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

use rustc_infer::traits::{TraitEngine, TraitEngineExt};

use crate::infer::canonical::OriginalQueryValues;
use crate::infer::InferCtxt;
use crate::traits::{EvaluationResult, OverflowError, PredicateObligation, SelectionContext};

#[extension(pub trait InferCtxtExt<'tcx>)]
impl<'tcx> InferCtxt<'tcx> {
    /// Evaluates whether the predicate can be satisfied (by any means)
    /// in the given `ParamEnv`.
    fn predicate_may_hold(&self, obligation: &PredicateObligation<'tcx>) -> bool {
        self.evaluate_obligation_no_overflow(obligation).may_apply()
    }

    /// Evaluates whether the predicate can be satisfied in the given
    /// `ParamEnv`, and returns `false` if not certain. However, this is
    /// not entirely accurate if inference variables are involved.
    ///
    /// This version may conservatively fail when outlives obligations
    /// are required. Therefore, this version should only be used for
    /// optimizations or diagnostics and be treated as if it can always
    /// return `false`.
    ///
    /// # Example
    ///
    /// ```
    /// # #![allow(dead_code)]
    /// trait Trait {}
    ///
    /// fn check<T: Trait>() {}
    ///
    /// fn foo<T: 'static>()
    /// where
    ///     &'static T: Trait,
    /// {
    ///     // Evaluating `&'?0 T: Trait` adds a `'?0: 'static` outlives obligation,
    ///     // which means that `predicate_must_hold_considering_regions` will return
    ///     // `false`.
    ///     check::<&'_ T>();
    /// }
    /// ```
    fn predicate_must_hold_considering_regions(
        &self,
        obligation: &PredicateObligation<'tcx>,
    ) -> bool {
        self.evaluate_obligation_no_overflow(obligation).must_apply_considering_regions()
    }

    /// Evaluates whether the predicate can be satisfied in the given
    /// `ParamEnv`, and returns `false` if not certain. However, this is
    /// not entirely accurate if inference variables are involved.
    ///
    /// This version ignores all outlives constraints.
    fn predicate_must_hold_modulo_regions(&self, obligation: &PredicateObligation<'tcx>) -> bool {
        self.evaluate_obligation_no_overflow(obligation).must_apply_modulo_regions()
    }

    /// Evaluate a given predicate, capturing overflow and propagating it back.
    fn evaluate_obligation(
        &self,
        obligation: &PredicateObligation<'tcx>,
    ) -> Result<EvaluationResult, OverflowError> {
        let mut _orig_values = OriginalQueryValues::default();

        let param_env = obligation.param_env;

        if self.next_trait_solver() {
            self.probe(|snapshot| {
                let mut fulfill_cx = crate::solve::FulfillmentCtxt::new(self);
                fulfill_cx.register_predicate_obligation(self, obligation.clone());
                // True errors
                // FIXME(-Znext-solver): Overflows are reported as ambig here, is that OK?
                if !fulfill_cx.select_where_possible(self).is_empty() {
                    Ok(EvaluationResult::EvaluatedToErr)
                } else if !fulfill_cx.select_all_or_error(self).is_empty() {
                    Ok(EvaluationResult::EvaluatedToAmbig)
                } else if self.opaque_types_added_in_snapshot(snapshot) {
                    Ok(EvaluationResult::EvaluatedToOkModuloOpaqueTypes)
                } else if self.region_constraints_added_in_snapshot(snapshot) {
                    Ok(EvaluationResult::EvaluatedToOkModuloRegions)
                } else {
                    Ok(EvaluationResult::EvaluatedToOk)
                }
            })
        } else {
            assert!(!self.intercrate);
            let c_pred =
                self.canonicalize_query(param_env.and(obligation.predicate), &mut _orig_values);
            self.tcx.at(obligation.cause.span()).evaluate_obligation(c_pred)
        }
    }

    /// Helper function that canonicalizes and runs the query. If an
    /// overflow results, we re-run it in the local context so we can
    /// report a nice error.
    fn evaluate_obligation_no_overflow(
        &self,
        obligation: &PredicateObligation<'tcx>,
    ) -> EvaluationResult {
        // Run canonical query. If overflow occurs, rerun from scratch but this time
        // in standard trait query mode so that overflow is handled appropriately
        // within `SelectionContext`.
        match self.evaluate_obligation(obligation) {
            Ok(result) => result,
            Err(OverflowError::Canonical) => {
                let mut selcx = SelectionContext::new(self);
                selcx.evaluate_root_obligation(obligation).unwrap_or_else(|r| match r {
                    OverflowError::Canonical => {
                        span_bug!(
                            obligation.cause.span,
                            "Overflow should be caught earlier in standard query mode: {:?}, {:?}",
                            obligation,
                            r,
                        )
                    }
                    OverflowError::Error(_) => EvaluationResult::EvaluatedToErr,
                })
            }
            Err(OverflowError::Error(_)) => EvaluationResult::EvaluatedToErr,
        }
    }
}