rust: init: consolidate init macros

Merges the implementations of `try_init!` and `try_pin_init!`. These two
macros are very similar, but use different traits. The new macro
`__init_internal!` that is now the implementation for both takes these
traits as parameters.

This change does not affect any users, as no public API has been
changed, but it should simplify maintaining the init macros.

Reviewed-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20230814084602.25699-2-benno.lossin@proton.me
[ Cleaned a couple trivial nits. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

2 files changed, 259 insertions, 366 deletions

diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index b4332a4ec1f4d5..d9a91950cba290 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -540,11 +540,14 @@ macro_rules! pin_init {
     ($(&$this:ident in)? $t:ident $(::<$($generics:ty),* $(,)?>)? {
         $($fields:tt)*
     }) => {
-        $crate::try_pin_init!(
+        $crate::__init_internal!(
             @this($($this)?),
             @typ($t $(::<$($generics),*>)?),
             @fields($($fields)*),
             @error(::core::convert::Infallible),
+            @data(PinData, use_data),
+            @has_data(HasPinData, __pin_data),
+            @construct_closure(pin_init_from_closure),
         )
     };
 }
@@ -593,205 +596,29 @@ macro_rules! try_pin_init {
     ($(&$this:ident in)? $t:ident $(::<$($generics:ty),* $(,)?>)? {
         $($fields:tt)*
     }) => {
-        $crate::try_pin_init!(
+        $crate::__init_internal!(
             @this($($this)?),
             @typ($t $(::<$($generics),*>)? ),
             @fields($($fields)*),
             @error($crate::error::Error),
+            @data(PinData, use_data),
+            @has_data(HasPinData, __pin_data),
+            @construct_closure(pin_init_from_closure),
         )
     };
     ($(&$this:ident in)? $t:ident $(::<$($generics:ty),* $(,)?>)? {
         $($fields:tt)*
     }? $err:ty) => {
-        $crate::try_pin_init!(
+        $crate::__init_internal!(
             @this($($this)?),
             @typ($t $(::<$($generics),*>)? ),
             @fields($($fields)*),
             @error($err),
+            @data(PinData, use_data),
+            @has_data(HasPinData, __pin_data),
+            @construct_closure(pin_init_from_closure),
         )
     };
-    (
-        @this($($this:ident)?),
-        @typ($t:ident $(::<$($generics:ty),*>)?),
-        @fields($($fields:tt)*),
-        @error($err:ty),
-    ) => {{
-        // We do not want to allow arbitrary returns, so we declare this type as the `Ok` return
-        // type and shadow it later when we insert the arbitrary user code. That way there will be
-        // no possibility of returning without `unsafe`.
-        struct __InitOk;
-        // Get the pin data from the supplied type.
-        let data = unsafe {
-            use $crate::init::__internal::HasPinData;
-            $t$(::<$($generics),*>)?::__pin_data()
-        };
-        // Ensure that `data` really is of type `PinData` and help with type inference:
-        let init = $crate::init::__internal::PinData::make_closure::<_, __InitOk, $err>(
-            data,
-            move |slot| {
-                {
-                    // Shadow the structure so it cannot be used to return early.
-                    struct __InitOk;
-                    // Create the `this` so it can be referenced by the user inside of the
-                    // expressions creating the individual fields.
-                    $(let $this = unsafe { ::core::ptr::NonNull::new_unchecked(slot) };)?
-                    // Initialize every field.
-                    $crate::try_pin_init!(init_slot:
-                        @data(data),
-                        @slot(slot),
-                        @munch_fields($($fields)*,),
-                    );
-                    // We use unreachable code to ensure that all fields have been mentioned exactly
-                    // once, this struct initializer will still be type-checked and complain with a
-                    // very natural error message if a field is forgotten/mentioned more than once.
-                    #[allow(unreachable_code, clippy::diverging_sub_expression)]
-                    if false {
-                        $crate::try_pin_init!(make_initializer:
-                            @slot(slot),
-                            @type_name($t),
-                            @munch_fields($($fields)*,),
-                            @acc(),
-                        );
-                    }
-                    // Forget all guards, since initialization was a success.
