Why does C# disallow readonly local variables?

Question

Having a friendly debate with a co-worker about this. We have some thoughts about this, but wondering what the SO crowd thinks about this?

Answer 1

c# already has a readonly var, albeit in a somewhat different syntax:

Consider the following lines:

var mutable = myImmutableCalculationMethod();
readonly var immutable = mutable; // not allowed in C# 8 and prior versions
return immutable;

Compare with:

var mutable = myImmutableCalculationMethod();
string immutable() => mutable; // allowed in C# 7
return immutable();

Admittedly, the first solution could possibly be less code to write. But the 2nd snippet will make the readonly explicit, when referencing the variable.