Message-ID: <CAJ-ks9=OsopMhr6Ui3PLD-ZkBo736ha9Ltkw=0ZaBzrQLC60Eg@mail.gmail.com>
Date: Thu, 29 May 2025 18:07:52 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Benno Lossin <lossin@...nel.org>
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Subject: Re: [PATCH v10 2/5] rust: support formatting of foreign types

On Tue, May 27, 2025 at 4:49 PM Benno Lossin <lossin@...nel.org> wrote:
>
> 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@...nel.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@...nel.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?

I was referring to your unwraps above.

> >> >> > +    };
> >> >> > +
> >> >> > +    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.

Let's adjudicate that on v11, where you can see the code.

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