Auto-import should respect std/extern/crate order

[matklad]

TraitId is misplaced, it should have been added to the second group. Before we fix import insertion infra to do this, it maybe make sense to move it to a separate module inside utils and in general to refactor code. I think it was introduced before we even had the modern make/edit APIs?

cc @SomeoneToIgnore

[SomeoneToIgnore]

Does that get fixed by the cargo fmt?
I'm not entirely sure that we need to dive that deep into determining the specific place for an import if it's possible to reformat it after importing.

[matklad]

Does that get fixed by the cargo fmt?

I don't think so. And, anyway, we should eventually subsume rustfmt, so this functionality needs to be implemented anyway ^^

[flodiebold]

cargo fmt just reorders imports within groups (i.e. where there's not a blank line in between), and only by ordering alphabetically, as far as I know.

[matklad]

I've moved code for inserting uses to the utils module. I haven't refactored it yet. But I've scetched an interface we want in the end:

pub fn insert_use(where_: SyntaxNode, path: ModPath) -> SyntaxNode {
    let use_item = make::use_item(path);

    let use_scope: UseScope = UseScope::cast(where_).unwrap();
    for existing_use in use_scope.imports() {
        if let Some(merged) = try_merge_two_imports(existing_use, &use_item) {
            return algo::replace_children(where_, existing_use, merged);
        }
    }

    let group: ImportGroup = ImportGroup::new(path);
    let insert_position: InsertPosition<SyntaxElement> = find_insert_position(&use_scope, group);

    algo::insert_children(&where_, insert_position, vec![use_item].into_iter())
}

enum UseScope {
    SourceFile(ast::SourceFile),
    ItemList(ast::ItemList),
}

impl UseScope {
    fn imports(&self) -> AstChildren<ast::UseItem> {
        todo!()
    }
}

fn try_merge_two_imports(lhs: &ast::UseItem, rhs: &ast::UseItem) -> Option<ast::UseItem> {
    todo!()
}

enum ImportGroup {
    Std,
    ExternCrate,
    ThisCrate,
    ThisModule,
    SuperModule,
}

fn find_insert_position(scope: &UseScope, group: ImportGroup) -> InsertPosition<SyntaxElement> {
    todo!()
}

try_merge_two_imports should also allow us to implement the eponymous assist.

[matklad]

in general to refactor code.

This is super hard actually!

[SomeoneToIgnore]

I suspected that :(

[matklad]

Some minimal groundwork in #3487