Enum rustc_borrowck::type_check::Locations
source · Expand description
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.
Variants§
All(Span)
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.
The span points to the place the constraint arose. For example, it points to the type in a user-given type annotation. If there’s no sensible span then it’s DUMMY_SP.
Single(Location)
An outlives constraint that only has to hold at a single location,
usually it represents a point where references flow from one spot to
another (e.g., x = y
)
Implementations§
Trait Implementations§
source§impl NormalizeLocation for Locations
impl NormalizeLocation for Locations
fn to_locations(self) -> Locations
source§impl Ord for Locations
impl Ord for Locations
source§impl PartialOrd<Locations> for Locations
impl PartialOrd<Locations> for Locations
1.0.0 · source§fn le(&self, other: &Rhs) -> bool
fn le(&self, other: &Rhs) -> bool
self
and other
) and is used by the <=
operator. Read moreimpl Copy for Locations
impl Eq for Locations
impl StructuralEq for Locations
impl StructuralPartialEq for Locations
Auto Trait Implementations§
impl RefUnwindSafe for Locations
impl !Send for Locations
impl !Sync for Locations
impl Unpin for Locations
impl UnwindSafe for Locations
Blanket Implementations§
Layout§
Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...)
attributes. Please see the Rust Reference’s “Type Layout” chapter for details on type layout guarantees.
Size: 16 bytes
Size for each variant:
All
: 8 bytesSingle
: 16 bytes