        c.curr_mark += 1;
+        c.saw_prev_marked = false;
+        key.for_each_child(&mut c);
+        assert!(c.saw_prev_marked);
+    }
+
+    if PRINT { println!(""); }
+
+    // Cycle 4: { h -> (hmk0,hmv0,hmk1,hmv1), {hmk0,hmv0,hmk1,hmv1} -> h }
+
+    let mut h: HashMap<HM,HM> = HashMap::new();
+    h.insert(Named::new("hmk0"), Named::new("hmv0"));
+    h.insert(Named::new("hmk0"), Named::new("hmv0"));
+    for (key, val) in h.iter() {
+        val.contents.set(Some(&h));
+        key.contents.set(Some(&h));
+    }
+
+    let mut c = c_orig.clone();
+    c.max_depth = 2;
+    c.curr_mark = 40;
+    for (key, _) in h.iter() {
+        c.curr_mark += 1;
+        c.saw_prev_marked = false;
+        key.for_each_child(&mut c);
+        assert!(c.saw_prev_marked);
+        // break;
+    }
+
+    if PRINT { println!(""); }
+
+    // Cycle 5: { vd[0] -> vd[1], vd[1] -> vd[0] };
+    // does not exercise vd itself
+    let mut vd: VecDeque<S> = VecDeque::new();
+    vd.push_back(Named::new("d0"));
+    vd.push_back(Named::new("d1"));
+    vd[0].next.set(Some(&vd[1]));
+    vd[1].next.set(Some(&vd[0]));
+
+    let mut c = c_orig.clone();
+    c.curr_mark = 50;
+    assert!(!c.saw_prev_marked);
+    vd[0].for_each_child(&mut c);
+    assert!(c.saw_prev_marked);
+
+    if PRINT { println!(""); }
+
+    // Cycle 6: { vd -> (vd0, vd1), {vd0, vd1} -> vd }
+    let mut vd: VecDeque<VD> = VecDeque::new();
+    vd.push_back(Named::new("vd0"));
+    vd.push_back(Named::new("vd1"));
+    vd[0].contents.set(Some(&vd));
+    vd[1].contents.set(Some(&vd));
+
+    let mut c = c_orig.clone();
+    c.curr_mark = 60;
+    assert!(!c.saw_prev_marked);
+    vd[0].for_each_child(&mut c);
+    assert!(c.saw_prev_marked);
+
+    if PRINT { println!(""); }
+
+    // Cycle 7: { vm -> (vm0, vm1), {vm0, vm1} -> vm }
+    let mut vm: VecMap<VM> = VecMap::new();
+    vm.insert(0, Named::new("vm0"));
+    vm.insert(1, Named::new("vm1"));
+    vm[0].contents.set(Some(&vm));
+    vm[1].contents.set(Some(&vm));
+
+    let mut c = c_orig.clone();
+    c.curr_mark = 70;
+    assert!(!c.saw_prev_marked);
+    vm[0].for_each_child(&mut c);
+    assert!(c.saw_prev_marked);
+
+    if PRINT { println!(""); }
+
+    // Cycle 8: { ll -> (ll0, ll1), {ll0, ll1} -> ll }
+    let mut ll: LinkedList<LL> = LinkedList::new();
+    ll.push_back(Named::new("ll0"));
+    ll.push_back(Named::new("ll1"));
+    for e in &ll {
+        e.contents.set(Some(&ll));
+    }
+
+    let mut c = c_orig.clone();
+    c.curr_mark = 80;
+    for e in &ll {
+        c.curr_mark += 1;
+        c.saw_prev_marked = false;
+        e.for_each_child(&mut c);
+        assert!(c.saw_prev_marked);
+        // break;
+    }
+
+    if PRINT { println!(""); }
+
+    // Cycle 9: { bh -> (bh0, bh1), {bh0, bh1} -> bh }
+    let mut bh: BinaryHeap<BH> = BinaryHeap::new();
+    bh.push(Named::new("bh0"));
+    bh.push(Named::new("bh1"));
+    for b in bh.iter() {
+        b.contents.set(Some(&bh));
+    }
+
+    let mut c = c_orig.clone();
+    c.curr_mark = 90;
+    for b in &bh {
+        c.curr_mark += 1;
+        c.saw_prev_marked = false;
+        b.for_each_child(&mut c);
+        assert!(c.saw_prev_marked);
+        // break;
+    }
+
+    if PRINT { println!(""); }
+
+    // Cycle 10: { btm -> (btk0, btv1), {bt0, bt1} -> btm }
+    let mut btm: BTreeMap<BTM, BTM> = BTreeMap::new();
