Add type of a let tait test impl trait straight in let

Author: spastorino

src/test/ui/type-alias-impl-trait/type_of_a_let2.rs

@@ -0,0 +1,25 @@
+#![feature(type_alias_impl_trait)]
+#![allow(dead_code)]
+
+// FIXME This should be under a feature flag
+
+use std::fmt::Debug;
+
+fn foo1() -> u32 {
+    let x: impl Debug = 22_u32;
+    //~^ ERROR: `impl Trait` not allowed outside of function and method return types [E0562]
+    x // ERROR: we only know x: Debug, we don't know x = u32
+}
+
+fn foo2() -> u32 {
+    let x: impl Debug = 22_u32;
+    //~^ ERROR: `impl Trait` not allowed outside of function and method return types [E0562]
+    let y: impl Debug = x;
+    //~^ ERROR: `impl Trait` not allowed outside of function and method return types [E0562]
+    same_type((x, y)); // ERROR
+    x
+}
+
+fn same_type<T>(x: (T, T)) {}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/type_of_a_let2.stderr

@@ -0,0 +1,21 @@
+error[E0562]: `impl Trait` not allowed outside of function and method return types
+  --> $DIR/type_of_a_let2.rs:9:12
+   |
+LL |     let x: impl Debug = 22_u32;
+   |            ^^^^^^^^^^
+
+error[E0562]: `impl Trait` not allowed outside of function and method return types
+  --> $DIR/type_of_a_let2.rs:15:12
+   |
+LL |     let x: impl Debug = 22_u32;
+   |            ^^^^^^^^^^
+
+error[E0562]: `impl Trait` not allowed outside of function and method return types
+  --> $DIR/type_of_a_let2.rs:17:12
+   |
+LL |     let y: impl Debug = x;
+   |            ^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0562`.