Avoid shared function bodies for now

Since it may take a while to implement all the "reverse validation" in
wasm-smith's instruction generator, this effectively cordons off that
bit of the fuzzer so that we can at least generate shared types in
modules. Later, once the instruction generator is smarter,  we will
remove this filter and start generating shared function bodies.

crates/wasm-smith/src/core.rs

@@ -1618,12 +1618,26 @@ impl Module {
             return Ok(());
         }
 
+        // For now, only define non-shared functions. Until we can update
+        // instruction generation to understand the additional sharedness
+        // validation, we don't want to generate instructions that touch
+        // unshared objects from a shared context (TODO: handle shared).
+        let unshared_func_types: Vec<_> = self
+            .func_types
+            .iter()
+            .copied()
+            .filter(|&i| !self.is_shared_type(i))
+            .collect();
+        if unshared_func_types.is_empty() {
+            return Ok(());
+        }
+
         arbitrary_loop(u, self.config.min_funcs, self.config.max_funcs, |u| {
             if !self.can_add_local_or_import_func() {
                 return Ok(false);
             }
-            let max = self.func_types.len() - 1;
-            let ty = self.func_types[u.int_in_range(0..=max)?];
+            let max = unshared_func_types.len() - 1;
+            let ty = unshared_func_types[u.int_in_range(0..=max)?];
             self.funcs.push((ty, self.func_type(ty).clone()));
             self.num_defined_funcs += 1;
             Ok(true)