+    btm.insert(Named::new("btk0"), Named::new("btv0"));
+    btm.insert(Named::new("btk1"), Named::new("btv1"));
+    for (k, v) in btm.iter() {
+        k.contents.set(Some(&btm));
+        v.contents.set(Some(&btm));
+    }
+
+    let mut c = c_orig.clone();
+    c.curr_mark = 100;
+    for (k, _) in &btm {
+        c.curr_mark += 1;
+        c.saw_prev_marked = false;
+        k.for_each_child(&mut c);
+        assert!(c.saw_prev_marked);
+        // break;
+    }
+
+    if PRINT { println!(""); }
+
+    // Cycle 10: { bts -> (bts0, bts1), {bts0, bts1} -> btm }
+    let mut bts: BTreeSet<BTS> = BTreeSet::new();
+    bts.insert(Named::new("bts0"));
+    bts.insert(Named::new("bts1"));
+    for v in bts.iter() {
+        v.contents.set(Some(&bts));
+    }
+
+    let mut c = c_orig.clone();
+    c.curr_mark = 100;
+    for b in &bts {
+        c.curr_mark += 1;
+        c.saw_prev_marked = false;
+        b.for_each_child(&mut c);
+        assert!(c.saw_prev_marked);
+        // break;
+    }
+}
+
+trait Named {
+    fn new(&'static str) -> Self;
+    fn name(&self) -> &str;
+}
+
+trait Marked<M> {
+    fn mark(&self) -> M;
+    fn set_mark(&self, mark: M);
+}
+
+struct S<'a> {
+    name: &'static str,
+    mark: Cell<u32>,
+    next: Cell<Option<&'a S<'a>>>,
+}
+
+impl<'a> Named for S<'a> {
+    fn new<'b>(name: &'static str) -> S<'b> {
+        S { name: name, mark: Cell::new(0), next: Cell::new(None) }
+    }
+    fn name(&self) -> &str { self.name }
+}
+
+impl<'a> Marked<u32> for S<'a> {
+    fn mark(&self) -> u32 { self.mark.get() }
+    fn set_mark(&self, mark: u32) { self.mark.set(mark); }
+}
+
+struct V<'a> {
+    name: &'static str,
+    mark: Cell<u32>,
+    contents: Vec<Cell<Option<&'a V<'a>>>>,
+}
+
+impl<'a> Named for V<'a> {
+    fn new<'b>(name: &'static str) -> V<'b> {
+        V { name: name,
+            mark: Cell::new(0),
+            contents: vec![Cell::new(None), Cell::new(None)]
+        }
+    }
+    fn name(&self) -> &str { self.name }
+}
+
+impl<'a> Marked<u32> for V<'a> {
+    fn mark(&self) -> u32 { self.mark.get() }
+    fn set_mark(&self, mark: u32) { self.mark.set(mark); }
+}
+
+#[derive(Eq)]
+struct H<'a> {
+    name: &'static str,
+    mark: Cell<u32>,
+    next: Cell<Option<&'a H<'a>>>,
+}
+
+impl<'a> Named for H<'a> {
+    fn new<'b>(name: &'static str) -> H<'b> {
+        H { name: name, mark: Cell::new(0), next: Cell::new(None) }
+    }
+    fn name(&self) -> &str { self.name }
+}
+
+impl<'a> Marked<u32> for H<'a> {
+    fn mark(&self) -> u32 { self.mark.get() }
+    fn set_mark(&self, mark: u32) { self.mark.set(mark); }
+}
+
+impl<'a> PartialEq for H<'a> {
+    fn eq(&self, rhs: &H<'a>) -> bool {
+        self.name == rhs.name
+    }
+}
+
+impl<'a> Hash for H<'a> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.name.hash(state)
+    }
+}
+
+#[derive(Eq)]
+struct HM<'a> {
+    name: &'static str,
+    mark: Cell<u32>,
+    contents: Cell<Option<&'a HashMap<HM<'a>, HM<'a>>>>,
+}
+
+impl<'a> Named for HM<'a> {
+    fn new<'b>(name: &'static str) -> HM<'b> {
+        HM { name: name,
+             mark: Cell::new(0),
+             contents: Cell::new(None)
+        }
+    }
+    fn name(&self) -> &str { self.name }
+}
+
+impl<'a> Marked<u32> for HM<'a> {
+    fn mark(&self) -> u32 { self.mark.get() }
+    fn set_mark(&self, mark: u32) { self.mark.set(mark); }
+}
+
