/-- Any linear form on a topological vector space over a nontrivially normed field is continuous if
    its kernel is closed. -/
theorem LinearMap.continuous_of_isClosed_ker (l : E →ₗ[𝕜] 𝕜)
    (hl : IsClosed (LinearMap.ker l : Set E)) :
    Continuous l := by
  -- `l` is either constant or surjective. If it is constant, the result is trivial.
  by_cases H : finrank 𝕜 (LinearMap.range l) = 0
  · rw [Submodule.finrank_eq_zero, LinearMap.range_eq_bot] at H
    rw [H]
    exact continuous_zero
  · -- In the case where `l` is surjective, we factor it as `φ : (E ⧸ l.ker) ≃ₗ[𝕜] 𝕜`. Note that
    -- `E ⧸ l.ker` is T2 since `l.ker` is closed.
    have : finrank 𝕜 (LinearMap.range l) = 1 :=
      le_antisymm (finrank_self 𝕜 ▸ l.range.finrank_le) (zero_lt_iff.mpr H)
    have hi : Function.Injective ((LinearMap.ker l).liftQ l (le_refl _)) := by
      rw [← LinearMap.ker_eq_bot]
      exact Submodule.ker_liftQ_eq_bot _ _ _ (le_refl _)
    have hs : Function.Surjective ((LinearMap.ker l).liftQ l (le_refl _)) := by
      rw [← LinearMap.range_eq_top, Submodule.range_liftQ]
      exact Submodule.eq_top_of_finrank_eq ((finrank_self 𝕜).symm ▸ this)
    let φ : (E ⧸ LinearMap.ker l) ≃ₗ[𝕜] 𝕜 :=
      LinearEquiv.ofBijective ((LinearMap.ker l).liftQ l (le_refl _)) ⟨hi, hs⟩
    have hlφ : (l : E → 𝕜) = φ ∘ (LinearMap.ker l).mkQ := by ext; rfl
    -- Since the quotient map `E →ₗ[𝕜] (E ⧸ l.ker)` is continuous, the continuity of `l` will follow
    -- form the continuity of `φ`.
    suffices Continuous φ.toEquiv by
      rw [hlφ]
      exact this.comp continuous_quot_mk
    -- The pullback by `φ.symm` of the quotient topology is a T2 topology on `𝕜`, because `φ.symm`
    -- is injective. Since `φ.symm` is linear, it is also a vector space topology.
    -- Hence, we know that it is equal to the topology induced by the norm.
    have : induced φ.toEquiv.symm inferInstance = hnorm.toUniformSpace.toTopologicalSpace := by
      refine'
        unique_topology_of_t2 (topologicalAddGroup_induced φ.symm.toLinearMap)
          (continuousSMul_induced φ.symm.toLinearMap) _
      -- Porting note: was `rw [t2Space_iff]`
      refine (@t2Space_iff 𝕜 (induced (↑(LinearEquiv.toEquiv φ).symm) inferInstance)).mpr ?_
      exact fun x y hxy =>
        @separated_by_continuous _ _ (induced _ _) _ _ _ continuous_induced_dom _ _
          (φ.toEquiv.symm.injective.ne hxy)
    -- Finally, the pullback by `φ.symm` is exactly the pushforward by `φ`, so we have to prove
    -- that `φ` is continuous when `𝕜` is endowed with the pushforward by `φ` of the quotient
    -- topology, which is trivial by definition of the pushforward.
    rw [this.symm, Equiv.induced_symm]
    

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'
l : E →ₗ[𝕜] 𝕜
hl : IsClosed ↑(ker l)
H : ¬finrank 𝕜 ↥(range l) = 0
this✝ : finrank 𝕜 ↥(range l) = 1
hi : Function.Injective ⇑(Submodule.liftQ (ker l) l (_ : ker l ≤ ker l))
hs : Function.Surjective ⇑(Submodule.liftQ (ker l) l (_ : ker l ≤ ker l))
φ : (E ⧸ ker l) ≃ₗ[𝕜] 𝕜 :=
  LinearEquiv.ofBijective (Submodule.liftQ (ker l) l (_ : ker l ≤ ker l))
    (_ :
      Function.Injective ⇑(Submodule.liftQ (ker l) l (_ : ker l ≤ ker l)) ∧
        Function.Surjective ⇑(Submodule.liftQ (ker l) l (_ : ker l ≤ ker l)))
hlφ : ⇑l = ⇑φ ∘ ⇑(Submodule.mkQ (ker l))
this : induced (⇑(LinearEquiv.toEquiv φ).symm) inferInstance = UniformSpace.toTopologicalSpace
⊢ Continuous ⇑(LinearEquiv.toEquiv φ)

Next tactic:
exact continuous_coinduced_rng