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79db96c717
Adds 'python/ql/lib/semmle/python/dataflow/new/internal/SsaImpl.qll', a
minimal Python SSA implementation built on the shared SSA library
('codeql.ssa.Ssa::Make<Location, Cfg, Input>'). The structure mirrors
Java's adapter at 'java/ql/lib/semmle/code/java/dataflow/internal/SsaImpl.qll'.
Key design choices:
* 'SourceVariable' wraps 'Py::Variable'. Only variables that are read
or deleted somewhere are tracked - write-only variables don't
benefit from SSA construction.
* Variable references are positional ('BasicBlock', 'int') pairs
looked up via 'Cfg::NameNode.defines'/'.uses'/'.deletes' (which
themselves are one-line bridges to AST-level 'Name.defines' etc.).
* Parameter writes are not synthesised: parameter Name nodes are
already wired into the CFG (per the earlier C#-style parameter
extension in 'AstNodeImpl.qll'), so the regular 'variableWrite'
path handles them at their natural CFG index.
* Non-local / captured / global / builtin variables read in a scope
but not written in it receive a synthetic entry definition at
index '-1' of the scope's entry basic block. This matches Java's
'hasEntryDef'.
* 'del x' is modelled as a certain write at the deletion site.
Includes an inline-expectations test under
'python/ql/test/library-tests/dataflow-new-ssa/' covering:
plain parameter pass-through, simple assignment + read, reassignment
with dead-write pruning, if/else with phi insertion at the join, and
an undefined-name read (currently a known limitation - no SSA flow
without an enclosing definition).
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>