Always capture slice when pattern requires checking the length

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -438,12 +438,19 @@ impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
                         // to borrow discr.
                         needs_to_be_read = true;
                     }
-                    PatKind::Or(_)
-                    | PatKind::Box(_)
-                    | PatKind::Slice(..)
-                    | PatKind::Ref(..)
-                    | PatKind::Wild => {
-                        // If the PatKind is Or, Box, Slice or Ref, the decision is made later
+                    PatKind::Slice(lhs, wild, rhs) => {
+                        // We don't need to test the length if the pattern is `[..]`
+                        if matches!((lhs, wild, rhs), (&[], Some(_), &[]))
+                            // Arrays have a statically known size, so
+                            // there is no need to read their length
+                            || discr_place.place.base_ty.is_array()
+                        {
+                        } else {
+                            needs_to_be_read = true;
+                        }
+                    }
+                    PatKind::Or(_) | PatKind::Box(_) | PatKind::Ref(..) | PatKind::Wild => {
+                        // If the PatKind is Or, Box, or Ref, the decision is made later
                         // as these patterns contains subpatterns
                         // If the PatKind is Wild, the decision is made based on the other patterns being
                         // examined

tests/ui/closures/2229_closure_analysis/match/patterns-capture-analysis.rs

@@ -1,16 +1,14 @@
 // edition:2021
 
 #![feature(rustc_attrs)]
+#![feature(stmt_expr_attributes)]
 
 // Should capture the discriminant since a variant of a multivariant enum is
 // mentioned in the match arm; the discriminant is captured by the closure regardless
 // of if it creates a binding
 fn test_1_should_capture() {
     let variant = Some(2229);
     let c =  #[rustc_capture_analysis]
-    //~^ ERROR: attributes on expressions are experimental
-    //~| NOTE: see issue #15701 <https://github.com/rust-lang/rust/issues/15701>
-
     || {
     //~^ First Pass analysis includes:
     //~| Min Capture analysis includes:
@@ -29,8 +27,6 @@ fn test_1_should_capture() {
 fn test_2_should_not_capture() {
     let variant = Some(2229);
     let c =  #[rustc_capture_analysis]
-    //~^ ERROR: attributes on expressions are experimental
-    //~| NOTE: see issue #15701 <https://github.com/rust-lang/rust/issues/15701>
     || {
     //~^ First Pass analysis includes:
         match variant {
@@ -50,8 +46,6 @@ enum SingleVariant {
 fn test_3_should_not_capture_single_variant() {
     let variant = SingleVariant::Points(1);
     let c =  #[rustc_capture_analysis]
-    //~^ ERROR: attributes on expressions are experimental
-    //~| NOTE: see issue #15701 <https://github.com/rust-lang/rust/issues/15701>
     || {
     //~^ First Pass analysis includes:
         match variant {
@@ -66,8 +60,6 @@ fn test_3_should_not_capture_single_variant() {
 fn test_6_should_capture_single_variant() {
     let variant = SingleVariant::Points(1);
     let c =  #[rustc_capture_analysis]
-    //~^ ERROR: attributes on expressions are experimental
-    //~| NOTE: see issue #15701 <https://github.com/rust-lang/rust/issues/15701>
     || {
     //~^ First Pass analysis includes:
     //~| Min Capture analysis includes:
@@ -88,8 +80,6 @@ fn test_6_should_capture_single_variant() {
 fn test_4_should_not_capture_array() {
     let array: [i32; 3] = [0; 3];
     let c =  #[rustc_capture_analysis]
-    //~^ ERROR: attributes on expressions are experimental
-    //~| NOTE: see issue #15701 <https://github.com/rust-lang/rust/issues/15701>
     || {
     //~^ First Pass analysis includes:
         match array {
@@ -112,8 +102,6 @@ enum MVariant {
 fn test_5_should_capture_multi_variant() {
     let variant = MVariant::A;
     let c =  #[rustc_capture_analysis]
-    //~^ ERROR: attributes on expressions are experimental
-    //~| NOTE: see issue #15701 <https://github.com/rust-lang/rust/issues/15701>
     || {
     //~^ First Pass analysis includes:
     //~| Min Capture analysis includes:
@@ -127,11 +115,69 @@ fn test_5_should_capture_multi_variant() {
     c();
 }
 
+// Even though all patterns are wild, we need to read the discriminant
+// in order to test the slice length
+fn test_7_should_capture_slice_len() {
+    let slice: &[i32] = &[1, 2, 3];
+    let c =  #[rustc_capture_analysis]
+    || {
+    //~^ First Pass analysis includes:
+    //~| Min Capture analysis includes:
+        match slice {
+            //~^ NOTE: Capturing slice[] -> ImmBorrow
+            //~| NOTE: Min Capture slice[] -> ImmBorrow
+            [_,_,_] => {},
+            _ => {}
+        }
+    };
+    c();
+    let c =  #[rustc_capture_analysis]
+    || {
+    //~^ First Pass analysis includes:
+    //~| Min Capture analysis includes:
+        match slice {
+            //~^ NOTE: Capturing slice[] -> ImmBorrow
+            //~| NOTE: Min Capture slice[] -> ImmBorrow
+            [] => {},
+            _ => {}
+        }
+    };
+    c();
+    let c =  #[rustc_capture_analysis]
+    || {
+    //~^ First Pass analysis includes:
+    //~| Min Capture analysis includes:
+        match slice {
+            //~^ NOTE: Capturing slice[] -> ImmBorrow
+            //~| NOTE: Min Capture slice[] -> ImmBorrow
+            [_, .. ,_] => {},
+            _ => {}
+        }
+    };
+    c();
+}
+
+// Wild pattern that doesn't bind, so no capture
+fn test_8_capture_slice_wild() {
+    let slice: &[i32] = &[1, 2, 3];
+    let c =  #[rustc_capture_analysis]
+    || {
+    //~^ First Pass analysis includes:
+        match slice {
+            [..] => {},
+            _ => {}
+        }
+    };
+    c();
+}
+
 fn main() {
     test_1_should_capture();
     test_2_should_not_capture();
     test_3_should_not_capture_single_variant();
     test_6_should_capture_single_variant();
     test_4_should_not_capture_array();
     test_5_should_capture_multi_variant();
+    test_7_should_capture_slice_len();
+    test_8_capture_slice_wild();
 }