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more comments and justify correctness

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This commit is contained in:
Niko Matsakis 2018-08-04 09:30:24 +02:00
parent 936fa6f67f
commit 5e2f337e6b
1 changed files with 16 additions and 4 deletions
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20
src/librustc_mir/borrow_check/nll/escaping_locals.rs
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@ -173,17 +173,22 @@ impl Visitor<'tcx> for GatherAssignedLocalsVisitor<'_, '_, 'tcx> {
) {
let local = find_local_in_place(place);
// Conservatively check a subset of `Rvalue`s we know our
// benchmarks track, for example `html5ever`.
// Find those cases where there is a `Place` consumed by
// `rvalue` and we know that all regions in its type will be
// incorporated into `place`, the `Place` we are assigning to.
match rvalue {
// `x = y` is the simplest possible case.
Rvalue::Use(op) => self.union_locals_if_needed(local, find_local_in_operand(op)),
// `X = &'r P` -- the type of `X` will be `&'r T_P`, where
// `T_P` is the type of `P`.
Rvalue::Ref(_, _, place) => {
// Special case: if you have `X = &*Y` (or `X = &**Y`
// etc), then the outlives relationships will ensure
// that all regions in `Y` are constrained by regions
// in `X` -- this is because the lifetimes of the
// references we deref through are required to outlive
// the borrow lifetime (which appears in `X`).
// the borrow lifetime `'r` (which appears in `X`).
//
// (We don't actually need to check the type of `Y`:
// since `ProjectionElem::Deref` represents a built-in
@ -204,16 +209,23 @@ impl Visitor<'tcx> for GatherAssignedLocalsVisitor<'_, '_, 'tcx> {
}
Rvalue::Cast(kind, op, _) => match kind {
CastKind::Unsize => self.union_locals_if_needed(local, find_local_in_operand(op)),
CastKind::Unsize => {
// Casting a `&[T; N]` to `&[T]` or `&Foo` to `&Trait` --
// in both cases, no regions are "lost".
self.union_locals_if_needed(local, find_local_in_operand(op))
}
_ => (),
},
// Constructing an aggregate like `(x,)` or `Foo { x }`
// includes the full type of `x`.
Rvalue::Aggregate(_, ops) => {
for rvalue in ops.iter().map(find_local_in_operand) {
self.union_locals_if_needed(local, rvalue);
}
}
// For other things, be conservative and do not union.
_ => (),
};