Add regression test for #79450

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

src/test/ui/rfc-2632-const-trait-impl/issue-79450.rs

@@ -0,0 +1,20 @@
+#![feature(const_fmt_arguments_new)]
+#![feature(const_trait_impl)]
+
+#[const_trait]
+trait Tr {
+    fn req(&self);
+
+    fn prov(&self) {
+        println!("lul"); //~ ERROR: cannot call non-const fn `_print` in constant functions
+        self.req();
+    }
+}
+
+struct S;
+
+impl const Tr for S {
+    fn req(&self) {}
+}
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/issue-79450.stderr

@@ -0,0 +1,12 @@
+error[E0015]: cannot call non-const fn `_print` in constant functions
+  --> $DIR/issue-79450.rs:9:9
+   |
+LL |         println!("lul");
+   |         ^^^^^^^^^^^^^^^
+   |
+   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+   = note: this error originates in the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0015`.