diff --git a/coroutine/asyncify/Context.h b/coroutine/asyncify/Context.h
index 71791a400492b5..4e1a804923a5ef 100644
--- a/coroutine/asyncify/Context.h
+++ b/coroutine/asyncify/Context.h
@@ -62,7 +62,9 @@ static inline void coroutine_initialize(struct coroutine_context *context, corou
 static inline struct coroutine_context * coroutine_transfer(struct coroutine_context * current, struct coroutine_context * target)
 {
     if (ASYNCIFY_CORO_DEBUG) fprintf(stderr, "[%s] entry (current = %p, target = %p)\n", __func__, current, target);
+#ifndef COROUTINE_TARGET_MAY_BE_FREED
     struct coroutine_context * previous = target->from;
+#endif
 
     target->from = current;
     if (ASYNCIFY_CORO_DEBUG) fprintf(stderr, "[%s] current->current_sp = %p -> %p\n", __func__, current->current_sp, rb_wasm_get_stack_pointer());
@@ -77,7 +79,11 @@ static inline struct coroutine_context * coroutine_transfer(struct coroutine_con
 
     rb_wasm_set_stack_pointer(current->current_sp);
 
+#ifndef COROUTINE_TARGET_MAY_BE_FREED
+    /* from is read only by coroutine_trampoline, when target starts, which has
+     * happened before we get here. */
     target->from = previous;
+#endif
 
     return target;
 }
diff --git a/coroutine/emscripten/Context.h b/coroutine/emscripten/Context.h
index 361e241685682f..c26b4b23bbde92 100644
--- a/coroutine/emscripten/Context.h
+++ b/coroutine/emscripten/Context.h
@@ -60,11 +60,17 @@ static inline void coroutine_initialize(
 
 static inline struct coroutine_context * coroutine_transfer(struct coroutine_context * current, struct coroutine_context * target)
 {
+#ifndef COROUTINE_TARGET_MAY_BE_FREED
     struct coroutine_context * previous = target->from;
+#endif
 
     target->from = current;
     emscripten_fiber_swap(&current->state, &target->state);
+#ifndef COROUTINE_TARGET_MAY_BE_FREED
+    /* from is read only by coroutine_trampoline, when target starts, which has
+     * happened before we get here. */
     target->from = previous;
+#endif
 
     return target;
 }
diff --git a/coroutine/pthread/Context.c b/coroutine/pthread/Context.c
index 38774cda0b409d..014abbd4b59b30 100644
--- a/coroutine/pthread/Context.c
+++ b/coroutine/pthread/Context.c
@@ -229,7 +229,11 @@ struct coroutine_context * coroutine_transfer(struct coroutine_context * current
     pthread_testcancel();
 #endif
 
+#ifndef COROUTINE_TARGET_MAY_BE_FREED
+    /* from is read only by coroutine_trampoline, when target starts, which has
+     * happened before we get here. */
     target->from = previous;
+#endif
 
     return target;
 }
diff --git a/coroutine/ucontext/Context.h b/coroutine/ucontext/Context.h
index d338d8de608c3f..0063f72b77beee 100644
--- a/coroutine/ucontext/Context.h
+++ b/coroutine/ucontext/Context.h
@@ -60,11 +60,17 @@ static inline void coroutine_initialize(
 
 static inline struct coroutine_context * coroutine_transfer(struct coroutine_context * current, struct coroutine_context * target)
 {
+#ifndef COROUTINE_TARGET_MAY_BE_FREED
     struct coroutine_context * previous = target->from;
+#endif
 
     target->from = current;
     swapcontext(&current->state, &target->state);
+#ifndef COROUTINE_TARGET_MAY_BE_FREED
+    /* from is read only by coroutine_trampoline, when target starts, which has
+     * happened before we get here. */
     target->from = previous;
+#endif
 
     return target;
 }
diff --git a/ext/digest/blake3/blake3_dispatch.c b/ext/digest/blake3/blake3_dispatch.c
index 14dfbbe0c8f385..f58eebdec12dbc 100644
--- a/ext/digest/blake3/blake3_dispatch.c
+++ b/ext/digest/blake3/blake3_dispatch.c
@@ -19,7 +19,8 @@
 #endif
 
 #if !defined(BLAKE3_ATOMICS)
-#if defined(__has_include)
+#if defined(__has_include) && defined(__STDC_VERSION__) && \
+    __STDC_VERSION__ >= 201112L
 #if __has_include(<stdatomic.h>) && !defined(_MSC_VER)
 #define BLAKE3_ATOMICS 1
 #else
@@ -27,7 +28,7 @@
 #endif /* __has_include(<stdatomic.h>) && !defined(_MSC_VER) */
 #else
 #define BLAKE3_ATOMICS 0
-#endif /* defined(__has_include) */
+#endif /* defined(__has_include) && C11 */
 #endif /* BLAKE3_ATOMICS */
 
 #if BLAKE3_ATOMICS
diff --git a/ext/digest/blake3/extconf.rb b/ext/digest/blake3/extconf.rb
index 6d1b8f54e34bc0..05c98507ef44fd 100644
--- a/ext/digest/blake3/extconf.rb
+++ b/ext/digest/blake3/extconf.rb
@@ -24,6 +24,10 @@
 # Extra per-object compiler flags, keyed by object basename.
 simd_cflags = {}
 
+def blake3_disable(macro)
+  $CPPFLAGS << " -D#{macro}"
+end
+
 # Probe used to confirm the compiler both accepts +flag+ and can compile the
 # intrinsics the backend relies on.
 def blake3_have_isa?(name, flag, snippet)
@@ -55,7 +59,7 @@ def blake3_have_isa?(name, flag, snippet)
       objs << obj
       simd_cflags[obj] = flag
     else
-      $defs << "-D#{no_macro}"
+      blake3_disable(no_macro)
     end
   end
 when /\A(aarch64|arm64)\z/i
@@ -67,11 +71,11 @@ def blake3_have_isa?(name, flag, snippet)
   # No optimized backend wired up for this architecture (e.g. 32-bit x86,
   # ppc): build portable-only.  Disabling every x86 ISA keeps the dispatcher
   # from referencing backends we didn't compile, and NEON is forced off.
-  $defs << "-DBLAKE3_NO_SSE2"
-  $defs << "-DBLAKE3_NO_SSE41"
-  $defs << "-DBLAKE3_NO_AVX2"
-  $defs << "-DBLAKE3_NO_AVX512"
-  $defs << "-DBLAKE3_USE_NEON=0"
+  blake3_disable("BLAKE3_NO_SSE2")
+  blake3_disable("BLAKE3_NO_SSE41")
+  blake3_disable("BLAKE3_NO_AVX2")
+  blake3_disable("BLAKE3_NO_AVX512")
+  blake3_disable("BLAKE3_USE_NEON=0")
 end
 
 $objs = objs.map { |o| "#{o}.#{$OBJEXT}" }
diff --git a/thread_pthread.c b/thread_pthread.c
index b5de62c37db882..2500e8998b8d48 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -62,6 +62,10 @@ static pthread_condattr_t *condattr_monotonic = &condattr_mono;
 static const void *const condattr_monotonic = NULL;
 #endif
 
+/* A retiring shared native thread frees its own context while the threads it
+ * parked are still suspended with that context as their target. */
+#define COROUTINE_TARGET_MAY_BE_FREED 1
+
 #include COROUTINE_H
 
 #ifndef HAVE_SYS_EVENT_H