-                    $crate::try_pin_init!(forget_guards:
-                        @munch_fields($($fields)*,),
-                    );
-                }
-                Ok(__InitOk)
-            }
-        );
-        let init = move |slot| -> ::core::result::Result<(), $err> {
-            init(slot).map(|__InitOk| ())
-        };
-        let init = unsafe { $crate::init::pin_init_from_closure::<_, $err>(init) };
-        init
-    }};
-    (init_slot:
-        @data($data:ident),
-        @slot($slot:ident),
-        @munch_fields($(,)?),
-    ) => {
-        // Endpoint of munching, no fields are left.
-    };
-    (init_slot:
-        @data($data:ident),
-        @slot($slot:ident),
-        // In-place initialization syntax.
-        @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
-    ) => {
-        let $field = $val;
-        // Call the initializer.
-        //
-        // SAFETY: `slot` is valid, because we are inside of an initializer closure, we
-        // return when an error/panic occurs.
-        // We also use the `data` to require the correct trait (`Init` or `PinInit`) for `$field`.
-        unsafe { $data.$field(::core::ptr::addr_of_mut!((*$slot).$field), $field)? };
-        // Create the drop guard.
-        //
-        // We only give access to `&DropGuard`, so it cannot be forgotten via safe code.
-        //
-        // SAFETY: We forget the guard later when initialization has succeeded.
-        let $field = &unsafe {
-            $crate::init::__internal::DropGuard::new(::core::ptr::addr_of_mut!((*$slot).$field))
-        };
-
-        $crate::try_pin_init!(init_slot:
-            @data($data),
-            @slot($slot),
-            @munch_fields($($rest)*),
-        );
-    };
-    (init_slot:
-        @data($data:ident),
-        @slot($slot:ident),
-        // Direct value init, this is safe for every field.
-        @munch_fields($field:ident $(: $val:expr)?, $($rest:tt)*),
-    ) => {
-        $(let $field = $val;)?
-        // Initialize the field.
-        //
-        // SAFETY: The memory at `slot` is uninitialized.
-        unsafe { ::core::ptr::write(::core::ptr::addr_of_mut!((*$slot).$field), $field) };
-        // Create the drop guard:
-        //
-        // We only give access to `&DropGuard`, so it cannot be accidentally forgotten.
-        //
-        // SAFETY: We forget the guard later when initialization has succeeded.
-        let $field = &unsafe {
-            $crate::init::__internal::DropGuard::new(::core::ptr::addr_of_mut!((*$slot).$field))
-        };
-
-        $crate::try_pin_init!(init_slot:
-            @data($data),
-            @slot($slot),
-            @munch_fields($($rest)*),
-        );
-    };
-    (make_initializer:
-        @slot($slot:ident),
-        @type_name($t:ident),
-        @munch_fields($(,)?),
-        @acc($($acc:tt)*),
-    ) => {
-        // Endpoint, nothing more to munch, create the initializer.
-        // Since we are in the `if false` branch, this will never get executed. We abuse `slot` to
-        // get the correct type inference here:
-        unsafe {
-            ::core::ptr::write($slot, $t {
-                $($acc)*
-            });
-        }
-    };
-    (make_initializer:
-        @slot($slot:ident),
-        @type_name($t:ident),
-        @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
-        @acc($($acc:tt)*),
-    ) => {
-        $crate::try_pin_init!(make_initializer:
-            @slot($slot),
-            @type_name($t),
-            @munch_fields($($rest)*),
-            @acc($($acc)* $field: ::core::panic!(),),
-        );
-    };
-    (make_initializer:
-        @slot($slot:ident),
-        @type_name($t:ident),
-        @munch_fields($field:ident $(: $val:expr)?, $($rest:tt)*),
-        @acc($($acc:tt)*),
-    ) => {
-        $crate::try_pin_init!(make_initializer:
-            @slot($slot),
-            @type_name($t),
-            @munch_fields($($rest)*),
-            @acc($($acc)* $field: ::core::panic!(),),
-        );
-    };
-    (forget_guards:
-        @munch_fields($(,)?),
-    ) => {
-        // Munching finished.
-    };
-    (forget_guards:
-        @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
-    ) => {
-        unsafe { $crate::init::__internal::DropGuard::forget($field) };
-
-        $crate::try_pin_init!(forget_guards:
-            @munch_fields($($rest)*),
-        );
-    };
-    (forget_guards:
-        @munch_fields($field:ident $(: $val:expr)?, $($rest:tt)*),
-    ) => {
-        unsafe { $crate::init::__internal::DropGuard::forget($field) };
-
-        $crate::try_pin_init!(forget_guards:
-            @munch_fields($($rest)*),
-        );
-    };
 }
 
 /// Construct an in-place initializer for `struct`s.
