C#: Remove splitting-awareness from data flow.

[aschackmull]

Now that splitting has been (almost) eradicated from C#, the code that makes C# data flow splitting-aware has become superfluous.

[Copilot]

Pull request overview

This PR removes the C#-specific “splitting-aware” control-flow reachability layer previously used to constrain local data-flow/taint steps, simplifying the internal C# data-flow implementation now that splitting is largely gone.

Changes:

• Deleted dataflow/internal/ControlFlowReachability.qll and removed its imports/uses across internal data-flow and taint tracking.
• Replaced reachability-configuration–based stepping with direct local step predicates and simplified reverse post-update handling.
• Updated config/identical-files.json to drop the “C# ControlFlowReachability” identical-files group.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
csharp/ql/lib/semmle/code/csharp/dataflow/internal/TaintTrackingPrivate.qll	Removes dependency on splitting-aware reachability and simplifies local taint expression stepping and post-update reverse steps.
csharp/ql/lib/semmle/code/csharp/dataflow/internal/DataFlowPrivate.qll	Removes reachability configuration scaffolding; refactors local flow/argument/store/read/lambda logic to not rely on splitting-aware reachability.
csharp/ql/lib/semmle/code/csharp/dataflow/internal/ControlFlowReachability.qll	Deletes the splitting-aware control-flow reachability helper module used by internal data-flow.
csharp/ql/consistency-queries/DataFlowConsistency.ql	Updates post-update node handling to match the refactored LocalFlow reverse-step API.
config/identical-files.json	Removes the identical-files entry tying the deleted file to its rangeanalysis counterpart.

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[hvitved]

LGTM (assuming CI is happy); nice simplification 🎉 DCA reports a slowdown on efcore-binlog; did you investigate?

[aschackmull]

DCA reports a slowdown on efcore-binlog; did you investigate?

I did not, actually, since stage timings indicate that the slowdown is in csharp/ql/src/Telemetry/SupportedExternalSources.ql, so at a first glance I figured it was spurious. I have checked now, and nothing particularly jumped out at me. The query does appear to have several slow predicates on that repo already on main, but nothing seemed to get worse.