Design trade-offs for completion item name shadowing in rust-analyzer

[CubeSugarCheese]

current problem

In rust-analyzer, there is a problem exist for many yaers
#18398 Don't auto import items when one is already in scope
#17164 Block specific items from being auto-imported
The most common examples as follows

fn foo() -> Result<()> {
    Ok$0
}

current ra will give auto-complete hint like this

Ok(...)(use anyhow::Ok)
Ok(...)

if you push enter to auto-complete, you will complete and import anyhow::Ok instead of core::result::Result::Ok. That's really annoying.
In #17164, Veykril mention that "we should filter out auto-importing completions if they shadow something that's in scope".
I try modify ra to impl this feature, but it casued now preblem.
Let's imagine a lib (just call it foo) provide a type alias type Result<T> = core::result::Result<T, FooError>;
User may use foo::Result to shadow core::result::Result, but we already filter it out of alternative list, so user lose there completions.

How about rustc?

For rustc, there is no import sequence, all imports are treat with same rules as follows.
Namespace is type of name, in current rust, only 3 types of namespace, they are Types, Values and Macros.
For simple, all name in follow list refer (name, namespace) pair.

• 1. if no same name item exist, allow import.
• 2. if there are two or more named import, forbid import and report error[E0252]: the name bar is defined multiple times
• 3. if only one named import, it will shadow all same name item from wildcard imports.
• 4. if only wildcard imports, allow all imports, but when use name, will report error[E0659]: ambiguous name, need disambiguation.

what rust-analyzer needs to solve?

After considering the order and the fact that RA will not use wildcard imports, there is 2 conditions needs to solve.

a.already exist named import

use ratatui::layout::Layout;

fn f () {
  let layout = Lay$0
}

ra will give hint use std::alloc::Layout.

b.already exist wildcard imports

// core::result::Result::Ok implict wildcard imported via prelude
fn foo() -> Result<()> {
    Ok$0
}

ra will give hint use anyhow::Ok.

possible solution

1. Reduce the priority of alternative options from completion list
2. Remove alternative options from completion list
3. named import and shadow current wildcard imports
4. using full path import to disambig like std::alloc::Layout
5. using long path import to disambig like alloc::Layout
6. auto rename item with number like use std::alloc::Layout as Layout1
7. auto rename item with super path like use std::alloc::Layout as AllocLayout

I prefer a with 1 and 4, b with 1 and 3.
Because full path import easy to implement, other methods require a loop check for conflicts in the super path names to obtain the shortest path.

[ChayimFriedman2]

How will this prevent Result aliases from being completed if we only filter autoimports?

[CubeSugarCheese]

How will this prevent Result aliases from being completed if we only filter autoimports?

Sorry for the misnomer, what I meant is that the completion list, not not just auto add import statements, I've already edited my wording.

[ChayimFriedman2]

Then the solution, of course, is to only filter them from autoimports.