Re: [PATCH v3 01/16] rust: io: register: reimplement as proc macro

["Gary Guo" <gary@garyguo.net>]

On Thu Aug 27, 2026 at 7:16 AM BST, Alexandre Courbot wrote:
> On Wed Aug 19, 2026 at 8:09 PM JST, Gary Guo wrote:
>> The existing `register!` macro is implemented as a declarative macro.
>> Reimplement it as proc macro instead, with no functional changes intended.
>>
>> The old implementation produces unhelpful diagnostics when things go wrong.
>> For example, for code like
>>
>>     register! {
>>         pub(crate) TESTREG(u32) {
>>             31:0    data;
>>         }
>>     }
>>
>> which misses out the "@ offset" part of the specification, and the
>> following error is produced:
>>
>> error: no rules expected `{`
>>    --> test.rs:42:5
>>     |
>>  42 | /     register! {
>>  43 | |         pub(crate) TESTREG(u32) {
>>  44 | |             31:0    data;
>> ...   |
>> 100 | |     }
>>     | |_____^ no rules expected this token in macro call
>>
>> which isn't very helpful. With the proc macro implementation, the following
>> error is produced:
>>
>> error: expected `@` or `=>`
>>   --> tests.rs:43:33
>>    |
>> 43 |         pub(crate) TESTREG(u32) {
>>    |                                 ^
>>
>> which is much more helpful. Apart from diagnostics, proc macro also has a
>> benefit of not having follow-set restrictions, which makes syntax like
>>
>>     register!(name: ty @ offset);
>>
>> possible; declarative macro will reject this as `@` is not in the
>> follow-set of "ty" metavariable kind.
>>
>> Signed-off-by: Gary Guo <gary@garyguo.net>
>
> Well that patch was a great way for me to finally learn how to write
> proc macros! :)
>
> The parser and generator are both simple enough, but the new macro
> module would read a bit better with some light high-level doc (i.e.
> module-level and most important types). But even for a beginner like me
> this is almost self-explanatory, so no need to go too deep.
>
> <...>
>> diff --git a/rust/macros/io/mod.rs b/rust/macros/io/mod.rs
>> new file mode 100644
>> index 000000000000..39fa5bc302ba
>> --- /dev/null
>> +++ b/rust/macros/io/mod.rs
>> @@ -0,0 +1,3 @@
>> +// SPDX-License-Identifier: Apache-2.0 OR MIT
>
> Other files use GPL-2.0 as license, is this on purpose?
>
> <...>
>> +pub(crate) fn register(def: RegDef) -> Result<TokenStream> {
>> +    let mut outputs = TokenStream::new();
>> +
>> +    for reg in def.regs {
>> +        let Reg {
>> +            attrs,
>> +            vis,
>> +            name,
>> +            storage,
>> +            array,
>> +            relative_base,
>> +            offset,
>> +            bitfield_args,
>> +        } = reg;
>> +
>> +        // Use register name's span for generated code, so error messages (if any) can point to it
>> +        // instead of the entire register allocation.
>> +        let span = name.span().resolved_at(Span::mixed_site());
>> +
>> +        let offset = match offset {
>> +            RegOffset::Fixed { offset } => quote!(#offset),
>> +            RegOffset::Alias { alias } => {
>> +                quote_spanned!(alias.span().resolved_at(span) =>
>> +                    <#alias as ::kernel::io::register::Register>::OFFSET
>> +                )
>> +            }
>> +            RegOffset::ElementAlias { alias, idx } => {
>> +                outputs.extend(quote_spanned!(idx.span().resolved_at(span) =>
>> +                    ::kernel::build_assert::static_assert!(
>> +                        #idx < <#alias as ::kernel::io::register::RegisterArray>::SIZE
>> +                    );
>> +                ));
>> +                quote_spanned!(alias.span().resolved_at(span) =>
>> +                    <#alias as ::kernel::io::register::Register>::OFFSET
>> +                        + #idx * <#alias as ::kernel::io::register::RegisterArray>::STRIDE
>
> Sashiko raised this but `#idx` should be within parentheses (this code
> is replaced later down the series but let's make it correct
> nonetheless).

Ack

>
> <...>
>> diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
>> index 24f96feaeb34..5807dee84747 100644
>> --- a/rust/macros/lib.rs
>> +++ b/rust/macros/lib.rs
>> @@ -19,6 +19,7 @@
>>  mod fmt;
>>  mod for_lt;
>>  mod helpers;
>> +mod io;
>>  mod kunit;
>>  mod module;
>>  mod paste;
>> @@ -481,6 +482,15 @@ pub fn paste(input: TokenStream) -> TokenStream {
>>          .into()
>>  }
>>  
>> +#[doc(hidden)] // Documented in `kernel` crate.
>> +#[proc_macro]
>> +#[allow(non_snake_case)]
>
> Is `non_snake_case` needed here?

You probably already guessed it but both this and the Apache license is from
copy-pasta :) Will fix in next version.

Best,
Gary