Credence.Semantic.UnusedVariable (credence v0.7.1)

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Fixes compiler warnings about unused variables by adding _ prefix.

LLMs often generate destructuring patterns where not all bound variables are used, causing --warnings-as-errors to fail compilation.

Example

# Warning: variable "current_sum" is unused
{current_sum, max_sum} = Enum.reduce(...)

# Fixed:
{_current_sum, max_sum} = Enum.reduce(...)

How the fix locates the binding

The compiler diagnostic carries a {line, col} position pointing at the first character of the unused binding (1-indexed). The fix uses that column strictly:

  • if var_name lives at that exact column with non-word characters on both sides, insert _ there;
  • otherwise refuse to act — silently mangling the wrong identifier is far worse than leaving the warning visible.

When the diagnostic carries only a line (no column), the fix falls back to "rewrite only if var_name appears exactly once on the line as a standalone identifier" — same safety principle.