Add additional const tests

Author: ecstatic-morse

src/test/ui/consts/const-if.rs

@@ -0,0 +1,5 @@
+const _X: i32 = if true { 5 } else { 6 };
+//~^ ERROR constant contains unimplemented expression type
+//~| ERROR constant contains unimplemented expression type
+
+fn main() {}

src/test/ui/consts/const-if.stderr

@@ -0,0 +1,15 @@
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/const-if.rs:1:20
+   |
+LL | const _X: i32 = if true { 5 } else { 6 };
+   |                    ^^^^
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/const-if.rs:1:17
+   |
+LL | const _X: i32 = if true { 5 } else { 6 };
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0019`.

src/test/ui/consts/const-multi-ref.rs

@@ -0,0 +1,11 @@
+const _X: i32 = {
+    let mut a = 5;
+    let p = &mut a;      //~ ERROR references in constants may only refer to immutable values
+
+    let reborrow = {p};  //~ ERROR references in constants may only refer to immutable values
+    let pp = &reborrow;
+    let ppp = &pp;
+    ***ppp
+};
+
+fn main() {}

src/test/ui/consts/const-multi-ref.stderr

@@ -0,0 +1,15 @@
+error[E0017]: references in constants may only refer to immutable values
+  --> $DIR/const-multi-ref.rs:3:13
+   |
+LL |     let p = &mut a;
+   |             ^^^^^^ constants require immutable values
+
+error[E0017]: references in constants may only refer to immutable values
+  --> $DIR/const-multi-ref.rs:5:21
+   |
+LL |     let reborrow = {p};
+   |                     ^ constants require immutable values
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0017`.