implement build.rs for rustc_public

Author: makai410

build.rs

@@ -0,0 +1,30 @@
+use std::process::{self, Command};
+use std::{env, str};
+
+use rustversion;
+
+const MSRV: &str = "2025-08-09";
+const SUPPORTED: bool = rustversion::cfg!(since(2025-08-09));
+
+fn main() {
+    if !SUPPORTED {
+        let current = rustc_version().unwrap_or(String::from("unknown"));
+        eprintln!(
+            "\nERROR: rustc_public requires rustc nightly-{MSRV} or newer\n\
+                current: {current}\n\
+                help: run `rustup update nightly`.\n"
+        );
+        process::exit(1);
+    }
+    println!("cargo:rerun-if-changed=build.rs");
+}
+
+fn rustc_version() -> Option<String> {
+    let rustc = env::var_os("RUSTC").unwrap_or_else(|| {
+        eprintln!("RUSTC is not set during build script execution.\n");
+        process::exit(1);
+    });
+    let output = Command::new(rustc).arg("--version").output().ok()?;
+    let version = str::from_utf8(&output.stdout).ok()?;
+    version.parse().ok()
+}

rust-toolchain.toml

@@ -1,3 +1,2 @@
 [toolchain]
-channel = "nightly"
 components = ["llvm-tools-preview", "rustc-dev", "rust-src", "rustfmt"]