Correctly fill default type parameters

Cargo.lock

@@ -936,6 +936,7 @@ dependencies = [
  "ra_db",
  "ra_fmt",
  "ra_hir",
+ "ra_hir_def",
  "ra_ide_db",
  "ra_prof",
  "ra_syntax",

crates/ra_assists/Cargo.toml

@@ -21,4 +21,5 @@ ra_prof = { path = "../ra_prof" }
 ra_db = { path = "../ra_db" }
 ra_ide_db = { path = "../ra_ide_db" }
 hir = { path = "../ra_hir", package = "ra_hir" }
+hir_def = { path = "../ra_hir_def", package = "ra_hir_def" }
 test_utils = { path = "../test_utils" }

crates/ra_assists/src/ast_transform.rs

@@ -2,6 +2,7 @@
 use rustc_hash::FxHashMap;
 
 use hir::{PathResolution, SemanticsScope};
+use hir_def::type_ref::TypeRef;
 use ra_ide_db::RootDatabase;
 use ra_syntax::{
     algo::SyntaxRewriter,
@@ -51,7 +52,24 @@ impl<'a> SubstituteTypeParams<'a> {
             .into_iter()
             // this is a trait impl, so we need to skip the first type parameter -- this is a bit hacky
             .skip(1)
-            .zip(substs.into_iter())
+            // The actual list of trait type parameters may be longer than the one
+            // used in the `impl` block due to trailing default type parametrs.
+            // For that case we extend the `substs` with an empty iterator so we
+            // can still hit those trailing values and check if they actually have
+            // a default type. If they do, go for that type from `hir` to `ast` so
+            // the resulting change can be applied correctly.
+            .zip(substs.into_iter().map(Some).chain(std::iter::repeat(None)))
+            .filter_map(|(k, v)| match v {
+                Some(v) => Some((k, v)),
+                None => match k.default(source_scope.db)? {
+                    TypeRef::Path(path) => Some((
+                        k,
+                        ast::make::type_arg(&format!("{}", path.mod_path().as_ident()?))
+                            .type_ref()?,
+                    )),
+                    _ => None,
+                },
+            })
             .collect();
         return SubstituteTypeParams {
             source_scope,

crates/ra_assists/src/handlers/add_missing_impl_members.rs

@@ -615,6 +615,56 @@ trait Foo {
 struct S;
 impl Foo for S {
     <|>fn valid(some: u32) -> bool { false }
+}"#,
+        )
+    }
+
+    #[test]
+    fn test_generic_single_default_parameter() {
+        check_assist(
+            add_missing_impl_members,
+            r#"
+trait Foo<T = Self> {
+    fn bar(&self, other: &T);
+}
+
+struct S;
+impl Foo for S { <|> }"#,
+            r#"
+trait Foo<T = Self> {
+    fn bar(&self, other: &T);
+}
+
+struct S;
+impl Foo for S {
+    <|>fn bar(&self, other: &Self) {
+        todo!()
+    }
+}"#,
+        )
+    }
+
+    #[test]
+    fn test_generic_default_parameter_is_second() {
+        check_assist(
+            add_missing_impl_members,
+            r#"
+trait Foo<T1, T2 = Self> {
+    fn bar(&self, this: &T1, that: &T2);
+}
+
+struct S<T>;
+impl Foo<T> for S<T> { <|> }"#,
+            r#"
+trait Foo<T1, T2 = Self> {
+    fn bar(&self, this: &T1, that: &T2);
+}
+
+struct S<T>;
+impl Foo<T> for S<T> {
+    <|>fn bar(&self, this: &T, that: &Self) {
+        todo!()
+    }
 }"#,
         )
     }

crates/ra_hir/src/code_model.rs

@@ -989,6 +989,11 @@ impl TypeParam {
             ty: InEnvironment { value: ty, environment },
         }
     }
+
+    pub fn default(self, db: &dyn HirDatabase) -> Option<TypeRef> {
+        let params = db.generic_params(self.id.parent);
+        params.types[self.id.local_id].default.clone()
+    }
 }
 
 // FIXME: rename from `ImplDef` to `Impl`

crates/ra_syntax/src/ast/make.rs

@@ -13,6 +13,10 @@ pub fn name_ref(text: &str) -> ast::NameRef {
     ast_from_text(&format!("fn f() {{ {}; }}", text))
 }
 
+pub fn type_arg(text: &str) -> ast::TypeArg {
+    ast_from_text(&format!("impl F<{}> for G {{}}", text))
+}
+
 pub fn path_segment(name_ref: ast::NameRef) -> ast::PathSegment {
     ast_from_text(&format!("use {};", name_ref))
 }