Test that stable const fn requires allow_internal_unstable

Author: ecstatic-morse

src/test/ui/internal/internal-unstable-const.rs

@@ -0,0 +1,10 @@
+#![feature(staged_api)]
+#![feature(const_if_match)]
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_stable(feature = "rust1", since = "1.0.0")]
+const fn foo() -> i32 {
+    if true { 4 } else { 5 } //~ loops and conditional expressions are not stable in const fn
+}
+
+fn main() {}

src/test/ui/internal/internal-unstable-const.stderr

@@ -0,0 +1,12 @@
+error[E0723]: loops and conditional expressions are not stable in const fn
+  --> $DIR/internal-unstable-const.rs:7:5
+   |
+LL |     if true { 4 } else { 5 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
+   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0723`.