rust: support formatting of foreign types

Introduce a `fmt!` macro which wraps all arguments in
`kernel::fmt::Adapter` and a `kernel::fmt::Display` trait. This enables
formatting of foreign types (like `core::ffi::CStr`) that do not
implement `core::fmt::Display` due to concerns around lossy conversions
which do not apply in the kernel.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/Custom.20formatting/with/516476467
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://patch.msgid.link/20251018-cstr-core-v18-15-9378a54385f8@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

3 files changed, 120 insertions, 0 deletions

diff --git a/rust/macros/fmt.rs b/rust/macros/fmt.rs
new file mode 100644
index 000000000000..2f4b9f6e2211
--- /dev/null
+++ b/rust/macros/fmt.rs
@@ -0,0 +1,94 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use proc_macro::{Ident, TokenStream, TokenTree};
+use std::collections::BTreeSet;
+
+/// 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_span = {
+        let Some((mut first_str, first_span)) = (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.span()))
+                })
+            }
+            _ => None,
+        }) else {
+            return first_opt.into_iter().chain(input).collect();
+        };
+
+        // Parse `identifier`s from the format string.
+        //
+        // See https://doc.rust-lang.org/std/fmt/index.html#syntax.
+        while let Some((_, rest)) = first_str.split_once('{') {
+            first_str = rest;
+            if let Some(rest) = first_str.strip_prefix('{') {
+                first_str = rest;
+                continue;
+            }
+            if let Some((name, rest)) = first_str.split_once('}') {
+                first_str = rest;
+                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);
+                }
+            }
+        }
+        first_span
+    };
+
+    let adapter = quote_spanned!(first_span => ::kernel::fmt::Adapter);
+
+    let mut args = TokenStream::from_iter(first_opt);
+    {
+        let mut flush = |args: &mut TokenStream, current: &mut TokenStream| {
+            let current = std::mem::take(current);
+            if !current.is_empty() {
+                let (lhs, rhs) = (|| {
+                    let mut current = current.into_iter();
+                    let mut acc = TokenStream::new();
+                    while let Some(tt) = current.next() {
+                        // Split on `=` only once to handle cases like `a = b = c`.
+                        if matches!(&tt, TokenTree::Punct(p) if p.as_char() == '=') {
+                            names.remove(acc.to_string().as_str());
+                            // Include the `=` itself to keep the handling below uniform.
+                            acc.extend([tt]);
+                            return (Some(acc), current.collect::<TokenStream>());
+                        }
+                        acc.extend([tt]);
+                    }
+                    (None, acc)
+                })();
+                args.extend(quote_spanned!(first_span => #lhs #adapter(&#rhs)));
+            }
+        };
+
+        let mut current = TokenStream::new();
+        for tt in input {
+            match &tt {
+                TokenTree::Punct(p) if p.as_char() == ',' => {
+                    flush(&mut args, &mut current);
+                    &mut args
+                }
+                _ => &mut current,
+            }
+            .extend([tt]);
+        }
+        flush(&mut args, &mut current);
+    }
+
+    for name in names {
+        let name = Ident::new(name, first_span);
+        args.extend(quote_spanned!(first_span => , #name = #adapter(&#name)));
+    }
+
+    quote_spanned!(first_span => ::core::format_args!(#args))
+}
diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
index fa847cf3a9b5..793f712dbf7c 100644
--- a/rust/macros/lib.rs
+++ b/rust/macros/lib.rs
@@ -15,6 +15,7 @@
 mod quote;
 mod concat_idents;
 mod export;
+mod fmt;
 mod helpers;
 mod kunit;
 mod module;
@@ -201,6 +202,24 @@ pub fn export(attr: TokenStream, ts: TokenStream) -> TokenStream {
     export::export(attr, ts)
 }
 
+/// Like [`core::format_args!`], but automatically wraps arguments in [`kernel::fmt::Adapter`].
+///
+/// This macro allows generating `fmt::Arguments` while ensuring that each argument is wrapped with
+/// `::kernel::fmt::Adapter`, which customizes formatting behavior for kernel logging.
+///
+/// Named arguments used in the format string (e.g. `{foo}`) are detected and resolved from local
+/// bindings. All positional and named arguments are automatically wrapped.
+///
+/// This macro is an implementation detail of other kernel logging macros like [`pr_info!`] and
+/// should not typically be used directly.
+///
+/// [`kernel::fmt::Adapter`]: ../kernel/fmt/struct.Adapter.html
+/// [`pr_info!`]: ../kernel/macro.pr_info.html
+#[proc_macro]
+pub fn fmt(input: TokenStream) -> TokenStream {
+    fmt::fmt(input)
+}
+
 /// Concatenate two identifiers.
 ///
 /// This is useful in macros that need to declare or reference items with names
diff --git a/rust/macros/quote.rs b/rust/macros/quote.rs
index acc140c18653..ddfc21577539 100644
--- a/rust/macros/quote.rs
+++ b/rust/macros/quote.rs
@@ -48,6 +48,7 @@ macro_rules! quote_spanned {
     ($span:expr => $($tt:tt)*) => {{
         let mut tokens = ::proc_macro::TokenStream::new();
         {
+            #[allow(unused_variables)]
             let span = $span;
             quote_spanned!(@proc tokens span $($tt)*);
         }
@@ -146,6 +147,12 @@ macro_rules! quote_spanned {
         )]);
         quote_spanned!(@proc $v $span $($tt)*);
     };
+    (@proc $v:ident $span:ident & $($tt:tt)*) => {
+        $v.extend([::proc_macro::TokenTree::Punct(
+            ::proc_macro::Punct::new('&', ::proc_macro::Spacing::Alone),
+        )]);
+        quote_spanned!(@proc $v $span $($tt)*);
+    };
     (@proc $v:ident $span:ident _ $($tt:tt)*) => {
         $v.extend([::proc_macro::TokenTree::Ident(
             ::proc_macro::Ident::new("_", $span),