Create type-stable `@generated` version of `_show_default` for `--trim`

[topolarity]

I was kind of hoping we'd define a decent --trim fallback for show_default, e.g. w/ @generated to iterate struct fields type-stably.

Originally posted by @topolarity in #59598 (review)

show_default is a "default implementation" in Base - it's an automatic implementation of printing based purely on structural / type information.

The current version in Base._show_default iterates the fields dynamically (simplified):

for i in 1:nf
    show(recur_io, getfield(x, i))
    # ...
end

which is inherently type-unstable (and is additionally marked @nospecialize, so getfield(x, i) is inferred ::Any for any argument anywhere). This is fine for typical Julia execution, where printing is not especially performance-sensitive, and instead it's beneficial to avoid generating specialized code for all structs (hence the @nospecialize)

For --trim compatibility, we need an @generated equivalent that behaves similarly to #[derive(Display)] in Rust and inspects the type structurally and generates a type-stable implementation of show_fallback for the type.

[0x-avi]

hey can i work on this? from what i gather #[derive(Display)] is exactly an example of what we need and i need to read the underlying implementation of @generated so i can figure out how to make a similiar function to #[derive(Display)]?

[nsajko]

Just do it, don't have to ask.

[nsajko]

But no, it is not necessary to read the implementation of @generated, just use it in the implementation of the solution.