From ccd92c2a4e5ed634bbbd6d3a5bd491c47b80f642 Mon Sep 17 00:00:00 2001 From: Niko Matsakis Date: Sun, 16 Jul 2017 08:49:21 -0400 Subject: [PATCH] correct subtle bug in the type variable code --- src/librustc/infer/type_variable.rs | 171 ++++++++++------------------ 1 file changed, 63 insertions(+), 108 deletions(-) diff --git a/src/librustc/infer/type_variable.rs b/src/librustc/infer/type_variable.rs index 261cd396fced7..9e98c16c819b9 100644 --- a/src/librustc/infer/type_variable.rs +++ b/src/librustc/infer/type_variable.rs @@ -12,15 +12,18 @@ use syntax::ast; use syntax_pos::Span; use ty::{self, Ty}; -use std::cmp::min; use std::marker::PhantomData; use std::u32; use rustc_data_structures::fx::FxHashMap; -use rustc_data_structures::snapshot_vec as sv; use rustc_data_structures::unify as ut; pub struct TypeVariableTable<'tcx> { - values: sv::SnapshotVec, + /// Extra data for each type variable, such as the origin. This is + /// not stored in the unification table since, when we inquire + /// after the origin of a variable X, we want the origin of **that + /// variable X**, not the origin of some other variable Y with + /// which X has been unified. + var_data: Vec, /// Two variables are unified in `eq_relations` when we have a /// constraint `?X == ?Y`. This table also stores, for each key, @@ -82,21 +85,20 @@ enum TypeVariableValue<'tcx> { } pub struct Snapshot<'tcx> { - snapshot: sv::Snapshot, + /// number of variables at the time of the snapshot + num_vars: usize, + + /// snapshot from the `eq_relations` table eq_snapshot: ut::Snapshot>>, - sub_snapshot: ut::Snapshot>, -} -struct Instantiate { - vid: ty::TyVid, + /// snapshot from the `sub_relations` table + sub_snapshot: ut::Snapshot>, } -struct Delegate; - impl<'tcx> TypeVariableTable<'tcx> { pub fn new() -> TypeVariableTable<'tcx> { TypeVariableTable { - values: sv::SnapshotVec::new(), + var_data: Vec::new(), eq_relations: ut::UnificationTable::new(), sub_relations: ut::UnificationTable::new(), } @@ -107,7 +109,7 @@ impl<'tcx> TypeVariableTable<'tcx> { /// Note that this function does not return care whether /// `vid` has been unified with something else or not. pub fn var_diverges<'a>(&'a self, vid: ty::TyVid) -> bool { - self.values.get(vid.index as usize).diverging + self.var_data[vid.index as usize].diverging } /// Returns the origin that was given when `vid` was created. @@ -115,7 +117,7 @@ impl<'tcx> TypeVariableTable<'tcx> { /// Note that this function does not return care whether /// `vid` has been unified with something else or not. pub fn var_origin(&self, vid: ty::TyVid) -> &TypeVariableOrigin { - &self.values.get(vid.index as usize).origin + &self.var_data[vid.index as usize].origin } /// Records that `a == b`, depending on `dir`. @@ -147,11 +149,6 @@ impl<'tcx> TypeVariableTable<'tcx> { "instantiating type variable `{:?}` twice: new-value = {:?}, old-value={:?}", vid, ty, self.eq_relations.probe_value(vid)); self.eq_relations.union_value(vid, TypeVariableValue::Known { value: ty }); - - // Hack: we only need this so that `types_escaping_snapshot` - // can see what has been unified; see the Delegate impl for - // more details. - self.values.record(Instantiate { vid: vid }); } /// Creates a new type variable. @@ -173,11 +170,8 @@ impl<'tcx> TypeVariableTable<'tcx> { let sub_key = self.sub_relations.new_key(()); assert_eq!(eq_key.vid, sub_key); - let index = self.values.push(TypeVariableData { - origin, - diverging, - }); - assert_eq!(eq_key.vid.index, index as u32); + assert_eq!(self.var_data.len(), sub_key.index as usize); + self.var_data.push(TypeVariableData { origin, diverging }); debug!("new_var(index={:?}, diverging={:?}, origin={:?