Allow hints after 'fn' keyword if it's an impl trait block

crates/ide/src/completion/completion_context.rs

@@ -18,8 +18,9 @@ use crate::{
         patterns::{
             fn_is_prev, for_is_prev2, has_bind_pat_parent, has_block_expr_parent,
             has_field_list_parent, has_impl_as_prev_sibling, has_impl_parent,
-            has_item_list_or_source_file_parent, has_ref_parent, has_trait_as_prev_sibling,
+            has_impl_trait_parent, has_item_list_or_source_file_parent, has_ref_parent,
-            has_trait_parent, if_is_prev, is_in_loop_body, is_match_arm, unsafe_is_prev,
+            has_trait_as_prev_sibling, has_trait_parent, if_is_prev, is_in_loop_body, is_match_arm,
+            unsafe_is_prev,
         },
         CompletionConfig,
     },
@@ -86,6 +87,7 @@ pub(crate) struct CompletionContext<'a> {
     pub(super) in_loop_body: bool,
     pub(super) has_trait_parent: bool,
     pub(super) has_impl_parent: bool,
+    pub(super) has_impl_trait_parent: bool,
     pub(super) has_field_list_parent: bool,
     pub(super) trait_as_prev_sibling: bool,
     pub(super) impl_as_prev_sibling: bool,
@@ -170,6 +172,7 @@ impl<'a> CompletionContext<'a> {
             block_expr_parent: false,
             has_trait_parent: false,
             has_impl_parent: false,
+            has_impl_trait_parent: false,
             has_field_list_parent: false,
             trait_as_prev_sibling: false,
             impl_as_prev_sibling: false,
@@ -228,9 +231,10 @@ impl<'a> CompletionContext<'a> {
     /// Checks whether completions in that particular case don't make much sense.
     /// Examples:
     /// - `fn <|>` -- we expect function name, it's unlikely that "hint" will be helpful.
+    ///   Exception for this case is `impl Trait for Foo`, where we would like to hint trait method names.
     /// - `for _ i<|>` -- obviously, it'll be "in" keyword.
     pub(crate) fn no_completion_required(&self) -> bool {
-        self.fn_is_prev || self.for_is_prev2
+        (self.fn_is_prev && !self.has_impl_trait_parent) || self.for_is_prev2
     }
 
     /// The range of the identifier that is being completed.
@@ -256,6 +260,7 @@ impl<'a> CompletionContext<'a> {
         self.in_loop_body = is_in_loop_body(syntax_element.clone());
         self.has_trait_parent = has_trait_parent(syntax_element.clone());
         self.has_impl_parent = has_impl_parent(syntax_element.clone());
+        self.has_impl_trait_parent = has_impl_trait_parent(syntax_element.clone());
         self.has_field_list_parent = has_field_list_parent(syntax_element.clone());
         self.impl_as_prev_sibling = has_impl_as_prev_sibling(syntax_element.clone());
         self.trait_as_prev_sibling = has_trait_as_prev_sibling(syntax_element.clone());

crates/ide/src/completion/patterns.rs

@@ -9,7 +9,7 @@ use syntax::{
 };
 
 #[cfg(test)]
-use crate::completion::test_utils::check_pattern_is_applicable;
+use crate::completion::test_utils::{check_pattern_is_applicable, check_pattern_is_not_applicable};
 
 pub(crate) fn has_trait_parent(element: SyntaxElement) -> bool {
     not_same_range_ancestor(element)
@@ -34,6 +34,22 @@ pub(crate) fn has_impl_parent(element: SyntaxElement) -> bool {
 fn test_has_impl_parent() {
     check_pattern_is_applicable(r"impl A { f<|> }", has_impl_parent);
 }
+
+pub(crate) fn has_impl_trait_parent(element: SyntaxElement) -> bool {
+    not_same_range_ancestor(element)
+        .filter(|it| it.kind() == ASSOC_ITEM_LIST)
+        .and_then(|it| it.parent())
+        .filter(|it| it.kind() == IMPL)
+        .map(|it| ast::Impl::cast(it).unwrap())
+        .map(|it| it.trait_().is_some())
+        .unwrap_or(false)
+}
+#[test]
+fn test_has_impl_trait_parent() {
+    check_pattern_is_applicable(r"impl Foo for Bar { f<|> }", has_impl_trait_parent);
+    check_pattern_is_not_applicable(r"impl A { f<|> }", has_impl_trait_parent);
+}
+
 pub(crate) fn has_field_list_parent(element: SyntaxElement) -> bool {
     not_same_range_ancestor(element).filter(|it| it.kind() == RECORD_FIELD_LIST).is_some()
 }

crates/ide/src/completion/test_utils.rs

@@ -104,6 +104,18 @@ pub(crate) fn check_pattern_is_applicable(code: &str, check: fn(SyntaxElement) -
         .unwrap();
 }
 
+pub(crate) fn check_pattern_is_not_applicable(code: &str, check: fn(SyntaxElement) -> bool) {
+    let (analysis, pos) = fixture::position(code);
+    analysis
+        .with_db(|db| {
+            let sema = Semantics::new(db);
+            let original_file = sema.parse(pos.file_id);
+            let token = original_file.syntax().token_at_offset(pos.offset).left_biased().unwrap();
+            assert!(!check(NodeOrToken::Token(token)));
+        })
+        .unwrap();
+}
+
 pub(crate) fn get_all_completion_items(
     config: CompletionConfig,
     code: &str,