@@ -816,11 +643,14 @@ macro_rules! init {
     ($(&$this:ident in)? $t:ident $(::<$($generics:ty),* $(,)?>)? {
         $($fields:tt)*
     }) => {
-        $crate::try_init!(
+        $crate::__init_internal!(
             @this($($this)?),
             @typ($t $(::<$($generics),*>)?),
             @fields($($fields)*),
             @error(::core::convert::Infallible),
+            @data(InitData, /*no use_data*/),
+            @has_data(HasInitData, __init_data),
+            @construct_closure(init_from_closure),
         )
     }
 }
@@ -863,199 +693,29 @@ macro_rules! try_init {
     ($(&$this:ident in)? $t:ident $(::<$($generics:ty),* $(,)?>)? {
         $($fields:tt)*
     }) => {
-        $crate::try_init!(
+        $crate::__init_internal!(
             @this($($this)?),
             @typ($t $(::<$($generics),*>)?),
             @fields($($fields)*),
             @error($crate::error::Error),
+            @data(InitData, /*no use_data*/),
+            @has_data(HasInitData, __init_data),
+            @construct_closure(init_from_closure),
         )
     };
     ($(&$this:ident in)? $t:ident $(::<$($generics:ty),* $(,)?>)? {
         $($fields:tt)*
     }? $err:ty) => {
-        $crate::try_init!(
+        $crate::__init_internal!(
             @this($($this)?),
             @typ($t $(::<$($generics),*>)?),
             @fields($($fields)*),
             @error($err),
+            @data(InitData, /*no use_data*/),
+            @has_data(HasInitData, __init_data),
+            @construct_closure(init_from_closure),
         )
     };
-    (
-        @this($($this:ident)?),
-        @typ($t:ident $(::<$($generics:ty),*>)?),
-        @fields($($fields:tt)*),
-        @error($err:ty),
-    ) => {{
-        // We do not want to allow arbitrary returns, so we declare this type as the `Ok` return
-        // type and shadow it later when we insert the arbitrary user code. That way there will be
-        // no possibility of returning without `unsafe`.
-        struct __InitOk;
-        // Get the init data from the supplied type.
-        let data = unsafe {
-            use $crate::init::__internal::HasInitData;
-            $t$(::<$($generics),*>)?::__init_data()
-        };
-        // Ensure that `data` really is of type `InitData` and help with type inference:
-        let init = $crate::init::__internal::InitData::make_closure::<_, __InitOk, $err>(
-            data,
-            move |slot| {
-                {
-                    // Shadow the structure so it cannot be used to return early.
-                    struct __InitOk;
-                    // Create the `this` so it can be referenced by the user inside of the
-                    // expressions creating the individual fields.
-                    $(let $this = unsafe { ::core::ptr::NonNull::new_unchecked(slot) };)?
-                    // Initialize every field.
-                    $crate::try_init!(init_slot:
-                        @slot(slot),
-                        @munch_fields($($fields)*,),
-                    );
-                    // We use unreachable code to ensure that all fields have been mentioned exactly
-                    // once, this struct initializer will still be type-checked and complain with a
-                    // very natural error message if a field is forgotten/mentioned more than once.
-                    #[allow(unreachable_code, clippy::diverging_sub_expression)]
-                    if false {
-                        $crate::try_init!(make_initializer:
-                            @slot(slot),
-                            @type_name($t),
-                            @munch_fields($($fields)*,),
-                            @acc(),
-                        );
-                    }
-                    // Forget all guards, since initialization was a success.
-                    $crate::try_init!(forget_guards:
-                        @munch_fields($($fields)*,),
-                    );
-                }
-                Ok(__InitOk)
-            }
-        );
-        let init = move |slot| -> ::core::result::Result<(), $err> {
-            init(slot).map(|__InitOk| ())
-        };
-        let init = unsafe { $crate::init::init_from_closure::<_, $err>(init) };
-        init
-    }};
-    (init_slot:
-        @slot($slot:ident),
-        @munch_fields( $(,)?),
-    ) => {
-        // Endpoint of munching, no fields are left.
-    };
-    (init_slot:
-        @slot($slot:ident),
-        @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
-    ) => {
-        let $field = $val;
-        // Call the initializer.
-        //
-        // SAFETY: `slot` is valid, because we are inside of an initializer closure, we
-        // return when an error/panic occurs.
