src/librustc_infer/infer/outlives/env.rs

   1 use crate::infer::free_regions::FreeRegionMap;
   2 use crate::infer::{GenericKind, InferCtxt};
   3 use crate::traits::query::OutlivesBound;
   4 use rustc_data_structures::fx::FxHashMap;
   5 use rustc_hir as hir;
   6 use rustc_middle::ty;
   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, for each body B that we are checking (that is, the fn
  35     // item, but also any nested closures), the set of implied region
  36     // bounds that are in scope in that particular body.
  37     //
  38     // Example:
  39     //
  40     // ```
  41     // fn foo<'a, 'b, T>(x: &'a T, y: &'b ()) {
  42     //   bar(x, y, |y: &'b T| { .. } // body B1)
  43     // } // body B0
  44     // ```
  45     //
  46     // Here, for body B0, the list would be `[T: 'a]`, because we
  47     // infer that `T` must outlive `'a` from the implied bounds on the
  48     // fn declaration.
  49     //
  50     // For the body B1, the list would be `[T: 'a, T: 'b]`, because we
  51     // also can see that -- within the closure body! -- `T` must
  52     // outlive `'b`. This is not necessarily true outside the closure
  53     // body, since the closure may never be called.
  54     //
  55     // We collect this map as we descend the tree. We then use the
  56     // results when proving outlives obligations like `T: 'x` later
  57     // (e.g., if `T: 'x` must be proven within the body B1, then we
  58     // know it is true if either `'a: 'x` or `'b: 'x`).
  59     region_bound_pairs_map: FxHashMap<hir::HirId, RegionBoundPairs<'tcx>>,
  60
  61     // Used to compute `region_bound_pairs_map`: contains the set of
  62     // in-scope region-bound pairs thus far.
  63     region_bound_pairs_accum: RegionBoundPairs<'tcx>,
  64 }
  65
  66 /// "Region-bound pairs" tracks outlives relations that are known to
  67 /// be true, either because of explicit where-clauses like `T: 'a` or
  68 /// because of implied bounds.
  69 pub type RegionBoundPairs<'tcx> = Vec<(ty::Region<'tcx>, GenericKind<'tcx>)>;
  70
  71 impl<'a, 'tcx> OutlivesEnvironment<'tcx> {
  72     pub fn new(param_env: ty::ParamEnv<'tcx>) -> Self {
  73         let mut env = OutlivesEnvironment {
  74             param_env,
  75             free_region_map: Default::default(),
  76             region_bound_pairs_map: Default::default(),
  77             region_bound_pairs_accum: vec![],
  78         };
  79
  80         env.add_outlives_bounds(None, explicit_outlives_bounds(param_env));
  81
  82         env
  83     }
  84
  85     /// Borrows current value of the `free_region_map`.
  86     pub fn free_region_map(&self) -> &FreeRegionMap<'tcx> {
  87         &self.free_region_map
  88     }
  89
  90     /// Borrows current value of the `region_bound_pairs`.
  91     pub fn region_bound_pairs_map(&self) -> &FxHashMap<hir::HirId, RegionBoundPairs<'tcx>> {
  92         &self.region_bound_pairs_map
  93     }
  94
  95     /// Returns ownership of the `free_region_map`.
  96     pub fn into_free_region_map(self) -> FreeRegionMap<'tcx> {
  97         self.free_region_map
  98     }
  99
 100     /// This is a hack to support the old-skool regionck, which
 101     /// processes region constraints from the main function and the
 102     /// closure together. In that context, when we enter a closure, we
 103     /// want to be able to "save" the state of the surrounding a
 104     /// function. We can then add implied bounds and the like from the
 105     /// closure arguments into the environment -- these should only
 106     /// apply in the closure body, so once we exit, we invoke
 107     /// `pop_snapshot_post_closure` to remove them.
