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;
8 use super::explicit_outlives_bounds;
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
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
30 pub struct OutlivesEnvironment<'tcx> {
31 pub param_env: ty::ParamEnv<'tcx>,
32 free_region_map: FreeRegionMap<'tcx>,
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.
41 // fn foo<'a, 'b, T>(x: &'a T, y: &'b ()) {
42 // bar(x, y, |y: &'b T| { .. } // body B1)
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
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.
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>>,
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>,
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>)>;
71 impl<'a, 'tcx> OutlivesEnvironment<'tcx> {
72 pub fn new(param_env: ty::ParamEnv<'tcx>) -> Self {
73 let mut env = OutlivesEnvironment {
75 free_region_map: Default::default(),
76 region_bound_pairs_map: Default::default(),
77 region_bound_pairs_accum: vec![],
80 env.add_outlives_bounds(None, explicit_outlives_bounds(param_env));
85 /// Borrows current value of the `free_region_map`.
86 pub fn free_region_map(&self) -> &FreeRegionMap<'tcx> {
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
95 /// This is a hack to support the old-skool regionck, which
96 /// processes region constraints from the main function and the
97 /// closure together. In that context, when we enter a closure, we
98 /// want to be able to "save" the state of the surrounding a
99 /// function. We can then add implied bounds and the like from the
100 /// closure arguments into the environment -- these should only
101 /// apply in the closure body, so once we exit, we invoke
102 /// `pop_snapshot_post_typeck_child` to remove them.
108 /// callback(for<'a> |x: &'a T| {
109 /// // ^^^^^^^ not legal syntax, but probably should be
110 /// // within this closure body, `T: 'a` holds
115 /// This "containment" of closure's effects only works so well. In
116 /// particular, we (intentionally) leak relationships between free
117 /// regions that are created by the closure's bounds. The case
118 /// where this is useful is when you have (e.g.) a closure with a
119 /// signature like `for<'a, 'b> fn(x: &'a &'b u32)` -- in this
120 /// case, we want to keep the relationship `'b: 'a` in the
121 /// free-region-map, so that later if we have to take `LUB('b,
122 /// 'a)` we can get the result `'b`.
124 /// I have opted to keep **all modifications** to the
125 /// free-region-map, however, and not just those that concern free
126 /// variables bound in the closure. The latter seems more correct,
127 /// but it is not the existing behavior, and I could not find a
128 /// case where the existing behavior went wrong. In any case, it
129 /// seems like it'd be readily fixed if we wanted. There are
130 /// similar leaks around givens that seem equally suspicious, to
131 /// be honest. --nmatsakis
132 pub fn push_snapshot_pre_typeck_child(&self) -> usize {
133 self.region_bound_pairs_accum.len()
136 /// See `push_snapshot_pre_typeck_child`.
137 pub fn pop_snapshot_post_typeck_child(&mut self, len: usize) {
138 self.region_bound_pairs_accum.truncate(len);
141 /// Save the current set of region-bound pairs under the given `body_id`.
142 pub fn save_implied_bounds(&mut self, body_id: hir::HirId) {
144 self.region_bound_pairs_map.insert(body_id, self.region_bound_pairs_accum.clone());
145 assert!(old.is_none());
148 /// Processes outlives bounds that are known to hold, whether from implied or other sources.
150 /// The `infcx` parameter is optional; if the implied bounds may
151 /// contain inference variables, it must be supplied, in which
152 /// case we will register "givens" on the inference context. (See
153 /// `RegionConstraintData`.)
154 pub fn add_outlives_bounds<I>(
156 infcx: Option<&InferCtxt<'a, 'tcx>>,
159 I: IntoIterator<Item = OutlivesBound<'tcx>>,
161 // Record relationships such as `T:'x` that don't go into the
162 // free-region-map but which we use here.
163 for outlives_bound in outlives_bounds {
164 debug!("add_outlives_bounds: outlives_bound={:?}", outlives_bound);
165 match outlives_bound {
166 OutlivesBound::RegionSubRegion(
167 r_a @ (&ty::ReEarlyBound(_) | &ty::ReFree(_)),
170 infcx.expect("no infcx provided but region vars found").add_given(r_a, vid_b);
172 OutlivesBound::RegionSubParam(r_a, param_b) => {
173 self.region_bound_pairs_accum.push((r_a, GenericKind::Param(param_b)));
175 OutlivesBound::RegionSubProjection(r_a, projection_b) => {
176 self.region_bound_pairs_accum
177 .push((r_a, GenericKind::Projection(projection_b)));
179 OutlivesBound::RegionSubRegion(r_a, r_b) => {
180 // In principle, we could record (and take
181 // advantage of) every relationship here, but
182 // we are also free not to -- it simply means
183 // strictly less that we can successfully type
184 // check. Right now we only look for things
185 // relationships between free regions. (It may
186 // also be that we should revise our inference
187 // system to be more general and to make use
188 // of *every* relationship that arises here,
189 // but presently we do not.)
190 self.free_region_map.relate_regions(r_a, r_b);