rustc_borrowck/
def_use.rs

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
use rustc_middle::bug;
use rustc_middle::mir::visit::{
    MutatingUseContext, NonMutatingUseContext, NonUseContext, PlaceContext,
};

#[derive(Eq, PartialEq, Clone)]
pub(crate) enum DefUse {
    Def,
    Use,
    Drop,
}

pub(crate) fn categorize(context: PlaceContext) -> Option<DefUse> {
    match context {
        ///////////////////////////////////////////////////////////////////////////
        // DEFS

        PlaceContext::MutatingUse(MutatingUseContext::Store) |

        // We let Call define the result in both the success and
        // unwind cases. This is not really correct, however it
        // does not seem to be observable due to the way that we
        // generate MIR. To do things properly, we would apply
        // the def in call only to the input from the success
        // path and not the unwind path. -nmatsakis
        PlaceContext::MutatingUse(MutatingUseContext::Call) |
        PlaceContext::MutatingUse(MutatingUseContext::AsmOutput) |
        PlaceContext::MutatingUse(MutatingUseContext::Yield) |

        // Storage live and storage dead aren't proper defines, but we can ignore
        // values that come before them.
        PlaceContext::NonUse(NonUseContext::StorageLive) |
        PlaceContext::NonUse(NonUseContext::StorageDead) => Some(DefUse::Def),

        ///////////////////////////////////////////////////////////////////////////
        // REGULAR USES
        //
        // These are uses that occur *outside* of a drop. For the
        // purposes of NLL, these are special in that **all** the
        // lifetimes appearing in the variable must be live for each regular use.

        PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) |
        PlaceContext::MutatingUse(MutatingUseContext::Projection) |

        // Borrows only consider their local used at the point of the borrow.
        // This won't affect the results since we use this analysis for coroutines
        // and we only care about the result at suspension points. Borrows cannot
        // cross suspension points so this behavior is unproblematic.
        PlaceContext::MutatingUse(MutatingUseContext::Borrow) |
        PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow) |
        PlaceContext::NonMutatingUse(NonMutatingUseContext::FakeBorrow) |

        // `PlaceMention` and `AscribeUserType` both evaluate the place, which must not
        // contain dangling references.
        PlaceContext::NonMutatingUse(NonMutatingUseContext::PlaceMention) |
        PlaceContext::NonUse(NonUseContext::AscribeUserTy(_)) |

        PlaceContext::MutatingUse(MutatingUseContext::RawBorrow) |
        PlaceContext::NonMutatingUse(NonMutatingUseContext::RawBorrow) |
        PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect) |
        PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
        PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) |
        PlaceContext::MutatingUse(MutatingUseContext::Retag) =>
            Some(DefUse::Use),

        ///////////////////////////////////////////////////////////////////////////
        // DROP USES
        //
        // These are uses that occur in a DROP (a MIR drop, not a
        // call to `std::mem::drop()`). For the purposes of NLL,
        // uses in drop are special because `#[may_dangle]`
        // attributes can affect whether lifetimes must be live.

        PlaceContext::MutatingUse(MutatingUseContext::Drop) =>
            Some(DefUse::Drop),

        // Debug info is neither def nor use.
        PlaceContext::NonUse(NonUseContext::VarDebugInfo) => None,

        PlaceContext::MutatingUse(MutatingUseContext::Deinit | MutatingUseContext::SetDiscriminant) => {
            bug!("These statements are not allowed in this MIR phase")
        }
    }
}