fixes alignment on opaque types as discueed with @tidwall

neco.c

@@ -7063,6 +7063,7 @@ struct neco_mutex {
 
 
 static_assert(sizeof(neco_mutex) >= sizeof(struct neco_mutex), "");
+static_assert(_Alignof(neco_mutex) == _Alignof(struct neco_mutex), "");
 
 static int mutex_init(neco_mutex *mutex) {
     struct neco_mutex *mu = (void*)mutex;
@@ -7319,6 +7320,7 @@ struct neco_waitgroup {
 };
 
 static_assert(sizeof(neco_waitgroup) >= sizeof(struct neco_waitgroup), "");
+static_assert(_Alignof(neco_waitgroup) == _Alignof(struct neco_waitgroup), "");
 
 inline
 static int check_waitgroup(struct neco_waitgroup *wg) {
@@ -7466,6 +7468,7 @@ struct neco_cond {
 };
 
 static_assert(sizeof(neco_cond) >= sizeof(struct neco_cond), "");
+static_assert(_Alignof(neco_cond) == _Alignof(struct neco_cond), "");
 
 static int cond_init0(struct neco_cond *cv) {
     memset(cv, 0, sizeof(struct neco_cond));

neco.h

@@ -106,7 +106,7 @@ int neco_gen_close(neco_gen *gen);
 /// to a variable or set of variables and helps to avoid data inconsistencies
 /// due to race conditions.
 /// @{
-typedef struct { char _[48]; } neco_mutex;
+typedef struct { _Alignas(16) char _[48]; } neco_mutex;
 
 #define NECO_MUTEX_INITIALIZER { 0 }
 
@@ -128,7 +128,7 @@ int neco_mutex_tryrdlock(neco_mutex *mutex);
 /// At the same time, neco_waitgroup_wait() can be used to block until all
 /// coroutines are completed.
 /// @{
-typedef struct { char _[48]; } neco_waitgroup;
+typedef struct { _Alignas(16) char _[48]; } neco_waitgroup;
 
 #define NECO_WAITGROUP_INITIALIZER { 0 }
 
@@ -143,7 +143,7 @@ int neco_waitgroup_wait_dl(neco_waitgroup *waitgroup, int64_t deadline);
 /// A condition variable is a synchronization mechanism that allows coroutines
 /// to suspend execution until some condition is true. 
 /// @{
-typedef struct { char _[48]; } neco_cond;
+typedef struct { _Alignas(16) char _[48]; } neco_cond;
 #define NECO_COND_INITIALIZER { 0 }
 
 int neco_cond_init(neco_cond *cond);