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
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145

use crate::infer::InferCtxt;
use crate::traits::{ObligationCause, ObligationCtxt};
use rustc_data_structures::fx::FxIndexSet;
use rustc_infer::infer::resolve::OpportunisticRegionResolver;
use rustc_infer::infer::InferOk;
use rustc_middle::infer::canonical::{OriginalQueryValues, QueryRegionConstraints};
use rustc_middle::ty::{self, ParamEnv, Ty, TypeFolder, TypeVisitableExt};
use rustc_span::def_id::LocalDefId;

pub use rustc_middle::traits::query::OutlivesBound;

pub type BoundsCompat<'a, 'tcx: 'a> = impl Iterator<Item = OutlivesBound<'tcx>> + 'a;
pub type Bounds<'a, 'tcx: 'a> = impl Iterator<Item = OutlivesBound<'tcx>> + 'a;

/// Implied bounds are region relationships that we deduce
/// automatically. The idea is that (e.g.) a caller must check that a
/// function's argument types are well-formed immediately before
/// calling that fn, and hence the *callee* can assume that its
/// argument types are well-formed. This may imply certain relationships
/// between generic parameters. For example:
/// ```
/// fn foo<T>(x: &T) {}
/// ```
/// can only be called with a `'a` and `T` such that `&'a T` is WF.
/// For `&'a T` to be WF, `T: 'a` must hold. So we can assume `T: 'a`.
///
/// # Parameters
///
/// - `param_env`, the where-clauses in scope
/// - `body_id`, the body-id to use when normalizing assoc types.
///   Note that this may cause outlives obligations to be injected
///   into the inference context with this body-id.
/// - `ty`, the type that we are supposed to assume is WF.
#[instrument(level = "debug", skip(infcx, param_env, body_id), ret)]
fn implied_outlives_bounds<'a, 'tcx>(
    infcx: &'a InferCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    body_id: LocalDefId,
    ty: Ty<'tcx>,
    compat: bool,
) -> Vec<OutlivesBound<'tcx>> {
    let ty = infcx.resolve_vars_if_possible(ty);
    let ty = OpportunisticRegionResolver::new(infcx).fold_ty(ty);

    // We do not expect existential variables in implied bounds.
    // We may however encounter unconstrained lifetime variables
    // in very rare cases.
    //
    // See `ui/implied-bounds/implied-bounds-unconstrained-2.rs` for
    // an example.
    assert!(!ty.has_non_region_infer());

    let mut canonical_var_values = OriginalQueryValues::default();
    let canonical_ty = infcx.canonicalize_query(param_env.and(ty), &mut canonical_var_values);
    let implied_bounds_result = if compat {
        infcx.tcx.implied_outlives_bounds_compat(canonical_ty)
    } else {
        infcx.tcx.implied_outlives_bounds(canonical_ty)
    };
    let Ok(canonical_result) = implied_bounds_result else {
        return vec![];
    };

    let mut constraints = QueryRegionConstraints::default();
    let span = infcx.tcx.def_span(body_id);
    let Ok(InferOk { value: mut bounds, obligations }) = infcx
        .instantiate_nll_query_response_and_region_obligations(
            &ObligationCause::dummy_with_span(span),
            param_env,
            &canonical_var_values,
            canonical_result,
            &mut constraints,
        )
    else {
        return vec![];
    };
    assert_eq!(&obligations, &[]);

    // Because of #109628, we may have unexpected placeholders. Ignore them!
    // FIXME(#109628): panic in this case once the issue is fixed.
    bounds.retain(|bound| !bound.has_placeholders());

    if !constraints.is_empty() {
        debug!(?constraints);
        if !constraints.member_constraints.is_empty() {
            span_bug!(span, "{:#?}", constraints.member_constraints);
        }

        // Instantiation may have produced new inference variables and constraints on those
        // variables. Process these constraints.
        let ocx = ObligationCtxt::new(infcx);
        let cause = ObligationCause::misc(span, body_id);
        for &constraint in &constraints.outlives {
            ocx.register_obligation(infcx.query_outlives_constraint_to_obligation(
                constraint,
                cause.clone(),
                param_env,
            ));
        }

        let errors = ocx.select_all_or_error();
        if !errors.is_empty() {
            infcx.dcx().span_bug(
                span,
                "implied_outlives_bounds failed to solve obligations from instantiation",
            );
        }
    };

    bounds
}

#[extension(pub trait InferCtxtExt<'a, 'tcx>)]
impl<'a, 'tcx: 'a> InferCtxt<'tcx> {
    /// Do *NOT* call this directly.
    fn implied_bounds_tys_compat(
        &'a self,
        param_env: ParamEnv<'tcx>,
        body_id: LocalDefId,
        tys: &'a FxIndexSet<Ty<'tcx>>,
        compat: bool,
    ) -> BoundsCompat<'a, 'tcx> {
        tys.iter()
            .flat_map(move |ty| implied_outlives_bounds(self, param_env, body_id, *ty, compat))
    }

    /// If `-Z no-implied-bounds-compat` is set, calls `implied_bounds_tys_compat`
    /// with `compat` set to `true`, otherwise `false`.
    fn implied_bounds_tys(
        &'a self,
        param_env: ParamEnv<'tcx>,
        body_id: LocalDefId,
        tys: &'a FxIndexSet<Ty<'tcx>>,
    ) -> Bounds<'a, 'tcx> {
        tys.iter().flat_map(move |ty| {
            implied_outlives_bounds(
                self,
                param_env,
                body_id,
                *ty,
                !self.tcx.sess.opts.unstable_opts.no_implied_bounds_compat,
            )
        })
    }
}