}", eq_key.vid, diverging, origin); @@ -186,7 +180,7 @@ impl<'tcx> TypeVariableTable<'tcx> { /// Returns the number of type variables created thus far. pub fn num_vars(&self) -> usize { - self.values.len() + self.var_data.len() } /// Returns the "root" variable of `vid` in the `eq_relations` @@ -246,7 +240,7 @@ impl<'tcx> TypeVariableTable<'tcx> { /// be processed in a stack-like fashion. pub fn snapshot(&mut self) -> Snapshot<'tcx> { Snapshot { - snapshot: self.values.start_snapshot(), + num_vars: self.var_data.len(), eq_snapshot: self.eq_relations.snapshot(), sub_snapshot: self.sub_relations.snapshot(), } @@ -256,21 +250,12 @@ impl<'tcx> TypeVariableTable<'tcx> { /// snapshots created since that point must already have been /// committed or rolled back. pub fn rollback_to(&mut self, s: Snapshot<'tcx>) { - debug!("rollback_to{:?}", { - for action in self.values.actions_since_snapshot(&s.snapshot) { - match *action { - sv::UndoLog::NewElem(index) => { - debug!("inference variable _#{}t popped", index) - } - _ => { } - } - } - }); - - let Snapshot { snapshot, eq_snapshot, sub_snapshot } = s; - self.values.rollback_to(snapshot); + let Snapshot { num_vars, eq_snapshot, sub_snapshot } = s; + debug!("type_variables::rollback_to(num_vars = {})", num_vars); + assert!(self.var_data.len() >= num_vars); self.eq_relations.rollback_to(eq_snapshot); self.sub_relations.rollback_to(sub_snapshot); + self.var_data.truncate(num_vars); } /// Commits all changes since the snapshot was created, making @@ -278,8 +263,8 @@ impl<'tcx> TypeVariableTable<'tcx> { /// another snapshot). Any snapshots created since that point /// must already have been committed or rolled back. pub fn commit(&mut self, s: Snapshot<'tcx>) { - let Snapshot { snapshot, eq_snapshot, sub_snapshot } = s; - self.values.commit(snapshot); + let Snapshot { num_vars, eq_snapshot, sub_snapshot } = s; + debug!("type_variables::commit(num_vars = {})", num_vars); self.eq_relations.commit(eq_snapshot); self.sub_relations.commit(sub_snapshot); } @@ -288,19 +273,12 @@ impl<'tcx> TypeVariableTable<'tcx> { /// ty-variables created during the snapshot, and the values /// `{V2}` are the root variables that they were unified with, /// along with their origin. - pub fn types_created_since_snapshot(&mut self, s: &Snapshot<'tcx>) -> TypeVariableMap { - let actions_since_snapshot = self.values.actions_since_snapshot(&s.snapshot); - - actions_since_snapshot + pub fn types_created_since_snapshot(&mut self, snapshot: &Snapshot<'tcx>) -> TypeVariableMap { + self.var_data .iter() - .filter_map(|action| match action { - &sv::UndoLog::NewElem(index) => Some(ty::TyVid { index: index as u32 }), - _ => None, - }) - .map(|vid| { - let origin = self.values.get(vid.index as usize).origin.clone(); - (vid, origin) - }) + .enumerate() + .skip(snapshot.num_vars) // skip those that existed when snapshot was taken + .map(|(index, data)| (ty::TyVid { index: index as u32 }, data.origin)) .collect() } @@ -310,47 +288,45 @@ impl<'tcx> TypeVariableTable<'tcx> { /// a type variable `V0`, then we started the snapshot, then we /// created a type variable `V1`, unifed `V0` with `T0`, and /// unified `V1` with `T1`, this function would return `{T0}`. - pub fn types_escaping_snapshot(&mut self, s: &Snapshot<'tcx>) -> Vec> { - let mut new_elem_threshold = u32::MAX; - let mut escaping_types = Vec::new(); - let actions_since_snapshot = self.values.actions_since_snapshot(&s.snapshot); - debug!("actions_since_snapshot.len() = {}", actions_since_snapshot.len()); - for action in actions_since_snapshot { - match *action { - sv::UndoLog::NewElem(index) => { - // if any new variables were created during the - // snapshot, remember the lower index (which will - // always be the first one we see). Note that this - // action must precede those variables being - // specified. - new_elem_threshold = min(new_elem_threshold, index as u32); - debug!