-        unsafe {
-            $crate::init::Init::__init($field, ::core::ptr::addr_of_mut!((*$slot).$field))?;
-        }
-        // Create the drop guard.
-        //
-        // We only give access to `&DropGuard`, so it cannot be accidentally forgotten.
-        //
-        // SAFETY: We forget the guard later when initialization has succeeded.
-        let $field = &unsafe {
-            $crate::init::__internal::DropGuard::new(::core::ptr::addr_of_mut!((*$slot).$field))
-        };
-
-        $crate::try_init!(init_slot:
-            @slot($slot),
-            @munch_fields($($rest)*),
-        );
-    };
-    (init_slot:
-        @slot($slot:ident),
-        // Direct value init.
-        @munch_fields($field:ident $(: $val:expr)?, $($rest:tt)*),
-    ) => {
-        $(let $field = $val;)?
-        // Call the initializer.
-        //
-        // SAFETY: The memory at `slot` is uninitialized.
-        unsafe { ::core::ptr::write(::core::ptr::addr_of_mut!((*$slot).$field), $field) };
-        // Create the drop guard.
-        //
-        // We only give access to `&DropGuard`, so it cannot be accidentally forgotten.
-        //
-        // SAFETY: We forget the guard later when initialization has succeeded.
-        let $field = &unsafe {
-            $crate::init::__internal::DropGuard::new(::core::ptr::addr_of_mut!((*$slot).$field))
-        };
-
-        $crate::try_init!(init_slot:
-            @slot($slot),
-            @munch_fields($($rest)*),
-        );
-    };
-    (make_initializer:
-        @slot($slot:ident),
-        @type_name($t:ident),
-        @munch_fields( $(,)?),
-        @acc($($acc:tt)*),
-    ) => {
-        // Endpoint, nothing more to munch, create the initializer.
-        // Since we are in the `if false` branch, this will never get executed. We abuse `slot` to
-        // get the correct type inference here:
-        unsafe {
-            ::core::ptr::write($slot, $t {
-                $($acc)*
-            });
-        }
-    };
-    (make_initializer:
-        @slot($slot:ident),
-        @type_name($t:ident),
-        @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
-        @acc($($acc:tt)*),
-    ) => {
-        $crate::try_init!(make_initializer:
-            @slot($slot),
-            @type_name($t),
-            @munch_fields($($rest)*),
-            @acc($($acc)*$field: ::core::panic!(),),
-        );
-    };
-    (make_initializer:
-        @slot($slot:ident),
-        @type_name($t:ident),
-        @munch_fields($field:ident $(: $val:expr)?, $($rest:tt)*),
-        @acc($($acc:tt)*),
-    ) => {
-        $crate::try_init!(make_initializer:
-            @slot($slot),
-            @type_name($t),
-            @munch_fields($($rest)*),
-            @acc($($acc)*$field: ::core::panic!(),),
-        );
-    };
-    (forget_guards:
-        @munch_fields($(,)?),
-    ) => {
-        // Munching finished.
-    };
-    (forget_guards:
-        @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
-    ) => {
-        unsafe { $crate::init::__internal::DropGuard::forget($field) };
-
-        $crate::try_init!(forget_guards:
-            @munch_fields($($rest)*),
-        );
-    };
-    (forget_guards:
-        @munch_fields($field:ident $(: $val:expr)?, $($rest:tt)*),
-    ) => {
-        unsafe { $crate::init::__internal::DropGuard::forget($field) };
-
-        $crate::try_init!(forget_guards:
-            @munch_fields($($rest)*),
-        );
-    };
 }
 
 /// A pin-initializer for the type `T`.
diff --git a/rust/kernel/init/macros.rs b/rust/kernel/init/macros.rs
index 00aa4e956c0ae9..ad78dd0d9d9ed4 100644
--- a/rust/kernel/init/macros.rs
+++ b/rust/kernel/init/macros.rs
@@ -1,10 +1,12 @@
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
 //! This module provides the macros that actually implement the proc-macros `pin_data` and
-//! `pinned_drop`.
+//! `pinned_drop`. It also contains `__init_internal` the implementation of the `{try_}{pin_}init!`
+//! macros.
 //!
 //! These macros should never be called directly, since they expect their input to be
-//! in a certain format which is internal. Use the proc-macros instead.
+//! in a certain format which is internal. If used incorrectly, these macros can lead to UB even in
+//! safe code! Use the public facing macros instead.
 //!
