Better error message when mocking structs with elided lifetimes (#622)

If a struct has an elided lifetime in its impl block, then the generics
list from the "impl" won't match the generics list from the struct.
Mockall needs more work to handle that, and I'm not ready to invest that
time right now.

mockall_derive/src/lib.rs

@@ -1556,6 +1556,15 @@ mod automock {
         assert_not_contains(&output, quote!(#[must_use] struct MockFoo));
     }
 
+    #[test]
+    #[should_panic(expected = "automock does not currently support structs with elided lifetimes")]
+    fn elided_lifetimes() {
+        let code = "impl X<'_> {}";
+        let ts = proc_macro2::TokenStream::from_str(code).unwrap();
+        let attrs_ts = proc_macro2::TokenStream::from_str("").unwrap();
+        do_automock(attrs_ts, ts).to_string();
+    }
+
     #[test]
     #[should_panic(expected = "can only mock inline modules")]
     fn external_module() {

mockall_derive/src/mockable_struct.rs

@@ -395,6 +395,17 @@ impl From<ItemImpl> for MockableStruct {
         let name = match &*item_impl.self_ty {
             Type::Path(type_path) => {
                 let n = find_ident_from_path(&type_path.path).0;
+                let self_generics = &type_path.path.segments.last().unwrap().arguments;
+                if let PathArguments::AngleBracketed(abga) = &self_generics {
+                    if item_impl.generics.params.len() != abga.args.len() {
+                        // If a struct's impl block has elided lifetimes, then
+                        // they won't show up in the impl block's generics
+                        // list.  automock can't currently handle that.
+                        // https://github.com/asomers/mockall/issues/610
+                        compile_error(item_impl.span(),
+                            "automock does not currently support structs with elided lifetimes");
+                    }
+                }
                 gen_mock_ident(&n)
             },
             x => {