Import assist for traits on unresolved trait item should offer importing anonymously

[Veykril]

That is in the following case

// foo.rs
trait Foo {
    fn f(&self) {}
}
impl Foo for u32 {}
// main.rs
mod foo;
fn main() {
    0u32.foo$0();
}

We should show the general Import `crate::foo::Foo` as well as a Import `crate::foo::Foo as _` that imports the trait without bringing the name of it into scope.

[Veykril]

What needs to be done is roughly the following:

rust-analyzer/crates/ide-assists/src/handlers/auto_import.rs

Line 91 in 8a23314

let (import_assets, syntax_under_caret) = find_importable_node(ctx)?;

Here we are fetching the ImportAssets which describe what kind of thing we want to import, it contains an ImportCandidate :

rust-analyzer/crates/ide-db/src/imports/import_assets.rs

Lines 26 to 37 in 8a23314

	pub enum ImportCandidate {
	/// A path, qualified (`std::collections::HashMap`) or not (`HashMap`).
	Path(PathImportCandidate),
	/// A trait associated function (with no self parameter) or an associated constant.
	/// For 'test_mod::TestEnum::test_function', `ty` is the `test_mod::TestEnum` expression type
	/// and `name` is the `test_function`
	TraitAssocItem(TraitImportCandidate),
	/// A trait method with self parameter.
	/// For 'test_enum.test_method()', `ty` is the `test_enum` expression type
	/// and `name` is the `test_method`
	TraitMethod(TraitImportCandidate),
	}

Leveraging that info we need to add additional import groups that will offer an _ import here

rust-analyzer/crates/ide-assists/src/handlers/auto_import.rs

Lines 129 to 146 in 8a23314

	for import in proposed_imports {
	let import_path = import.import_path;
	acc.add_group(
	&group_label,
	AssistId("auto_import", AssistKind::QuickFix),
	format!("Import `{}`", import_path.display(ctx.db())),
	range,
	|builder| {
	let scope = match scope.clone() {
	ImportScope::File(it) => ImportScope::File(builder.make_mut(it)),
	ImportScope::Module(it) => ImportScope::Module(builder.make_mut(it)),
	ImportScope::Block(it) => ImportScope::Block(builder.make_mut(it)),
	};
	insert_use(&scope, mod_path_to_ast(&import_path), &ctx.config.insert_use);
	},
	);
	}

To able to add the as _ part to the inserted use item, we need to modify the insert_use function and have it set the rename component appropriately here

rust-analyzer/crates/ide-db/src/imports/insert_use.rs

Lines 177 to 179 in 8a23314

	let use_item =
	make::use_(None, make::use_tree(path.clone(), None, None, false)).clone_for_update();

It's probably easiest to split out the function into two, one for the previous behavior, one for the underscore behavior and then have those wrap a function that takes an extra param to not having to edit all the current call sites of the function.

[Young-Flash]

I'd like to have a try.

It's probably easiest to split out the function into two, one for the previous behavior, one for the underscore behavior and then have those wrap a function that takes an extra param to not having to edit all the current call sites of the function.

But how could we have a new function with an extra param without edit other call of insert_use? There's not something like default param in Rust.

[Young-Flash]

But how could we have a new function with an extra param without edit other call of insert_use? There's not something like default param in Rust.

oh maybe I got it.

pub fn insert_use(scope: &ImportScope, path: ast::Path, cfg: &InsertUseConfig) {
    insert_use_with_alias_option(scope, path, cfg, None);
}

pub fn insert_use_as_alias(scope: &ImportScope, path: ast::Path, cfg: &InsertUseConfig) {
    let alias = ...;
    insert_use_with_alias_option(scope, path, cfg, alias);
}

fn insert_use_with_alias_option(scope: &ImportScope, path: ast::Path, cfg: &InsertUseConfig, alias: Option<ast::Rename>) {
    ...

    let use_item = if alias.is_some() {
        make::use_(None, make::use_tree(path.clone(), None, alias, false)).clone_for_update()
    } else {
        make::use_(None, make::use_tree(path.clone(), None, None, false)).clone_for_update()
    };

    ...
}