theorem finiteType_ofLocalizationSpanTarget : OfLocalizationSpanTarget @FiniteType := by
  -- Setup algebra intances.
  rw [ofLocalizationSpanTarget_iff_finite]
  introv R hs H
  classical
  letI := f.toAlgebra
  replace H : ∀ r : s, Algebra.FiniteType R (Localization.Away (r : S))
  · intro r; simp_rw [RingHom.FiniteType] at H; convert H r; ext; simp_rw [Algebra.smul_def]; rfl
  replace H := fun r => (H r).1
  constructor
  -- Suppose `s : Finset S` spans `S`, and each `Sᵣ` is finitely generated as an `R`-algebra.
  -- Say `t r : Finset Sᵣ` generates `Sᵣ`. By assumption, we may find `lᵢ` such that
  -- `∑ lᵢ * sᵢ = 1`. I claim that all `s` and `l` and the numerators of `t` and generates `S`.
  choose t ht using H
  obtain ⟨l, hl⟩ :=
    (Finsupp.mem_span_iff_total S (s : Set S) 1).mp
      (show (1 : S) ∈ Ideal.span (s : Set S) by rw [hs]; trivial)
  let sf := fun x : s => IsLocalization.finsetIntegerMultiple (Submonoid.powers (x : S)) (t x)
  use s.attach.biUnion sf ∪ s ∪ l.support.image l
  rw [eq_top_iff]
  -- We need to show that every `x` falls in the subalgebra generated by those elements.
  -- Since all `s` and `l` are in the subalgebra, it suffices to check that `sᵢ ^ nᵢ • x` falls in
  -- the algebra for each `sᵢ` and some `nᵢ`.
  rintro x -
  apply Subalgebra.mem_of_span_eq_top_of_smul_pow_mem _ (s : Set S) l hl _ _ x _
  · intro x hx
    apply Algebra.subset_adjoin
    rw [Finset.coe_union, Finset.coe_union]
    exact Or.inl (Or.inr hx)
  · intro i
    by_cases h : l i = 0; · rw [h]; exact zero_mem _
    apply Algebra.subset_adjoin
    rw [Finset.coe_union, Finset.coe_image]
    exact Or.inr (Set.mem_image_of_mem _ (Finsupp.mem_support_iff.mpr h))
  · intro r
    rw [Finset.coe_union, Finset.coe_union, Finset.coe_biUnion]
    -- Since all `sᵢ` and numerators of `t r` are in the algebra, it suffices to show that the
    -- image of `x` in `Sᵣ` falls in the `R`-adjoin of `t r`, which is of course true.
    -- Porting note: The following `obtain` fails because Lean wants to know right away what the
    -- placeholders are, so we need to provide a little more guidance
    -- obtain ⟨⟨_, n₂, rfl⟩, hn₂⟩ := IsLocalization.exists_smul_mem_of_mem_adjoin
    --   (Submonoid.powers (r : S)) x (t r) (Algebra.adjoin R _) _ _ _
    rw [show ∀ A : Set S, (∃ n, (r : S) ^ n • x ∈ Algebra.adjoin R A) ↔
      (∃ m : (Submonoid.powers (r : S)), (m : S) • x ∈ Algebra.adjoin R A) by
      { exact fun _ => by simp [Submonoid.mem_powers_iff] }]
    refine IsLocalization.exists_smul_mem_of_mem_adjoin
      (Submonoid.powers (r : S)) x (t r) (Algebra.adjoin R _) ?_ ?_ ?_
    · intro x hx
      apply Algebra.subset_adjoin
      

Proof state:
case refine_1.a
R S : Type u_1
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Finset S
hs : Ideal.span ↑s = ⊤
this : Algebra R S := toAlgebra f
t : (r : { x // x ∈ s }) → Finset (Localization.Away ↑r)
ht : ∀ (r : { x // x ∈ s }), Algebra.adjoin R ↑(t r) = ⊤
l : ↑↑s →₀ S
hl : (Finsupp.total (↑↑s) S S Subtype.val) l = 1
sf : { x // x ∈ s } → Finset S := fun x => IsLocalization.finsetIntegerMultiple (Submonoid.powers ↑x) (t x)
x✝ : S
r : ↑↑s
x : S
hx : x ∈ ↑(IsLocalization.finsetIntegerMultiple (Submonoid.powers ↑r) (t r))
⊢ x ∈ (⋃ x ∈ ↑(Finset.attach s), ↑(sf x)) ∪ ↑s ∪ ↑(Finset.image (⇑l) l.support)

Next tactic:
exact Or.inl (Or.inl ⟨_, ⟨r, rfl⟩, _, ⟨s.mem_attach r, rfl⟩, hx⟩)