+impl<'a> PartialEq for HM<'a> {
+    fn eq(&self, rhs: &HM<'a>) -> bool {
+        self.name == rhs.name
+    }
+}
+
+impl<'a> Hash for HM<'a> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.name.hash(state)
+    }
+}
+
+
+struct VD<'a> {
+    name: &'static str,
+    mark: Cell<u32>,
+    contents: Cell<Option<&'a VecDeque<VD<'a>>>>,
+}
+
+impl<'a> Named for VD<'a> {
+    fn new<'b>(name: &'static str) -> VD<'b> {
+        VD { name: name,
+             mark: Cell::new(0),
+             contents: Cell::new(None)
+        }
+    }
+    fn name(&self) -> &str { self.name }
+}
+
+impl<'a> Marked<u32> for VD<'a> {
+    fn mark(&self) -> u32 { self.mark.get() }
+    fn set_mark(&self, mark: u32) { self.mark.set(mark); }
+}
+
+struct VM<'a> {
+    name: &'static str,
+    mark: Cell<u32>,
+    contents: Cell<Option<&'a VecMap<VM<'a>>>>,
+}
+
+impl<'a> Named for VM<'a> {
+    fn new<'b>(name: &'static str) -> VM<'b> {
+        VM { name: name,
+             mark: Cell::new(0),
+             contents: Cell::new(None)
+        }
+    }
+    fn name(&self) -> &str { self.name }
+}
+
+impl<'a> Marked<u32> for VM<'a> {
+    fn mark(&self) -> u32 { self.mark.get() }
+    fn set_mark(&self, mark: u32) { self.mark.set(mark); }
+}
+
+struct LL<'a> {
+    name: &'static str,
+    mark: Cell<u32>,
+    contents: Cell<Option<&'a LinkedList<LL<'a>>>>,
+}
+
+impl<'a> Named for LL<'a> {
+    fn new<'b>(name: &'static str) -> LL<'b> {
+        LL { name: name,
+             mark: Cell::new(0),
+             contents: Cell::new(None)
+        }
+    }
+    fn name(&self) -> &str { self.name }
+}
+
+impl<'a> Marked<u32> for LL<'a> {
+    fn mark(&self) -> u32 { self.mark.get() }
+    fn set_mark(&self, mark: u32) { self.mark.set(mark); }
+}
+
+struct BH<'a> {
+    name: &'static str,
+    mark: Cell<u32>,
+    contents: Cell<Option<&'a BinaryHeap<BH<'a>>>>,
+}
+
+impl<'a> Named for BH<'a> {
+    fn new<'b>(name: &'static str) -> BH<'b> {
+        BH { name: name,
+             mark: Cell::new(0),
+             contents: Cell::new(None)
+        }
+    }
+    fn name(&self) -> &str { self.name }
+}
+
+impl<'a> Marked<u32> for BH<'a> {
+    fn mark(&self) -> u32 { self.mark.get() }
+    fn set_mark(&self, mark: u32) { self.mark.set(mark); }
+}
+
+impl<'a> Eq for BH<'a> { }
+
+impl<'a> PartialEq for BH<'a> {
+    fn eq(&self, rhs: &BH<'a>) -> bool {
+        self.name == rhs.name
+    }
+}
+
+impl<'a> PartialOrd for BH<'a> {
+    fn partial_cmp(&self, rhs: &BH<'a>) -> Option<Ordering> {
+        Some(self.cmp(rhs))
+    }
+}
+
+impl<'a> Ord for BH<'a> {
+    fn cmp(&self, rhs: &BH<'a>) -> Ordering {
+        self.name.cmp(rhs.name)
+    }
+}
+
+struct BTM<'a> {
+    name: &'static str,
+    mark: Cell<u32>,
+    contents: Cell<Option<&'a BTreeMap<BTM<'a>, BTM<'a>>>>,
+}
+
+impl<'a> Named for BTM<'a> {
+    fn new<'b>(name: &'static str) -> BTM<'b> {
+        BTM { name: name,
+             mark: Cell::new(0),
+             contents: Cell::new(None)
+        }
+    }
+    fn name(&self) -> &str { self.name }
+}
+
+impl<'a> Marked<u32> for BTM<'a> {
+    fn mark(&self) -> u32 { self.mark.get() }
+    fn set_mark(&self, mark: u32) { self.mark.set(mark); }
+}
+
+impl<'a> Eq for BTM<'a> { }
+
+impl<'a> PartialEq for BTM<'a> {
+    fn eq(&self, rhs: &BTM<'a>) -> bool {
+        self.name == rhs.name
+    }
+}
+
+impl<'a> PartialOrd for BTM<'a> {
