[PATCH] rust: pin-init: add code blocks to `[try_][pin_]init!` macros

[Benno Lossin <lossin@kernel.org>]

Allow writing `_: { /* any number of statements */ }` in initializers to
run arbitrary code during initialization.

    try_init!(MyStruct {
        _: {
            if check_something() {
                return Err(MyError);
            }
        },
        foo: Foo::new(val),
        _: {
            println!("successfully initialized `MyStruct`");
        },
    })

Link: https://github.com/Rust-for-Linux/pin-init/pull/84/commits/2880a9b898336e2d54f80715f00ce00f21f74d2f
Signed-off-by: Benno Lossin <lossin@kernel.org>
---
I originally wanted to do some more modifications to the syntax of
initializer macros, but I didn't have the time this cycle. See

    https://github.com/Rust-for-Linux/pin-init/pull/69

For the development of that syntax, it's probably going to be like a
closure where in order to support formatting via rustfmt. But it still
needs some disucssion around removing `<-`.

That change would allow having a `let` that binds a value for use in the
initializer. IIRC @Alice, you needed that for something.
---
 rust/pin-init/src/lib.rs    |  2 ++
 rust/pin-init/src/macros.rs | 29 +++++++++++++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
index 62e013a5cc20..8b556b0e106a 100644
--- a/rust/pin-init/src/lib.rs
+++ b/rust/pin-init/src/lib.rs
@@ -740,6 +740,8 @@ macro_rules! stack_try_pin_init {
 /// As already mentioned in the examples above, inside of `pin_init!` a `struct` initializer with
 /// the following modifications is expected:
 /// - Fields that you want to initialize in-place have to use `<-` instead of `:`.
+/// - You can use `_: { /* run any user-code here */ },` anywhere where you can place fields in
+///   order to run arbitrary code.
 /// - In front of the initializer you can write `&this in` to have access to a [`NonNull<Self>`]
 ///   pointer named `this` inside of the initializer.
 /// - Using struct update syntax one can place `..Zeroable::init_zeroed()` at the very end of the
diff --git a/rust/pin-init/src/macros.rs b/rust/pin-init/src/macros.rs
index 9ced630737b8..752e77db998c 100644
--- a/rust/pin-init/src/macros.rs
+++ b/rust/pin-init/src/macros.rs
@@ -1202,6 +1202,21 @@ fn assert_zeroable<T: $crate::Zeroable>(_: *mut T) {}
         // have been initialized. Therefore we can now dismiss the guards by forgetting them.
         $(::core::mem::forget($guards);)*
     };
+    (init_slot($($use_data:ident)?):
+        @data($data:ident),
+        @slot($slot:ident),
+        @guards($($guards:ident,)*),
+        // arbitrary code block
+        @munch_fields(_: { $($code:tt)* }, $($rest:tt)*),
+    ) => {
+        { $($code)* }
+        $crate::__init_internal!(init_slot($($use_data)?):
+            @data($data),
+            @slot($slot),
+            @guards($($guards,)*),
+            @munch_fields($($rest)*),
+        );
+    };
     (init_slot($use_data:ident): // `use_data` is present, so we use the `data` to init fields.
         @data($data:ident),
         @slot($slot:ident),
@@ -1351,6 +1366,20 @@ fn assert_zeroable<T: $crate::Zeroable>(_: *mut T) {}
             );
         }
     };
+    (make_initializer:
+        @slot($slot:ident),
+        @type_name($t:path),
+        @munch_fields(_: { $($code:tt)* }, $($rest:tt)*),
+        @acc($($acc:tt)*),
+    ) => {
+        // code blocks are ignored for the initializer check
+        $crate::__init_internal!(make_initializer:
+            @slot($slot),
+            @type_name($t),
+            @munch_fields($($rest)*),
+            @acc($($acc)*),
+        );
+    };
     (make_initializer:
         @slot($slot:ident),
         @type_name($t:path),

base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
-- 
2.50.1