1 //! This module handles the relationships between "free regions", i.e., lifetime parameters.
2 //! Ordinarily, free regions are unrelated to one another, but they can be related via implied
3 //! or explicit bounds. In that case, we track the bounds using the `TransitiveRelation` type,
4 //! and use that to decide when one free region outlives another, and so forth.
6 use rustc_data_structures::transitive_relation::TransitiveRelation;
7 use rustc_middle::ty::{Lift, Region, TyCtxt};
9 /// Combines a `FreeRegionMap` and a `TyCtxt`.
11 /// This stuff is a bit convoluted and should be refactored, but as we
12 /// transition to NLL, it'll all go away anyhow.
13 pub(crate) struct RegionRelations<'a, 'tcx> {
14 pub tcx: TyCtxt<'tcx>,
16 /// Free-region relationships.
17 pub free_regions: &'a FreeRegionMap<'tcx>,
20 impl<'a, 'tcx> RegionRelations<'a, 'tcx> {
21 pub fn new(tcx: TyCtxt<'tcx>, free_regions: &'a FreeRegionMap<'tcx>) -> Self {
22 Self { tcx, free_regions }
25 pub fn lub_free_regions(&self, r_a: Region<'tcx>, r_b: Region<'tcx>) -> Region<'tcx> {
26 self.free_regions.lub_free_regions(self.tcx, r_a, r_b)
30 #[derive(Clone, Debug)]
31 pub struct FreeRegionMap<'tcx> {
32 // Stores the relation `a < b`, where `a` and `b` are regions.
34 // Invariant: only free regions like `'x` or `'static` are stored
35 // in this relation, not scopes.
36 pub(crate) relation: TransitiveRelation<Region<'tcx>>,
39 impl<'tcx> FreeRegionMap<'tcx> {
40 pub fn elements(&self) -> impl Iterator<Item = Region<'tcx>> + '_ {
41 self.relation.elements().copied()
44 pub fn is_empty(&self) -> bool {
45 self.relation.is_empty()
48 /// Tests whether `r_a <= r_b`.
50 /// Both regions must meet `is_free_or_static`.
52 /// Subtle: one tricky case that this code gets correct is as
53 /// follows. If we know that `r_b: 'static`, then this function
54 /// will return true, even though we don't know anything that
55 /// directly relates `r_a` and `r_b`.
56 pub fn sub_free_regions(
62 assert!(r_a.is_free_or_static() && r_b.is_free_or_static());
63 let re_static = tcx.lifetimes.re_static;
64 if self.check_relation(re_static, r_b) {
65 // `'a <= 'static` is always true, and not stored in the
66 // relation explicitly, so check if `'b` is `'static` (or
70 self.check_relation(r_a, r_b)
74 /// Check whether `r_a <= r_b` is found in the relation.
75 fn check_relation(&self, r_a: Region<'tcx>, r_b: Region<'tcx>) -> bool {
76 r_a == r_b || self.relation.contains(r_a, r_b)
79 /// Computes the least-upper-bound of two free regions. In some
80 /// cases, this is more conservative than necessary, in order to
81 /// avoid making arbitrary choices. See
82 /// `TransitiveRelation::postdom_upper_bound` for more details.
83 pub fn lub_free_regions(
89 debug!("lub_free_regions(r_a={:?}, r_b={:?})", r_a, r_b);
90 assert!(r_a.is_free());
91 assert!(r_b.is_free());
92 let result = if r_a == r_b {
95 match self.relation.postdom_upper_bound(r_a, r_b) {
96 None => tcx.lifetimes.re_static,
100 debug!("lub_free_regions(r_a={:?}, r_b={:?}) = {:?}", r_a, r_b, result);
105 impl<'a, 'tcx> Lift<'tcx> for FreeRegionMap<'a> {
106 type Lifted = FreeRegionMap<'tcx>;
107 fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<FreeRegionMap<'tcx>> {
108 self.relation.maybe_map(|fr| tcx.lift(fr)).map(|relation| FreeRegionMap { relation })