binderNameHint.{u, v, w} {α : Sort u} {β : Sort v} {γ : Sort w} (v : α)
  (binder : β) (e : γ) : γ
binderNameHint.{u, v, w} {α : Sort u}
  {β : Sort v} {γ : Sort w} (v : α)
  (binder : β) (e : γ) : γ

The expression binderNameHint v binder e defined to be e.

If it is used on the right-hand side of an equation that is used for rewriting by rw or simp, and v is a local variable, and binder is an expression that (after beta-reduction) is a binder (fun w => … or ∀ w, …), then it will rename v to the name used in that binder, and remove the binderNameHint.

A typical use of this gadget would be as follows; the gadget ensures that after rewriting, the local variable is still name, and not x:

theorem all_eq_not_any_not (l : List α) (p : α → Bool) :
    l.all p = !l.any fun x => binderNameHint x p (!p x) := sorry

example (names : List String) : names.all (fun name => "Waldo".isPrefixOf name) = true := by
  rw [all_eq_not_any_not]
  -- ⊢ (!names.any fun name => !"Waldo".isPrefixOf name) = true

If binder is not a binder, then the name of v attains a macro scope. This only matters when the resulting term is used in a non-hygienic way, e.g. in termination proofs for well-founded recursion.

This gadget is supported by

• simp, dsimp and rw in the right-hand-side of an equation

• simp in the assumptions of congruence rules

It is ineffective in other positions (hypotheses of rewrite rules) or when used by other tactics (e.g. apply).