Add ui test to for unimplementaed if let guard

Author: tesuji

src/test/ui/rfc-2294-if-let-guard/non-implemented.rs

@@ -0,0 +1,13 @@
+// This function is to check whether `if-let` guard error points to
+// `let_chains` feature gate, which is wrong and confuse people.
+pub fn foo(a: &[u8], b: bool) -> bool {
+    match b {
+        true if let [1, 2, 3, ..] = a => true,
+        //~^    ERROR `let` expressions are not supported here
+        //~^^   NOTE only supported directly in conditions of `if`- and `while`-expressions
+        //~^^^  NOTE as well as when nested within `&&` and parenthesis in those conditions
+        _ => false,
+    }
+}
+
+fn main() {}

src/test/ui/rfc-2294-if-let-guard/non-implemented.stderr

@@ -0,0 +1,11 @@
+error: `let` expressions are not supported here
+  --> $DIR/non-implemented.rs:5:17
+   |
+LL |         true if let [1, 2, 3, ..] = a => true,
+   |                 ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: aborting due to previous error
+