refactor: rename DifferentialWasmiModuleConfig to SingleFunctionModuleConfig

Since we plan to reuse this configuration, we rename it and ensure it
has at least 1 type (this resulted in invalid modules).

crates/fuzzing/src/oracles.rs

@@ -563,15 +563,20 @@ pub fn table_ops(
 }
 
 /// Configuration options for wasm-smith such that generated modules always
-/// conform to certain specifications.
+/// conform to certain specifications: one exported function, one exported
+/// memory.
 #[derive(Default, Debug, Arbitrary, Clone)]
-pub struct DifferentialWasmiModuleConfig;
+pub struct SingleFunctionModuleConfig;
 
-impl wasm_smith::Config for DifferentialWasmiModuleConfig {
+impl wasm_smith::Config for SingleFunctionModuleConfig {
     fn allow_start_export(&self) -> bool {
         false
     }
 
+    fn min_types(&self) -> usize {
+        1
+    }
+
     fn min_funcs(&self) -> usize {
         1
     }
@@ -609,7 +614,7 @@ impl wasm_smith::Config for DifferentialWasmiModuleConfig {
 /// resulting memory image when execution terminates. This relies on the
 /// module-under-test to be instrumented to bound the execution time. Invoke
 /// with a module generated by `wasm-smith` using the
-/// `DiferentialWasmiModuleConfig` configuration type for best results.
+/// `SingleFunctionModuleConfig` configuration type for best results.
 ///
 /// May return `None` if we early-out due to a rejected fuzz config; these
 /// should be rare if modules are generated appropriately.

fuzz/fuzz_targets/differential_wasmi.rs

@@ -5,7 +5,7 @@ use wasmtime_fuzzing::{generators, oracles};
 
 fuzz_target!(|data: (
     generators::Config,
-    wasm_smith::ConfiguredModule<oracles::DifferentialWasmiModuleConfig>
+    wasm_smith::ConfiguredModule<oracles::SingleFunctionModuleConfig>
 )| {
     let (config, mut wasm) = data;
     wasm.ensure_termination(1000);