Small improvement discussion: Use `let` to lock variable previously declared as `var`

[geekrelief]

Using a view 
<https://nim-lang.github.io/Nim/manual_experimental.html#view-types> the 
generated code boils down to one pointer copy.
    
    
    import std / [macros, genasts]
    {.experimental: "views"}
    macro immut(n: typed) =
      if n.kind notin {nnkSym, nnkHiddenDeref}:
        error("Expected a variable", n)
      let
        (n, name) =
          if n.kind == nnkHiddenDeref:
            (n[0], ident($n[0]))
          else:
            (n, ident($n))
        typ = block:
          let typ = n.getType
          if typ[0].eqident"var":
            typ[^1]
          else:
            typ
      
      result = genast(typ, name = ident(n.repr), n):
        let name: lent typ = n
    
    proc doThing(i: var int) =
      immut i
      echo i
    
    
    Run
    
    
    N_LIB_PRIVATE N_NIMCALL(void, doThing__immut_49)(NI* i) {
            NI* i_2;
            tyArray__nHXaesL0DJZHyVS07ARPRA T1_;
            i_2 = i;
            nimZeroMem((void*)T1_, sizeof(tyArray__nHXaesL0DJZHyVS07ARPRA));
            T1_[0] = dollar___systemZdollars_3((*i_2));
            echoBinSafe(T1_, 1);
    }
    
    
    Run

I had to google for this, but at least in C casting in this case probably 
doesn't have any runtime cost because the fundamental type isn't changing. It's 
hard to say for sure without looking at the assembly.