2 files changed, 38 insertions, 3 deletions

diff --git a/crates/ide_assists/src/handlers/fill_match_arms.rs b/crates/ide_assists/src/handlers/fill_match_arms.rs
index 3532759bf..97435f021 100644
--- a/crates/ide_assists/src/handlers/fill_match_arms.rs
+++ b/crates/ide_assists/src/handlers/fill_match_arms.rs
@@ -105,6 +105,7 @@ pub(crate) fn fill_match_arms(acc: &mut Assists, ctx: &AssistContext) -> Option<
         let missing_pats = variants_of_enums
             .into_iter()
             .multi_cartesian_product()
+            .inspect(|_| cov_mark::hit!(fill_match_arms_lazy_computation))
             .map(|variants| {
                 let patterns =
                     variants.into_iter().filter_map(|variant| build_pat(ctx.db(), module, variant));
@@ -279,7 +280,9 @@ fn build_pat(db: &RootDatabase, module: hir::Module, var: ExtendedVariant) -> Op
 mod tests {
     use ide_db::helpers::FamousDefs;
 
-    use crate::tests::{check_assist, check_assist_not_applicable, check_assist_target};
+    use crate::tests::{
+        check_assist, check_assist_not_applicable, check_assist_target, check_assist_unresolved,
+    };
 
     use super::fill_match_arms;
 
@@ -1058,4 +1061,19 @@ fn foo(t: Test) {
 }"#,
         );
     }
+
+    #[test]
+    fn lazy_computation() {
+        // Computing a single missing arm is enough to determine applicability of the assist.
+        cov_mark::check_count!(fill_match_arms_lazy_computation, 1);
+        check_assist_unresolved(
+            fill_match_arms,
+            r#"
+enum A { One, Two, }
+fn foo(tuple: (A, A)) {
+    match $0tuple {};
+}
+"#,
+        );
+    }
 }
diff --git a/crates/ide_assists/src/tests.rs b/crates/ide_assists/src/tests.rs
index 0d3969c36..b091ab91a 100644
--- a/crates/ide_assists/src/tests.rs
+++ b/crates/ide_assists/src/tests.rs
@@ -61,6 +61,12 @@ pub(crate) fn check_assist_not_applicable(assist: Handler, ra_fixture: &str) {
     check(assist, ra_fixture, ExpectedResult::NotApplicable, None);
 }
 
+/// Check assist in unresolved state. Useful to check assists for lazy computation.
+#[track_caller]
+pub(crate) fn check_assist_unresolved(assist: Handler, ra_fixture: &str) {
+    check(assist, ra_fixture, ExpectedResult::Unresolved, None);
+}
+
 #[track_caller]
 fn check_doc_test(assist_id: &str, before: &str, after: &str) {
     let after = trim_indent(after);
@@ -97,6 +103,7 @@ fn check_doc_test(assist_id: &str, before: &str, after: &str) {
 
 enum ExpectedResult<'a> {
     NotApplicable,
+    Unresolved,
     After(&'a str),
     Target(&'a str),
 }
@@ -111,7 +118,11 @@ fn check(handler: Handler, before: &str, expected: ExpectedResult, assist_label:
     let sema = Semantics::new(&db);
     let config = TEST_CONFIG;
     let ctx = AssistContext::new(sema, &config, frange);
-    let mut acc = Assists::new(&ctx, AssistResolveStrategy::All);
+    let resolve = match expected {
+        ExpectedResult::Unresolved => AssistResolveStrategy::None,
+        _ => AssistResolveStrategy::All,
+    };
+    let mut acc = Assists::new(&ctx, resolve);
     handler(&mut acc, &ctx);
     let mut res = acc.finish();
 
@@ -159,8 +170,14 @@ fn check(handler: Handler, before: &str, expected: ExpectedResult, assist_label:
             let range = assist.target;
             assert_eq_text!(&text_without_caret[range], target);
         }
+        (Some(assist), ExpectedResult::Unresolved) => assert!(
+            assist.source_change.is_none(),
+            "unresolved assist should not contain source changes"
+        ),
         (Some(_), ExpectedResult::NotApplicable) => panic!("assist should not be applicable!"),
-        (None, ExpectedResult::After(_)) | (None, ExpectedResult::Target(_)) => {
+        (None, ExpectedResult::After(_))
+        | (None, ExpectedResult::Target(_))
+        | (None, ExpectedResult::Unresolved) => {
             panic!("code action is not applicable")
         }
         (None, ExpectedResult::NotApplicable) => (),