save-analysis: don't leak information about constructor ids

[aochagavia]

Note: the paths in this issue assume that #33370 is merged

Problem

external_data::StructData contains a ctor_id field that represents the Id of the struct constructor. However, this adds no value to the client of the API, since decl_id is enough to identify a struct.

Currently, some type references contain a ref_id that corresponds to the ctor_id of the type, instead of the decl_id. This forces clients of the API to keep a hashtable mapping from ctor_id to decl_id in case they want to get the original struct declaration.

Note: I have observed this only in tuple structs. It seems like normal structs don't even have a valid ctor_id

Example

Imagine you want to get all references to MyStruct in the following program:

struct MyStruct; // decl_id: 1, ctor_id: 2
fn main() {
    let x = foo();
}

fn foo() -> MyStruct { // Here, the reference to MyStruct has ref_id: 1
    MyStruct // Here, the reference to MyStruct has ref_id: 2 (the constructor)
}

Ideally, both references should have ref_id = 1

Solution

Remove the ctor_id field and modify the save code in such a way that references always point to a decl_id instead of a ctor_id.

cc @nrc do you think this makes sense?

[Mark-Simulacrum]

The save-analysis output appears to no longer have a ctor_id field, so I'm going to close this.