Re: [PATCH v10 2/5] rust: support formatting of foreign types

["Benno Lossin" <lossin@kernel.org>]

On Tue May 27, 2025 at 5:02 PM CEST, Tamir Duberstein wrote:
> On Mon, May 26, 2025 at 7:01 PM Benno Lossin <lossin@kernel.org> wrote:
>> On Tue May 27, 2025 at 12:17 AM CEST, Tamir Duberstein wrote:
>> > On Mon, May 26, 2025 at 10:48 AM Benno Lossin <lossin@kernel.org> wrote:
>> >> On Sat May 24, 2025 at 10:33 PM CEST, Tamir Duberstein wrote:
>> >> > +impl_display_forward!(
>> >> > +    bool,
>> >> > +    char,
>> >> > +    core::panic::PanicInfo<'_>,
>> >> > +    crate::str::BStr,
>> >> > +    fmt::Arguments<'_>,
>> >> > +    i128,
>> >> > +    i16,
>> >> > +    i32,
>> >> > +    i64,
>> >> > +    i8,
>> >> > +    isize,
>> >> > +    str,
>> >> > +    u128,
>> >> > +    u16,
>> >> > +    u32,
>> >> > +    u64,
>> >> > +    u8,
>> >> > +    usize,
>> >> > +    {<T: ?Sized>} crate::sync::Arc<T> {where crate::sync::Arc<T>: fmt::Display},
>> >> > +    {<T: ?Sized>} crate::sync::UniqueArc<T> {where crate::sync::UniqueArc<T>: fmt::Display},
>> >> > +);
>> >>
>> >> If we use `{}` instead of `()`, then we can format the contents
>> >> differently:
>> >>
>> >>     impl_display_forward! {
>> >>         i8, i16, i32, i64, i128, isize,
>> >>         u8, u16, u32, u64, u128, usize,
>> >>         bool, char, str,
>> >>         crate::str::BStr,
>> >>         fmt::Arguments<'_>,
>> >>         core::panic::PanicInfo<'_>,
>> >>         {<T: ?Sized>} crate::sync::Arc<T> {where Self: fmt::Display},
>> >>         {<T: ?Sized>} crate::sync::UniqueArc<T> {where Self: fmt::Display},
>> >>     }
>> >
>> > Is that formatting better? rustfmt refuses to touch it either way.
>>
>> Yeah rustfmt doesn't touch macro parameters enclosed in `{}`. I think
>> it's better.
>
> OK, but why? This seems entirely subjective.

If more types are added to the list, it will grow over one screen size.
With my formatting, leaving related types on a single line, that will
only happen much later.

>> >> > +/// Please see [`crate::fmt`] for documentation.
>> >> > +pub(crate) fn fmt(input: TokenStream) -> TokenStream {
>> >> > +    let mut input = input.into_iter();
>> >> > +
>> >> > +    let first_opt = input.next();
>> >> > +    let first_owned_str;
>> >> > +    let mut names = BTreeSet::new();
>> >> > +    let first_lit = {
>> >> > +        let Some((mut first_str, first_lit)) = (match first_opt.as_ref() {
>> >> > +            Some(TokenTree::Literal(first_lit)) => {
>> >> > +                first_owned_str = first_lit.to_string();
>> >> > +                Some(first_owned_str.as_str()).and_then(|first| {
>> >> > +                    let first = first.strip_prefix('"')?;
>> >> > +                    let first = first.strip_suffix('"')?;
>> >> > +                    Some((first, first_lit))
>> >> > +                })
>> >> > +            }
>> >> > +            _ => None,
>> >> > +        }) else {
>> >> > +            return first_opt.into_iter().chain(input).collect();
>> >> > +        };
>> >>
>> >> This usage of let-else + match is pretty confusing and could just be a
>> >> single match statement.
>> >
>> > I don't think so. Can you try rewriting it into the form you like?
>>
>>     let (mut first_str, first_lit) match first_opt.as_ref() {
>>         Some(TokenTree::Literal(lit)) if lit.to_string().starts_with('"') => {
>>             let contents = lit.to_string();
>>             let contents = contents.strip_prefix('"').unwrap().strip_suffix('"').unwrap();
>>             ((contents, lit))
>>         }
>>         _ => return first_opt.into_iter().chain(input).collect(),
>>     };
>
> What happens if the invocation is utterly malformed, e.g.
> `fmt!("hello)`? You're unwrapping here, which I intentionally avoid.

That example won't even survive lexing (macros always will get valid
rust tokens as input). If a literal begins with a `"`, it also will end
with one AFAIK.

>> Yes it will error like that, but if we do the replacement only when the
>> syntax is correct, there also will be compile errors because of a
>> missing `Display` impl, or is that not the case?
>
> I'm not sure - I would guess syntax errors "mask" typeck errors.

I checked and it seems to be so, that's good.

>> >> > +                    first_str = rest;
>> >> > +                    continue;
>> >> > +                }
>> >> > +                let name = name.split_once(':').map_or(name, |(name, _)| name);
>> >> > +                if !name.is_empty() && !name.chars().all(|c| c.is_ascii_digit()) {
>> >> > +                    names.insert(name);
>> >> > +                }
>> >> > +                break;
>> >> > +            }
>> >> > +        }
>> >> > +        first_lit
>> >>
>> >> `first_lit` is not modified, so could we just the code above it into a
>> >> block instead of keeping it in the expr for `first_lit`?
>> >
>> > As above, can you suggest the alternate form you like better? The
>> > gymnastics here are all in service of being able to let malformed
>> > input fall through to core::format_args which will do the hard work of
>> > producing good diagnostics.
>>
>> I don't see how this is hard, just do:
>>
>>     let (first_str, first_lit) = ...;
>
> It requires you to unwrap, like you did above, which is what I'm
> trying to avoid.

How so? What do you need to unwrap?

>> >> > +    };
>> >> > +
>> >> > +    let first_span = first_lit.span();
>> >> > +    let adapt = |expr| {
>> >> > +        let mut borrow =
>> >> > +            TokenStream::from_iter([TokenTree::Punct(Punct::new('&', Spacing::Alone))]);
>> >> > +        borrow.extend(expr);
>> >> > +        make_ident(first_span, ["kernel", "fmt", "Adapter"])
>> >> > +            .chain([TokenTree::Group(Group::new(Delimiter::Parenthesis, borrow))])
>> >>
>> >> This should be fine with using `quote!`:
>> >>
>> >>     quote!(::kernel::fmt::Adapter(&#expr))
>> >
>> > Yeah, I have a local commit that uses quote_spanned to remove all the
>> > manual constructions.
>>
>> I don't think that you need `quote_spanned` here at all. If you do, then
>> let me know, something weird with spans is going on then.
>
> You need to give idents a span, so each of `kernel`, `fmt`, and
> `adapter` need a span. I *could* use `quote!` and get whatever span it
> uses (mixed_site) but I'd rather retain control.

Please use `quote!` if it works. No need to make this more complex than
it already is. If it doesn't work then that's another story.

---
Cheers,
Benno