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_hir::def_id::DefId;
8 use rustc_middle::ty::{self, Lift, Region, TyCtxt};
10 /// Combines a `region::ScopeTree` (which governs relationships between
11 /// scopes) and a `FreeRegionMap` (which governs relationships between
12 /// free regions) to yield a complete relation between concrete
15 /// This stuff is a bit convoluted and should be refactored, but as we
16 /// transition to NLL, it'll all go away anyhow.
17 pub struct RegionRelations<'a, 'tcx> {
18 pub tcx: TyCtxt<'tcx>,
20 /// The context used to fetch the region maps.
23 /// Free-region relationships.
24 pub free_regions: &'a FreeRegionMap<'tcx>,
27 impl<'a, 'tcx> RegionRelations<'a, 'tcx> {
28 pub fn new(tcx: TyCtxt<'tcx>, context: DefId, free_regions: &'a FreeRegionMap<'tcx>) -> Self {
29 Self { tcx, context, free_regions }
32 pub fn lub_free_regions(&self, r_a: Region<'tcx>, r_b: Region<'tcx>) -> Region<'tcx> {
33 self.free_regions.lub_free_regions(self.tcx, r_a, r_b)
37 #[derive(Clone, RustcEncodable, RustcDecodable, Debug, Default, HashStable)]
38 pub struct FreeRegionMap<'tcx> {
39 // Stores the relation `a < b`, where `a` and `b` are regions.
41 // Invariant: only free regions like `'x` or `'static` are stored
42 // in this relation, not scopes.
43 relation: TransitiveRelation<Region<'tcx>>,
46 impl<'tcx> FreeRegionMap<'tcx> {
47 pub fn elements(&self) -> impl Iterator<Item = &Region<'tcx>> {
48 self.relation.elements()
51 pub fn is_empty(&self) -> bool {
52 self.relation.is_empty()
55 // Record that `'sup:'sub`. Or, put another way, `'sub <= 'sup`.
56 // (with the exception that `'static: 'x` is not notable)
57 pub fn relate_regions(&mut self, sub: Region<'tcx>, sup: Region<'tcx>) {
58 debug!("relate_regions(sub={:?}, sup={:?})", sub, sup);
59 if self.is_free_or_static(sub) && self.is_free(sup) {
60 self.relation.add(sub, sup)
64 /// Tests whether `r_a <= r_b`.
66 /// Both regions must meet `is_free_or_static`.
68 /// Subtle: one tricky case that this code gets correct is as
69 /// follows. If we know that `r_b: 'static`, then this function
70 /// will return true, even though we don't know anything that
71 /// directly relates `r_a` and `r_b`.
73 /// Also available through the `FreeRegionRelations` trait below.
74 pub fn sub_free_regions(
80 assert!(self.is_free_or_static(r_a) && self.is_free_or_static(r_b));
81 let re_static = tcx.lifetimes.re_static;
82 if self.check_relation(re_static, r_b) {
83 // `'a <= 'static` is always true, and not stored in the
84 // relation explicitly, so check if `'b` is `'static` (or
88 self.check_relation(r_a, r_b)
92 /// Check whether `r_a <= r_b` is found in the relation.
93 fn check_relation(&self, r_a: Region<'tcx>, r_b: Region<'tcx>) -> bool {
94 r_a == r_b || self.relation.contains(&r_a, &r_b)
97 /// True for free regions other than `'static`.
98 pub fn is_free(&self, r: Region<'_>) -> bool {
100 ty::ReEarlyBound(_) | ty::ReFree(_) => true,
105 /// True if `r` is a free region or static of the sort that this
106 /// free region map can be used with.
107 pub fn is_free_or_static(&self, r: Region<'_>) -> bool {
109 ty::ReStatic => true,
110 _ => self.is_free(r),
114 /// Computes the least-upper-bound of two free regions. In some
115 /// cases, this is more conservative than necessary, in order to
116 /// avoid making arbitrary choices. See
117 /// `TransitiveRelation::postdom_upper_bound` for more details.
118 pub fn lub_free_regions(
124 debug!("lub_free_regions(r_a={:?}, r_b={:?})", r_a, r_b);
125 assert!(self.is_free(r_a));
126 assert!(self.is_free(r_b));
127 let result = if r_a == r_b {
130 match self.relation.postdom_upper_bound(&r_a, &r_b) {
131 None => tcx.lifetimes.re_static,
135 debug!("lub_free_regions(r_a={:?}, r_b={:?}) = {:?}", r_a, r_b, result);
140 /// The NLL region handling code represents free region relations in a
141 /// slightly different way; this trait allows functions to be abstract
142 /// over which version is in use.
143 pub trait FreeRegionRelations<'tcx> {
144 /// Tests whether `r_a <= r_b`. Both must be free regions or
149 shorter: ty::Region<'tcx>,
150 longer: ty::Region<'tcx>,
154 impl<'tcx> FreeRegionRelations<'tcx> for FreeRegionMap<'tcx> {
155 fn sub_free_regions(&self, tcx: TyCtxt<'tcx>, r_a: Region<'tcx>, r_b: Region<'tcx>) -> bool {
156 // invoke the "inherent method"
157 self.sub_free_regions(tcx, r_a, r_b)
161 impl<'a, 'tcx> Lift<'tcx> for FreeRegionMap<'a> {
162 type Lifted = FreeRegionMap<'tcx>;
163 fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<FreeRegionMap<'tcx>> {
164 self.relation.maybe_map(|&fr| tcx.lift(&fr)).map(|relation| FreeRegionMap { relation })