3 files changed, 183 insertions, 0 deletions

diff --git a/crates/assists/src/handlers/replace_impl_trait_with_generic.rs b/crates/assists/src/handlers/replace_impl_trait_with_generic.rs
new file mode 100644
index 000000000..6738bc134
--- /dev/null
+++ b/crates/assists/src/handlers/replace_impl_trait_with_generic.rs
@@ -0,0 +1,168 @@
+use syntax::ast::{self, edit::AstNodeEdit, make, AstNode, GenericParamsOwner};
+
+use crate::{AssistContext, AssistId, AssistKind, Assists};
+
+// Assist: replace_impl_trait_with_generic
+//
+// Replaces `impl Trait` function argument with the named generic.
+//
+// ```
+// fn foo(bar: <|>impl Bar) {}
+// ```
+// ->
+// ```
+// fn foo<B: Bar>(bar: B) {}
+// ```
+pub(crate) fn replace_impl_trait_with_generic(
+    acc: &mut Assists,
+    ctx: &AssistContext,
+) -> Option<()> {
+    let type_impl_trait = ctx.find_node_at_offset::<ast::ImplTraitType>()?;
+    let type_param = type_impl_trait.syntax().parent().and_then(ast::Param::cast)?;
+    let type_fn = type_param.syntax().ancestors().find_map(ast::Fn::cast)?;
+
+    let impl_trait_ty = type_impl_trait.type_bound_list()?;
+
+    let target = type_fn.syntax().text_range();
+    acc.add(
+        AssistId("replace_impl_trait_with_generic", AssistKind::RefactorRewrite),
+        "Replace impl trait with generic",
+        target,
+        |edit| {
+            let generic_letter = impl_trait_ty.to_string().chars().next().unwrap().to_string();
+
+            let generic_param_list = type_fn
+                .generic_param_list()
+                .unwrap_or_else(|| make::generic_param_list(None))
+                .append_param(make::generic_param(generic_letter.clone(), Some(impl_trait_ty)));
+
+            let new_type_fn = type_fn
+                .replace_descendant::<ast::Type>(type_impl_trait.into(), make::ty(&generic_letter))
+                .with_generic_param_list(generic_param_list);
+
+            edit.replace_ast(type_fn.clone(), new_type_fn);
+        },
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::tests::check_assist;
+
+    #[test]
+    fn replace_impl_trait_with_generic_params() {
+        check_assist(
+            replace_impl_trait_with_generic,
+            r#"
+            fn foo<G>(bar: <|>impl Bar) {}
+            "#,
+            r#"
+            fn foo<G, B: Bar>(bar: B) {}
+            "#,
+        );
+    }
+
+    #[test]
+    fn replace_impl_trait_without_generic_params() {
+        check_assist(
+            replace_impl_trait_with_generic,
+            r#"
+            fn foo(bar: <|>impl Bar) {}
+            "#,
+            r#"
+            fn foo<B: Bar>(bar: B) {}
+            "#,
+        );
+    }
+
+    #[test]
+    fn replace_two_impl_trait_with_generic_params() {
+        check_assist(
+            replace_impl_trait_with_generic,
+            r#"
+            fn foo<G>(foo: impl Foo, bar: <|>impl Bar) {}
+            "#,
+            r#"
+            fn foo<G, B: Bar>(foo: impl Foo, bar: B) {}
+            "#,
+        );
+    }
+
+    #[test]
+    fn replace_impl_trait_with_empty_generic_params() {
+        check_assist(
+            replace_impl_trait_with_generic,
+            r#"
+            fn foo<>(bar: <|>impl Bar) {}
+            "#,
+            r#"
+            fn foo<B: Bar>(bar: B) {}
+            "#,
+        );
+    }
+
+    #[test]
+    fn replace_impl_trait_with_empty_multiline_generic_params() {
+        check_assist(
+            replace_impl_trait_with_generic,
+            r#"
+            fn foo<
+            >(bar: <|>impl Bar) {}
+            "#,
+            r#"
+            fn foo<B: Bar
+            >(bar: B) {}
+            "#,
+        );
+    }
+
+    #[test]
+    #[ignore = "This case is very rare but there is no simple solutions to fix it."]
+    fn replace_impl_trait_with_exist_generic_letter() {
+        check_assist(
+            replace_impl_trait_with_generic,
+            r#"
+            fn foo<B>(bar: <|>impl Bar) {}
+            "#,
+            r#"
+            fn foo<B, C: Bar>(bar: C) {}
+            "#,
+        );
+    }
+
+    #[test]
+    fn replace_impl_trait_with_multiline_generic_params() {
+        check_assist(
+            replace_impl_trait_with_generic,
+            r#"
+            fn foo<
+                G: Foo,
+                F,
+                H,
+            >(bar: <|>impl Bar) {}
+            "#,
+            r#"
+            fn foo<
+                G: Foo,
+                F,
+                H, B: Bar
+            >(bar: B) {}
+            "#,
+        );
+    }
+
+    #[test]
+    fn replace_impl_trait_multiple() {
+        check_assist(
+            replace_impl_trait_with_generic,
+            r#"
+            fn foo(bar: <|>impl Foo + Bar) {}
+            "#,
+            r#"
+            fn foo<F: Foo + Bar>(bar: F) {}
+            "#,
+        );
+    }
+}
diff --git a/crates/assists/src/lib.rs b/crates/assists/src/lib.rs
index 2e0d191a6..cbac53e71 100644
--- a/crates/assists/src/lib.rs
+++ b/crates/assists/src/lib.rs
@@ -155,6 +155,7 @@ mod handlers {
     mod remove_unused_param;
     mod reorder_fields;
     mod replace_if_let_with_match;
+    mod replace_impl_trait_with_generic;
     mod replace_let_with_if_let;
     mod replace_qualified_name_with_use;
     mod replace_unwrap_with_match;
@@ -202,6 +203,7 @@ mod handlers {
             remove_unused_param::remove_unused_param,
             reorder_fields::reorder_fields,
             replace_if_let_with_match::replace_if_let_with_match,
+            replace_impl_trait_with_generic::replace_impl_trait_with_generic,
             replace_let_with_if_let::replace_let_with_if_let,
             replace_qualified_name_with_use::replace_qualified_name_with_use,
             replace_unwrap_with_match::replace_unwrap_with_match,
diff --git a/crates/assists/src/tests/generated.rs b/crates/assists/src/tests/generated.rs
index 04c8fd1f9..27d15adb0 100644
--- a/crates/assists/src/tests/generated.rs
+++ b/crates/assists/src/tests/generated.rs
@@ -815,6 +815,19 @@ fn handle(action: Action) {
 }
 
 #[test]
+fn doctest_replace_impl_trait_with_generic() {
+    check_doc_test(
+        "replace_impl_trait_with_generic",
+        r#####"
+fn foo(bar: <|>impl Bar) {}
+"#####,
+        r#####"
+fn foo<B: Bar>(bar: B) {}
+"#####,
+    )
+}
+
+#[test]
 fn doctest_replace_let_with_if_let() {
     check_doc_test(
         "replace_let_with_if_let",