Make `SaveSolutionCallback` immutable

[DanielDoehring]

This will probably crash multiple times until I figured out what may be specified as immutable.

[github-actions[bot]]

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[vchuravy]

Where is the reset macro from? Is there a prior PR that introduced it?

[JoshuaLampert]

Where is the reset macro from? Is there a prior PR that introduced it?

https://github.com/trixi-framework/Trixi.jl/blob/5498f7846a2a570f1ab1829b989d7b57d7238bb7/src/Trixi.jl#L27

[DanielDoehring]

Where is the reset macro from? Is there a prior PR that introduced it?

Here: https://github.com/trixi-framework/Trixi.jl/pull/2052

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Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 96.83%. Comparing base (edde724) to head (763b63c).

Additional details and impacted files

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  Coverage   96.83%   96.83%           
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  Files         550      550           
  Lines       43455    43455           
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  Hits        42079    42079           
  Misses       1376     1376           

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[DanielDoehring]

My motivation was mainly to make the code more self-documenting in the sense that a struct such only be mutable if it is really necessary.

[DanielDoehring]

I guess I could split everything except for the change to SaveSolutionCallback apart to make this non-breaking.

[JoshuaLampert]

I guess I could split everything except for the change to SaveSolutionCallback apart to make this non-breaking.

I'm wondering whether the breaking change to the SaveSolutionCallback is really necessary (especially because it has no performance benefit). Your explanation above for making structs immutable was

My motivation was mainly to make the code more self-documenting in the sense that a struct such only be mutable if it is really necessary.

But if we do mutate instances of a SaveSolutionCallback I would consider it "really necessary". As I understand, the examples of Trixi.jl are considered to be part of the project itself. So if we utilize mutability in the examples, I would make the necessary fields mutable (the fields we don't mutate, can be made const). This would not only avoid breaking code and avoid confusion for users who wonder why they need Accessors.jl and @reset in the examples, but IMHO would also make the code more self-documenting as you want to achieve with this PR because the code tells that save_initial_solution is a field that we do want to mutate sometimes in officially supported code.

[DanielDoehring]

So in my opinion, the code in the examples is just laziness: One could have just constructed a new instance of the callback (this is what @reset does to my understanding) with a different setting.