do the same for Arrays

Author: ada4a

clippy_lints/src/no_effect.rs

@@ -340,7 +340,15 @@ fn reduce_expression<'a>(
         ExprKind::Binary(ref binop, a, b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
             Some(vec![a, b])
         },
-        ExprKind::Array(v) | ExprKind::Tup(v) => Some(v.iter().collect()),
+        ExprKind::Array(elems) => {
+            if elems.iter().any(|elem| !expr_type_is_certain(cx, elem)) {
+                // there's a risk that if we take the elem exprs out of the context of the array,
+                // their types might become ambiguous
+                *applicability = Applicability::MaybeIncorrect;
+            }
+            Some(elems.iter().collect())
+        },
+        ExprKind::Tup(v) => Some(v.iter().collect()),
         ExprKind::Repeat(inner, _)
         | ExprKind::Type(inner, _)
         | ExprKind::Unary(_, inner)

tests/ui/unnecessary_operation_unfixable.rs

@@ -25,4 +25,9 @@ fn issue15381() {
     struct Tuple(Vec<u32>);
     Tuple(Vec::new());
     //~^ unnecessary_operation
+
+    // the type of the second slice gets inferred based on it needing to be the same to that of the
+    // first one, but that doesn't happen when they're outside the array
+    [[1, 2, 3].as_slice(), [].as_slice()];
+    //~^ unnecessary_operation
 }

tests/ui/unnecessary_operation_unfixable.stderr

@@ -25,5 +25,11 @@ error: unnecessary operation
 LL |     Tuple(Vec::new());
    |     ^^^^^^^^^^^^^^^^^^ help: statement can be reduced to: `Vec::new();`
 
-error: aborting due to 4 previous errors
+error: unnecessary operation
+  --> tests/ui/unnecessary_operation_unfixable.rs:31:5
+   |
+LL |     [[1, 2, 3].as_slice(), [].as_slice()];
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: statement can be reduced to: `[1, 2, 3].as_slice();[].as_slice();`
+
+error: aborting due to 5 previous errors