Merge #6476

6476: Add missing AssocItems in add_custom_impl assist r=matklad a=Veykril

```rust
use std::fmt;

#[derive(Debu<|>g)]
struct Foo {
    bar: String,
}
```
->
```rust
use std::fmt;

struct Foo {
    bar: String,
}

impl fmt::Debug for Foo {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        ${0:todo!()}
    }
}
```

Co-authored-by: Lukas Wirth <lukastw97@gmail.com>

crates/syntax/src/ast/make.rs

@@ -25,6 +25,10 @@ pub fn assoc_item_list() -> ast::AssocItemList {
     ast_from_text("impl C for D {};")
 }
 
+pub fn impl_trait(trait_: ast::Path, ty: ast::Path) -> ast::Impl {
+    ast_from_text(&format!("impl {} for {} {{}}", trait_, ty))
+}
+
 pub fn path_segment(name_ref: ast::NameRef) -> ast::PathSegment {
     ast_from_text(&format!("use {};", name_ref))
 }