-- Mathlib/RingTheory/Spectrum/Prime/FreeLocus.lean
lemma isLocallyConstant_rankAtStalk_freeLocus [Module.FinitePresentation R M] :
    IsLocallyConstant (fun x : freeLocus R M ↦ rankAtStalk M x.1)

Proof state:
case intro.intro.intro.mk
R : Type uR
M : Type uM
inst✝³ : CommRing R
inst✝² : AddCommGroup M
inst✝¹ : Module R M
inst✝ : FinitePresentation R M
x✝ : ↑(freeLocus R M)
x : PrimeSpectrum R
hx : x ∈ freeLocus R M
this✝⁴ : Free (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
f : R
hf : f ∈ x.asIdeal.primeCompl
hf' : Free (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M)
hf'' :
  finrank (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M) =
    finrank (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
p : PrimeSpectrum R
hp'' : p ∈ freeLocus R M
hp : ⟨p, hp''⟩ ∈ Subtype.val ⁻¹' ↑(basicOpen f)
p' : Submonoid (Localization (Submonoid.powers f)) :=
  Algebra.algebraMapSubmonoid (Localization (Submonoid.powers f)) p.asIdeal.primeCompl
hp' : Submonoid.powers f ≤ p.asIdeal.primeCompl
Rₚ : Type uR := Localization.AtPrime p.asIdeal
Mₚ : Type (max uR uM) := LocalizedModule p.asIdeal.primeCompl M
this✝³ : Algebra (Localization.Away f) Rₚ :=
  IsLocalization.localizationAlgebraOfSubmonoidLe (Localization.Away f) Rₚ (Submonoid.powers f) p.asIdeal.primeCompl hp'
this✝² : IsScalarTower R (Localization.Away f) Rₚ
this✝¹ : Module (Localization.Away f) Mₚ := compHom Mₚ (algebraMap (Localization.Away f) Rₚ)
this✝ : IsScalarTower R (Localization.Away f) Mₚ
this : IsScalarTower (Localization.Away f) Rₚ Mₚ
⊢ rankAtStalk M ↑⟨p, hp''⟩ = rankAtStalk M ↑⟨x, hx⟩

Tactic:
let l := (IsLocalizedModule.liftOfLE _ _ hp' (LocalizedModule.mkLinearMap (.powers f) M)
  (LocalizedModule.mkLinearMap p.asIdeal.primeCompl M)).extendScalarsOfIsLocalization (.powers f)
  (Localization.Away f)

Result:
case intro.intro.intro.mk
R : Type uR
M : Type uM
inst✝³ : CommRing R
inst✝² : AddCommGroup M
inst✝¹ : Module R M
inst✝ : FinitePresentation R M
x✝ : ↑(freeLocus R M)
x : PrimeSpectrum R
hx : x ∈ freeLocus R M
this✝⁴ : Free (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
f : R
hf : f ∈ x.asIdeal.primeCompl
hf' : Free (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M)
hf'' :
  finrank (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M) =
    finrank (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
p : PrimeSpectrum R
hp'' : p ∈ freeLocus R M
hp : ⟨p, hp''⟩ ∈ Subtype.val ⁻¹' ↑(basicOpen f)
p' : Submonoid (Localization (Submonoid.powers f)) :=
  Algebra.algebraMapSubmonoid (Localization (Submonoid.powers f)) p.asIdeal.primeCompl
hp' : Submonoid.powers f ≤ p.asIdeal.primeCompl
Rₚ : Type uR := Localization.AtPrime p.asIdeal
Mₚ : Type (max uR uM) := LocalizedModule p.asIdeal.primeCompl M
this✝³ : Algebra (Localization.Away f) Rₚ :=
  IsLocalization.localizationAlgebraOfSubmonoidLe (Localization.Away f) Rₚ (Submonoid.powers f) p.asIdeal.primeCompl hp'
this✝² : IsScalarTower R (Localization.Away f) Rₚ
this✝¹ : Module (Localization.Away f) Mₚ := compHom Mₚ (algebraMap (Localization.Away f) Rₚ)
this✝ : IsScalarTower R (Localization.Away f) Mₚ
this : IsScalarTower (Localization.Away f) Rₚ Mₚ
l : LocalizedModule (Submonoid.powers f) M →ₗ[Localization.Away f] LocalizedModule p.asIdeal.primeCompl M :=
  LinearMap.extendScalarsOfIsLocalization (Submonoid.powers f) (Localization.Away f)
    (IsLocalizedModule.liftOfLE (Submonoid.powers f) p.asIdeal.primeCompl hp'
      (LocalizedModule.mkLinearMap (Submonoid.powers f) M) (LocalizedModule.mkLinearMap p.asIdeal.primeCompl M))
⊢ rankAtStalk M ↑⟨p, hp''⟩ = rankAtStalk M ↑⟨x, hx⟩