+    fn partial_cmp(&self, rhs: &BTM<'a>) -> Option<Ordering> {
+        Some(self.cmp(rhs))
+    }
+}
+
+impl<'a> Ord for BTM<'a> {
+    fn cmp(&self, rhs: &BTM<'a>) -> Ordering {
+        self.name.cmp(rhs.name)
+    }
+}
+
+struct BTS<'a> {
+    name: &'static str,
+    mark: Cell<u32>,
+    contents: Cell<Option<&'a BTreeSet<BTS<'a>>>>,
+}
+
+impl<'a> Named for BTS<'a> {
+    fn new<'b>(name: &'static str) -> BTS<'b> {
+        BTS { name: name,
+             mark: Cell::new(0),
+             contents: Cell::new(None)
+        }
+    }
+    fn name(&self) -> &str { self.name }
+}
+
+impl<'a> Marked<u32> for BTS<'a> {
+    fn mark(&self) -> u32 { self.mark.get() }
+    fn set_mark(&self, mark: u32) { self.mark.set(mark); }
+}
+
+impl<'a> Eq for BTS<'a> { }
+
+impl<'a> PartialEq for BTS<'a> {
+    fn eq(&self, rhs: &BTS<'a>) -> bool {
+        self.name == rhs.name
+    }
+}
+
+impl<'a> PartialOrd for BTS<'a> {
+    fn partial_cmp(&self, rhs: &BTS<'a>) -> Option<Ordering> {
+        Some(self.cmp(rhs))
+    }
+}
+
+impl<'a> Ord for BTS<'a> {
+    fn cmp(&self, rhs: &BTS<'a>) -> Ordering {
+        self.name.cmp(rhs.name)
+    }
+}
+
+
+trait Context {
+    fn should_act(&self) -> bool;
+    fn increase_visited(&mut self);
+    fn increase_skipped(&mut self);
+    fn increase_depth(&mut self);
+    fn decrease_depth(&mut self);
+}
+
+trait PrePost<T> {
+    fn pre(&mut self, &T);
+    fn post(&mut self, &T);
+    fn hit_limit(&mut self, &T);
+}
+
+trait Children<'a> {
+    fn for_each_child<C>(&self, context: &mut C)
+        where C: Context + PrePost<Self>, Self: Sized;
+
+    fn descend_into_self<C>(&self, context: &mut C)
+        where C: Context + PrePost<Self>, Self: Sized
+    {
+        context.pre(self);
+        if context.should_act() {
+            context.increase_visited();
+            context.increase_depth();
+            self.for_each_child(context);
+            context.decrease_depth();
+        } else {
+            context.hit_limit(self);
+            context.increase_skipped();
+        }
+        context.post(self);
+    }
+
+    fn descend<'b, C>(&self, c: &Cell<Option<&'b Self>>, context: &mut C)
+        where C: Context + PrePost<Self>, Self: Sized
+    {
+        if let Some(r) = c.get() {
+            r.descend_into_self(context);
+        }
+    }
+}
+
+impl<'a> Children<'a> for S<'a> {
+    fn for_each_child<C>(&self, context: &mut C)
+        where C: Context + PrePost<S<'a>>
+    {
+        self.descend(&self.next, context);
+    }
+}
+
+impl<'a> Children<'a> for V<'a> {
+    fn for_each_child<C>(&self, context: &mut C)
+        where C: Context + PrePost<V<'a>>
+    {
+        for r in &self.contents {
+            self.descend(r, context);
+        }
+    }
+}
+
+impl<'a> Children<'a> for H<'a> {
+    fn for_each_child<C>(&self, context: &mut C)
+        where C: Context + PrePost<H<'a>>
+    {
+        self.descend(&self.next, context);
+    }
+}
+
+impl<'a> Children<'a> for HM<'a> {
+    fn for_each_child<C>(&self, context: &mut C)
+        where C: Context + PrePost<HM<'a>>
+    {
+        if let Some(ref hm) = self.contents.get() {
+            for (k, v) in hm.iter() {
+                for r in &[k, v] {
+                    r.descend_into_self(context);
+                }
+            }
+        }
+    }
+}
+
+impl<'a> Children<'a> for VD<'a> {
+    fn for_each_child<C>(&self, context: &mut C)
