Add tests for struct literals in if let/while let

tests/ui/parser/struct-literal-in-if.rs

@@ -14,4 +14,9 @@ fn main() {
     }.hi() {
         println!("yo");
     }
+    if let true = Foo { //~ ERROR struct literals are not allowed here
+        x: 3
+    }.hi() {
+        println!("yo");
+    }
 }

tests/ui/parser/struct-literal-in-if.stderr

@@ -14,5 +14,21 @@ LL |         x: 3
 LL ~     }).hi() {
    |
 
-error: aborting due to previous error
+error: struct literals are not allowed here
+  --> $DIR/struct-literal-in-if.rs:17:19
+   |
+LL |       if let true = Foo {
+   |  ___________________^
+LL | |         x: 3
+LL | |     }.hi() {
+   | |_____^
+   |
+help: surround the struct literal with parentheses
+   |
+LL ~     if let true = (Foo {
+LL |         x: 3
+LL ~     }).hi() {
+   |
+
+error: aborting due to 2 previous errors
 

tests/ui/parser/struct-literal-in-while.rs

@@ -14,4 +14,9 @@ fn main() {
     }.hi() {
         println!("yo");
     }
+    while let true = Foo { //~ ERROR struct literals are not allowed here
+        x: 3
+    }.hi() {
+        println!("yo");
+    }
 }

tests/ui/parser/struct-literal-in-while.stderr

@@ -14,5 +14,21 @@ LL |         x: 3
 LL ~     }).hi() {
    |
 
-error: aborting due to previous error
+error: struct literals are not allowed here
+  --> $DIR/struct-literal-in-while.rs:17:22
+   |
+LL |       while let true = Foo {
+   |  ______________________^
+LL | |         x: 3
+LL | |     }.hi() {
+   | |_____^
+   |
+help: surround the struct literal with parentheses
+   |
+LL ~     while let true = (Foo {
+LL |         x: 3
+LL ~     }).hi() {
+   |
+
+error: aborting due to 2 previous errors