Qiskit / qiskit / 33649949845

88%

main: 88%

Build Type

push

github

Committed by web-flow

Commit Message

Implement `Drop` for `SymbolExpr` iteratively (#16775)

* Implement `Drop` for `SymbolExpr` iteratively

The derived `Drop` impl is naturally recursive, which will take a long
time and overflow the stack for deeply nested expressions.  Instead, we
do an iterative depth-first post-order traversal of any tree of `Binary`
nodes, dropping the children and following temporary backrefs back up
the tree to the next node.

This implementation is specifically intending to avoid large-scale
allocation within the `Drop` implementation, for performance.

The testing strategy here is intended to cause stack overflows in the
test suite in various cases if we ever failed to break the recursion.
The assertions against the `strong_count` are for completeness and
indicate what should happen at every point, but realistically, since
there's no interior mutability in `SymbolExpr` and no `unsafe` in the
`Drop` implementation, Rust's ownership guarantees should prevent us
from creating a true reference cycle; the risk is much more that we
accidentally fail to become fully iterative.

* Fix typo

Coverage Stats

261 of 262 new or added lines in 1 file covered. (99.62%)

5 existing lines in 2 files now uncovered.

115745 of 131798 relevant lines covered (87.82%)

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