[PATCH v2 15/19] rust: zerocopy-derive: avoid generating non-ASCII identifiers

[Miguel Ojeda <ojeda@kernel.org>]

Linux is built with `-Dnon_ascii_idents`. However, `zerocopy-derive`
uses a non-ASCII character (`ẕ`) internally, which in turn triggers
the lint when attempting to use derives like `FromBytes`:

    error: identifier contains non-ASCII characters
       --> rust/kernel/lib.rs:153:9
        |
    153 |         a: u32,
        |         ^
        |
        = note: requested on the command line with `-D non-ascii-idents`

This was already noticed by another project using
`#![deny(non_ascii_idents)]` [1]. `zerocopy` added an
`#[allow(non_ascii_idents)]` [2], but it does not work since, at the
moment, the `non_ascii_idents` lint is a `crate_level_only` one, and thus
`allow`s only work at the crate root level.

Due to this, an issue about relaxing this restriction was created in
upstream Rust [3] some months ago.

Thus work around it here by using another prefix. The likelihood of a
collision is very small for us, since we control the callers, and this
will hopefully be fixed soon at either the `zerocopy` or the Rust level.

I filed an issue [4] about it with upstream `zerocopy` as requested
and we discussed this with upstream Rust and `zerocopy`: the Rust issue
got nominated and a PR [5] to relax the restriction was submitted by
Joshua. Upstream `zerocopy` prefers that approach, so if Rust merges it,
then it means we will be able to remove the workaround when we bump the
MSRV, thus likely late 2027, since we follow Debian Stable.

Cc: Joshua Liebow-Feeser <joshlf@google.com>
Cc: Jack Wrenn <jswrenn@google.com>
Link: https://github.com/google/zerocopy/issues/2880 [1]
Link: https://github.com/google/zerocopy/pull/2882 [2]
Link: https://github.com/rust-lang/rust/issues/151025 [3]
Link: https://github.com/google/zerocopy/issues/3427 [4]
Link: https://github.com/rust-lang/rust/pull/157497 [5]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/zerocopy-derive/derive/try_from_bytes.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/rust/zerocopy-derive/derive/try_from_bytes.rs b/rust/zerocopy-derive/derive/try_from_bytes.rs
index ce9c926d5b8e..a3e4a75631a5 100644
--- a/rust/zerocopy-derive/derive/try_from_bytes.rs
+++ b/rust/zerocopy-derive/derive/try_from_bytes.rs
@@ -457,7 +457,7 @@ fn derive_has_field_struct_union(ctx: &Ctx, data: &dyn DataExt) -> TokenStream {
     }
 
     let field_tokens = fields.iter().map(|(vis, ident, _)| {
-        let ident = ident!(("ẕ{}", ident), ident.span());
+        let ident = ident!(("__z{}", ident), ident.span());
         quote!(
             #vis enum #ident {}
         )
@@ -487,7 +487,7 @@ fn derive_has_field_struct_union(ctx: &Ctx, data: &dyn DataExt) -> TokenStream {
         })
         .build();
     let has_fields = fields.iter().map(move |(_, ident, ty)| {
-        let field_token = ident!(("ẕ{}", ident), ident.span());
+        let field_token = ident!(("__z{}", ident), ident.span());
         let field: Box<Type> = parse_quote!(#field_token);
         let field_id: Box<Expr> = parse_quote!({ #zerocopy_crate::ident_id!(#ident) });
         let has_field_trait = Trait::HasField {
-- 
2.54.0