Test `fill_match_arms` for lazy computation.

[rust.git] / crates / ide_assists / src / handlers / fill_match_arms.rs

diff --git a/crates/ide_assists/src/handlers/fill_match_arms.rs b/crates/ide_assists/src/handlers/fill_match_arms.rs
index 12ea17d7a22db12653941afd4140d7a4c4354f44..97435f021137e5279a4a8cbdaddfac3927999924 100644 (file)
--- a/crates/ide_assists/src/handlers/fill_match_arms.rs
+++ b/crates/ide_assists/src/handlers/fill_match_arms.rs
@@ -84,15 +84,28 @@ pub(crate) fn fill_match_arms(acc: &mut Assists, ctx: &AssistContext) -> Option<
         };
         missing_pats.peekable()
     } else if let Some(enum_defs) = resolve_tuple_of_enum_def(&ctx.sema, &expr) {
+        let mut n_arms = 1;
+        let variants_of_enums: Vec<Vec<ExtendedVariant>> = enum_defs
+            .into_iter()
+            .map(|enum_def| enum_def.variants(ctx.db()))
+            .inspect(|variants| n_arms *= variants.len())
+            .collect();
+
         // When calculating the match arms for a tuple of enums, we want
         // to create a match arm for each possible combination of enum
         // values. The `multi_cartesian_product` method transforms
         // Vec<Vec<EnumVariant>> into Vec<(EnumVariant, .., EnumVariant)>
         // where each tuple represents a proposed match arm.
-        let missing_pats = enum_defs
+
+        // A number of arms grows very fast on even a small tuple of large enums.
+        // We skip the assist beyond an arbitrary threshold.
+        if n_arms > 256 {
+            return None;
+        }
+        let missing_pats = variants_of_enums
             .into_iter()
-            .map(|enum_def| enum_def.variants(ctx.db()))
             .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));
@@ -267,7 +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;
 
@@ -1046,4 +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 {};
+}
+"#,
+        );
+    }
 }