docs: describe new feature 'add custom impl for derived trait'

crates/ra_assists/src/assists/add_custom_impl.rs

@@ -12,6 +12,23 @@ use ra_syntax::{
 
 const DERIVE_TRAIT: &'static str = "derive";
 
+// Assist: add_custom_impl
+//
+// Adds impl block for derived trait.
+//
+// ```
+// #[derive(Deb<|>ug, Display)]
+// struct S;
+// ```
+// ->
+// ```
+// #[derive(Display)]
+// struct S;
+//
+// impl Debug for S {
+//
+// }
+// ```
 pub(crate) fn add_custom_impl(ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
     let input = ctx.find_node_at_offset::<ast::AttrInput>()?;
     let attr = input.syntax().parent().and_then(ast::Attr::cast)?;

crates/ra_assists/src/doc_tests/generated.rs

@@ -2,6 +2,25 @@
 
 use super::check;
 
+#[test]
+fn doctest_add_custom_impl() {
+    check(
+        "add_custom_impl",
+        r#####"
+#[derive(Deb<|>ug, Display)]
+struct S;
+"#####,
+        r#####"
+#[derive(Display)]
+struct S;
+
+impl Debug for S {
+
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_add_derive() {
     check(

docs/user/assists.md

@@ -3,6 +3,24 @@
 Cursor position or selection is signified by `┃` character.
 
 
+## `add_custom_impl`
+
+Adds impl block for derived trait.
+
+```rust
+// BEFORE
+#[derive(Deb┃ug, Display)]
+struct S;
+
+// AFTER
+#[derive(Display)]
+struct S;
+
+impl Debug for S {
+
+}
+```
+
 ## `add_derive`
 
 Adds a new `#[derive()]` clause to a struct or enum.