wasm: review-pass cleanup

- Rebase onto main (dissolves the example bzl_library targets removed by
  #1826 and the embedded file_prefix_map disables removed by #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

Author: AttilaTheFun

examples/cross_compilation/wasm/BUILD.bazel

@@ -66,45 +66,13 @@ sh_test(
 alias(
     name = "wasmtime",
     actual = select({
-        ":darwin_arm64": "@wasmtime_darwin_aarch64//:wasmtime",
-        ":darwin_x86_64": "@wasmtime_darwin_x86_64//:wasmtime",
-        ":linux_arm64": "@wasmtime_linux_aarch64//:wasmtime",
-        ":linux_x86_64": "@wasmtime_linux_x86_64//:wasmtime",
+        "@bazel_tools//src/conditions:darwin_arm64": "@wasmtime_darwin_aarch64//:wasmtime",
+        "@bazel_tools//src/conditions:darwin_x86_64": "@wasmtime_darwin_x86_64//:wasmtime",
+        "@bazel_tools//src/conditions:linux_aarch64": "@wasmtime_linux_aarch64//:wasmtime",
+        "@bazel_tools//src/conditions:linux_x86_64": "@wasmtime_linux_x86_64//:wasmtime",
     }),
 )
 
-config_setting(
-    name = "darwin_arm64",
-    constraint_values = [
-        "@platforms//cpu:arm64",
-        "@platforms//os:macos",
-    ],
-)
-
-config_setting(
-    name = "darwin_x86_64",
-    constraint_values = [
-        "@platforms//cpu:x86_64",
-        "@platforms//os:macos",
-    ],
-)
-
-config_setting(
-    name = "linux_arm64",
-    constraint_values = [
-        "@platforms//cpu:arm64",
-        "@platforms//os:linux",
-    ],
-)
-
-config_setting(
-    name = "linux_x86_64",
-    constraint_values = [
-        "@platforms//cpu:x86_64",
-        "@platforms//os:linux",
-    ],
-)
-
 # A static web app embedding the reactor: `index.html` + `Reactor.wasm` in one
 # directory. Serve it (e.g. `python3 -m http.server -d
 # bazel-bin/examples/cross_compilation/wasm/web_app`) and open it — the page

examples/cross_compilation/wasm/web/README.md

@@ -377,14 +377,10 @@ def _swift_wasm_sdk_impl(repository_ctx):
             "-lwasi-emulated-mman",
             "-lwasi-emulated-signal",
             "-lwasi-emulated-process-clocks",
-            # Place the linear-memory data and the indirect function table at the
-            # same bases `swiftc` uses for its own wasm links. The Swift driver
-            # always passes these to wasm-ld; in particular `--table-base=4096`
-            # is required — optimized (`-O`) Swift relies on the indirect
-            # function table starting where the runtime/codegen expects it, and
-            # without it generic-metadata instantiation reads out of bounds at
-            # runtime (`__swift_instantiateGenericMetadata` faults). `-Onone`
-            # happens to tolerate the default base, which masks the bug.
+            # The Swift driver always passes these bases to wasm-ld.
+            # `--table-base=4096` in particular is required: without it,
+            # optimized (`-O`) generic-metadata instantiation reads out of
+            # bounds at runtime (`-Onone` happens to tolerate the default).
             "-Wl,--global-base=4096",
             "-Wl,--table-base=4096",
         ]),

swift/internal/extensions/swift_sdks.bzl

@@ -55,9 +55,6 @@ toolchain(
 )
 """
 
-# Unlike Android (whose C/C++ toolchain comes from a separately registered
-# Android cc toolchain), the WebAssembly SDK repository also generates a
-# rules_cc cc_toolchain that drives the paired toolchain's clang.
 _WASM_SDK_TOOLCHAIN_PLATFORM = """
 # Swift SDK toolchains from repository: `{sdk_repository}`
 toolchain(

swift/internal/extensions/toolchains.bzl

@@ -203,12 +203,10 @@ def _swift_binary_impl(ctx):
     else:
         name = ctx.label.name
 
-    # When targeting WebAssembly a `linkshared` binary is a "reactor" module: it
-    # is still produced as an `executable`-shaped wasm file (not a `-shared`
-    # dynamic library), but linked with the reactor execution model so it has no
-    # `_start`/`main` and instead exports functions for a host to call.
-    # Everywhere else, `linkshared` produces a real dynamic library
-    # (`lib<name>.so` / `.dylib`), matching `cc_binary`'s `linkshared`.
+    # On WebAssembly a `linkshared` binary is a "reactor": an executable-shaped
+    # wasm module linked with `-mexec-model=reactor` (no `main`; it exports
+    # functions for a host to call). Everywhere else `linkshared` produces a
+    # real dynamic library, matching `cc_binary`.
     shared_link_flags = []
     if ctx.attr.linkshared and not is_wasm:
         output_type = "dynamic_library"