 //! This architecture has been chosen because the kernel does not yet have access to `syn` which
 //! would make matters a lot easier for implementing these as proc-macros.
@@ -980,3 +982,234 @@ macro_rules! __pin_data {
         }
     };
 }
+
+/// The internal init macro. Do not call manually!
+///
+/// This is called by the `{try_}{pin_}init!` macros with various inputs.
+///
+/// This macro has multiple internal call configurations, these are always the very first ident:
+/// - nothing: this is the base case and called by the `{try_}{pin_}init!` macros.
+/// - `init_slot`: recursively creates the code that initializes all fields in `slot`.
+/// - `make_initializer`: recursively create the struct initializer that guarantees that every
+///   field has been initialized exactly once.
+/// - `forget_guards`: recursively forget the drop guards for every field.
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __init_internal {
+    (
+        @this($($this:ident)?),
+        @typ($t:ident $(::<$($generics:ty),*>)?),
+        @fields($($fields:tt)*),
+        @error($err:ty),
+        // Either `PinData` or `InitData`, `$use_data` should only be present in the `PinData`
+        // case.
+        @data($data:ident, $($use_data:ident)?),
+        // `HasPinData` or `HasInitData`.
+        @has_data($has_data:ident, $get_data:ident),
+        // `pin_init_from_closure` or `init_from_closure`.
+        @construct_closure($construct_closure:ident),
+    ) => {{
+        // We do not want to allow arbitrary returns, so we declare this type as the `Ok` return
+        // type and shadow it later when we insert the arbitrary user code. That way there will be
+        // no possibility of returning without `unsafe`.
+        struct __InitOk;
+        // Get the data about fields from the supplied type.
+        let data = unsafe {
+            use $crate::init::__internal::$has_data;
+            $t$(::<$($generics),*>)?::$get_data()
+        };
+        // Ensure that `data` really is of type `$data` and help with type inference:
+        let init = $crate::init::__internal::$data::make_closure::<_, __InitOk, $err>(
+            data,
+            move |slot| {
+                {
+                    // Shadow the structure so it cannot be used to return early.
+                    struct __InitOk;
+                    // Create the `this` so it can be referenced by the user inside of the
+                    // expressions creating the individual fields.
+                    $(let $this = unsafe { ::core::ptr::NonNull::new_unchecked(slot) };)?
+                    // Initialize every field.
+                    $crate::__init_internal!(init_slot($($use_data)?):
+                        @data(data),
+                        @slot(slot),
+                        @munch_fields($($fields)*,),
+                    );
+                    // We use unreachable code to ensure that all fields have been mentioned exactly
+                    // once, this struct initializer will still be type-checked and complain with a
+                    // very natural error message if a field is forgotten/mentioned more than once.
+                    #[allow(unreachable_code, clippy::diverging_sub_expression)]
+                    if false {
+                        $crate::__init_internal!(make_initializer:
+                            @slot(slot),
+                            @type_name($t),
+                            @munch_fields($($fields)*,),
+                            @acc(),
+                        );
+                    }
+                    // Forget all guards, since initialization was a success.
+                    $crate::__init_internal!(forget_guards:
+                        @munch_fields($($fields)*,),
+                    );
+                }
+                Ok(__InitOk)
+            }
+        );
+        let init = move |slot| -> ::core::result::Result<(), $err> {
+            init(slot).map(|__InitOk| ())
+        };
+        let init = unsafe { $crate::init::$construct_closure::<_, $err>(init) };
+        init
+    }};
+    (init_slot($($use_data:ident)?):
+        @data($data:ident),
+        @slot($slot:ident),
+        @munch_fields($(,)?),
+    ) => {
+        // Endpoint of munching, no fields are left.
+    };
+    (init_slot($use_data:ident): // `use_data` is present, so we use the `data` to init fields.
+        @data($data:ident),
+        @slot($slot:ident),
+        // In-place initialization syntax.
+        @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
+    ) => {
+        let $field = $val;
+        // Call the initializer.
+        //
+        // SAFETY: `slot` is valid, because we are inside of an initializer closure, we
+        // return when an error/panic occurs.
+        // We also use the `data` to require the correct trait (`Init` or `PinInit`) for `$field`.
+        unsafe { $data.$field(::core::ptr::addr_of_mut!((*$slot).$field), $field)? };
+        // Create the drop guard.
+        //
+        // We only give access to `&DropGuard`, so it cannot be forgotten via safe code.
+        //
+        // SAFETY: We forget the guard later when initialization has succeeded.
