R19SAFEfunction

Parameter Mutability Widened

Flags when a parameter gains a readonly constraint. Callers passing mutable data automatically satisfy the new read-only constraint.

Applies to

TypeScriptRust

Why it matters

This is a safe change. A function that now accepts readonly T[] will happily accept mutable T[] arrays too.

Example

Before (mutable)

// processor.ts
export function process(data: number[]): number { ... }

After (readonly)

// processor.ts (SAFE)
export function process(data: readonly number[]): number { ... }

What you see in the terminal

$ npx dg check

  [SAFE] process (signature_change)
  src/processor.ts:2
  Parameter 'data' now accepts readonly data. Existing mutable callers are unaffected.

How detection works

The classifier detects when a parameter's type text gains 'readonly', 'Readonly', or 'ReadonlyArray' keywords.

Real-world scenario

A function adds readonly to its parameter to signal it won't mutate the input. All existing callers with mutable arrays still work.

Back to all rules