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Merge pull request #718 from RalfJung/stacked-borrows-2

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SharedReadOnly reborrows are never weak
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Ralf Jung 2019-04-30 20:03:16 +02:00 • committed by GitHub
commit 784233573f
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1 changed files with 4 additions and 1 deletions
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5
src/stacked_borrows.rs
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@ -400,6 +400,9 @@ impl<'tcx> Stack {
// Either way, we ensure that we insert the new item in a way that between
// `derived_from` and the new one, there are only items *compatible with* `derived_from`.
let new_idx = if weak {
// A weak SharedReadOnly reborrow might be added below other items, violating the
// invariant that only SharedReadOnly can sit on top of SharedReadOnly.
assert!(new.perm != Permission::SharedReadOnly, "Weak SharedReadOnly reborrows don't work");
// A very liberal reborrow because the new pointer does not expect any kind of aliasing guarantee.
// Just insert new permission as child of old permission, and maintain everything else.
// This inserts "as far down as possible", which is good because it makes this pointer as
@ -581,8 +584,8 @@ trait EvalContextPrivExt<'a, 'mir, 'tcx: 'a+'mir>: crate::MiriEvalContextExt<'a,
// We need a frozen-sensitive reborrow.
return this.visit_freeze_sensitive(place, size, |cur_ptr, size, frozen| {
// We are only ever `SharedReadOnly` inside the frozen bits.
let weak = !frozen || kind != RefKind::Shared; // `RefKind::Raw` is always weak, as is `SharedReadWrite`.
let perm = if frozen { Permission::SharedReadOnly } else { Permission::SharedReadWrite };
let weak = perm == Permission::SharedReadWrite;
let item = Item { perm, tag: new_tag, protector };
alloc.extra.for_each(cur_ptr, size, |stack, global| {
stack.reborrow(cur_ptr.tag, force_weak || weak, item, global)