3 files changed, 82 insertions, 5 deletions

diff --git a/crates/ide/src/completion/completion_context.rs b/crates/ide/src/completion/completion_context.rs
index 8dea8a4bf..d9f90477c 100644
--- a/crates/ide/src/completion/completion_context.rs
+++ b/crates/ide/src/completion/completion_context.rs
@@ -16,10 +16,11 @@ use crate::{
     call_info::ActiveParameter,
     completion::{
         patterns::{
-            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_trait_parent, if_is_prev,
-            is_in_loop_body, is_match_arm, unsafe_is_prev,
+            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_trait_parent, if_is_prev, inside_impl_trait_block, is_in_loop_body, is_match_arm,
+            unsafe_is_prev,
         },
         CompletionConfig,
     },
@@ -86,11 +87,14 @@ 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) inside_impl_trait_block: bool,
     pub(super) has_field_list_parent: bool,
     pub(super) trait_as_prev_sibling: bool,
     pub(super) impl_as_prev_sibling: bool,
     pub(super) is_match_arm: bool,
     pub(super) has_item_list_or_source_file_parent: bool,
+    pub(super) for_is_prev2: bool,
+    pub(super) fn_is_prev: bool,
     pub(super) locals: Vec<(String, Local)>,
 }
 
@@ -168,12 +172,15 @@ impl<'a> CompletionContext<'a> {
             block_expr_parent: false,
             has_trait_parent: false,
             has_impl_parent: false,
+            inside_impl_trait_block: false,
             has_field_list_parent: false,
             trait_as_prev_sibling: false,
             impl_as_prev_sibling: false,
             if_is_prev: false,
             is_match_arm: false,
             has_item_list_or_source_file_parent: false,
+            for_is_prev2: false,
+            fn_is_prev: false,
             locals,
         };
 
@@ -221,6 +228,15 @@ impl<'a> CompletionContext<'a> {
         Some(ctx)
     }
 
+    /// 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.inside_impl_trait_block) || self.for_is_prev2
+    }
+
     /// The range of the identifier that is being completed.
     pub(crate) fn source_range(&self) -> TextRange {
         // check kind of macro-expanded token, but use range of original token
@@ -244,6 +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.inside_impl_trait_block = inside_impl_trait_block(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());
@@ -253,6 +270,8 @@ impl<'a> CompletionContext<'a> {
         self.mod_declaration_under_caret =
             find_node_at_offset::<ast::Module>(&file_with_fake_ident, offset)
                 .filter(|module| module.item_list().is_none());
+        self.for_is_prev2 = for_is_prev2(syntax_element.clone());
+        self.fn_is_prev = fn_is_prev(syntax_element.clone());
     }
 
     fn fill(
diff --git a/crates/ide/src/completion/patterns.rs b/crates/ide/src/completion/patterns.rs
index b17ddf133..cf6d5947d 100644
--- a/crates/ide/src/completion/patterns.rs
+++ b/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 +34,25 @@ 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 inside_impl_trait_block(element: SyntaxElement) -> bool {
+    // Here we search `impl` keyword up through the all ancestors, unlike in `has_impl_parent`,
+    // where we only check the first parent with different text range.
+    element
+        .ancestors()
+        .find(|it| it.kind() == IMPL)
+        .map(|it| ast::Impl::cast(it).unwrap())
+        .map(|it| it.trait_().is_some())
+        .unwrap_or(false)
+}
+#[test]
+fn test_inside_impl_trait_block() {
+    check_pattern_is_applicable(r"impl Foo for Bar { f<|> }", inside_impl_trait_block);
+    check_pattern_is_applicable(r"impl Foo for Bar { fn f<|> }", inside_impl_trait_block);
+    check_pattern_is_not_applicable(r"impl A { f<|> }", inside_impl_trait_block);
+    check_pattern_is_not_applicable(r"impl A { fn f<|> }", inside_impl_trait_block);
+}
+
 pub(crate) fn has_field_list_parent(element: SyntaxElement) -> bool {
     not_same_range_ancestor(element).filter(|it| it.kind() == RECORD_FIELD_LIST).is_some()
 }
@@ -116,6 +135,33 @@ pub(crate) fn if_is_prev(element: SyntaxElement) -> bool {
         .is_some()
 }
 
+pub(crate) fn fn_is_prev(element: SyntaxElement) -> bool {
+    element
+        .into_token()
+        .and_then(|it| previous_non_trivia_token(it))
+        .filter(|it| it.kind() == FN_KW)
+        .is_some()
+}
+#[test]
+fn test_fn_is_prev() {
+    check_pattern_is_applicable(r"fn l<|>", fn_is_prev);
+}
+
+/// Check if the token previous to the previous one is `for`.
+/// For example, `for _ i<|>` => true.
+pub(crate) fn for_is_prev2(element: SyntaxElement) -> bool {
+    element
+        .into_token()
+        .and_then(|it| previous_non_trivia_token(it))
+        .and_then(|it| previous_non_trivia_token(it))
+        .filter(|it| it.kind() == FOR_KW)
+        .is_some()
+}
+#[test]
+fn test_for_is_prev2() {
+    check_pattern_is_applicable(r"for i i<|>", for_is_prev2);
+}
+
 #[test]
 fn test_if_is_prev() {
     check_pattern_is_applicable(r"if l<|>", if_is_prev);
diff --git a/crates/ide/src/completion/test_utils.rs b/crates/ide/src/completion/test_utils.rs
index feb8cd2a6..dabbef888 100644
--- a/crates/ide/src/completion/test_utils.rs
+++ b/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,