1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
|
//! This module handles the relationships between "free regions", i.e., lifetime parameters.
//! Ordinarily, free regions are unrelated to one another, but they can be related via implied
//! or explicit bounds. In that case, we track the bounds using the `TransitiveRelation` type,
//! and use that to decide when one free region outlives another, and so forth.
use rustc_data_structures::transitive_relation::TransitiveRelation;
use rustc_middle::ty::{Lift, Region, TyCtxt};
/// Combines a `FreeRegionMap` and a `TyCtxt`.
///
/// This stuff is a bit convoluted and should be refactored, but as we
/// transition to NLL, it'll all go away anyhow.
pub(crate) struct RegionRelations<'a, 'tcx> {
pub tcx: TyCtxt<'tcx>,
/// Free-region relationships.
pub free_regions: &'a FreeRegionMap<'tcx>,
}
impl<'a, 'tcx> RegionRelations<'a, 'tcx> {
pub fn new(tcx: TyCtxt<'tcx>, free_regions: &'a FreeRegionMap<'tcx>) -> Self {
Self { tcx, free_regions }
}
pub fn lub_free_regions(&self, r_a: Region<'tcx>, r_b: Region<'tcx>) -> Region<'tcx> {
self.free_regions.lub_free_regions(self.tcx, r_a, r_b)
}
}
#[derive(Clone, Debug)]
pub struct FreeRegionMap<'tcx> {
/// Stores the relation `a < b`, where `a` and `b` are regions.
///
/// Invariant: only free regions like `'x` or `'static` are stored
/// in this relation, not scopes.
pub(crate) relation: TransitiveRelation<Region<'tcx>>,
}
impl<'tcx> FreeRegionMap<'tcx> {
pub fn elements(&self) -> impl Iterator<Item = Region<'tcx>> + '_ {
self.relation.elements().copied()
}
pub fn is_empty(&self) -> bool {
self.relation.is_empty()
}
/// Tests whether `r_a <= r_b`.
///
/// Both regions must meet `is_free_or_static`.
///
/// Subtle: one tricky case that this code gets correct is as
/// follows. If we know that `r_b: 'static`, then this function
/// will return true, even though we don't know anything that
/// directly relates `r_a` and `r_b`.
pub fn sub_free_regions(
&self,
tcx: TyCtxt<'tcx>,
r_a: Region<'tcx>,
r_b: Region<'tcx>,
) -> bool {
assert!(r_a.is_free_or_static() && r_b.is_free_or_static());
let re_static = tcx.lifetimes.re_static;
if self.check_relation(re_static, r_b) {
// `'a <= 'static` is always true, and not stored in the
// relation explicitly, so check if `'b` is `'static` (or
// equivalent to it)
true
} else {
self.check_relation(r_a, r_b)
}
}
/// Check whether `r_a <= r_b` is found in the relation.
fn check_relation(&self, r_a: Region<'tcx>, r_b: Region<'tcx>) -> bool {
r_a == r_b || self.relation.contains(r_a, r_b)
}
/// Computes the least-upper-bound of two free regions. In some
/// cases, this is more conservative than necessary, in order to
/// avoid making arbitrary choices. See
/// `TransitiveRelation::postdom_upper_bound` for more details.
pub fn lub_free_regions(
&self,
tcx: TyCtxt<'tcx>,
r_a: Region<'tcx>,
r_b: Region<'tcx>,
) -> Region<'tcx> {
debug!("lub_free_regions(r_a={:?}, r_b={:?})", r_a, r_b);
assert!(r_a.is_free());
assert!(r_b.is_free());
let result = if r_a == r_b {
r_a
} else {
match self.relation.postdom_upper_bound(r_a, r_b) {
None => tcx.lifetimes.re_static,
Some(r) => r,
}
};
debug!("lub_free_regions(r_a={:?}, r_b={:?}) = {:?}", r_a, r_b, result);
result
}
}
impl<'a, 'tcx> Lift<'tcx> for FreeRegionMap<'a> {
type Lifted = FreeRegionMap<'tcx>;
fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<FreeRegionMap<'tcx>> {
self.relation.maybe_map(|fr| tcx.lift(fr)).map(|relation| FreeRegionMap { relation })
}
}
|
