add code blocks to [try_][pin_]init! macros

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`");
        },
    })

Signed-off-by: Benno Lossin <lossin@kernel.org>

Author: BennoLossin

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Add initializer code blocks to `[try_][pin_]init!` macros: make initializer
+  macros accept any number of `_: {/* arbitrary code */},` & make them run the
+  code at that point.
+
 ## [0.0.10] - 2025-08-19
 
 ### Added

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

src/macros.rs

@@ -1202,6 +1202,21 @@ macro_rules! __init_internal {
         // 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 @@ macro_rules! __init_internal {
             );
         }
     };
+    (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),

tests/ui/compile-fail/init/wrong_generics2.stderr

@@ -12,7 +12,7 @@ help: you might have forgotten to add the struct literal inside the block
  --> src/macros.rs
   |
   ~                 ::core::ptr::write($slot, $t { SomeStruct {
-  |9                    $($acc)*
+  |4                    $($acc)*
   ~                 } });
   |