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.)


Intermediate format to store information needed to generate a note in the migration lint.

Reasons that we might issue a migration warning.

Intermediate format to store the hir id of the root variable and a HashSet containing information on why the root variable should be fully captured


Describe the relationship between the paths of two places eg:

Intermediate format to store the hir_id pointing to the use that resulted in the corresponding place being captured and a String which contains the captured value’s name (i.e: a.b.c)


Truncate deref of any reference.

Adjust closure capture just that if taking ownership of data, only move data from enclosing stack frame.

Returns a Ty that applies the specified capture kind on the provided capture Ty

Helper function to determine if we need to escalate CaptureKind from CaptureInfo A to B and returns the escalated CaptureInfo. (Note: CaptureInfo contains CaptureKind and an expression that led to capture it in that way)

Determines the Ancestry relationship of Place A relative to Place B

Returns the Span of where the value with the provided HirId would be dropped

Precise capture is enabled if the feature gate capture_disjoint_fields is enabled or if user is using Rust Edition 2021 or higher.

Return a two string tuple (s1, s2)

Truncate projections so that following rules are obeyed by the captured place:

Rust doesn’t permit moving fields out of a type that implements drop

Truncate place so that an unsafe block isn’t required to capture it.

Truncate the capture so that the place being borrowed is in accordance with RFC 1240, which states that it’s unsafe to take a reference into a struct marked repr(packed).

Reduces the precision of the captured place when the precision doesn’t yield any benefit from borrow checking perspective, allowing us to save us on the size of the capture.

Truncates 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.

var_name 🔒

Type Definitions

Intermediate format to store a captured Place and associated ty::CaptureInfo during capture analysis. Information in this map feeds into the minimum capture analysis pass.