Fix error messages reporting number of expected vs actual params

We previously had some off-by-one errors in our error messages and this led to
very confusing messages like "expected 0 types, found 0" that were quite
annoying to debug as an API consumer.

crates/wasmtime/src/func/typed.rs

@@ -347,17 +347,23 @@ macro_rules! impl_wasm_params {
 
             fn typecheck(mut params: impl ExactSizeIterator<Item = crate::ValType>) -> Result<()> {
                 let mut _n = 0;
+
                 $(
                     match params.next() {
-                        Some(t) => $t::typecheck(t)?,
+                        Some(t) => {
-                        None => bail!("expected {} types, found {}", $n, _n),
-                    }
                             _n += 1;
+                            $t::typecheck(t)?
+                        },
+                        None => bail!("expected {} types, found {}", $n, params.len() + _n),
+                    }
                 )*
 
                 match params.next() {
                     None => Ok(()),
-                    Some(_) => bail!("expected {} types, found {}", $n, params.len() + _n),
+                    Some(_) => {
+                        _n += 1;
+                        bail!("expected {} types, found {}", $n, params.len() + _n)
+                    },
                 }
             }
 

tests/all/func.rs

@@ -884,3 +884,45 @@ fn wasm_ty_roundtrip() -> Result<(), anyhow::Error> {
     foo.call(&mut store, (-1, 1, 2.0, -3, 3, 4.0))?;
     Ok(())
 }
+
+#[test]
+fn typed_funcs_count_params_correctly_in_error_messages() -> anyhow::Result<()> {
+    let mut store = Store::<()>::default();
+    let module = Module::new(
+        store.engine(),
+        r#"
+            (module
+                (func (export "f") (param i32 i32))
+            )
+
+        "#,
+    )?;
+    let instance = Instance::new(&mut store, &module, &[])?;
+
+    // Too few parameters.
+    match instance.get_typed_func::<(), (), _>(&mut store, "f") {
+        Ok(_) => panic!("should be wrong signature"),
+        Err(e) => {
+            let msg = format!("{:?}", e);
+            assert!(dbg!(msg).contains("expected 0 types, found 2"))
+        }
+    }
+    match instance.get_typed_func::<(i32,), (), _>(&mut store, "f") {
+        Ok(_) => panic!("should be wrong signature"),
+        Err(e) => {
+            let msg = format!("{:?}", e);
+            assert!(dbg!(msg).contains("expected 1 types, found 2"))
+        }
+    }
+
+    // Too many parameters.
+    match instance.get_typed_func::<(i32, i32, i32), (), _>(&mut store, "f") {
+        Ok(_) => panic!("should be wrong signature"),
+        Err(e) => {
+            let msg = format!("{:?}", e);
+            assert!(dbg!(msg).contains("expected 3 types, found 2"))
+        }
+    }
+
+    Ok(())
+}