[v3,09/13] rust: init: add support for arbitrary paths in init macros

  Previously only `ident` and generic types were supported in the
`{try_}{pin_}init!` macros. This patch allows arbitrary path fragments,
so for example `Foo::Bar` but also very complex paths such as
`<Foo as Baz>::Bar::<0, i32>`.

Internally this is accomplished by using `path` fragments. Due to some
peculiar declarative macro limitations, we have to "forget" certain
additional parsing information in the token trees. This is achieved by
using the `paste!` proc macro. It does not actually modify the input,
since no `[< >]` will be present in the input, so it just strips the
information held by declarative macros. For example, if a declarative
macro takes `$t:path` as its input, it cannot sensibly propagate this to
a macro that takes `$($p:tt)*` as its input, since the `$t` token will
only be considered one `tt` token for the second macro. If we first pipe
the tokens through `paste!`, then it parses as expected.

Suggested-by: Asahi Lina <lina@asahilina.net>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
---
v2 -> v3:
- added Reviewed-by's from Martin and Alice.

v1 -> v2:
- use Gary's `paste!` macro instead of `retokenize`,
- remove the retokenize macro.

 rust/kernel/init/macros.rs | 54 ++++++++++++++++++++++++--------------
 1 file changed, 35 insertions(+), 19 deletions(-)
  

On Sat, 29 Jul 2023 09:10:16 +0000
Benno Lossin <benno.lossin@proton.me> wrote:

