Permit shrinking transmutes in transmute machinery #2912

joshlf · 2026-01-22T20:37:43Z

With this change, some TryTransmuteFromPtr impls become conditionally
sound – specifically, they are sound for size-preserving casts, but not
for size-shrinking casts. In order to ensure they remain sound, we make
the cast a type parameter on TryTransmuteFromPtr, which allows us to
add the appropriate bounds on these impls. This also prepares us for
more complex transmutes such as "overwriting/tearing" transmutes, as we
can require overwriting/tearing transmutability only for shrikning
casts, but not for size-preserving casts.

Makes progress on #2701, #1940, #1852

👉 Permit shrinking transmutes in transmute machinery #2912

Latest Update: v33 — Compare vs v32

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gemini-code-assist · 2026-01-22T20:37:59Z

Summary of Changes

Hello @joshlf, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the transmute machinery by introducing a mechanism to safely permit 'shrinking transmutes' under specific conditions. It modifies the rules for reverse transmutation, primarily by enforcing a CastExact constraint on the destination type's cast mechanism. This ensures that despite a potential 'shrinking' in type, the underlying memory footprint remains identical, thereby maintaining memory safety and expanding the flexibility of transmute operations.

Highlights

Permitting Shrinking Transmutes: The core change allows for 'shrinking transmutes' in the transmute machinery, which were previously explicitly banned. This is achieved by introducing a CastExact constraint, ensuring that the source and destination types occupy the same memory size during reverse transmutation.
Refined Reverse Transmutation Conditions: The documentation for 'Reverse transmutation' has been updated to reflect the new conditions. It now states that if Dst::CastFrom implements CastExact, and the set of DV-valid referents of dst is a subset of SV-valid referents of src, then the transmutation is permitted.
Trait Constraint Addition: A new where clause constraint, Dst::CastFrom: cast::CastExact<Src, Dst>, has been added to the TryTransmuteFromPtr trait, enforcing the size equality requirement for safe shrinking transmutes.
SizeEq Trait Adjustment: The SizeEq trait's associated type CastFrom has been relaxed from cast::CastExact to cast::Cast, with the CastExact requirement now being applied directly as a trait bound on TryTransmuteFromPtr.

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Code Review

This pull request refactors the transmutation machinery to enable future support for shrinking transmutes. The core change is relaxing the bound on SizeEq::CastFrom from CastExact to Cast, which allows it to be used for non-size-preserving casts. To maintain safety, existing code that relied on the implicit size-equality guarantee of SizeEq has been updated to explicitly require CastExact. The changes are well-structured, consistent, and the documentation has been updated accordingly. This is a good step towards more flexible transmutation capabilities. I have no further comments.

src/pointer/transmute.rs

codecov-commenter · 2026-01-22T21:10:58Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.31%. Comparing base (5fcf372) to head (ad5d704).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2912   +/-   ##
=======================================
  Coverage   91.31%   91.31%           
=======================================
  Files          20       20           
  Lines        5897     5897           
=======================================
  Hits         5385     5385           
  Misses        512      512

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With this change, some `TryTransmuteFromPtr` impls become conditionally sound – specifically, they are sound for size-preserving casts, but not for size-shrinking casts. In order to ensure they remain sound, we make the cast a type parameter on `TryTransmuteFromPtr`, which allows us to add the appropriate bounds on these impls. This also prepares us for more complex transmutes such as "overwriting/tearing" transmutes, as we can require overwriting/tearing transmutability *only* for shrikning casts, but not for size-preserving casts. Makes progress on #2701, #1940, #1852 gherrit-pr-id: Gd9d1c0de851091ffe6161889d21ca2167845a591

This was referenced Jan 22, 2026

Clarify semantics of size-preserving transmutes #2911

Merged

Make SizeEq safe #2907

Merged

joshlf force-pushed the Gda7c06c9a8e54f5afd6aa5a093406a7b71259fd3 branch from a4ef307 to 591b556 Compare January 22, 2026 20:38

joshlf force-pushed the Gd9d1c0de851091ffe6161889d21ca2167845a591 branch from 8529fd1 to 5b89e27 Compare January 22, 2026 20:38

gemini-code-assist bot reviewed Jan 22, 2026

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joshlf force-pushed the Gda7c06c9a8e54f5afd6aa5a093406a7b71259fd3 branch from 591b556 to 62ea699 Compare January 22, 2026 20:44

joshlf force-pushed the Gd9d1c0de851091ffe6161889d21ca2167845a591 branch from 5b89e27 to 5635968 Compare January 22, 2026 20:44

joshlf force-pushed the Gda7c06c9a8e54f5afd6aa5a093406a7b71259fd3 branch from 62ea699 to 522141c Compare January 22, 2026 20:47

joshlf force-pushed the Gd9d1c0de851091ffe6161889d21ca2167845a591 branch from 5635968 to 46f2fd0 Compare January 22, 2026 20:47

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joshlf commented Jan 22, 2026

View reviewed changes

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joshlf mentioned this pull request Jan 22, 2026

Model shrinking and overwriting transmutes in pointer::transmute #2910

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joshlf force-pushed the Gd9d1c0de851091ffe6161889d21ca2167845a591 branch 3 times, most recently from 77aedb3 to a86e3dd Compare January 22, 2026 21:36

joshlf force-pushed the Gda7c06c9a8e54f5afd6aa5a093406a7b71259fd3 branch from f0a5b8c to 4cb7174 Compare January 22, 2026 21:45

joshlf force-pushed the Gd9d1c0de851091ffe6161889d21ca2167845a591 branch from a86e3dd to 7a46add Compare January 22, 2026 21:45

Base automatically changed from Gda7c06c9a8e54f5afd6aa5a093406a7b71259fd3 to main January 22, 2026 22:32

joshlf force-pushed the Gd9d1c0de851091ffe6161889d21ca2167845a591 branch from 7a46add to 975631d Compare January 23, 2026 20:14

joshlf mentioned this pull request Jan 23, 2026

[transmute] Remove SizeEq super-trait bound #2920

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joshlf changed the base branch from main to G0973aa3261bb811a53f7d99b8d38df5d6c328d52 January 23, 2026 20:14

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joshlf changed the base branch from G0973aa3261bb811a53f7d99b8d38df5d6c328d52 to G1cb98a9a25bc3bcf02d17e63b5389c2d66e9b4d8 January 23, 2026 20:59

joshlf mentioned this pull request Jan 23, 2026

[transmute] Encode cast in TryTransmuteFromPtr params #2921

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joshlf force-pushed the Gd9d1c0de851091ffe6161889d21ca2167845a591 branch from 15386c4 to 8fa19c4 Compare January 24, 2026 05:07

joshlf force-pushed the Gd9d1c0de851091ffe6161889d21ca2167845a591 branch from 97a08f6 to 857d03c Compare January 24, 2026 05:59

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joshlf mentioned this pull request Jan 24, 2026

[transmute] Remove some SizeEq impls #2922

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joshlf mentioned this pull request Jan 24, 2026

Replace unsafe_with_size_eq! with transmute_with #2923

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joshlf force-pushed the G1cb98a9a25bc3bcf02d17e63b5389c2d66e9b4d8 branch from 918103f to 565194b Compare January 26, 2026 04:18

joshlf force-pushed the Gd9d1c0de851091ffe6161889d21ca2167845a591 branch from 0744864 to 1371b00 Compare January 26, 2026 04:18

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Permit shrinking transmutes in transmute machinery #2912

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