Module rustc_typeck::check::upvar
source · [−]Expand description
Inferring borrow kinds for upvars
Whenever there is a closure expression, we need to determine how each
upvar is used. We do this by initially assigning each upvar an
immutable “borrow kind” (see ty::BorrowKind
for details) and then
“escalating” the kind as needed. The borrow kind proceeds according to
the following lattice:
ty::ImmBorrow -> ty::UniqueImmBorrow -> ty::MutBorrow
So, for example, if we see an assignment x = 5
to an upvar x
, we
will promote its borrow kind to mutable borrow. If we see an &mut x
we’ll do the same. Naturally, this applies not just to the upvar, but
to everything owned by x
, so the result is the same for something
like x.f = 5
and so on (presuming x
is not a borrowed pointer to a
struct). These adjustments are performed in
adjust_upvar_borrow_kind()
(you can trace backwards through the code
from there).
The fact that we are inferring borrow kinds as we go results in a
semi-hacky interaction with mem-categorization. In particular,
mem-categorization will query the current borrow kind as it
categorizes, and we’ll return the current value, but this may get
adjusted later. Therefore, in this module, we generally ignore the
borrow kind (and derived mutabilities) that are returned from
mem-categorization, since they may be inaccurate. (Another option
would be to use a unification scheme, where instead of returning a
concrete borrow kind like ty::ImmBorrow
, we return a
ty::InferBorrow(upvar_id)
or something like that, but this would
then mean that all later passes would have to check for these figments
and report an error, and it just seems like more mess in the end.)
Structs
Enums
Functions
capture_disjoint_fields
is enabled or if
user is using Rust Edition 2021 or higher.place
:place
so that an unsafe
block isn’t required to capture it.repr(packed)
.place
to have up to len
projections.
curr_mode
is the current required capture kind for the place.
Returns the truncated place
and the updated required capture kind.Type Definitions
Place
and associated ty::CaptureInfo
during capture analysis. Information in this map feeds into the minimum capture
analysis pass.