From b0b7c9a2fb7cf7cbce21673ab1d41dbf1821fc22 Mon Sep 17 00:00:00 2001
From: Jack Wrenn <jswrenn@google.com>
Date: Wed, 4 Feb 2026 15:54:49 +0000
Subject: [PATCH] [wip] Introduce `Ptr::project_map`

Enables piecewise initialization.

gherrit-pr-id: Gc3e9ee9afa6945e10c35be84d30c970894395c96
---
 src/impls.rs             | 22 +++++++++++++++-------
 src/lib.rs               |  5 +++++
 src/pointer/invariant.rs | 10 ++++++++++
 src/pointer/ptr.rs       | 32 ++++++++++++++++++++++++++++++++
 4 files changed, 62 insertions(+), 7 deletions(-)

diff --git a/src/impls.rs b/src/impls.rs
index d1344098cf..46c66f7e73 100644
--- a/src/impls.rs
+++ b/src/impls.rs
@@ -1080,16 +1080,24 @@ mod tuples {
             // The types and indices at and after the current index.
             [$CurrT:ident $CurrV:ident $CurrI:tt $($AfterT:ident $AfterV:ident $AfterI:tt)*]
         ) => {
+            impl<Replacement, $($AllT),+> crate::invariant::Map<Replacement, crate::Idx<{ $CurrI }>> for ($($AllT,)+)
+            where
+                Replacement: crate::invariant::Validity,
+                $($AllT: crate::invariant::Validity,)*
+            {
+                type Result = ($($BeforeT,)* Replacement, $($AfterT,)*);
+            }
+
             // SAFETY:
             // - `Self` is a struct (albeit anonymous), so `VARIANT_ID` is
             //   `STRUCT_VARIANT_ID`.
             // - `$CurrI` is the field at index `$CurrI`, so `FIELD_ID` is
             //   `zerocopy::ident_id!($CurrI)`
-            // - `()` has the same visibility as the `.$CurrI` field (ie, `.0`,
-            //   `.1`, etc)
+            // - `crate::Idx` has the same visibility as the `.$CurrI` field
+            //   (ie, `.0`, `.1`, etc)
             // - `Type` has the same type as `$CurrI`; i.e., `$CurrT`.
             unsafe impl<$($AllT),+> crate::HasField<
-                (),
+                crate::Idx<{ $CurrI }>,
                 { crate::STRUCT_VARIANT_ID },
                 { crate::ident_id!($CurrI)}
             > for ($($AllT,)+) {
@@ -1115,7 +1123,7 @@ mod tuples {
 
             // SAFETY: See comments on items.
             unsafe impl<Aliasing, Alignment, $($AllT),+> crate::ProjectField<
-                (),
+                crate::Idx<{ $CurrI }>,
                 (Aliasing, Alignment, crate::invariant::Uninit),
                 { crate::STRUCT_VARIANT_ID },
                 { crate::ident_id!($CurrI)}
@@ -1141,7 +1149,7 @@ mod tuples {
 
             // SAFETY: See comments on items.
             unsafe impl<Aliasing, Alignment, $($AllT),+> crate::ProjectField<
-                (),
+                crate::Idx<{ $CurrI }>,
                 (Aliasing, Alignment, crate::invariant::Initialized),
                 { crate::STRUCT_VARIANT_ID },
                 { crate::ident_id!($CurrI)}
@@ -1167,7 +1175,7 @@ mod tuples {
 
             // SAFETY: See comments on items.
             unsafe impl<Aliasing, Alignment, $($AllT),+> crate::ProjectField<
-                (),
+                crate::Idx<{ $CurrI }>,
                 (Aliasing, Alignment, crate::invariant::Valid),
                 { crate::STRUCT_VARIANT_ID },
                 { crate::ident_id!($CurrI)}
@@ -1193,7 +1201,7 @@ mod tuples {
 
             // SAFETY: See comments on items.
             unsafe impl<Aliasing, Alignment, $($AllT,)+ $($AllV),+> crate::ProjectField<
-                (),
+                crate::Idx<{ $CurrI }>,
                 (Aliasing, Alignment, ($($AllV,)+)),
                 { crate::STRUCT_VARIANT_ID },
                 { crate::ident_id!($CurrI)}
diff --git a/src/lib.rs b/src/lib.rs
index 23793e3882..0603659c5a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1123,6 +1123,11 @@ pub const UNION_VARIANT_ID: i128 = -2;
 #[doc(hidden)]
 pub const REPR_C_UNION_VARIANT_ID: i128 = -3;
 
+/// A field visibility token for public tuple fields.
+#[allow(missing_copy_implementations, missing_debug_implementations)]
+#[doc(hidden)]
+pub struct Idx<const INDEX: u16>;
+
 /// # Safety
 ///
 /// `Self::ProjectToTag` must satisfy its safety invariant.
diff --git a/src/pointer/invariant.rs b/src/pointer/invariant.rs
index 0ce795f260..cff8a98c23 100644
--- a/src/pointer/invariant.rs
+++ b/src/pointer/invariant.rs
@@ -277,6 +277,16 @@ pub enum BecauseExclusive {}
 #[doc(hidden)]
 pub enum BecauseImmutable {}
 
+/// Replaces a validity component.
+///
+/// The [`Self::Result`] is a validity identical `Self` except that the
+/// component corresponding to `Field` component has been replaced with
+/// `Replacement`.
+#[doc(hidden)]
+pub trait Map<Replacement: Validity, Field> {
+    type Result: Validity;
+}
+
 use sealed::Sealed;
 pub(crate) mod sealed {
     use super::*;
diff --git a/src/pointer/ptr.rs b/src/pointer/ptr.rs
index 83c116747b..5e3781567e 100644
--- a/src/pointer/ptr.rs
+++ b/src/pointer/ptr.rs
@@ -914,6 +914,38 @@ mod _casts {
             tag.unify_invariants()
         }
 
+        #[inline(always)]
+        pub fn project_map<F, V, const VARIANT_ID: i128, const FIELD_ID: i128>(
+            mut self,
+            map: impl for<'b> FnOnce(
+                Ptr<'b, T::Type, T::Invariants>,
+            ) -> Ptr<
+                'b,
+                T::Type,
+                (
+                    <T::Invariants as Invariants>::Aliasing,
+                    <T::Invariants as Invariants>::Alignment,
+                    V,
+                ),
+            >,
+        ) -> Result<
+            Ptr<'a, T, (I::Aliasing, I::Alignment, <I::Validity as Map<V, F>>::Result)>,
+            T::Error,
+        >
+        where
+            T: ProjectField<F, I, VARIANT_ID, FIELD_ID>,
+            I::Aliasing: Reference,
+            I::Validity: Map<V, F>,
+            V: Validity,
+        {
+            let _mapped = match self.reborrow().project() {
+                Ok(field) => map(field),
+                Err(err) => return Err(err),
+            };
+            // SAFETY: TODO
+            Ok(unsafe { self.assume_validity() })
+        }
+
         /// Attempts to transform the pointer, restoring the original on
         /// failure.
         ///