print_help() et al. are conceptually read-only ops and should not take &mut self

[djeedai]

print_help() and other similar variants conceptually just print information to some output based on the already-configured App, and therefore it is quite unexpected for them to be mutating the App and take a &mut self reference. For a typical command-line tool which might want to print-help-and-exit deeper inside the program due to other errors not directly handled by clap's argument validation, this forces App to be passed by &mut self everywhere for no good apparent reason, and thereby makes other unrelated parts of the code more complex.

I looked at the implementation, and it seems this is due to some "propagation of globals and settings", which I understand to be lazy operations which need to be done once and could have been done at another time and/or because they are conceptually not mutating the app should be hidden behind a &self read-only reference. I might be wrong here, as I am no clap expert, but I see no good argument from a user standpoint as to why a print function would mutate anything.

Version

• Rust: rustc 1.48.0 (7eac88abb 2020-11-16)
• Clap: 2.33.3

Actual Behavior Summary

pub fn print_help(&mut self) -> Result<()>

Expected Behavior Summary

pub fn print_help(&self) -> Result<()>

[pksunkara]

For better performance. There should be an issue like this already.

[djeedai]

I couldn't find any other issue sorry. I've searched for print_help but nothing seems to match.

I understand that how the implementation is lazily doing some ops may be a performance reason, but in my opinion that doesn't warrant making the public interface mutable. Isn't that precisely what interior mutability is for? I can't find the official reference I took that from, but here's another one : https://ricardomartins.cc/2016/06/08/interior-mutability

There are a few general cases that call for interior mutability, such as:
3. Implementation details of logically immutable methods

[epage]

Closing this in favor of #2911 which has more details on the plan going forward.