rust: preserve WASIp3 TLS for no-std task context (#1745)

* rust: preserve WASIp3 TLS for no-std task context

The prebuilt task-context helper was compiled with -mcpu=mvp, which
lowers its _Thread_local storage to a plain global. Overlapping async
exports on WASIp3 then share the same task state and panic in start_task.

Keep wasip3_task_set in the MVP archive used by every wasm32 target.
Move the versioned context accessors into a separate archive compiled
for wasm32-wasip3 and link it only for that target. Regenerate the C,
object, and archive files and check the TLS-base import during rebuilds.

Validation:
- Stock nightly-2026-10-12 and Wasmtime 47.0.2: the unpatched HTTP
  handler returns 20/20 concurrent 500s; the fix returns 20/20 200s.
- Rust 1.99.0 builds binaries and cdylibs for WASIp1, WASIp2, and
  wasm32-unknown-unknown without introducing TLS-base imports.
- WASIp1/WASIp2 binaries and the unknown-target probe run successfully;
  native unit tests and the generated doctest pass.
- SDK 34 and wasm-tools 1.258.0 regenerate artifacts identically on
  macOS arm64. Reintroducing the MVP flag makes the rebuild guard fail.
- cargo fmt --all --check and git diff --check pass.

* rust: link the WASIp3 TLS helper only without std

Author: macovedj

ci/rebuild-libwit-bindgen-cabi.sh

@@ -48,6 +48,7 @@ wasip3_set=wit_bindgen_${version}_set_task_context
 rm -f crates/guest-rust/src/rt/wit_bindgen_*.{rs,o,c}
 rm -f crates/guest-rust/src/rt/libwit_bindgen_cabi_realloc.a
 rm -f crates/guest-rust/src/rt/libwit_bindgen_cabi_wasip3.a
+rm -f crates/guest-rust/src/rt/libwit_bindgen_cabi_wasip3_tls.a
 
 cat >./crates/guest-rust/src/rt/wit_bindgen_cabi_realloc.rs <<-EOF
 // This file is generated by $0
@@ -100,6 +101,15 @@ void *wasip3_task_set(void *ptr) {
   WASIP3_TASK = ptr;
   return ret;
 }
+EOF
+
+# On WASIp3, wasi-libc switches TLS blocks between tasks. Compile this helper
+# for that target so `_Thread_local` remains TLS; compiling it as MVP would
+# silently turn `task_context` into a global shared by all tasks.
+cat >./crates/guest-rust/src/rt/wit_bindgen_cabi_wasip3_tls.c <<-EOF
+// This file is generated by $0
+
+#include <stddef.h>
 
 static _Thread_local void *task_context = NULL;
 
@@ -114,19 +124,30 @@ EOF
 
 build() {
   file=$1
-  $WASI_SDK_PATH/bin/clang crates/guest-rust/src/rt/$1.c \
-    -O -c -o crates/guest-rust/src/rt/$1.o -mcpu=mvp -fPIC
+  shift
+  $WASI_SDK_PATH/bin/clang crates/guest-rust/src/rt/$file.c \
+    -O -c -o crates/guest-rust/src/rt/$file.o -fPIC "$@"
   # Remove the `producers` section. This appears to differ whether the host for
   # clang is either macOS or Linux. Not needed here anyway, so discard it to help
   # either host produce the same object.
-  wasm-tools strip -d producers ./crates/guest-rust/src/rt/$1.o \
-    -o ./crates/guest-rust/src/rt/$1.o
+  wasm-tools strip -d producers ./crates/guest-rust/src/rt/$file.o \
+    -o ./crates/guest-rust/src/rt/$file.o
 }
 
-build wit_bindgen_cabi_realloc
-build wit_bindgen_cabi_wasip3
+build wit_bindgen_cabi_realloc -mcpu=mvp
+build wit_bindgen_cabi_wasip3 -mcpu=mvp
+build wit_bindgen_cabi_wasip3_tls --target=wasm32-wasip3
+
+# Catch accidentally compiling the task-local state as a plain global again.
+if ! wasm-tools print crates/guest-rust/src/rt/wit_bindgen_cabi_wasip3_tls.o \
+  | grep -qF '(import "env" "__wasm_get_tls_base"'; then
+  echo "error: WASIp3 task context must use TLS" >&2
+  exit 1
+fi
 
 $WASI_SDK_PATH/bin/llvm-ar crus crates/guest-rust/src/rt/libwit_bindgen_cabi_realloc.a \
   crates/guest-rust/src/rt/wit_bindgen_cabi_realloc.o
 $WASI_SDK_PATH/bin/llvm-ar crus crates/guest-rust/src/rt/libwit_bindgen_cabi_wasip3.a \
   crates/guest-rust/src/rt/wit_bindgen_cabi_wasip3.o
+$WASI_SDK_PATH/bin/llvm-ar crus crates/guest-rust/src/rt/libwit_bindgen_cabi_wasip3_tls.a \
+  crates/guest-rust/src/rt/wit_bindgen_cabi_wasip3_tls.o

crates/guest-rust/build.rs

@@ -18,6 +18,12 @@ fn main() {
         link_lib("cabi_realloc");
     }
     link_lib("cabi_wasip3");
+
+    // The no-std async runtime's task context uses WASIp3 TLS. Keep this
+    // separate from the MVP task helper needed by every wasm32 target.
+    if target_env == "p3" && !cfg!(feature = "std") {
+        link_lib("cabi_wasip3_tls");
+    }
 }
 
 fn link_lib(name: &str) {

crates/guest-rust/src/rt/libwit_bindgen_cabi_wasip3.a

@@ -10,13 +10,3 @@ void *wasip3_task_set(void *ptr) {
   WASIP3_TASK = ptr;
   return ret;
 }
-
-static _Thread_local void *task_context = NULL;
-
-void *wit_bindgen_0_62_0_get_task_context(void) {
-  return task_context;
-}
-
-void wit_bindgen_0_62_0_set_task_context(void *ptr) {
-  task_context = ptr;
-}

crates/guest-rust/src/rt/libwit_bindgen_cabi_wasip3_tls.a

@@ -0,0 +1,13 @@
+// This file is generated by ./ci/rebuild-libwit-bindgen-cabi.sh
+
+#include <stddef.h>
+
+static _Thread_local void *task_context = NULL;
+
+void *wit_bindgen_0_62_0_get_task_context(void) {
+  return task_context;
+}
+
+void wit_bindgen_0_62_0_set_task_context(void *ptr) {
+  task_context = ptr;
+}