Add a check for a minimum rustc version in ff

ff/build.rs

@@ -1,5 +1,5 @@
 extern crate rustc_version;
-use rustc_version::{version_meta, Channel};
+use rustc_version::{version, version_meta, Channel, Version};
 
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
@@ -15,4 +15,10 @@ fn main() {
     if should_use_asm {
         println!("cargo:rustc-cfg=use_asm");
     }
+
+    // TODO: remove this once RFC 2495 ships
+    if version().expect("Installed rustc version unparseable!") < Version::parse("1.51.0").unwrap()
+    {
+        panic!("This code base uses const generics and requires a Rust compiler version greater or equal to 1.51.0");
+    }
 }