Fix VF2 layout allocation with idle qubits (backport #14938)#14940
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The two VF2 passes failed to allocate completely idle qubits into the `Layout` objects they returned. While the mapping doesn't really _mean_ much, it's important for the rest of the pipeline that a `Layout` object is a complete layout for the incoming virtuals, even if some of the choices are arbitrary. This typically appeared to users via calls to `TranspileLayout.initial_index_layout(filter_ancillas=True)` bugging out, but really anything inspecting the `Layout` before that was susceptible. (cherry picked from commit 2577bc1)
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Changelog: Fixed
jakelishman
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Aug 22, 2025
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Summary
The two VF2 passes failed to allocate completely idle qubits into the
Layoutobjects they returned. While the mapping doesn't really mean much, it's important for the rest of the pipeline that aLayoutobject is a complete layout for the incoming virtuals, even if some of the choices are arbitrary.This typically appeared to users via calls to
TranspileLayout.initial_index_layout(filter_ancillas=True)bugging out, but really anything inspecting theLayoutbefore that was susceptible.Details and comments
This should fix the bug detected in #14904 (comment).
This is an automatic backport of pull request #14938 done by [Mergify](https://mergify.com).