+        let $field = &unsafe {
+            $crate::init::__internal::DropGuard::new(::core::ptr::addr_of_mut!((*$slot).$field))
+        };
+
+        $crate::__init_internal!(init_slot($use_data):
+            @data($data),
+            @slot($slot),
+            @munch_fields($($rest)*),
+        );
+    };
+    (init_slot(): // No `use_data`, so we use `Init::__init` directly.
+        @data($data:ident),
+        @slot($slot:ident),
+        // In-place initialization syntax.
+        @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
+    ) => {
+        let $field = $val;
+        // Call the initializer.
+        //
+        // SAFETY: `slot` is valid, because we are inside of an initializer closure, we
+        // return when an error/panic occurs.
+        unsafe { $crate::init::Init::__init($field, ::core::ptr::addr_of_mut!((*$slot).$field))? };
+        // Create the drop guard.
+        //
+        // We only give access to `&DropGuard`, so it cannot be forgotten via safe code.
+        //
+        // SAFETY: We forget the guard later when initialization has succeeded.
+        let $field = &unsafe {
+            $crate::init::__internal::DropGuard::new(::core::ptr::addr_of_mut!((*$slot).$field))
+        };
+
+        $crate::__init_internal!(init_slot():
+            @data($data),
+            @slot($slot),
+            @munch_fields($($rest)*),
+        );
+    };
+    (init_slot($($use_data:ident)?):
+        @data($data:ident),
+        @slot($slot:ident),
+        // Init by-value.
+        @munch_fields($field:ident $(: $val:expr)?, $($rest:tt)*),
+    ) => {
+        $(let $field = $val;)?
+        // Initialize the field.
+        //
+        // SAFETY: The memory at `slot` is uninitialized.
+        unsafe { ::core::ptr::write(::core::ptr::addr_of_mut!((*$slot).$field), $field) };
+        // Create the drop guard:
+        //
+        // We only give access to `&DropGuard`, so it cannot be accidentally forgotten.
+        //
+        // SAFETY: We forget the guard later when initialization has succeeded.
+        let $field = &unsafe {
+            $crate::init::__internal::DropGuard::new(::core::ptr::addr_of_mut!((*$slot).$field))
+        };
+
+        $crate::__init_internal!(init_slot($($use_data)?):
+            @data($data),
+            @slot($slot),
+            @munch_fields($($rest)*),
+        );
+    };
+    (make_initializer:
+        @slot($slot:ident),
+        @type_name($t:ident),
+        @munch_fields($(,)?),
+        @acc($($acc:tt)*),
+    ) => {
+        // Endpoint, nothing more to munch, create the initializer.
+        // Since we are in the `if false` branch, this will never get executed. We abuse `slot` to
+        // get the correct type inference here:
+        unsafe {
+            ::core::ptr::write($slot, $t {
+                $($acc)*
+            });
+        }
+    };
+    (make_initializer:
+        @slot($slot:ident),
+        @type_name($t:ident),
+        @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
+        @acc($($acc:tt)*),
+    ) => {
+        $crate::__init_internal!(make_initializer:
+            @slot($slot),
+            @type_name($t),
+            @munch_fields($($rest)*),
+            @acc($($acc)* $field: ::core::panic!(),),
+        );
+    };
+    (make_initializer:
+        @slot($slot:ident),
+        @type_name($t:ident),
+        @munch_fields($field:ident $(: $val:expr)?, $($rest:tt)*),
+        @acc($($acc:tt)*),
+    ) => {
+        $crate::__init_internal!(make_initializer:
+            @slot($slot),
+            @type_name($t),
+            @munch_fields($($rest)*),
+            @acc($($acc)* $field: ::core::panic!(),),
+        );
+    };
+    (forget_guards:
+        @munch_fields($(,)?),
+    ) => {
+        // Munching finished.
+    };
+    (forget_guards:
+        @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
+    ) => {
+        unsafe { $crate::init::__internal::DropGuard::forget($field) };
+
+        $crate::__init_internal!(forget_guards:
+            @munch_fields($($rest)*),
+        );
+    };
+    (forget_guards:
+        @munch_fields($field:ident $(: $val:expr)?, $($rest:tt)*),
+    ) => {
+        unsafe { $crate::init::__internal::DropGuard::forget($field) };
+
+        $crate::__init_internal!(forget_guards:
+            @munch_fields($($rest)*),
+        );
+    };
+}