Debug help - Programming in D - extending tail slice terminates sharing unexpectedly. Page 69, 70

[monkyyy]

On Sunday, 19 October 2025 at 20:44:39 UTC, Brother Bill wrote:
> The program starts with a dynamic slice of odd integers 1 thru 
> 15.
> Append 17 and reassign to slice.
> Declare dynamic slice tailSlice of the second half of slice.
> Check the length of tailSlice, which is 5; and capacity of 
> tailSlice, which is 7.
>
> Since capacity > length, we ought to be able to add two 
> integers to tailSlice without terminating sharing of slices 
> with slice.
> But incrementing the length of tailSlice by 1 breaks sharing.
> This is unexpected.
>
> Am I missing something, or have I (unlikely) found a bug in the 
> D compiler?
> What is the explanation of this behavior?
> I would expect that increasing the length of tailSlice ought to 
> continue to have sharing with slice.
>
> source/app.d
> ```
> import std.stdio;
>
> void main()
> {
> 	int[] slice = [1, 3, 5, 7, 9, 11, 13, 15];
>
> 	slice = slice ~ 17;
> 	writeln("slice: ", slice, " length: ", slice.length, " 
> capacity: ", slice.capacity);
> 	writeln();
>
> 	int[] tailSlice = slice[$ / 2 .. $];
> 	writeln("slice: ", slice, " length: ", slice.length, " 
> capacity: ", slice.capacity);
> 	writeln("tailSlice: ", tailSlice, " length: ", 
> tailSlice.length, " capacity: ", tailSlice.capacity, " 
> &tailSlice[0]: ", &tailSlice[0]);
> 	tailSlice.length += 1; // fails here
> 	// tailSlice ~= 0; // also fails
> 	writeln("tailSlice after incrementing length by 1: ", 
> tailSlice, " length: ", tailSlice.length, " capacity: ", 
> tailSlice.capacity, " &tailSlice[0]: ", &tailSlice[0]);
> 	tailSlice[0] = 888;
> 	writeln("After tail slice length increase and changing 
> tailSlice[0] to 888. ", " length: ", tailSlice.length, " 
> capacity: ", tailSlice.capacity);
> 	writeln("tailSlice: ", tailSlice);
> 	writeln("slice    : ", slice);
> }
>
> ```
>
> Console output:
> ```
> slice: [1, 3, 5, 7, 9, 11, 13, 15, 17] length: 9 capacity: 11
>
> slice: [1, 3, 5, 7, 9, 11, 13, 15, 17] length: 9 capacity: 11
> tailSlice: [9, 11, 13, 15, 17] length: 5 capacity: 7 
> &tailSlice[0]: 26AAAA31040
> tailSlice after incrementing length by 1: [9, 11, 13, 15, 17, 
> 0] length: 6 capacity: 11 &tailSlice[0]: 26AAAA31060
> After tail slice length increase and changing tailSlice[0] to 
> 888.  length: 6 capacity: 11
> tailSlice: [888, 11, 13, 15, 17, 0]
> slice    : [1, 3, 5, 7, 9, 11, 13, 15, 17]
>
> ```

Appending one slice sets the capacity to zero on other related 
slices; its best to treat slices as dynamic arrays or slices, not 
both. It will just be spooki action at a distance.