Auto merge of #14948 - alibektas:14386, r=Veykril

feature : assist delegate impl

This PR ( fixes #14386 ) introduces a new IDE assist that generates a trait impl for a struct that delegates a field. This is a draft because the current `ide_db::path_transform::PathTransform` produces some unwanted results when it deals with extern crates, an example of which I attach as a GIF.

GIFs :
1. A general case
![14386-functional](https://github.com/rust-lang/rust-analyzer/assets/20956650/22114959-caa6-45ec-a154-b4b2f458f6b1)

2. A case where `ide_db::path_transform::PathTransform` fails to correctly resolve a property ( take `Allocator` as an example ) to its full path, thus causing an error to occur. ( Not to even mention that resolving this causes another error `use of unstable library feature 'allocator_api'` to occur
![14386-erroneous](https://github.com/rust-lang/rust-analyzer/assets/20956650/922ca715-594e-4168-a579-7c5c006f93aa)

crates/ide-assists/src/lib.rs

@@ -145,6 +145,7 @@ mod handlers {
     mod generate_constant;
     mod generate_default_from_enum_variant;
     mod generate_default_from_new;
+    mod generate_delegate_trait;
     mod generate_deref;
     mod generate_derive;
     mod generate_documentation_template;
@@ -251,6 +252,7 @@ pub(crate) fn all() -> &'static [Handler] {
             generate_constant::generate_constant,
             generate_default_from_enum_variant::generate_default_from_enum_variant,
             generate_default_from_new::generate_default_from_new,
+            generate_delegate_trait::generate_delegate_trait,
             generate_derive::generate_derive,
             generate_documentation_template::generate_documentation_template,
             generate_documentation_template::generate_doc_example,

crates/ide-assists/src/tests/generated.rs

@@ -1015,6 +1015,69 @@ impl Person {
     )
 }
 
+#[test]
+fn doctest_generate_delegate_trait() {
+    check_doc_test(
+        "generate_delegate_trait",
+        r#####"
+trait SomeTrait {
+    type T;
+    fn fn_(arg: u32) -> u32;
+    fn method_(&mut self) -> bool;
+}
+struct A;
+impl SomeTrait for A {
+    type T = u32;
+
+    fn fn_(arg: u32) -> u32 {
+        42
+    }
+
+    fn method_(&mut self) -> bool {
+        false
+    }
+}
+struct B {
+    a$0: A,
+}
+"#####,
+        r#####"
+trait SomeTrait {
+    type T;
+    fn fn_(arg: u32) -> u32;
+    fn method_(&mut self) -> bool;
+}
+struct A;
+impl SomeTrait for A {
+    type T = u32;
+
+    fn fn_(arg: u32) -> u32 {
+        42
+    }
+
+    fn method_(&mut self) -> bool {
+        false
+    }
+}
+struct B {
+    a: A,
+}
+
+impl SomeTrait for B {
+    type T = <A as SomeTrait>::T;
+
+    fn fn_(arg: u32) -> u32 {
+        <A as SomeTrait>::fn_(arg)
+    }
+
+    fn method_(&mut self) -> bool {
+        <A as SomeTrait>::method_( &mut self.a )
+    }
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_generate_deref() {
     check_doc_test(