> Previously only `ident` and generic types were supported in the
> `{try_}{pin_}init!` macros. This patch allows arbitrary path fragments,
> so for example `Foo::Bar` but also very complex paths such as
> `<Foo as Baz>::Bar::<0, i32>`.
> 
> Internally this is accomplished by using `path` fragments. Due to some
> peculiar declarative macro limitations, we have to "forget" certain
> additional parsing information in the token trees. This is achieved by
> using the `paste!` proc macro. It does not actually modify the input,
> since no `[< >]` will be present in the input, so it just strips the
> information held by declarative macros. For example, if a declarative
> macro takes `$t:path` as its input, it cannot sensibly propagate this to
> a macro that takes `$($p:tt)*` as its input, since the `$t` token will
> only be considered one `tt` token for the second macro. If we first pipe
> the tokens through `paste!`, then it parses as expected.
> 
> Suggested-by: Asahi Lina <lina@asahilina.net>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
> v2 -> v3:
> - added Reviewed-by's from Martin and Alice.
> 
> v1 -> v2:
> - use Gary's `paste!` macro instead of `retokenize`,
> - remove the retokenize macro.
> 
>  rust/kernel/init/macros.rs | 54 ++++++++++++++++++++++++--------------
>  1 file changed, 35 insertions(+), 19 deletions(-)
> 
> diff --git a/rust/kernel/init/macros.rs b/rust/kernel/init/macros.rs
> index 4c86281301d8..d54243cd3c82 100644
> --- a/rust/kernel/init/macros.rs
> +++ b/rust/kernel/init/macros.rs
> @@ -1000,7 +1000,7 @@ impl<$($impl_generics)*> $pin_data<$($ty_generics)*>
>  macro_rules! __init_internal {
>      (
>          @this($($this:ident)?),
> -        @typ($t:ident $(::<$($generics:ty),*>)?),
> +        @typ($t:path),
>          @fields($($fields:tt)*),
>          @error($err:ty),
>          // Either `PinData` or `InitData`, `$use_data` should only be present in the `PinData`
> @@ -1014,7 +1014,7 @@ macro_rules! __init_internal {
>      ) => {
>          $crate::__init_internal!(with_update_parsed:
>              @this($($this)?),
> -            @typ($t $(::<$($generics),*>)? ),
> +            @typ($t),
>              @fields($($fields)*),
>              @error($err),
>              @data($data, $($use_data)?),
> @@ -1025,7 +1025,7 @@ macro_rules! __init_internal {
>      };
>      (
>          @this($($this:ident)?),
> -        @typ($t:ident $(::<$($generics:ty),*>)?),
> +        @typ($t:path),
>          @fields($($fields:tt)*),
>          @error($err:ty),
>          // Either `PinData` or `InitData`, `$use_data` should only be present in the `PinData`
> @@ -1039,7 +1039,7 @@ macro_rules! __init_internal {
>      ) => {
>          $crate::__init_internal!(with_update_parsed:
>              @this($($this)?),
> -            @typ($t $(::<$($generics),*>)? ),
> +            @typ($t),
>              @fields($($fields)*),
>              @error($err),
>              @data($data, $($use_data)?),
> @@ -1050,7 +1050,7 @@ macro_rules! __init_internal {
>      };
>      (
>          @this($($this:ident)?),
> -        @typ($t:ident $(::<$($generics:ty),*>)?),
> +        @typ($t:path),
>          @fields($($fields:tt)*),
>          @error($err:ty),
>          // Either `PinData` or `InitData`, `$use_data` should only be present in the `PinData`
> @@ -1064,7 +1064,7 @@ macro_rules! __init_internal {
>      ) => {
>          $crate::__init_internal!(
>              @this($($this)?),
> -            @typ($t $(::<$($generics),*>)? ),
> +            @typ($t),
>              @fields($($fields)*),
>              @error($err),
>              @data($data, $($use_data)?),
> @@ -1075,7 +1075,7 @@ macro_rules! __init_internal {
>      };
>      (with_update_parsed:
>          @this($($this:ident)?),
> -        @typ($t:ident $(::<$($generics:ty),*>)?),
> +        @typ($t:path),
>          @fields($($fields:tt)*),
>          @error($err:ty),
>          // Either `PinData` or `InitData`, `$use_data` should only be present in the `PinData`
> @@ -1094,7 +1094,11 @@ macro_rules! __init_internal {
>          // Get the data about fields from the supplied type.
>          let data = unsafe {
>              use $crate::init::__internal::$has_data;
> -            $t$(::<$($generics),*>)?::$get_data()
> +            // Here we abuse `paste!` to retokenize `$t`. Declarative macros have some internal
> +            // information that is associated to already parsed fragments, so a path fragment
> +            // cannot be used in this position. Doing the retokenization results in valid rust
> +            // code.
> +            ::kernel::macros::paste!($t::$get_data())
>          };
>          // Ensure that `data` really is of type `$data` and help with type inference:
>          let init = $crate::init::__internal::$data::make_closure::<_, __InitOk, $err>(
> @@ -1253,7 +1257,7 @@ fn assert_zeroable<T: $crate::init::Zeroable>(_: *mut T) {}
>      };
>      (make_initializer:
>          @slot($slot:ident),
> -        @type_name($t:ident),
> +        @type_name($t:path),
>          @munch_fields(..Zeroable::zeroed() $(,)?),
>          @acc($($acc:tt)*),
>      ) => {
> @@ -1270,15 +1274,21 @@ fn assert_zeroable<T: $crate::init::Zeroable>(_: *mut T) {}
>              // not get executed, so it has no effect.
>              ::core::ptr::write($slot, zeroed);
>              zeroed = ::core::mem::zeroed();
> -            ::core::ptr::write($slot, $t {
> -                $($acc)*
> -                ..zeroed
> -            });
> +            // Here we abuse `paste!` to retokenize `$t`. Declarative macros have some internal
> +            // information that is associated to already parsed fragments, so a path fragment
> +            // cannot be used in this position. Doing the retokenization results in valid rust
> +            // code.
> +            ::kernel::macros::paste!(
> +                ::core::ptr::write($slot, $t {
> +                    $($acc)*
> +                    ..zeroed
> +                });
> +            );
>          }
>      };
>      (make_initializer:
>          @slot($slot:ident),
> -        @type_name($t:ident),
> +        @type_name($t:path),
>          @munch_fields($(,)?),
>          @acc($($acc:tt)*),
>      ) => {
> @@ -1286,14 +1296,20 @@ fn assert_zeroable<T: $crate::init::Zeroable>(_: *mut T) {}
>          // Since we are in the closure that is never called, this will never get executed.
>          // We abuse `slot` to get the correct type inference here:
>          unsafe {
> -            ::core::ptr::write($slot, $t {
> -                $($acc)*
> -            });
> +            // Here we abuse `paste!` to retokenize `$t`. Declarative macros have some internal
> +            // information that is associated to already parsed fragments, so a path fragment
> +            // cannot be used in this position. Doing the retokenization results in valid rust
> +            // code.
> +            ::kernel::macros::paste!(
> +                ::core::ptr::write($slot, $t {
> +                    $($acc)*
> +                });
> +            );
>          }
>      };
>      (make_initializer:
>          @slot($slot:ident),
> -        @type_name($t:ident),
> +        @type_name($t:path),
>          @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
>          @acc($($acc:tt)*),
>      ) => {
> @@ -1306,7 +1322,7 @@ fn assert_zeroable<T: $crate::init::Zeroable>(_: *mut T) {}
>      };
>      (make_initializer:
>          @slot($slot:ident),
> -        @type_name($t:ident),
> +        @type_name($t:path),
>          @munch_fields($field:ident $(: $val:expr)?, $($rest:tt)*),
>          @acc($($acc:tt)*),
>      ) => {