/-- If `𝕜` is a nontrivially normed field, any T2 topology on `𝕜` which makes it a topological
vector space over itself (with the norm topology) is *equal* to the norm topology. -/
theorem unique_topology_of_t2 {t : TopologicalSpace 𝕜} (h₁ : @TopologicalAddGroup 𝕜 t _)
    (h₂ : @ContinuousSMul 𝕜 𝕜 _ hnorm.toUniformSpace.toTopologicalSpace t) (h₃ : @T2Space 𝕜 t) :
    t = hnorm.toUniformSpace.toTopologicalSpace := by
  -- Let `𝓣₀` denote the topology on `𝕜` induced by the norm, and `𝓣` be any T2 vector
  -- topology on `𝕜`. To show that `𝓣₀ = 𝓣`, it suffices to show that they have the same
  -- neighborhoods of 0.
  refine' TopologicalAddGroup.ext h₁ inferInstance (le_antisymm _ _)
  · -- To show `𝓣 ≤ 𝓣₀`, we have to show that closed balls are `𝓣`-neighborhoods of 0.
    rw [Metric.nhds_basis_closedBall.ge_iff]
    -- Let `ε > 0`. Since `𝕜` is nontrivially normed, we have `0 < ‖ξ₀‖ < ε` for some `ξ₀ : 𝕜`.
    intro ε hε
    rcases NormedField.exists_norm_lt 𝕜 hε with ⟨ξ₀, hξ₀, hξ₀ε⟩
    -- Since `ξ₀ ≠ 0` and `𝓣` is T2, we know that `{ξ₀}ᶜ` is a `𝓣`-neighborhood of 0.
    -- Porting note: added `mem_compl_singleton_iff.mpr`
    have : {ξ₀}ᶜ ∈ @nhds 𝕜 t 0 := IsOpen.mem_nhds isOpen_compl_singleton <|
      mem_compl_singleton_iff.mpr <| Ne.symm <| norm_ne_zero_iff.mp hξ₀.ne.symm
    -- Thus, its balanced core `𝓑` is too. Let's show that the closed ball of radius `ε` contains
    -- `𝓑`, which will imply that the closed ball is indeed a `𝓣`-neighborhood of 0.
    have : balancedCore 𝕜 {ξ₀}ᶜ ∈ @nhds 𝕜 t 0 := balancedCore_mem_nhds_zero this
    refine' mem_of_superset this fun ξ hξ => _
    -- Let `ξ ∈ 𝓑`. We want to show `‖ξ‖ < ε`. If `ξ = 0`, this is trivial.
    by_cases hξ0 : ξ = 0
    · rw [hξ0]
      exact Metric.mem_closedBall_self hε.le
    · rw [mem_closedBall_zero_iff]
      -- Now suppose `ξ ≠ 0`. By contradiction, let's assume `ε < ‖ξ‖`, and show that
      -- `ξ₀ ∈ 𝓑 ⊆ {ξ₀}ᶜ`, which is a contradiction.
      by_contra! h
      suffices (ξ₀ * ξ⁻¹) • ξ ∈ balancedCore 𝕜 {ξ₀}ᶜ by
        rw [smul_eq_mul 𝕜, mul_assoc, inv_mul_cancel hξ0, mul_one] at this
        exact not_mem_compl_iff.mpr (mem_singleton ξ₀) ((balancedCore_subset _) this)
      -- For that, we use that `𝓑` is balanced : since `‖ξ₀‖ < ε < ‖ξ‖`, we have `‖ξ₀ / ξ‖ ≤ 1`,
      -- hence `ξ₀ = (ξ₀ / ξ) • ξ ∈ 𝓑` because `ξ ∈ 𝓑`.
      refine' (balancedCore_balanced _).smul_mem _ hξ
      

Proof state:
case neg
𝕜 : Type u
hnorm : NontriviallyNormedField 𝕜
E : Type v
inst✝¹⁴ : AddCommGroup E
inst✝¹³ : Module 𝕜 E
inst✝¹² : TopologicalSpace E
inst✝¹¹ : TopologicalAddGroup E
inst✝¹⁰ : ContinuousSMul 𝕜 E
F : Type w
inst✝⁹ : AddCommGroup F
inst✝⁸ : Module 𝕜 F
inst✝⁷ : TopologicalSpace F
inst✝⁶ : TopologicalAddGroup F
inst✝⁵ : ContinuousSMul 𝕜 F
F' : Type x
inst✝⁴ : AddCommGroup F'
inst✝³ : Module 𝕜 F'
inst✝² : TopologicalSpace F'
inst✝¹ : TopologicalAddGroup F'
inst✝ : ContinuousSMul 𝕜 F'
t : TopologicalSpace 𝕜
h₁ : TopologicalAddGroup 𝕜
h₂ : ContinuousSMul 𝕜 𝕜
h₃ : T2Space 𝕜
ε : ℝ
hε : 0 < ε
ξ₀ : 𝕜
hξ₀ : 0 < ‖ξ₀‖
hξ₀ε : ‖ξ₀‖ < ε
this✝ : {ξ₀}ᶜ ∈ nhds 0
this : balancedCore 𝕜 {ξ₀}ᶜ ∈ nhds 0
ξ : 𝕜
hξ : ξ ∈ balancedCore 𝕜 {ξ₀}ᶜ
hξ0 : ¬ξ = 0
h : ε < ‖ξ‖
⊢ ‖ξ₀ * ξ⁻¹‖ ≤ 1

Next tactic:
rw [norm_mul, norm_inv, mul_inv_le_iff (norm_pos_iff.mpr hξ0), mul_one]