Fix unsized fields usage in derive(Display) (#432)

[tyranron]

Resolves #432

Synopsis

See #432 (comment):

use derive_more::Display;

#[derive(Display, Debug)]
#[display("{head}.{tail}")]
struct Struct {
    head: char,
    tail: str,
}

fn main() {
    let dat = [51i32, 3028017];
    let s = unsafe { &*(&raw const dat as *const [i32] as *const Struct) };
    println!("{:?}", s);
    println!("{}", s);
}

The derived Debug impl prints Struct { head: '3', tail: "14" } but the Display impl does not compile.

error[E0277]: the size for values of type `str` cannot be known at compilation time
   --> src/main.rs:3:10
    |
3   | #[derive(Display, Debug)]
    |          ^^^^^^^ doesn't have a size known at compile-time
4   | #[display("{head}.{tail}")]
    |                   ------ required by a bound introduced by this call
    |
    = help: the trait `Sized` is not implemented for `str`
note: required by an implicit `Sized` bound in `derive_more::core::fmt::rt::Argument::<'_>::new_display`
   --> $RUSTUP_HOME/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/fmt/rt.rs:113:24
    |
113 |     pub fn new_display<T: Display>(x: &T) -> Argument<'_> {
    |                        ^ required by the implicit `Sized` requirement on this type parameter in `Argument::<'_>::new_display`
    = note: this error originates in the macro `$crate::format_args` which comes from the expansion of the derive macro `Display` (in Nightly builds, run with -Z macro-backtrace for more info)

The problem is introduced by #381, and its cause is that the FmtAttribute::additional_deref_args() implementation is too aggressive and is enabled for any fmt trait, not just fmt::Pointer.

#[allow(unreachable_code)]
#[automatically_derived]
impl derive_more::core::fmt::Display for Struct2 {
    fn fmt(&self, __derive_more_f: &mut derive_more::core::fmt::Formatter<'_>) -> derive_more::core::fmt::Result {
        let head = &self.head;
        let tail = &self.tail;
        derive_more::core::write!(__derive_more_f, "{head}.{tail}", head = *head, tail = *tail)
    }
}

Solution

Apply FmtAttribute::additional_deref_args() only for fmt::Pointer placeholders.

Checklist

• Documentation is updated (not required)
• Tests are added/updated
• CHANGELOG entry is added

[tyranron]

@JelteF after merging this, we can finally cut the 2.0.0 release! 🎉