fix: rewrite code_action generate_delegate_trait

[roife]

I've made substantial enhancements to the "generate delegate trait" code action in rust-analyzer. Here's a summary of the changes:

Resolved the "Can’t find CONST_ARG@158..159 in AstIdMap" error

Fix #15804, fix #15968, fix #15108

The issue stemmed from an incorrect application of PathTransform in the original code. Previously, a new 'impl' was generated first and then transformed, causing PathTransform to fail in locating the correct AST node, resulting in an error. I rectified this by performing the transformation before generating the new 'impl' (using make::impl_trait), ensuring a step-by-step transformation of associated items.

Rectified generation of Self type

generate_delegate_trait is unable to properly handle trait with Self type.

Let's take the following code as an example:

trait Trait {
    fn f() -> Self;
}

struct B {}
impl Trait for B {
    fn f() -> B { B{} }
}

struct S {
    b: B,
}

Here, if we implement Trait for S, the type of f should be () -> Self, i.e. () -> S. However we cannot automatically generate a function that constructs S.

To ensure that the code action doesn't generate delegate traits for traits with Self types, I add a function named has_self_type to handle it.

Extended support for generics in structs and fields within this code action

The former version of generate_delegate_trait cannot handle structs with generics properly. Here's an example:

struct B<T> {
    a: T
}

trait Trait<T> {
    fn f(a: T);
}

impl<T1, T2> Trait<T1> for B<T2> {
    fn f(a: T1) -> T2 { self.a }
}

struct A {}
struct S {
    b$0 : B<A>,
}

The former version will generates improper code:

impl<T1, T2> Trait<T1, T2> for S {
    fn f(&self, a: T1) -> T1 {
        <B as Trait<T1, T2>>::f( &self.b , a)
    }
}

The rewritten version can handle generics properly:

impl<T1> Trait<T1> for S {
    fn f(&self, a: T1) -> T1 {
        <B<A> as Trait<T1>>::f(&self.b, a)
    }
}

See more examples in added unit tests.

I enabled support for generic structs in generate_delegate_trait through the following steps (using the code example provided):

1. Initially, to prevent conflicts between the generic parameters in struct S and the ones in the impl of B, I renamed the generic parameters of S.
2. Then, since B's parameters are instantiated within S, the original generic parameters of B needed removal within S (to avoid errors from redundant parameters). An important consideration here arises when Trait and B share parameters in B's impl. In such cases, these shared generic parameters cannot be removed.
3. Next, I addressed the matching of types between B's type in S and its type in the impl. Given that some generic parameters in the impl are instantiated in B, I replaced these parameters with their instantiated results using PathTransform. For instance, in the example provided, matching B<A> and B<T2>, where T2 is instantiated as A, I replaced all occurrences of T2 in the impl with A (i.e. apply the instantiated generic arguments to the params).
4. Finally, I performed transformations on each assoc item (also to prevent the initial issue) and handled redundant where clauses.

For a more detailed explanation, please refer to the code and comments. I welcome suggestions and any further questions!

[roife]

Here's an simple example explaining the third step mentioned above. Say we have the following code:

struct B<T> {
    a: T
}

trait Trait<T> {
    fn f(&self) -> &T;
}

impl<T> Trait<T> for B<T> {
    fn f(&self) -> &T { &self.a }
}

struct S<T> {
    a: B<T>
}

To generate a delegate impl for a in S<T>, the proper implementation is as following:

// generated after rewritten
impl<T> Trait<T> for S<T> {
    fn f(&self) -> &T {
        <B<T> as Trait<T0>>::f(&self.a)
    }
}

However, if we do not match and replace instantiated generic arguments, we will get

impl<T, T0> Trait<T> for S<T0> {
    fn f(&self) -> &T {
        <B<T0> as Trait<T>>::f(&self.a)
    }
}

which is not correct.

[Veykril]

There maybe more than one generic arg list in a type, Qux<(Foo<A, B>, Bar<C, D>>) for example, also generic args might apear in other spots like [T], (T, U, V) etc.

[roife]

Thank you for pointing this out! I hadn't considered this situation before.

These situations might make the analysis a bit more complex; I need to reorganize my current approach and think about how to elegantly address this issue. ❤️

[Veykril]

Small comment, but thats something that could be addressed as a follow up

[Veykril]

Thanks!
@bors r+

[bors]

📌 Commit 5070534 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 5070534 with merge 34df296...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 34df296 to master...