diff options
| -rw-r--r-- | src/librustc_mir/borrow_check/nll/type_check/mod.rs | 36 |
1 files changed, 36 insertions, 0 deletions
diff --git a/src/librustc_mir/borrow_check/nll/type_check/mod.rs b/src/librustc_mir/borrow_check/nll/type_check/mod.rs index ec98a0a02c0..42a1745addf 100644 --- a/src/librustc_mir/borrow_check/nll/type_check/mod.rs +++ b/src/librustc_mir/borrow_check/nll/type_check/mod.rs @@ -618,9 +618,45 @@ pub struct OutlivesSet<'tcx> { pub data: RegionConstraintData<'tcx>, } +/// The `Locations` type summarizes *where* region constraints are +/// required to hold. Normally, this is at a particular point which +/// created the obligation, but for constraints that the user gave, we +/// want the constraint to hold at all points. #[derive(Copy, Clone, Debug)] pub enum Locations { + /// Indicates that a type constraint should always be true. This + /// is particularly important in the new borrowck analysis for + /// things like the type of the return slot. Consider this + /// example: + /// + /// ``` + /// fn foo<'a>(x: &'a u32) -> &'a u32 { + /// let y = 22; + /// return &y; // error + /// } + /// ``` + /// + /// Here, we wind up with the signature from the return type being + /// something like `&'1 u32` where `'1` is a universal region. But + /// the type of the return slot `_0` is something like `&'2 u32` + /// where `'2` is an existential region variable. The type checker + /// requires that `&'2 u32 = &'1 u32` -- but at what point? In the + /// older NLL analysis, we required this only at the entry point + /// to the function. By the nature of the constraints, this wound + /// up propagating to all points reachable from start (because + /// `'1` -- as a universal region -- is live everywhere). In the + /// newer analysis, though, this doesn't work: `_0` is considered + /// dead at the start (it has no usable value) and hence this type + /// equality is basically a no-op. Then, later on, when we do `_0 + /// = &'3 y`, that region `'3` never winds up related to the + /// universal region `'1` and hence no error occurs. Therefore, we + /// use Locations::All instead, which ensures that the `'1` and + /// `'2` are equal everything. We also use this for other + /// user-given type annotations; e.g., if the user wrote `let mut + /// x: &'static u32 = ...`, we would ensure that all values + /// assigned to `x` are of `'static` lifetime. All, + Pair { /// The location in the MIR that generated these constraints. /// This is intended for error reporting and diagnosis; the |