diff --git a/library/test/build.rs b/library/test/build.rs
index 8e31adbf2ca74..3f7a5c16e5b1e 100644
--- a/library/test/build.rs
+++ b/library/test/build.rs
@@ -2,11 +2,16 @@ fn main() {
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rustc-check-cfg=cfg(enable_unstable_features)");
 
-    let rustc = std::env::var("RUSTC").unwrap_or_else(|_| "rustc".into());
-    let version = std::process::Command::new(rustc).arg("-vV").output().unwrap();
-    let stdout = String::from_utf8(version.stdout).unwrap();
+    // Miri testing uses unstable features, so always enable that for its sysroot.
+    // Otherwise, only enable unstable if rustc looks like a nightly or dev build.
+    let enable_unstable_features = std::env::var("MIRI_CALLED_FROM_SETUP").is_ok() || {
+        let rustc = std::env::var("RUSTC").unwrap_or_else(|_| "rustc".into());
+        let version = std::process::Command::new(rustc).arg("-vV").output().unwrap();
+        let stdout = String::from_utf8(version.stdout).unwrap();
+        stdout.contains("nightly") || stdout.contains("dev")
+    };
 
-    if stdout.contains("nightly") || stdout.contains("dev") {
+    if enable_unstable_features {
         println!("cargo:rustc-cfg=enable_unstable_features");
     }
 }