Add test forbidding const parameters in const fn

Author: varkor

src/test/ui/const-generics/const-fn-with-const-param.rs

@@ -0,0 +1,12 @@
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+const fn const_u32_identity<const X: u32>() -> u32 {
+    //~^ ERROR const parameters are not permitted in `const fn`
+    //~^^ ERROR const generics in any position are currently unsupported
+    X
+}
+
+fn main() {
+    println!("{:?}", const_u32_identity::<18>());
+}

src/test/ui/const-generics/const-fn-with-const-param.stderr

@@ -0,0 +1,24 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/const-fn-with-const-param.rs:1:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+
+error: const parameters are not permitted in `const fn`
+  --> $DIR/const-fn-with-const-param.rs:4:1
+   |
+LL | / const fn const_u32_identity<const X: u32>() -> u32 {
+LL | |     //~^ ERROR const parameters are not permitted in `const fn`
+LL | |     //~^^ ERROR const generics in any position are currently unsupported
+LL | |     X
+LL | | }
+   | |_^
+
+error: const generics in any position are currently unsupported
+  --> $DIR/const-fn-with-const-param.rs:4:35
+   |
+LL | const fn const_u32_identity<const X: u32>() -> u32 {
+   |                                   ^
+
+error: aborting due to 2 previous errors
+