("NewElem({}) new_elem_threshold={}", index, new_elem_threshold); - } - - sv::UndoLog::Other(Instantiate { vid, .. }) => { - if vid.index < new_elem_threshold { - // quick check to see if this variable was - // created since the snapshot started or not. - let escaping_type = match self.eq_relations.probe_value(vid) { - TypeVariableValue::Unknown => bug!(), - TypeVariableValue::Known { value } => value, - }; - escaping_types.push(escaping_type); - } - debug!("SpecifyVar({:?}) new_elem_threshold={}", vid, new_elem_threshold); - } - - _ => { } - } - } - + pub fn types_escaping_snapshot(&mut self, snapshot: &Snapshot<'tcx>) -> Vec> { + // We want to select only those instantiations that have + // occurred since the snapshot *and* which affect some + // variable that existed prior to the snapshot. This code just + // affects all instantiatons that ever occurred which affect + // variables prior to the snapshot. + // + // It's hard to do better than this, though, without changing + // the unification table to prefer "lower" vids -- the problem + // is that we may have a variable X (from before the snapshot) + // and Y (from after the snapshot) which get unified, with Y + // chosen as the new root. Now we are "instantiating" Y with a + // value, but it escapes into X, but we wouldn't readily see + // that. (In fact, earlier revisions of this code had this + // bug; it was introduced when we added the `eq_relations` + // table, but it's hard to create rust code that triggers it.) + // + // We could tell the table to prefer lower vids, and then we would + // see the case above, but we would get less-well-balanced trees. + // + // Since I hope to kill the leak-check in this branch, and + // that's the code which uses this logic anyway, I'm going to + // use the less efficient algorithm for now. + let mut escaping_types = Vec::with_capacity(snapshot.num_vars); + escaping_types.extend( + (0..snapshot.num_vars) // for all variables that pre-exist the snapshot... + .map(|i| ty::TyVid { index: i as u32 }) + .filter_map(|vid| match self.eq_relations.probe_value(vid) { + TypeVariableValue::Unknown => None, + TypeVariableValue::Known { value } => Some(value), + })); // ...collect what types they've been instantiated with. + debug!("types_escaping_snapshot = {:?}", escaping_types); escaping_types } /// Returns indices of all variables that are not yet /// instantiated. pub fn unsolved_variables(&mut self) -> Vec { - (0..self.values.len()) + (0..self.var_data.len()) .filter_map(|i| { let vid = ty::TyVid { index: i as u32 }; if self.probe(vid).is_some() { @@ -362,27 +338,6 @@ impl<'tcx> TypeVariableTable<'tcx> { .collect() } } - -impl sv::SnapshotVecDelegate for Delegate { - type Value = TypeVariableData; - type Undo = Instantiate; - - fn reverse(_values: &mut Vec, _action: Instantiate) { - // We don't actually have to *do* anything to reverse an - // instanation; the value for a variable is stored in the - // `eq_relations` and hence its rollback code will handle - // it. In fact, we could *almost* just remove the - // `SnapshotVec` entirely, except that we would have to - // reproduce *some* of its logic, since we want to know which - // type variables have been instantiated since the snapshot - // was started, so we can implement `types_escaping_snapshot`. - // - // (If we extended the `UnificationTable` to let us see which - // values have been unified and so forth, that might also - // suffice.) - } -} - /////////////////////////////////////////////////////////////////////////// /// These structs (a newtyped TyVid) are used as the unification key