/-- In a noncompact locally compact group, a left-invariant measure which is positive
on open sets has infinite mass. -/
@[to_additive (attr := simp)
"In a noncompact locally compact additive group, a left-invariant measure which is positive on open
sets has infinite mass."]
theorem measure_univ_of_isMulLeftInvariant [WeaklyLocallyCompactSpace G] [NoncompactSpace G]
    (μ : Measure G) [IsOpenPosMeasure μ] [μ.IsMulLeftInvariant] : μ univ = ∞ := by
  /- Consider a closed compact set `K` with nonempty interior. For any compact set `L`, one may
    find `g = g (L)` such that `L` is disjoint from `g • K`. Iterating this, one finds
    infinitely many translates of `K` which are disjoint from each other. As they all have the
    same positive mass, it follows that the space has infinite measure. -/
  obtain ⟨K, hK, Kclosed, K1⟩ : ∃ K : Set G, IsCompact K ∧ IsClosed K ∧ K ∈ 𝓝 1 :=
    exists_isCompact_isClosed_nhds_one G
  have K_pos : 0 < μ K := measure_pos_of_nonempty_interior _ ⟨_, mem_interior_iff_mem_nhds.2 K1⟩
  have A : ∀ L : Set G, IsCompact L → ∃ g : G, Disjoint L (g • K) := fun L hL =>
    exists_disjoint_smul_of_isCompact hL hK
  choose! g hg using A
  set L : ℕ → Set G := fun n => (fun T => T ∪ g T • K)^[n] K
  have Lcompact : ∀ n, IsCompact (L n) := by
    intro n
    induction' n with n IH
    · exact hK
    · simp_rw [iterate_succ']
      apply IsCompact.union IH (hK.smul (g (L n)))
  have Lclosed : ∀ n, IsClosed (L n) := by
    intro n
    induction' n with n IH
    · exact Kclosed
    · simp_rw [iterate_succ']
      apply IsClosed.union IH (Kclosed.smul (g (L n)))
  have M : ∀ n, μ (L n) = (n + 1 : ℕ) * μ K := by
    intro n
    induction' n with n IH
    · simp only [one_mul, Nat.cast_one, iterate_zero, id.def, Nat.zero_eq, Nat.zero_add]
    · calc
        μ (L (n + 1)) = μ (L n) + μ (g (L n) • K) := by
          simp_rw [iterate_succ']
          exact measure_union' (hg _ (Lcompact _)) (Lclosed _).measurableSet
        _ = (n + 1 + 1 : ℕ) * μ K := by
          simp only [IH, measure_smul, add_mul, Nat.cast_add, Nat.cast_one, one_mul]
  have N : Tendsto (fun n => μ (L n)) atTop (𝓝 (∞ * μ K)) := by
    simp_rw [M]
    apply ENNReal.Tendsto.mul_const _ (Or.inl ENNReal.top_ne_zero)
    exact ENNReal.tendsto_nat_nhds_top.comp (tendsto_add_atTop_nat _)
  simp only [ENNReal.top_mul', K_pos.ne', if_false] at N
  apply top_le_iff.1
  

Proof state:
case intro.intro.intro
𝕜 : Type u_1
G : Type u_2
H : Type u_3
inst✝¹⁰ : MeasurableSpace G
inst✝⁹ : MeasurableSpace H
inst✝⁸ : TopologicalSpace G
inst✝⁷ : BorelSpace G
μ✝ : Measure G
inst✝⁶ : Group G
inst✝⁵ : TopologicalGroup G
inst✝⁴ : IsMulLeftInvariant μ✝
inst✝³ : WeaklyLocallyCompactSpace G
inst✝² : NoncompactSpace G
μ : Measure G
inst✝¹ : IsOpenPosMeasure μ
inst✝ : IsMulLeftInvariant μ
K : Set G
hK : IsCompact K
Kclosed : IsClosed K
K1 : K ∈ 𝓝 1
K_pos : 0 < ↑↑μ K
g : Set G → G
hg : ∀ (L : Set G), IsCompact L → Disjoint L (g L • K)
L : ℕ → Set G := fun n => (fun T => T ∪ g T • K)^[n] K
Lcompact : ∀ (n : ℕ), IsCompact (L n)
Lclosed : ∀ (n : ℕ), IsClosed (L n)
M : ∀ (n : ℕ), ↑↑μ (L n) = ↑(n + 1) * ↑↑μ K
N : Tendsto (fun n => ↑↑μ ((fun T => T ∪ g T • K)^[n] K)) atTop (𝓝 ⊤)
⊢ ⊤ ≤ ↑↑μ univ

Next tactic:
exact le_of_tendsto' N fun n => measure_mono (subset_univ _)