Add tests for the issue#120605

Author: long-long-float

tests/ui/suggestions/issue120605.rs

@@ -0,0 +1,20 @@
+pub struct Struct {
+    a: Vec<Struct>,
+}
+
+impl Struct {
+    pub fn test(&self) {
+        if let [Struct { a: [] }] = &self.a {
+            //~^ ERROR expected an array or slice
+            //~| ERROR expected an array or slice
+            println!("matches!")
+        }
+
+        if let [Struct { a: [] }] = &self.a[..] {
+            //~^ ERROR expected an array or slice
+            println!("matches!")
+        }
+    }
+}
+
+fn main() {}

tests/ui/suggestions/issue120605.stderr

@@ -0,0 +1,23 @@
+error[E0529]: expected an array or slice, found `Vec<Struct>`
+  --> $DIR/issue120605.rs:7:16
+   |
+LL |         if let [Struct { a: [] }] = &self.a {
+   |                ^^^^^^^^^^^^^^^^^^   ------- help: consider slicing here: `&self.a[..]`
+   |                |
+   |                pattern cannot match with input type `Vec<Struct>`
+
+error[E0529]: expected an array or slice, found `Vec<Struct>`
+  --> $DIR/issue120605.rs:7:29
+   |
+LL |         if let [Struct { a: [] }] = &self.a {
+   |                             ^^ pattern cannot match with input type `Vec<Struct>`
+
+error[E0529]: expected an array or slice, found `Vec<Struct>`
+  --> $DIR/issue120605.rs:13:29
+   |
+LL |         if let [Struct { a: [] }] = &self.a[..] {
+   |                             ^^ pattern cannot match with input type `Vec<Struct>`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0529`.