compiler/rustc_trait_selection/src/solve/search_graph/cache.rs

   1 //! This module both handles the global cache which stores "finished" goals,
   2 //! and the provisional cache which contains partially computed goals.
   3 //!
   4 //! The provisional cache is necessary when dealing with coinductive cycles.
   5 //!
   6 //! For more information about the provisional cache and coinduction in general,
   7 //! check out the relevant section of the rustc-dev-guide.
   8 //!
   9 //! FIXME(@lcnr): Write that section, feel free to ping me if you need help here
  10 //! before then or if I still haven't done that before January 2023.
  11 use super::overflow::OverflowData;
  12 use super::StackDepth;
  13 use crate::solve::{CanonicalGoal, QueryResult};
  14 use rustc_data_structures::fx::FxHashMap;
  15 use rustc_index::vec::IndexVec;
  16 use rustc_middle::ty::TyCtxt;
  17
  18 rustc_index::newtype_index! {
  19     pub struct EntryIndex {}
  20 }
  21
  22 #[derive(Debug, Clone)]
  23 pub(super) struct ProvisionalEntry<'tcx> {
  24     // In case we have a coinductive cycle, this is the
  25     // the currently least restrictive result of this goal.
  26     pub(super) response: QueryResult<'tcx>,
  27     // In case of a cycle, the position of deepest stack entry involved
  28     // in that cycle. This is monotonically decreasing in the stack as all
  29     // elements between the current stack element in the deepest stack entry
  30     // involved have to also be involved in that cycle.
  31     //
  32     // We can only move entries to the global cache once we're complete done
  33     // with the cycle. If this entry has not been involved in a cycle,
  34     // this is just its own depth.
  35     pub(super) depth: StackDepth,
  36
  37     // The goal for this entry. Should always be equal to the corresponding goal
  38     // in the lookup table.
  39     pub(super) goal: CanonicalGoal<'tcx>,
  40 }
  41
  42 pub(super) struct ProvisionalCache<'tcx> {
  43     pub(super) entries: IndexVec<EntryIndex, ProvisionalEntry<'tcx>>,
  44     // FIXME: This is only used to quickly check whether a given goal
  45     // is in the cache. We should experiment with using something like
  46     // `SsoHashSet` here because in most cases there are only a few entries.
  47     pub(super) lookup_table: FxHashMap<CanonicalGoal<'tcx>, EntryIndex>,
  48 }
  49
  50 impl<'tcx> ProvisionalCache<'tcx> {
  51     pub(super) fn empty() -> ProvisionalCache<'tcx> {
  52         ProvisionalCache { entries: Default::default(), lookup_table: Default::default() }
  53     }
  54
  55     pub(super) fn is_empty(&self) -> bool {
  56         self.entries.is_empty() && self.lookup_table.is_empty()
  57     }
  58
  59     /// Adds a dependency from the current leaf to `target` in the cache
  60     /// to prevent us from moving any goals which depend on the current leaf
  61     /// to the global cache while we're still computing `target`.
  62     ///
  63     /// Its important to note that `target` may already be part of a different cycle.
  64     /// In this case we have to ensure that we also depend on all other goals
  65     /// in the existing cycle in addition to the potentially direct cycle with `target`.
  66     pub(super) fn add_dependency_of_leaf_on(&mut self, target: EntryIndex) {
  67         let depth = self.entries[target].depth;
  68         for provisional_entry in &mut self.entries.raw[target.index()..] {
  69             // The depth of `target` is the position of the deepest goal in the stack
  70             // on which `target` depends. That goal is the `root` of this cycle.
  71             //
  72             // Any entry which was added after `target` is either on the stack itself
  73             // at which point its depth is definitely at least as high as the depth of
  74             // `root`. If it's not on the stack itself it has to depend on a goal
  75             // between `root` and `leaf`. If it were to depend on a goal deeper in the
  76             // stack than `root`, then `root` would also depend on that goal, at which
  77             // point `root` wouldn't be the root anymore.
  78             debug_assert!(provisional_entry.depth >= depth);
  79             provisional_entry.depth = depth;
  80         }
  81
  82         // We only update entries which were added after `target` as no other
  83         // entry should have a higher depth.
  84         //
  85         // Any entry which previously had a higher depth than target has to
  86         // be between `target` and `root`. Because of this we would have updated
  87         // its depth when calling `add_dependency_of_leaf_on(root)` for `target`.
  88         if cfg!(debug_assertions) {
  89             self.entries.iter().all(|e| e.depth <= depth);
  90         }
  91     }
  92
  93     pub(super) fn depth(&self, entry_index: EntryIndex) -> StackDepth {
  94         self.entries[entry_index].depth
  95     }
  96
  97     pub(super) fn provisional_result(&self, entry_index: EntryIndex) -> QueryResult<'tcx> {
  98         self.entries[entry_index].response
  99     }
 100 }
 101
 102 pub(super) fn try_move_finished_goal_to_global_cache<'tcx>(
 103     tcx: TyCtxt<'tcx>,
 104     overflow_data: &mut OverflowData,
 105     stack: &IndexVec<super::StackDepth, super::StackElem<'tcx>>,
 106     goal: CanonicalGoal<'tcx>,
 107     response: QueryResult<'tcx>,
 108 ) {
 109     // We move goals to the global cache if we either did not hit an overflow or if it's
 110     // the root goal as that will now always hit the same overflow limit.
 111     //
 112     // NOTE: We cannot move any non-root goals to the global cache even if their final result
 113     // isn't impacted by the overflow as that goal still has unstable query dependencies
 114     // because it didn't go its full depth.
 115     //
 116     // FIXME(@lcnr): We could still cache subtrees which are not impacted by overflow though.
 117     // Tracking that info correctly isn't trivial, so I haven't implemented it for now.
 118     let should_cache_globally = !overflow_data.did_overflow() || stack.is_empty();
 119     if should_cache_globally {
 120         // FIXME: move the provisional entry to the global cache.
 121         let _ = (tcx, goal, response);
 122     }
 123 }