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UnusedVariable: Local variable is declared but never used

Reports local variables that are declared in a POU but never read or written anywhere in its body.

Why is it bad?​

A local variable that is never used is dead weight: it reserves PLC memory, clutters the variable table, and — most importantly — usually points at a real bug. A typo in a variable reference, a refactor that removed the only use site, or copy-pasted boilerplate from another POU all look exactly like an unused variable from the outside.

This rule walks every PROGRAM, FUNCTION, and FUNCTION_BLOCK and reports the names declared in their VAR* blocks that do not appear in any expression inside the same POU.

Example​

PROGRAM program0
  VAR
    counter : INT := 0;
    scratch : INT := 0;   (* UnusedVariable — never read or written *)
  END_VAR
  counter := counter + 1;
END_PROGRAM

Use instead — drop the dead variable, or wire it into the logic that needs it:

PROGRAM program0
  VAR
    counter : INT := 0;
  END_VAR
  counter := counter + 1;
END_PROGRAM