fix: let LSP autocompletion work in more contexts (#5719)

# Description

## Problem

There were a couple of contexts where autocompletion wouldn't trigger.
This PR fixes some of these.

## Summary

### Autocompletion inside if condition without following body

Before:


![lsp-autocomplete-if-before](https://github.com/user-attachments/assets/e4d96573-9c96-4ab7-b5b7-7c99a87c4b3a)

After:


![lsp-autocomplete-if-after](https://github.com/user-attachments/assets/318569b2-9556-41f1-bd63-c68eda3e7371)

### Autocompletion in nested expression

Before:


![lsp-autocomplete-nested-before](https://github.com/user-attachments/assets/ef1aa9f8-4c30-4796-9672-dc54af913b69)

The reason is that is was autocompleting for the type of `foo.bar & foo`
instead of just `foo`.

After:


![lsp-autocomplete-nested-after](https://github.com/user-attachments/assets/98c28c43-029d-4305-ba41-18f2697ba58f)


### Autocompletion in call chains

Before:


![lsp-autocomplete-chain-before](https://github.com/user-attachments/assets/d2d3e4f5-3b00-4f80-bd37-25cf76288d8e)

After:


![lsp-autocomplete-chain-after](https://github.com/user-attachments/assets/bead94ba-55d2-40f1-896e-58625fd7f542)

### Autocompletion when assignment follows

Before:


![lsp-autocomplete-assignment-follows-before](https://github.com/user-attachments/assets/80a5db63-bcd3-421a-85e4-9acdfec79763)

After:


![lsp-autocomplete-assignment-follows-after](https://github.com/user-attachments/assets/328a2a83-959a-4252-b974-17f547f7fc13)

## Additional Context

I might send a follow-up PR to split `completion.rs` into several files
because it's getting pretty big. For example tests could go in a
separate file.

## Documentation

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_frontend/src/elaborator/expressions.rs

@@ -385,7 +385,8 @@ impl<'context> Elaborator<'context> {
                     function_args.push((typ, arg, span));
                 }
 
-                let location = Location::new(span, self.file);
+                let call_span = Span::from(object_span.start()..method_name_span.end());
+                let location = Location::new(call_span, self.file);
                 let method = method_call.method_name;
                 let turbofish_generics = generics.clone();
                 let is_macro_call = method_call.is_macro_call;

compiler/noirc_frontend/src/parser/errors.rs

@@ -20,6 +20,8 @@ pub enum ParserErrorReason {
     ExpectedIdentifierAfterDot,
     #[error("expected an identifier after ::")]
     ExpectedIdentifierAfterColons,
+    #[error("expected {{ after if condition")]
+    ExpectedLeftBraceAfterIfCondition,
     #[error("Expected a ; separating these two statements")]
     MissingSeparatingSemi,
     #[error("constrain keyword is deprecated")]

compiler/noirc_frontend/src/parser/parser.rs

@@ -954,10 +954,32 @@ where
 
         keyword(Keyword::If)
             .ignore_then(expr_no_constructors)
-            .then(if_block)
-            .then(keyword(Keyword::Else).ignore_then(else_block).or_not())
-            .map(|((condition, consequence), alternative)| {
-                ExpressionKind::If(Box::new(IfExpression { condition, consequence, alternative }))
+            .then(if_block.then(keyword(Keyword::Else).ignore_then(else_block).or_not()).or_not())
+            .validate(|(condition, consequence_and_alternative), span, emit_error| {
+                if let Some((consequence, alternative)) = consequence_and_alternative {
+                    ExpressionKind::If(Box::new(IfExpression {
+                        condition,
+                        consequence,
+                        alternative,
+                    }))
+                } else {
+                    // We allow `if cond` without a block mainly for LSP, so that it parses right
+                    // and autocompletion works there.
+                    emit_error(ParserError::with_reason(
+                        ParserErrorReason::ExpectedLeftBraceAfterIfCondition,
+                        span,
+                    ));
+
+                    let span_end = condition.span.end();
+                    ExpressionKind::If(Box::new(IfExpression {
+                        condition,
+                        consequence: Expression::new(
+                            ExpressionKind::Error,
+                            Span::from(span_end..span_end),
+                        ),
+                        alternative: None,
+                    }))
+                }
             })
     })
 }
@@ -1431,6 +1453,24 @@ mod test {
         );
     }
 
+    #[test]
+    fn parse_if_without_block() {
+        let src = "if foo";
+        let parser = if_expr(expression_no_constructors(expression()), fresh_statement());
+        let (expression_kind, errors) = parse_recover(parser, src);
+
+        let expression_kind = expression_kind.unwrap();
+        let ExpressionKind::If(if_expression) = expression_kind else {
+            panic!("Expected an if expression, got {:?}", expression_kind);
+        };
+
+        assert_eq!(if_expression.consequence.kind, ExpressionKind::Error);
+        assert_eq!(if_expression.alternative, None);
+
+        assert_eq!(errors.len(), 1);
+        assert_eq!(errors[0].message, "expected { after if condition");
+    }
+
     #[test]
     fn parse_module_declaration() {
         parse_with(module_declaration(), "mod foo").unwrap();

