clippy: fix ptr_as_ptr lint

[xtqqczze]

Follow-up to #730.

https://rust-lang.github.io/rust-clippy/master/index.html#ptr_as_ptr

[lhecker]

I'm not super sure yet if I want to accept fixes for pedantic warnings (this one is quite larger after all). Is there a benefit to doing so?

[xtqqczze]

The benefit of this lint is that pointer::cast cannot accidentally change the pointer’s mutability, which forces you to be explicit when casting mutability.

I also think .cast() is a bit more readable from a style perspective since it follows the usual postfix method-call pattern, making chains of pointer operations easier to read. It also avoids some extra parentheses that are often needed with as casts.

[xtqqczze]

@lhecker e.g. this is more readable

[lhecker]

"from_ref cast mut cast" is not as readable to me as "as text buffer as whatever", personally. It'd be great to hear the opinion of others but I doubt anyone just randomly shows up haha. (I mean people that aren't too opinionated on Rust-correctness, otherwise that'd be unfair.)

[xtqqczze]

Being explicit when casting mutability makes the intent clearer and, in my opinion, improves readability.

In the case of .cast_mut(), this looks like a potentially suspicious operation, so I looked into this instance further. It seems the context field may be incorrectly typed, as the corresponding C definition uses const void *, while the Rust definition uses *mut c_void:

edit/crates/edit/src/icu.rs

Line 1266 in 3e20615

pub context: *mut c_void,

https://github.com/unicode-org/icu/blob/bbb41e0bc320e2d5515d337c0cd9e9703a1942ee/icu4c/source/common/unicode/utext.h#L1443

[xtqqczze]

• fix icu_ffi::UText definition #908