Add test case for array false positive

Author: y21

tests/ui/vec.fixed

@@ -242,3 +242,12 @@ fn issue_12101() {
     for a in &[1, 2] {}
     //~^ useless_vec
 }
+
+fn issue_14531() {
+    // The lint used to suggest using an array rather than a reference to a slice.
+
+    fn requires_ref_slice(v: &[()]) {}
+    let v = &[];
+    //~^ useless_vec
+    requires_ref_slice(v);
+}

tests/ui/vec.rs

@@ -242,3 +242,12 @@ fn issue_12101() {
     for a in &(vec![1, 2]) {}
     //~^ useless_vec
 }
+
+fn issue_14531() {
+    // The lint used to suggest using an array rather than a reference to a slice.
+
+    fn requires_ref_slice(v: &[()]) {}
+    let v = &vec![];
+    //~^ useless_vec
+    requires_ref_slice(v);
+}

tests/ui/vec.stderr

@@ -127,5 +127,11 @@ error: useless use of `vec!`
 LL |     for a in &(vec![1, 2]) {}
    |              ^^^^^^^^^^^^^ help: you can use a slice directly: `&[1, 2]`
 
-error: aborting due to 21 previous errors
+error: useless use of `vec!`
+  --> tests/ui/vec.rs:250:13
+   |
+LL |     let v = &vec![];
+   |             ^^^^^^^ help: you can use a slice directly: `&[]`
+
+error: aborting due to 22 previous errors