test.sass

!foo = 12

// Within this mixin, !foo refers to the argument,
// not to the global variable
=test-shadow(!foo)
  !foo = !foo + 1
  mixin-foo = !foo

a
  +test-shadow(1)
  non-mixin-foo = !foo


// Within the mixin, !foo refers to the global variable,
// even when it's called from a scope that defines !foo as a
// local variable.
=test-scope
  child-foo = !foo

b
  @for !foo from 0 to 1
    +test-scope