Add test for uninhabited std::mem::uninitialized uses.

Author: cjgillot

tests/ui/uninhabited/void-branch.rs

@@ -0,0 +1,30 @@
+#![deny(unreachable_code)]
+#![allow(deprecated, invalid_value)]
+
+enum Void {}
+
+fn with_void() {
+    if false {
+        unsafe {
+            //~^ ERROR unreachable expression
+            std::mem::uninitialized::<Void>();
+        }
+    }
+
+    println!();
+}
+
+fn infallible() -> std::convert::Infallible {
+    loop {}
+}
+
+fn with_infallible() {
+    if false {
+        //~^ ERROR unreachable expression
+        infallible();
+    }
+
+    println!()
+}
+
+fn main() {}

tests/ui/uninhabited/void-branch.stderr

@@ -0,0 +1,40 @@
+error: unreachable expression
+  --> $DIR/void-branch.rs:8:9
+   |
+LL | /         unsafe {
+LL | |
+LL | |             std::mem::uninitialized::<Void>();
+   | |             --------------------------------- any code following this expression is unreachable
+LL | |         }
+   | |_________^ unreachable expression
+   |
+note: this expression has type `Void`, which is uninhabited
+  --> $DIR/void-branch.rs:10:13
+   |
+LL |             std::mem::uninitialized::<Void>();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: the lint level is defined here
+  --> $DIR/void-branch.rs:1:9
+   |
+LL | #![deny(unreachable_code)]
+   |         ^^^^^^^^^^^^^^^^
+
+error: unreachable expression
+  --> $DIR/void-branch.rs:22:14
+   |
+LL |       if false {
+   |  ______________^
+LL | |
+LL | |         infallible();
+   | |         ------------ any code following this expression is unreachable
+LL | |     }
+   | |_____^ unreachable expression
+   |
+note: this expression has type `Infallible`, which is uninhabited
+  --> $DIR/void-branch.rs:24:9
+   |
+LL |         infallible();
+   |         ^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+