tooling/lsp/src/requests/completion.rs

@@ -450,7 +450,18 @@ impl<'a> NodeFinder<'a> {
 
     fn find_in_lvalue(&mut self, lvalue: &LValue) {
         match lvalue {
-            LValue::Ident(_) => (),
+            LValue::Ident(ident) => {
+                if self.byte == Some(b'.') && ident.span().end() as usize == self.byte_index - 1 {
+                    let location = Location::new(ident.span(), self.file);
+                    if let Some(ReferenceId::Local(definition_id)) =
+                        self.interner.find_referenced(location)
+                    {
+                        let typ = self.interner.definition_type(definition_id);
+                        let prefix = "";
+                        self.complete_type_fields_and_methods(&typ, prefix);
+                    }
+                }
+            }
             LValue::MemberAccess { object, field_name: _, span: _ } => self.find_in_lvalue(object),
             LValue::Index { array, index, span: _ } => {
                 self.find_in_lvalue(array);
@@ -477,19 +488,6 @@ impl<'a> NodeFinder<'a> {
     }
 
     fn find_in_expression(&mut self, expression: &Expression) {
-        // "foo." (no identifier afterwards) is parsed as the expression on the left hand-side of the dot.
-        // Here we check if there's a dot at the completion position, and if the expression
-        // ends right before the dot. If so, it means we want to complete the expression's type fields and methods.
-        if self.byte == Some(b'.') && expression.span.end() as usize == self.byte_index - 1 {
-            let location = Location::new(expression.span, self.file);
-            if let Some(typ) = self.interner.type_at_location(location) {
-                let typ = typ.follow_bindings();
-                let prefix = "";
-                self.complete_type_fields_and_methods(&typ, prefix);
-                return;
-            }
-        }
-
         match &expression.kind {
             noirc_frontend::ast::ExpressionKind::Literal(literal) => self.find_in_literal(literal),
             noirc_frontend::ast::ExpressionKind::Block(block_expression) => {
@@ -554,6 +552,23 @@ impl<'a> NodeFinder<'a> {
             | noirc_frontend::ast::ExpressionKind::Resolved(_)
             | noirc_frontend::ast::ExpressionKind::Error => (),
         }
+
+        // "foo." (no identifier afterwards) is parsed as the expression on the left hand-side of the dot.
+        // Here we check if there's a dot at the completion position, and if the expression
+        // ends right before the dot. If so, it means we want to complete the expression's type fields and methods.
+        // We only do this after visiting nested expressions, because in an expression like `foo & bar.` we want
+        // to complete for `bar`, not for `foo & bar`.
+        if self.completion_items.is_empty()
+            && self.byte == Some(b'.')
+            && expression.span.end() as usize == self.byte_index - 1
+        {
+            let location = Location::new(expression.span, self.file);
+            if let Some(typ) = self.interner.type_at_location(location) {
+                let typ = typ.follow_bindings();
+                let prefix = "";
+                self.complete_type_fields_and_methods(&typ, prefix);
+            }
+        }
     }
 
     fn find_in_literal(&mut self, literal: &Literal) {
@@ -2883,4 +2898,86 @@ mod completion_tests {
         )
         .await;
     }
+
+    #[test]
+    async fn test_completes_in_broken_if_after_dot() {
+        let src = r#"
+            struct Some {
+                foo: i32,
+            }
+
+            fn foo(s: Some) {
+                if s.>|<
+            }
+        "#;
+        assert_completion(
+            src,
+            vec![simple_completion_item("foo", CompletionItemKind::FIELD, Some("i32".to_string()))],
+        )
+        .await;
+    }
+
+    #[test]
+    async fn test_completes_in_nested_expression() {
+        let src = r#"
+            struct Foo { bar: Bar }
+            struct Bar { baz: i32 }
+
+            fn foo(f: Foo) {
+                f.bar & f.>|<
+            }
+        "#;
+        assert_completion(
+            src,
+            vec![simple_completion_item("bar", CompletionItemKind::FIELD, Some("Bar".to_string()))],
+        )
+        .await;
+    }
+
+    #[test]
+    async fn test_completes_in_call_chain() {
+        let src = r#"
+            struct Foo {}
+
+            impl Foo {
+                fn foo(self) -> Foo { self }
+            }
+
+            fn foo(f: Foo) {
+                f.foo().>|<
+            }
+        "#;
+        assert_completion(
+            src,
+            vec![snippet_completion_item(
+                "foo()",
+                CompletionItemKind::FUNCTION,
+                "foo()",
+                Some("fn(self) -> Foo".to_string()),
+            )],
+        )
+        .await;
+    }
+
+    #[test]
+    async fn test_completes_when_assignment_follows() {
+        let src = r#"
+            struct Foo {
+                bar: i32,
+            }
+
+            fn foo(f: Foo) {
+                let mut x = 1;
+
+                f.>|<
+
+                x = 2;
+            }
+        "#;
+        assert_completion(
+            src,
+            vec![simple_completion_item("bar", CompletionItemKind::FIELD, Some("i32".to_string()))],
+        )
+        .await;
+    }
 }