bug: LSP completion dialog should only recommend members of a struct

[0xNeshi]

Bug Report

Cairo version:
2.6.3

Current behavior:

When creating a struct instance, there is no completion dialog support - the one that does appear contains some values that can only be described as random.

Example 1: struct in the same module:

Example 2: public struct in a different module with a public field:

Example 3: public struct in a different module with a private field:

Expected behavior:

Completion dialog for struct instances should only list its available fields and nothing else.

Expected for examples above:
Example 1: struct in the same module - should only list field.
Example 2: public struct in a different module with a public field - should only list field_2.
Example 3: public struct in a different module with a private field - should list nothing.

Steps to reproduce:

• create a new Scarb package
• open lib.cairo in VS Code (ensure Cairo 1.0 extension is installed)
• paste the code from "Related code" section below into the opened file
• put the cursor inside any of the struct instance creation expressions (between curly brackets)
• prompt the completion dialog to open (usually CTRL + Space)

Related code:

#[derive(Drop)]
struct Type {
    field: felt252,
}

mod some_mod {
    #[derive(Drop, Debug)]
    pub struct SomeType {
        pub field_2: felt252
    }
}

mod another_mod {
    #[derive(Drop, Debug)]
    pub struct AnotherType {
        field_3: felt252
    }
}

fn main() {
    let struct_instance = Type {
        
    };
    let struct_instance_2 = some_mod::SomeType {
        
    };
    let struct_instance_3 = another_mod::AnotherType {
        
    };
}

Other information:

Related issue: #5660

[danielcdz]

Hello @misicnenad, I would like to help with this one if possible!

[0xNeshi]

@danielcdz awesome, please check with @mkaput if he can assign this to you

[danielcdz]

ty @misicnenad, hey @mkaput can I help with this?

[mkaput]

@danielcdz yes of course! Task is assigned to you :) Thanks 💖

[danielcdz]

Hey @mkaput this is similar to #5660, can I follow the same instructions you left in the comments?

[mkaput]

Hey @mkaput this is similar to #5660, can I follow the same instructions you left in the comments?

I think so

[mkaput]

@danielcdz hey, how's it going? Would you like some assistance from my side in this area?

[danielcdz]

@mkaput that would be great! I would write some questions below 👇

[danielcdz]

I'm trying to look where the completions for a struct are being made but I didn't find anything useful, I saw that on issue #5660 you left some comments with examples of where completions are computed, can you help me with something similar for this 🙏

[mkaput]

It looks like you have to add a new completion kind for struct fields, here:

cairo/crates/cairo-lang-language-server/src/ide/completion/mod.rs

Lines 68 to 149 in 88f0506

	match node.kind(db) {
	SyntaxKind::TerminalDot => {
	let parent = node.parent().unwrap();
	if parent.kind(db) == SyntaxKind::ExprBinary {
	return CompletionKind::Dot(ast::ExprBinary::from_syntax_node(db, parent));
	}
	}
	SyntaxKind::TerminalColonColon => {
	let parent = node.parent().unwrap();
	debug!("parent.kind: {:#?}", parent.kind(db));
	if parent.kind(db) == SyntaxKind::ExprPath {
	return completion_kind_from_path_node(db, parent);
	}
	let grandparent = parent.parent().unwrap();
	debug!("grandparent.kind: {:#?}", grandparent.kind(db));
	if grandparent.kind(db) == SyntaxKind::ExprPath {
	return completion_kind_from_path_node(db, grandparent);
	}
	let (use_ast, should_pop) = if parent.kind(db) == SyntaxKind::UsePathLeaf {
	(ast::UsePath::Leaf(ast::UsePathLeaf::from_syntax_node(db, parent)), true)
	} else if grandparent.kind(db) == SyntaxKind::UsePathLeaf {
	(ast::UsePath::Leaf(ast::UsePathLeaf::from_syntax_node(db, grandparent)), true)
	} else if parent.kind(db) == SyntaxKind::UsePathSingle {
	(ast::UsePath::Single(ast::UsePathSingle::from_syntax_node(db, parent)), false)
	} else if grandparent.kind(db) == SyntaxKind::UsePathSingle {
	(ast::UsePath::Single(ast::UsePathSingle::from_syntax_node(db, grandparent)), false)
	} else {
	debug!("Generic");
	return CompletionKind::ColonColon(vec![]);
	};
	let mut segments = vec![];
	let Ok(()) = get_use_segments(db.upcast(), &use_ast, &mut segments) else {
	debug!("Generic");
	return CompletionKind::ColonColon(vec![]);
	};
	if should_pop {
	segments.pop();
	}
	debug!("ColonColon");
	return CompletionKind::ColonColon(segments);
	}
	SyntaxKind::TerminalIdentifier => {
	let parent = node.parent().unwrap();
	debug!("parent.kind: {:#?}", parent.kind(db));
	let grandparent = parent.parent().unwrap();
	debug!("grandparent.kind: {:#?}", grandparent.kind(db));
	if grandparent.kind(db) == SyntaxKind::ExprPath {
	if db.get_children(grandparent.clone())[0].stable_ptr() != parent.stable_ptr() {
	// Not the first segment.
	debug!("Not first segment");
	return completion_kind_from_path_node(db, grandparent);
	}
	// First segment.
	let grandgrandparent = grandparent.parent().unwrap();
	debug!("grandgrandparent.kind: {:#?}", grandgrandparent.kind(db));
	if grandgrandparent.kind(db) == SyntaxKind::ExprBinary {
	let expr = ast::ExprBinary::from_syntax_node(db, grandgrandparent.clone());
	if matches!(
	ast::ExprBinary::from_syntax_node(db, grandgrandparent).op(db),
	ast::BinaryOperator::Dot(_)
	) {
	debug!("Dot");
	return CompletionKind::Dot(expr);
	}
	}
	}
	if grandparent.kind(db) == SyntaxKind::UsePathLeaf {
	let use_ast = ast::UsePathLeaf::from_syntax_node(db, grandparent);
	let mut segments = vec![];
	let Ok(()) =
	get_use_segments(db.upcast(), &ast::UsePath::Leaf(use_ast), &mut segments)
	else {
	debug!("Generic");
	return CompletionKind::ColonColon(vec![]);
	};
	segments.pop();
	debug!("ColonColon");
	return CompletionKind::ColonColon(segments);
	}
	}
	_ => (),
	}

This is basically traversing AST up to learn where you are. So you have to check out if you are within braces ({}) of struct construction expression. I guess either the left brace, or comma would be the nodes that you get at the input, but this is an exercise for you to investigate :)

Then, you have to hook completion implementation, starting here add a clause for your new completion kind:

cairo/crates/cairo-lang-language-server/src/ide/completion/mod.rs

Lines 45 to 57 in 88f0506

	match completion_kind(db, node) {
	CompletionKind::Dot(expr) => {
	dot_completions(db, file_id, lookup_items, expr).map(CompletionResponse::Array)
	}
	CompletionKind::ColonColon(segments) if !segments.is_empty() => {
	colon_colon_completions(db, module_file_id, lookup_items, segments)
	.map(CompletionResponse::Array)
	}
	_ if trigger_kind == CompletionTriggerKind::INVOKED => {
	Some(CompletionResponse::Array(generic_completions(db, module_file_id, lookup_items)))
	}
	_ => None,
	}

The body of this function is pretty wild west now. You have to get the semantic model of the struct that you are completing. I think you can find a lot of logic to copy in dot completions (i.e. foobar.<complete>), here:

cairo/crates/cairo-lang-language-server/src/ide/completion/completions.rs

Lines 182 to 254 in 57ffe18

	#[tracing::instrument(level = "trace", skip_all)]
	pub fn dot_completions(
	db: &(dyn SemanticGroup + 'static),
	file_id: FileId,
	lookup_items: Vec<LookupItemId>,
	expr: ast::ExprBinary,
	) -> Option<Vec<CompletionItem>> {
	let syntax_db = db.upcast();
	// Get a resolver in the current context.
	let lookup_item_id = lookup_items.into_iter().next()?;
	let function_with_body = lookup_item_id.function_with_body()?;
	let module_id = function_with_body.module_file_id(db.upcast()).0;
	let resolver_data = lookup_item_id.resolver_data(db).ok()?;
	let resolver = Resolver::with_data(
	db,
	resolver_data.as_ref().clone_with_inference_id(db, InferenceId::NoContext),
	);
	// Extract lhs node.
	let node = expr.lhs(syntax_db);
	let stable_ptr = node.stable_ptr().untyped();
	// Get its semantic model.
	let expr_id = db.lookup_expr_by_ptr(function_with_body, node.stable_ptr()).ok()?;
	let semantic_expr = db.expr_semantic(function_with_body, expr_id);
	// Get the type.
	let ty = semantic_expr.ty();
	if ty.is_missing(db) {
	debug!("type is missing");
	return None;
	}
	// Find relevant methods for type.
	let offset = if let Some(ModuleId::Submodule(submodule_id)) =
	db.find_module_containing_node(&expr.as_syntax_node())
	{
	let module_def_ast = submodule_id.stable_ptr(db.upcast()).lookup(syntax_db);
	if let ast::MaybeModuleBody::Some(body) = module_def_ast.body(syntax_db) {
	body.items(syntax_db).as_syntax_node().span_start_without_trivia(syntax_db)
	} else {
	TextOffset::default()
	}
	} else {
	TextOffset::default()
	};
	let position = offset.position_in_file(db.upcast(), file_id).unwrap().to_lsp();
	let relevant_methods = find_methods_for_type(db, resolver, ty, stable_ptr);
	let mut completions = Vec::new();
	for trait_function in relevant_methods {
	let Some(completion) = completion_for_method(db, module_id, trait_function, position)
	else {
	continue;
	};
	completions.push(completion);
	}
	// Find members of the type.
	let (_, long_ty) = peel_snapshots(db, ty);
	if let TypeLongId::Concrete(ConcreteTypeId::Struct(concrete_struct_id)) = long_ty {
	db.concrete_struct_members(concrete_struct_id).ok()?.into_iter().for_each(
	|(name, member)| {
	let completion = CompletionItem {
	label: name.to_string(),
	detail: Some(member.ty.format(db.upcast())),
	kind: Some(CompletionItemKind::FIELD),
	..CompletionItem::default()
	};
	completions.push(completion);
	},
	);
	}
	Some(completions)
	}

For example, this is the function to get struct members (concrete, i.e. with generic parameters resolved to concrete types according to type inference context):

cairo/crates/cairo-lang-semantic/src/items/structure.rs

Lines 260 to 284 in 448c234

	pub trait SemanticStructEx<'a>: Upcast<dyn SemanticGroup + 'a> {
	fn concrete_struct_members(
	&self,
	concrete_struct_id: ConcreteStructId,
	) -> Maybe<OrderedHashMap<SmolStr, semantic::Member>> {
	// TODO(spapini): Uphold the invariant that constructed ConcreteEnumId instances
	// always have the correct number of generic arguments.
	let db = self.upcast();
	let generic_params = db.struct_generic_params(concrete_struct_id.struct_id(db))?;
	let generic_args = concrete_struct_id.lookup_intern(db).generic_args;
	let substitution = GenericSubstitution::new(&generic_params, &generic_args);
	let generic_members =
	self.upcast().struct_members(concrete_struct_id.struct_id(self.upcast()))?;
	generic_members
	.into_iter()
	.map(|(name, member)| {
	let ty =
	SubstitutionRewriter { db, substitution: &substitution }.rewrite(member.ty)?;
	let member = semantic::Member { ty, ..member };
	Ok((name, member))
	})
	.collect::<Maybe<_>>()
	}
	}

[danielcdz]

Hey @mkaput ty for all the explanations you gave me, I think I got the main idea on how to solve it, but I prefer to let this issue to a person more experienced in Rust, also I'm participating in a Hackathon and I don't have enough time to solve this issue at the moment, I'm sorry for this 😞

[mkaput]

@danielcdz understood :)

I think I got the main idea on how to solve it

Do you think you can add your own two cents to this? This would be a great contribution on its own! 😍

[mkaput]

@KevinMB0220 feel free to submit a PR. We're not assigning tasks to external contributors anymore