 108     ///
 109     /// Example:
 110     ///
 111     /// ```
 112     /// fn foo<T>() {
 113     ///    callback(for<'a> |x: &'a T| {
 114     ///         // ^^^^^^^ not legal syntax, but probably should be
 115     ///         // within this closure body, `T: 'a` holds
 116     ///    })
 117     /// }
 118     /// ```
 119     ///
 120     /// This "containment" of closure's effects only works so well. In
 121     /// particular, we (intentionally) leak relationships between free
 122     /// regions that are created by the closure's bounds. The case
 123     /// where this is useful is when you have (e.g.) a closure with a
 124     /// signature like `for<'a, 'b> fn(x: &'a &'b u32)` -- in this
 125     /// case, we want to keep the relationship `'b: 'a` in the
 126     /// free-region-map, so that later if we have to take `LUB('b,
 127     /// 'a)` we can get the result `'b`.
 128     ///
 129     /// I have opted to keep **all modifications** to the
 130     /// free-region-map, however, and not just those that concern free
 131     /// variables bound in the closure. The latter seems more correct,
 132     /// but it is not the existing behavior, and I could not find a
 133     /// case where the existing behavior went wrong. In any case, it
 134     /// seems like it'd be readily fixed if we wanted. There are
 135     /// similar leaks around givens that seem equally suspicious, to
 136     /// be honest. --nmatsakis
 137     pub fn push_snapshot_pre_closure(&self) -> usize {
 138         self.region_bound_pairs_accum.len()
 139     }
 140
 141     /// See `push_snapshot_pre_closure`.
 142     pub fn pop_snapshot_post_closure(&mut self, len: usize) {
 143         self.region_bound_pairs_accum.truncate(len);
 144     }
 145
 146     /// Save the current set of region-bound pairs under the given `body_id`.
 147     pub fn save_implied_bounds(&mut self, body_id: hir::HirId) {
 148         let old =
 149             self.region_bound_pairs_map.insert(body_id, self.region_bound_pairs_accum.clone());
 150         assert!(old.is_none());
 151     }
 152
 153     /// Processes outlives bounds that are known to hold, whether from implied or other sources.
 154     ///
 155     /// The `infcx` parameter is optional; if the implied bounds may
 156     /// contain inference variables, it must be supplied, in which
 157     /// case we will register "givens" on the inference context. (See
 158     /// `RegionConstraintData`.)
 159     pub fn add_outlives_bounds<I>(
 160         &mut self,
 161         infcx: Option<&InferCtxt<'a, 'tcx>>,
 162         outlives_bounds: I,
 163     ) where
 164         I: IntoIterator<Item = OutlivesBound<'tcx>>,
 165     {
 166         // Record relationships such as `T:'x` that don't go into the
 167         // free-region-map but which we use here.
 168         for outlives_bound in outlives_bounds {
 169             debug!("add_outlives_bounds: outlives_bound={:?}", outlives_bound);
 170             match outlives_bound {
 171                 OutlivesBound::RegionSubRegion(
 172                     r_a @ (&ty::ReEarlyBound(_) | &ty::ReFree(_)),
 173                     &ty::ReVar(vid_b),
 174                 ) => {
 175                     infcx.expect("no infcx provided but region vars found").add_given(r_a, vid_b);
 176                 }
 177                 OutlivesBound::RegionSubParam(r_a, param_b) => {
 178                     self.region_bound_pairs_accum.push((r_a, GenericKind::Param(param_b)));
 179                 }
 180                 OutlivesBound::RegionSubProjection(r_a, projection_b) => {
 181                     self.region_bound_pairs_accum
 182                         .push((r_a, GenericKind::Projection(projection_b)));
 183                 }
 184                 OutlivesBound::RegionSubRegion(r_a, r_b) => {
 185                     // In principle, we could record (and take
 186                     // advantage of) every relationship here, but
 187                     // we are also free not to -- it simply means
 188                     // strictly less that we can successfully type
 189                     // check. Right now we only look for things
 190                     // relationships between free regions. (It may
 191                     // also be that we should revise our inference
 192                     // system to be more general and to make use
 193                     // of *every* relationship that arises here,
 194                     // but presently we do not.)
 195                     self.free_region_map.relate_regions(r_a, r_b);
 196                 }
 197             }
 198         }
 199     }
 200 }