Proof state:
case intro.intro.intro.mk
R : Type uR
M : Type uM
inst✝³ : CommRing R
inst✝² : AddCommGroup M
inst✝¹ : Module R M
inst✝ : FinitePresentation R M
x✝ : ↑(freeLocus R M)
x : PrimeSpectrum R
hx : x ∈ freeLocus R M
this✝¹ : Free (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
f : R
hf : f ∈ x.asIdeal.primeCompl
hf' : Free (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M)
hf'' :
  finrank (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M) =
    finrank (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
p : PrimeSpectrum R
hp'' : p ∈ freeLocus R M
hp : ⟨p, hp''⟩ ∈ Subtype.val ⁻¹' ↑(basicOpen f)
p' : Submonoid (Localization (Submonoid.powers f)) :=
  Algebra.algebraMapSubmonoid (Localization (Submonoid.powers f)) p.asIdeal.primeCompl
hp' : Submonoid.powers f ≤ p.asIdeal.primeCompl
Rₚ : Type uR := Localization.AtPrime p.asIdeal
Mₚ : Type (max uR uM) := LocalizedModule p.asIdeal.primeCompl M
this✝ : Algebra (Localization.Away f) Rₚ :=
  IsLocalization.localizationAlgebraOfSubmonoidLe (Localization.Away f) Rₚ (Submonoid.powers f) p.asIdeal.primeCompl hp'
this : IsScalarTower R (Localization.Away f) Rₚ
⊢ rankAtStalk M ↑⟨p, hp''⟩ = rankAtStalk M ↑⟨x, hx⟩

Tactic:
letI : Module (Localization.Away f) Mₚ := Module.compHom Mₚ (algebraMap _ Rₚ)

Result:
case intro.intro.intro.mk
R : Type uR
M : Type uM
inst✝³ : CommRing R
inst✝² : AddCommGroup M
inst✝¹ : Module R M
inst✝ : FinitePresentation R M
x✝ : ↑(freeLocus R M)
x : PrimeSpectrum R
hx : x ∈ freeLocus R M
this✝² : Free (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
f : R
hf : f ∈ x.asIdeal.primeCompl
hf' : Free (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M)
hf'' :
  finrank (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M) =
    finrank (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
p : PrimeSpectrum R
hp'' : p ∈ freeLocus R M
hp : ⟨p, hp''⟩ ∈ Subtype.val ⁻¹' ↑(basicOpen f)
p' : Submonoid (Localization (Submonoid.powers f)) :=
  Algebra.algebraMapSubmonoid (Localization (Submonoid.powers f)) p.asIdeal.primeCompl
hp' : Submonoid.powers f ≤ p.asIdeal.primeCompl
Rₚ : Type uR := Localization.AtPrime p.asIdeal
Mₚ : Type (max uR uM) := LocalizedModule p.asIdeal.primeCompl M
this✝¹ : Algebra (Localization.Away f) Rₚ :=
  IsLocalization.localizationAlgebraOfSubmonoidLe (Localization.Away f) Rₚ (Submonoid.powers f) p.asIdeal.primeCompl hp'
this✝ : IsScalarTower R (Localization.Away f) Rₚ
this : Module (Localization.Away f) Mₚ := compHom Mₚ (algebraMap (Localization.Away f) Rₚ)
⊢ rankAtStalk M ↑⟨p, hp''⟩ = rankAtStalk M ↑⟨x, hx⟩