code style: guideline for const parameters

[justinmk]

Clarify project guideline on use of const for non-pointer parameters.

ref: neovim/neovim#8684 (comment)
ref: neovim/neovim#8142 (comment)

[justinmk]

cc @bfredl @janlazo @aktau @ZyX-I

[bfredl]

I agree, non-target const just trivially repeats local information, unlike const target which actually changes the invariants between the caller and the callee.

[bfredl]

Perhaps include a bad example of const pointer to non-const target as well (int *const ptr)

[janlazo]

What's the alternative? Create a new local variable at the top of the function to avoid modifying the parameter even if Vim source doesn't have this variable? If it's about the API (ie. src/nvim/api/) and its doxygen documentation, then it's preferable to avoid changes to the function signature.

Is the issue on parameter style on function declaration? I don't like the second style; either the function declaration fits in one line or each parameter gets its own line, making it easier to add/remove const.

int foo(const int a, const int b,
        const int c)

[justinmk]

The link is https://neovim.io/develop/style-guide.xml#Use_of_const .

If it's about the API (ie. src/nvim/api/) and its doxygen documentation, then it's preferable to avoid changes to the function signature.

The main goals of this recommendation are to avoid verbosity/clutter where it isn't serving a strong purpose. aktau's comment in neovim/neovim#8142 (comment) may help.