Support cross-compiling to WebAssembly with a Swift SDK#1817
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AttilaTheFun
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This was referenced Jun 23, 2026
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didn't get all the way through yet but leaving what i have so far
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Builds on the linkshared and Swift SDK framework PRs. Adds the swift.wasm_sdk module extension tag: it downloads the swift.org WebAssembly Swift SDK artifact bundle matching a swift.toolchain and, using the shared framework, defines Swift and C/C++ toolchains targeting wasm32-unknown-wasip1, so plain swift_library / swift_binary targets build for WebAssembly under --platforms. swift_binary(linkshared = True) gains the WebAssembly reactor output here (-mexec-model=reactor, no main, exported functions). examples/cross_compilation builds a reactor and a static browser web app that drives it from JS.
- Drop the manual tags and the dedicated CI task; the example now builds and tests in the normal //examples/... wildcard on macOS and Linux. The wasm-only Reactor is target_compatible_with os:wasi so host wildcards skip it while the transition still builds it. - Make the example a real executed test: reactor_test runs the reactor under a hermetic wasmtime (dev dependency, downloaded per host) and asserts on the value Swift computes. - Re-enable swift.file_prefix_map for the wasm toolchain: the developer-dir remap moved behind a feature the wasm toolchain doesn't enable (bazelbuild#1825), so the worker no longer requires DEVELOPER_DIR and debug info stays hermetic. - Toss the top-level example README and move the example to a wasm/ subdirectory (done as part of the rebase; the old shared layout was reorganized by the Android PR), with its dev MODULE wiring split into wasm.MODULE.bazel following the Android pattern. Claude-Session: https://claude.ai/code/session_01SmG1kqA3qB4WsLGU2xavuJ
CI passes --test_env=HOME, and linux-sandbox mounts the agent's HOME read-only, so wasmtime's default compilation cache hard-failed with 'failed to create cache directory'. Set XDG_CACHE_HOME to the test tmpdir, and stop swallowing wasmtime's stderr so failures like this are visible in the test log. Claude-Session: https://claude.ai/code/session_01SmG1kqA3qB4WsLGU2xavuJ
- Rebase onto main (dissolves the example bzl_library targets removed by bazelbuild#1826 and the embedded file_prefix_map disables removed by bazelbuild#1824). - Use @bazel_tools//src/conditions for the wasmtime host select instead of bespoke config_settings. - Drop web/README.md (covered by the BUILD file) and trim comments. Claude-Session: https://claude.ai/code/session_01SmG1kqA3qB4WsLGU2xavuJ
Drop the wasm example's redundant header comment and default_visibility (nothing outside the package references its targets), and document why the SDK repository pairs a rules_cc toolchain with the Swift one: it drives the bundle's own clang, resolves only for the wasm target platform, and a consumer's root-module registration takes precedence over it.
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Rebased onto main (post-#1819) and addressed the remaining comments — the example BUILD nits are applied, and the wasm cc-toolchain registration is kept with the rationale documented inline (wasm-platform-only resolution; root-module registrations take precedence for anyone bringing their own). |
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WebAssembly support, part of the #1809 split, on top of the now-merged foundation
PRs #1820 (
swift_binary(linkshared)) and #1821 (Swift-SDK framework). A siblingof the Android PR (#1818).
What this adds
swift.wasm_sdk: downloads the swift.org WebAssembly Swift SDK bundle matching aswift.toolchainand, using the shared framework (#1821), defines Swift + C/C++toolchains targeting
wasm32-unknown-wasip1.swift_binary(linkshared = True)gains the WebAssembly reactor output here (
-mexec-model=reactor, nomain,exported functions) — the wasm-specific bit that keith asked to keep out of the
generic
swift_binary.examples/cross_compilationbuilds a reactor and abrowser web app that drives it from JS.
Open questions
-swift.file_prefix_mapis disabled for the wasm toolchain. With it on, theworker's developer-dir remap hard-requires
DEVELOPER_DIRon Apple hosts. Theclean fix is to gate that remap on
swift._supports_developer_dir(theanalysis-time config already is); it touches the shared Apple worker path, so I
wanted your read first.