tmp2 = x^2 + 1; f[y_] := Module[{x = 1}, Evaluate[y tmp2]]

tmp2 = x^2 + 1; Module[{x = 1}, Evaluate[tmp2]]

ClearAll[withRules]
SetAttributes[withRules, HoldAll]
withRules[rules_, expr_] :=
 Internal`InheritedBlock[{Rule, RuleDelayed},
  SetAttributes[{Rule, RuleDelayed}, HoldFirst];
  Unevaluated[expr] /. rules]
ClearAll[f, tmp2]
tmp2[y_] = y (x^2 + 1)
withRules[{expr -> tmp2}, f[y_] := Module[{x = 1}, expr[y]]]

ClearAll[f, x];
With[{tmp = tmp2},
   f[y_] := Module[{x = 1}, y tmp]]

?f
Global`f
f[y$_]:=Module[{x$=1},y$ (1+x^2)]

ClearAll[f, x];
Block[{tmp}, Unevaluated[f[y_] := Module[{x = 1}, y*tmp]] /. tmp -> tmp2]

SetAttributes[withTransforms,HoldAll];
withTransforms[code_]:=
 Block[{kx=k*Sin[a]*Cos[b],ky=k*Sin[a]*Sin[b],kz=k*Cos[b]},
   code
 ]

withTransforms[Integrate[kx^2+ky^2,{a,0,2 Pi}]]

tmp2 = x^2 + 1;

With[{expr = tmp2},
 f[y_] := Module[{x = 1}, y expr]
]

?f

Global`f
f[y$_]:=Module[{x$=1},y$ (1+x^2)]

ClearAll[withRules]
SetAttributes[withRules, HoldAll]
withRules[rules_, expr_] :=
  PreemptProtect@Internal`InheritedBlock[
    {Rule, RuleDelayed},
    SetAttributes[{Rule, RuleDelayed}, HoldFirst];
    Unevaluated[expr] /. rules
  ]

Clear[f]
withRules[{expr -> tmp2},
 f[y_] := Module[{x = 1}, y expr]
]

?f

Global`f
f[y_]:=Module[{x=1},y (1+x^2)]