Idea: Immutable blocks within functions

[Xinok]

For me, one of the annoyances of working with immutable data is 
simply initializing data. Immutable variables have to be 
initialized upon declaration and cannot be modified after that 
point. This is made more difficult if you have several immutable 
variables to initialize in conjunction with one another

Currently, the following syntax is not allowed within functions:
immutable{ }

I propose enabling this syntax to have special usage within 
functions for initializing immutable variables. Variables would 
be mutable within the scope of the immutable block, and immutable 
outside of that scope. That scope would be limited to the same 
access restrictions as pure functions to ensure mutable 
references to the data can't escape.

While something similar could be accomplished using nested 
functions, they're limited to initializing one immutable variable 
at a time and less efficient since they must utilize a context 
pointer to access outside data. I think my idea has a nice allure 
for greater simplicity and flexibility for initializing immutable 
variables.


An impractical example:

void foo()
{
	immutable
	{
		int[] arr = ...;
		int[] arr2 = arr.dup;
		sort(arr); // arr is still mutable
		
		int[] arr2 = arr.dup;
		reverse(arr2);
		arr ~= arr2;
	}
	
	sort(arr); // Error is thrown because arr is now immutable
}


Considerations:
* Global and thread-local variables within an immutable block 
would be immutable; otherwise, the control flow could re-enter 
the immutable scope and change the values of those variables.
* There may be issues with reflection since the type of the 
variable suddenly changes from mutable to immutable.

So what do nested immutable blocks imply? The same as having a 
pure function within a pure function.

void foo()
{
	immutable
	{
		int[] arr = ...;
		
		immutable
		{
			// This statement would throw an error
			int value = arr[0];
			// Here, arr is still mutable.
			// So like a pure function, access to outside
			// mutable variables isn't allowed.
		}
	}
}