/-- If a locally integrable function `f` on a finite-dimensional real manifold has zero integral
when multiplied by any smooth compactly supported function, then `f` vanishes almost everywhere. -/
theorem ae_eq_zero_of_integral_smooth_smul_eq_zero (hf : LocallyIntegrable f μ)
    (h : ∀ g : M → ℝ, Smooth I 𝓘(ℝ) g → HasCompactSupport g → ∫ x, g x • f x ∂μ = 0) :
    ∀ᵐ x ∂μ, f x = 0 := by
  -- record topological properties of `M`
  have := I.locallyCompactSpace
  have := ChartedSpace.locallyCompactSpace H M
  have := I.secondCountableTopology
  have := ChartedSpace.secondCountable_of_sigma_compact H M
  have := ManifoldWithCorners.metrizableSpace I M
  let _ : MetricSpace M := TopologicalSpace.metrizableSpaceMetric M
  -- it suffices to show that the integral of the function vanishes on any compact set `s`
  apply ae_eq_zero_of_forall_set_integral_isCompact_eq_zero' hf (fun s hs ↦ Eq.symm ?_)
  obtain ⟨δ, δpos, hδ⟩ : ∃ δ, 0 < δ ∧ IsCompact (cthickening δ s) := hs.exists_isCompact_cthickening
  -- choose a sequence of smooth functions `gₙ` equal to `1` on `s` and vanishing outside of the
  -- `uₙ`-neighborhood of `s`, where `uₙ` tends to zero. Then each integral `∫ gₙ f` vanishes,
  -- and by dominated convergence these integrals converge to `∫ x in s, f`.
  obtain ⟨u, -, u_pos, u_lim⟩ : ∃ u, StrictAnti u ∧ (∀ (n : ℕ), u n ∈ Ioo 0 δ)
    ∧ Tendsto u atTop (𝓝 0) := exists_seq_strictAnti_tendsto' δpos
  let v : ℕ → Set M := fun n ↦ thickening (u n) s
  obtain ⟨K, K_compact, vK⟩ : ∃ K, IsCompact K ∧ ∀ n, v n ⊆ K :=
    ⟨_, hδ, fun n ↦ thickening_subset_cthickening_of_le (u_pos n).2.le _⟩
  have : ∀ n, ∃ (g : M → ℝ), support g = v n ∧ Smooth I 𝓘(ℝ) g ∧ Set.range g ⊆ Set.Icc 0 1
          ∧ ∀ x ∈ s, g x = 1 := by
    intro n
    rcases exists_msmooth_support_eq_eq_one_iff I isOpen_thickening hs.isClosed
      (self_subset_thickening (u_pos n).1 s) with ⟨g, g_smooth, g_range, g_supp, hg⟩
    exact ⟨g, g_supp, g_smooth, g_range, fun x hx ↦ (hg x).1 hx⟩
  choose g g_supp g_diff g_range hg using this
  -- main fact: the integral of `∫ gₙ f` tends to `∫ x in s, f`.
  have L : Tendsto (fun n ↦ ∫ x, g n x • f x ∂μ) atTop (𝓝 (∫ x in s, f x ∂μ)) := by
    rw [← integral_indicator hs.measurableSet]
    let bound : M → ℝ := K.indicator (fun x ↦ ‖f x‖)
    have A : ∀ n, AEStronglyMeasurable (fun x ↦ g n x • f x) μ :=
      fun n ↦ (g_diff n).continuous.aestronglyMeasurable.smul hf.aestronglyMeasurable
    have B : Integrable bound μ := by
      rw [integrable_indicator_iff K_compact.measurableSet]
      exact (hf.integrableOn_isCompact K_compact).norm
    have C : ∀ n, ∀ᵐ x ∂μ, ‖g n x • f x‖ ≤ bound x := by
      intro n
      apply eventually_of_forall (fun x ↦ ?_)
      rw [norm_smul]
      refine le_indicator_apply (fun _ ↦ ?_) (fun hxK ↦ ?_)
      · have : ‖g n x‖ ≤ 1 := by
          have := g_range n (mem_range_self (f := g n) x)
          

Proof state:
E : Type u_1
inst✝¹³ : NormedAddCommGroup E
inst✝¹² : NormedSpace ℝ E
inst✝¹¹ : FiniteDimensional ℝ E
F : Type u_2
inst✝¹⁰ : NormedAddCommGroup F
inst✝⁹ : NormedSpace ℝ F
inst✝⁸ : CompleteSpace F
H : Type u_3
inst✝⁷ : TopologicalSpace H
I : ModelWithCorners ℝ E H
M : Type u_4
inst✝⁶ : TopologicalSpace M
inst✝⁵ : ChartedSpace H M
inst✝⁴ : SmoothManifoldWithCorners I M
inst✝³ : MeasurableSpace M
inst✝² : BorelSpace M
inst✝¹ : SigmaCompactSpace M
inst✝ : T2Space M
f f' : M → F
μ : Measure M
hf : LocallyIntegrable f
h : ∀ (g : M → ℝ), Smooth I 𝓘(ℝ, ℝ) g → HasCompactSupport g → ∫ (x : M), g x • f x ∂μ = 0
this✝⁴ : LocallyCompactSpace H
this✝³ : LocallyCompactSpace M
this✝² : SecondCountableTopology H
this✝¹ : SecondCountableTopology M
this✝ : MetrizableSpace M
x✝¹ : MetricSpace M := metrizableSpaceMetric M
s : Set M
hs : IsCompact s
δ : ℝ
δpos : 0 < δ
hδ : IsCompact (cthickening δ s)
u : ℕ → ℝ
u_pos : ∀ (n : ℕ), u n ∈ Ioo 0 δ
u_lim : Tendsto u atTop (𝓝 0)
v : ℕ → Set M := fun n => thickening (u n) s
K : Set M
K_compact : IsCompact K
vK : ∀ (n : ℕ), v n ⊆ K
g : ℕ → M → ℝ
g_supp : ∀ (n : ℕ), support (g n) = v n
g_diff : ∀ (n : ℕ), Smooth I 𝓘(ℝ, ℝ) (g n)
g_range : ∀ (n : ℕ), range (g n) ⊆ Icc 0 1
hg : ∀ (n : ℕ), ∀ x ∈ s, g n x = 1
bound : M → ℝ := indicator K fun x => ‖f x‖
A : ∀ (n : ℕ), AEStronglyMeasurable (fun x => g n x • f x) μ
B : Integrable bound
n : ℕ
x : M
x✝ : x ∈ K
this : g n x ∈ Icc 0 1
⊢ ‖g n x‖ ≤ 1

Next tactic:
rw [Real.norm_of_nonneg this.1]