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

// Representing terms
//
// Terms are structured as a straightforward tree. Rather than rely on
// GC, we allocate terms out of a bounded arena (the lifetime of this
// arena is the lifetime 'a that is threaded around).
//
// We assign a unique index to each type/region parameter whose variance
// is to be inferred. We refer to such variables as "inferreds". An
// `InferredIndex` is a newtype'd int representing the index of such
// a variable.

use rustc_arena::DroplessArena;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::{LocalDefId, LocalDefIdMap};
use rustc_middle::ty::{self, TyCtxt};
use std::fmt;

use self::VarianceTerm::*;

pub type VarianceTermPtr<'a> = &'a VarianceTerm<'a>;

#[derive(Copy, Clone, Debug)]
pub struct InferredIndex(pub usize);

#[derive(Copy, Clone)]
pub enum VarianceTerm<'a> {
    ConstantTerm(ty::Variance),
    TransformTerm(VarianceTermPtr<'a>, VarianceTermPtr<'a>),
    InferredTerm(InferredIndex),
}

impl<'a> fmt::Debug for VarianceTerm<'a> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            ConstantTerm(c1) => write!(f, "{:?}", c1),
            TransformTerm(v1, v2) => write!(f, "({:?} \u{00D7} {:?})", v1, v2),
            InferredTerm(id) => write!(f, "[{}]", {
                let InferredIndex(i) = id;
                i
            }),
        }
    }
}

/// The first pass over the crate simply builds up the set of inferreds.

pub struct TermsContext<'a, 'tcx> {
    pub tcx: TyCtxt<'tcx>,
    pub arena: &'a DroplessArena,

    /// For marker types, `UnsafeCell`, and other lang items where
    /// variance is hardcoded, records the item-id and the hardcoded
    /// variance.
    pub lang_items: Vec<(LocalDefId, Vec<ty::Variance>)>,

    /// Maps from the node id of an item to the first inferred index
    /// used for its type & region parameters.
    pub inferred_starts: LocalDefIdMap<InferredIndex>,

    /// Maps from an InferredIndex to the term for that variable.
    pub inferred_terms: Vec<VarianceTermPtr<'a>>,
}

pub fn determine_parameters_to_be_inferred<'a, 'tcx>(
    tcx: TyCtxt<'tcx>,
    arena: &'a DroplessArena,
) -> TermsContext<'a, 'tcx> {
    let mut terms_cx = TermsContext {
        tcx,
        arena,
        inferred_starts: Default::default(),
        inferred_terms: vec![],

        lang_items: lang_items(tcx),
    };

    // See the following for a discussion on dep-graph management.
    //
    // - https://rustc-dev-guide.rust-lang.org/query.html
    // - https://rustc-dev-guide.rust-lang.org/variance.html
    let crate_items = tcx.hir_crate_items(());

    for def_id in crate_items.definitions() {
        debug!("add_inferreds for item {:?}", def_id);

        let def_kind = tcx.def_kind(def_id);

        match def_kind {
            DefKind::Struct | DefKind::Union | DefKind::Enum => {
                terms_cx.add_inferreds_for_item(def_id);

                let adt = tcx.adt_def(def_id);
                for variant in adt.variants() {
                    if let Some(ctor_def_id) = variant.ctor_def_id() {
                        terms_cx.add_inferreds_for_item(ctor_def_id.expect_local());
                    }
                }
            }
            DefKind::Fn | DefKind::AssocFn => terms_cx.add_inferreds_for_item(def_id),
            _ => {}
        }
    }

    terms_cx
}

fn lang_items(tcx: TyCtxt<'_>) -> Vec<(LocalDefId, Vec<ty::Variance>)> {
    let lang_items = tcx.lang_items();
    let all = [
        (lang_items.phantom_data(), vec![ty::Covariant]),
        (lang_items.unsafe_cell_type(), vec![ty::Invariant]),
    ];

    all.into_iter() // iterating over (Option<DefId>, Variance)
        .filter_map(|(d, v)| {
            let def_id = d?.as_local()?; // LocalDefId
            Some((def_id, v))
        })
        .collect()
}

impl<'a, 'tcx> TermsContext<'a, 'tcx> {
    fn add_inferreds_for_item(&mut self, def_id: LocalDefId) {
        let tcx = self.tcx;
        let count = tcx.generics_of(def_id).count();

        if count == 0 {
            return;
        }

        // Record the start of this item's inferreds.
        let start = self.inferred_terms.len();
        let newly_added = self.inferred_starts.insert(def_id, InferredIndex(start)).is_none();
        assert!(newly_added);

        // N.B., in the code below for writing the results back into the
        // `CrateVariancesMap`, we rely on the fact that all inferreds
        // for a particular item are assigned continuous indices.

        let arena = self.arena;
        self.inferred_terms.extend(
            (start..(start + count)).map(|i| &*arena.alloc(InferredTerm(InferredIndex(i)))),
        );
    }
}