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Alexander Liberov edited this page Nov 8, 2026 · 1 revision

Data-flow rules that know a parameter's mode

Three OFF rules for what the default data-flow rules cannot see. write-only-local and overwrite-before-read count every call argument as a use, and used-before-assignment asks a parameter's mode for bare-name calls only. So a local passed to G.GetOut(X) looks read, and a local passed to G.Take(X) before it is assigned goes unnoticed.

drag-lint lint <file.pas> --db <project.sqlite> --enable local-only-out-arg,dead-store-before-out-arg,by-value-arg-before-assignment
drag-lint lint-all --db <project.sqlite> --rule dead-store-before-out-arg
Rule Default Reports
local-only-out-arg OFF, info A local passed as an out argument that nothing in the routine reads
dead-store-before-out-arg OFF, warning X := 1; G.GetOut(X); Use(X);: the store is overwritten before any read
by-value-arg-before-assignment OFF, error or warning G.Take(X); with X not assigned. Error when no path assigned it, warning when some path did

Where a mode comes from

A bare callee: its signature, from the index or from this unit. A method: the receiver's static type in ref_types, the method's overload set over the type's whole ancestry, and every applicable overload agreeing. So on an index older than 1.44, or with no index, only the bare out shapes report, and by-value-arg-before-assignment reports nothing.

What keeps a rule silent

  • The mode is not proved: a type name with two declarations, an ancestor the index cannot follow, a helper with a method of that name, a member that is not a method, overloads that disagree, a with.
  • A var argument: the callee may read it.
  • An argument that is not a bare local: G.GetOut(R.A) writes part of R.
  • A second mention of the local in the same statement: G.Two(X, X).
  • An out argument that may not run: right of and / or, or in a branch of an if expression.
  • A local a nested routine captures, and the stores overwrite-before-read also forgives: a nil initialisation before a try, X := nil on an interface.
  • For by-value-arg-before-assignment: a managed local, a static array (Delphi passes its address, and the callee may fill it), an untyped parameter, a pointer-typed parameter (^T, Pointer, PChar, P + capital, LP + capital), an assignment under the same guard.

Limits

An out is taken at its word: a callee that reads what it was handed is not seen. A by-value call still counts as a possible definition for used-before-assignment, so a later read of the same local stays silent; the rule reports the first call only.

Why these are separate ids, and how they could fold into the default rules: docs\superpowers\specs\2026-11-06-liveness-out-args-by-value.md.

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