+        where C: Context + PrePost<VD<'a>>
+    {
+        if let Some(ref vd) = self.contents.get() {
+            for r in vd.iter() {
+                r.descend_into_self(context);
+            }
+        }
+    }
+}
+
+impl<'a> Children<'a> for VM<'a> {
+    fn for_each_child<C>(&self, context: &mut C)
+        where C: Context + PrePost<VM<'a>>
+    {
+        if let Some(ref vd) = self.contents.get() {
+            for (_idx, r) in vd.iter() {
+                r.descend_into_self(context);
+            }
+        }
+    }
+}
+
+impl<'a> Children<'a> for LL<'a> {
+    fn for_each_child<C>(&self, context: &mut C)
+        where C: Context + PrePost<LL<'a>>
+    {
+        if let Some(ref ll) = self.contents.get() {
+            for r in ll.iter() {
+                r.descend_into_self(context);
+            }
+        }
+    }
+}
+
+impl<'a> Children<'a> for BH<'a> {
+    fn for_each_child<C>(&self, context: &mut C)
+        where C: Context + PrePost<BH<'a>>
+    {
+        if let Some(ref bh) = self.contents.get() {
+            for r in bh.iter() {
+                r.descend_into_self(context);
+            }
+        }
+    }
+}
+
+impl<'a> Children<'a> for BTM<'a> {
+    fn for_each_child<C>(&self, context: &mut C)
+        where C: Context + PrePost<BTM<'a>>
+    {
+        if let Some(ref bh) = self.contents.get() {
+            for (k, v) in bh.iter() {
+                for r in &[k, v] {
+                    r.descend_into_self(context);
+                }
+            }
+        }
+    }
+}
+
+impl<'a> Children<'a> for BTS<'a> {
+    fn for_each_child<C>(&self, context: &mut C)
+        where C: Context + PrePost<BTS<'a>>
+    {
+        if let Some(ref bh) = self.contents.get() {
+            for r in bh.iter() {
+                r.descend_into_self(context);
+            }
+        }
+    }
+}
+
+#[derive(Copy, Clone)]
+struct ContextData {
+    curr_depth: usize,
+    max_depth: usize,
+    visited: usize,
+    max_visits: usize,
+    skipped: usize,
+    curr_mark: u32,
+    saw_prev_marked: bool,
+}
+
+impl Context for ContextData {
+    fn should_act(&self) -> bool {
+        self.curr_depth < self.max_depth && self.visited < self.max_visits
+    }
+    fn increase_visited(&mut self) { self.visited += 1; }
+    fn increase_skipped(&mut self) { self.skipped += 1; }
+    fn increase_depth(&mut self) {  self.curr_depth += 1; }
+    fn decrease_depth(&mut self) {  self.curr_depth -= 1; }
+}
+
+impl<T:Named+Marked<u32>> PrePost<T> for ContextData {
+    fn pre(&mut self, t: &T) {
+        for _ in 0..self.curr_depth {
+            if PRINT { print!(" "); }
+        }
+        if PRINT { println!("prev {}", t.name()); }
+        if t.mark() == self.curr_mark {
+            for _ in 0..self.curr_depth {
+                if PRINT { print!(" "); }
+            }
+            if PRINT { println!("(probably previously marked)"); }
+            self.saw_prev_marked = true;
+        }
+        t.set_mark(self.curr_mark);
+    }
+    fn post(&mut self, t: &T) {
+        for _ in 0..self.curr_depth {
+            if PRINT { print!(" "); }
+        }
+        if PRINT { println!("post {}", t.name()); }
+    }
+    fn hit_limit(&mut self, t: &T) {
+        for _ in 0..self.curr_depth {
+            if PRINT { print!(" "); }
+        }
+        if PRINT { println!("LIMIT {}", t.name()); }
+    }
+}