Remove redundant completions

crates/ide/src/completion.rs

@@ -112,6 +112,11 @@ pub(crate) fn completions(
 ) -> Option<Completions> {
     let ctx = CompletionContext::new(db, position, config)?;
 
+    if ctx.no_completion_required() {
+        // No work required here.
+        return None;
+    }
+
     let mut acc = Completions::default();
     complete_attribute::complete_attribute(&mut acc, &ctx);
     complete_fn_param::complete_fn_param(&mut acc, &ctx);
@@ -157,6 +162,23 @@ fn check_detail_and_documentation(ra_fixture: &str, expected: DetailAndDocumenta
         panic!("completion detail not found: {}", expected.detail)
     }
 
+    fn check_no_completion(ra_fixture: &str) {
+        let (analysis, position) = fixture::position(ra_fixture);
+        let config = CompletionConfig::default();
+        analysis.completions(&config, position).unwrap();
+
+        let completions: Option<Vec<String>> = analysis
+            .completions(&config, position)
+            .unwrap()
+            .and_then(|completions| if completions.is_empty() { None } else { Some(completions) })
+            .map(|completions| {
+                completions.into_iter().map(|completion| format!("{:?}", completion)).collect()
+            });
+
+        // `assert_eq` instead of `assert!(completions.is_none())` to get the list of completions if test will panic.
+        assert_eq!(completions, None, "Completions were generated, but weren't expected");
+    }
+
     #[test]
     fn test_completion_detail_from_macro_generated_struct_fn_doc_attr() {
         check_detail_and_documentation(
@@ -208,4 +230,15 @@ fn foo() {
             DetailAndDocumentation { detail: "fn foo(&self)", documentation: " Do the foo" },
         );
     }
+
+    #[test]
+    fn test_no_completions_required() {
+        check_no_completion(
+            r#"
+            fn foo() {
+                for i i<|>
+            }
+            "#,
+        )
+    }
 }

crates/ide/src/completion/completion_context.rs

@@ -16,10 +16,10 @@
     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, is_in_loop_body, is_match_arm, unsafe_is_prev,
         },
         CompletionConfig,
     },
@@ -91,6 +91,8 @@ pub(crate) struct CompletionContext<'a> {
     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)>,
 }
 
@@ -174,6 +176,8 @@ pub(super) fn new(
             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 +225,14 @@ pub(super) fn new(
         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.
+    /// - `for _ i<|>` -- obviously, it'll be "in" keyword.
+    pub(crate) fn no_completion_required(&self) -> bool {
+        self.fn_is_prev || 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
@@ -253,6 +265,8 @@ fn fill_keyword_patterns(&mut self, file_with_fake_ident: &SyntaxNode, offset: T
         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(

crates/ide/src/completion/patterns.rs

@@ -116,6 +116,25 @@ 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()
+}
+
+/// 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_if_is_prev() {
     check_pattern_is_applicable(r"if l<|>", if_is_prev);