compiler/rustc_infer/src/infer/outlives/env.rs

   1 use crate::infer::free_regions::FreeRegionMap;
   2 use crate::infer::GenericKind;
   3 use crate::traits::query::OutlivesBound;
   4 use rustc_data_structures::fx::FxIndexSet;
   5 use rustc_data_structures::transitive_relation::TransitiveRelationBuilder;
   6 use rustc_middle::ty::{self, Region};
   7
   8 use super::explicit_outlives_bounds;
   9
  10 /// The `OutlivesEnvironment` collects information about what outlives
  11 /// what in a given type-checking setting. For example, if we have a
  12 /// where-clause like `where T: 'a` in scope, then the
  13 /// `OutlivesEnvironment` would record that (in its
  14 /// `region_bound_pairs` field). Similarly, it contains methods for
  15 /// processing and adding implied bounds into the outlives
  16 /// environment.
  17 ///
  18 /// Other code at present does not typically take a
  19 /// `&OutlivesEnvironment`, but rather takes some of its fields (e.g.,
  20 /// `process_registered_region_obligations` wants the
  21 /// region-bound-pairs). There is no mistaking it: the current setup
  22 /// of tracking region information is quite scattered! The
  23 /// `OutlivesEnvironment`, for example, needs to sometimes be combined
  24 /// with the `middle::RegionRelations`, to yield a full picture of how
  25 /// (lexical) lifetimes interact. However, I'm reluctant to do more
  26 /// refactoring here, since the setup with NLL is quite different.
  27 /// For example, NLL has no need of `RegionRelations`, and is solely
  28 /// interested in the `OutlivesEnvironment`. -nmatsakis
  29 #[derive(Clone)]
  30 pub struct OutlivesEnvironment<'tcx> {
  31     pub param_env: ty::ParamEnv<'tcx>,
  32     free_region_map: FreeRegionMap<'tcx>,
  33
  34     // Contains the implied region bounds in scope for our current body.
  35     //
  36     // Example:
  37     //
  38     // ```
  39     // fn foo<'a, 'b, T>(x: &'a T, y: &'b ()) {
  40     //   bar(x, y, |y: &'b T| { .. } // body B1)
  41     // } // body B0
  42     // ```
  43     //
  44     // Here, when checking the body B0, the list would be `[T: 'a]`, because we
  45     // infer that `T` must outlive `'a` from the implied bounds on the
  46     // fn declaration.
  47     //
  48     // For the body B1 however, the list would be `[T: 'a, T: 'b]`, because we
  49     // also can see that -- within the closure body! -- `T` must
  50     // outlive `'b`. This is not necessarily true outside the closure
  51     // body, since the closure may never be called.
  52     region_bound_pairs: RegionBoundPairs<'tcx>,
  53 }
  54
  55 /// Builder of OutlivesEnvironment.
  56 #[derive(Debug)]
  57 struct OutlivesEnvironmentBuilder<'tcx> {
  58     param_env: ty::ParamEnv<'tcx>,
  59     region_relation: TransitiveRelationBuilder<Region<'tcx>>,
  60     region_bound_pairs: RegionBoundPairs<'tcx>,
  61 }
  62
  63 /// "Region-bound pairs" tracks outlives relations that are known to
  64 /// be true, either because of explicit where-clauses like `T: 'a` or
  65 /// because of implied bounds.
  66 pub type RegionBoundPairs<'tcx> =
  67     FxIndexSet<ty::OutlivesPredicate<GenericKind<'tcx>, Region<'tcx>>>;
  68
  69 impl<'tcx> OutlivesEnvironment<'tcx> {
  70     /// Create a builder using `ParamEnv` and add explicit outlives bounds into it.
  71     fn builder(param_env: ty::ParamEnv<'tcx>) -> OutlivesEnvironmentBuilder<'tcx> {
  72         let mut builder = OutlivesEnvironmentBuilder {
  73             param_env,
  74             region_relation: Default::default(),
  75             region_bound_pairs: Default::default(),
  76         };
  77
  78         builder.add_outlives_bounds(explicit_outlives_bounds(param_env));
  79
  80         builder
  81     }
  82
  83     #[inline]
  84     /// Create a new `OutlivesEnvironment` without extra outlives bounds.
  85     pub fn new(param_env: ty::ParamEnv<'tcx>) -> Self {
  86         Self::builder(param_env).build()
  87     }
  88
  89     /// Create a new `OutlivesEnvironment` with extra outlives bounds.
  90     pub fn with_bounds(
  91         param_env: ty::ParamEnv<'tcx>,
  92         extra_bounds: impl IntoIterator<Item = OutlivesBound<'tcx>>,
  93     ) -> Self {
  94         let mut builder = Self::builder(param_env);
  95         builder.add_outlives_bounds(extra_bounds);
  96         builder.build()
  97     }
  98
  99     /// Borrows current value of the `free_region_map`.
 100     pub fn free_region_map(&self) -> &FreeRegionMap<'tcx> {
 101         &self.free_region_map
 102     }
 103
 104     /// Borrows current `region_bound_pairs`.
 105     pub fn region_bound_pairs(&self) -> &RegionBoundPairs<'tcx> {
 106         &self.region_bound_pairs
 107     }
 108 }
 109
 110 impl<'tcx> OutlivesEnvironmentBuilder<'tcx> {
 111     #[inline]
 112     #[instrument(level = "debug")]
 113     fn build(self) -> OutlivesEnvironment<'tcx> {
 114         OutlivesEnvironment {
 115             param_env: self.param_env,
 116             free_region_map: FreeRegionMap { relation: self.region_relation.freeze() },
 117             region_bound_pairs: self.region_bound_pairs,
 118         }
 119     }
 120
 121     /// Processes outlives bounds that are known to hold, whether from implied or other sources.
 122     fn add_outlives_bounds<I>(&mut self, outlives_bounds: I)
 123     where
 124         I: IntoIterator<Item = OutlivesBound<'tcx>>,
 125     {
 126         // Record relationships such as `T:'x` that don't go into the
 127         // free-region-map but which we use here.
 128         for outlives_bound in outlives_bounds {
 129             debug!("add_outlives_bounds: outlives_bound={:?}", outlives_bound);
 130             match outlives_bound {
 131                 OutlivesBound::RegionSubParam(r_a, param_b) => {
 132                     self.region_bound_pairs
 133                         .insert(ty::OutlivesPredicate(GenericKind::Param(param_b), r_a));
 134                 }
 135                 OutlivesBound::RegionSubAlias(r_a, alias_b) => {
 136                     self.region_bound_pairs
 137                         .insert(ty::OutlivesPredicate(GenericKind::Alias(alias_b), r_a));
 138                 }
 139                 OutlivesBound::RegionSubRegion(r_a, r_b) => match (*r_a, *r_b) {
 140                     (
 141                         ty::ReStatic | ty::ReEarlyBound(_) | ty::ReFree(_),
 142                         ty::ReStatic | ty::ReEarlyBound(_) | ty::ReFree(_),
 143                     ) => self.region_relation.add(r_a, r_b),
 144                     (ty::ReError(_), _) | (_, ty::ReError(_)) => {}
 145                     // FIXME(#109628): We shouldn't have existential variables in implied bounds.
 146                     // Panic here once the linked issue is resolved!
 147                     (ty::ReVar(_), _) | (_, ty::ReVar(_)) => {}
 148                     _ => bug!("add_outlives_bounds: unexpected regions: ({r_a:?}, {r_b:?})"),
 149                 },
 150             }
 151         }
 152     }
 153 }