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macOS Metal dependency and Rust uninhabited-static future incompatibility

Document status: verified. Canonical source.

Summary

The previous macOS editor graph resolved masonry_winit -> imaging_wgpu -> wgpu 28.0.0 -> wgpu-hal 28.0.1 -> metal 0.33.0 -> block 0.1.6. Rust 1.98 compiled that graph but reported block as future-incompatible. wgpu pull request 5641 replaced metal-rs with objc2-metal and block2; wgpu 29 contains that migration. The reviewed refpath/xilem commit eabfe0a updates the NUIF Masonry revision to the wgpu 29 API without patching the Objective-C blocks ABI. NUIF pins its immediate descendant 1b96eb8 by full SHA; that descendant replaces abandoned font dependencies without changing the graphics migration.

The active editor graph resolves imaging_wgpu 0.0.2 -> wgpu 29.0.4 -> objc2-metal 0.3.2 -> block2 0.6.2. block 0.1.6 and metal-rs are absent from the lock file and active dependency graph. The fork remains a maintenance boundary until the equivalent migration is available from the selected upstream Xilem revision.

Evidence

  • wgpu pull request 5641 replaced metal with objc2-metal, block with block2, and core-graphics-types with objc2-core-graphics. The pull request merged on 2026-01-28. Locator: dependency and source-file changes, retrieved 2026-08-30: https://github.com/gfx-rs/wgpu/pull/5641.
  • wgpu-hal 29.0.4 declares block2 0.6.2, objc2-metal 0.3.2, and objc2-quartz-core 0.3.2 for the Metal backend. Locator: the macOS target dependencies in wgpu-hal/Cargo.toml at tag v29.0.4, retrieved 2026-08-30: https://github.com/gfx-rs/wgpu/blob/v29.0.4/wgpu-hal/Cargo.toml.
  • refpath/xilem commit eabfe0a92ff5ab0e26515383fdeaf288672b3e88 updates the imaging dependencies and the three affected wgpu API call sites. Locator: commit diff, retrieved 2026-08-30: https://github.com/refpath/xilem/commit/eabfe0a92ff5ab0e26515383fdeaf288672b3e88.
  • The active pin 1b96eb8db3f88f85db1a3594d80d3480b29392fb has eabfe0a as its sole parent and replaces abandoned font dependencies. Locator: commit metadata and diff, retrieved 2026-08-30: https://github.com/refpath/xilem/commit/1b96eb8db3f88f85db1a3594d80d3480b29392fb.
  • cargo tree -p nuif-editor -i block reports no matching package after the migration. cargo tree -p nuif-editor resolves wgpu 29.0.4, objc2-metal 0.3.2, and block2 0.6.2 from the active lock file (2026-08-30).
  • cargo report future-incompatibilities reports that no reports are available after rebuilding nuif-editor against the migrated graph with rustc 1.98.0 (2026-08-30).
  • cargo test -p nuif-editor --features editor-automation passes 14 tests against the pinned fork commit. A macOS Metal window smoke test reaches the event loop and presents without a surface error (2026-08-30).
  • Upstream metal-rs commit 9ed9fe9 still declares block 0.1.6 and its README now deprecates the crate in favor of objc2-metal. The review-only refpath/metal-rs branch move-to-block2, commit 7e0a178, replaces the production dependency with block2 0.6.2, migrates the typed callbacks, and replaces the shared-event layout mutation with block2’s explicit ABI encoding. No pull request was opened and NUIF does not depend on this fork. Locator: https://github.com/refpath/metal-rs/tree/move-to-block2, retrieved 2026-08-30.
  • The review fork passes the upstream crate checks and tests on Rust 1.82, its declared MSRV. Real macOS probes completed both an MTLSharedEvent notification and a command-buffer completion handler. Its normal dependency graph contains block2 and no rust-block; the all-target development graph still reaches rust-block through the deprecated cocoa 0.26 dev dependency. This makes the branch useful for review, but not a complete modernization of the deprecated objc stack.

Mechanism

masonry_winit constructs the wgpu instance, acquires each surface texture, and accesses the Metal presentation layer during macOS live resize. wgpu 29 changes the instance descriptor from a borrowed value to an owned value and returns CurrentSurfaceTexture instead of Result<SurfaceTexture, SurfaceError>. The fork updates those call sites and maps Timeout and Occluded to a skipped frame, as specified by the wgpu 29 variant documentation. The live-resize path calls the objc2 selector spelling exposed by objc2-quartz-core. The dependency update selects wgpu-hal’s objc2 Metal backend, so metal-rs and rust-block no longer participate in compilation or linking. Locator: masonry_winit/src/vello_util.rs and masonry_winit/src/event_loop_runner.rs in refpath/xilem commit eabfe0a; CurrentSurfaceTexture in wgpu 29.0.4, retrieved 2026-08-30: https://docs.rs/wgpu/29.0.4/wgpu/enum.CurrentSurfaceTexture.html.

Decision boundary

Borrow wgpu 29’s objc2 Metal backend as the maintained replacement for metal-rs and rust-block.

Adapt the selected Xilem revision through a full-SHA fork pin. Each fork update includes the NUIF editor tests, the reverse dependency trace, and a macOS Metal window smoke test.

Contain the direct metal-rs experiment in the review-only refpath/metal-rs branch. It demonstrates a small block2 migration but changes callback-facing Rust types and cannot remove rust-block from the legacy Cocoa development graph. NUIF therefore does not pin or ship the experiment.

NUIF relevance

Borrow Cargo’s future-incompatibility report and reverse dependency tree as the reproducible diagnostics for the pinned toolchain and lock file.

Adapt dependency-update review so a macOS editor-stack change verifies the absence of block, the presence of the expected block2 path, and a package smoke test in addition to the workspace checks.

Reject treating the fork as an indefinite divergence. The pin is removed when the selected upstream Xilem revision provides an equivalent wgpu version.

Open questions

  • Which upstream Xilem revision first provides a compatible wgpu 29 or later dependency graph?
  • When does imaging_skia publish a release that no longer requires wgpu 28 for its optional GPU backend?