diff --git a/library/test/build.rs b/library/test/build.rs
index 3f7a5c16e5b1e..8e31adbf2ca74 100644
--- a/library/test/build.rs
+++ b/library/test/build.rs
@@ -2,16 +2,11 @@ fn main() {
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rustc-check-cfg=cfg(enable_unstable_features)");
 
-    // 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")
-    };
+    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();
 
-    if enable_unstable_features {
+    if stdout.contains("nightly") || stdout.contains("dev") {
         println!("cargo:rustc-cfg=enable_unstable_features");
     }
 }
diff --git a/src/bootstrap/src/core/build_steps/test.rs b/src/bootstrap/src/core/build_steps/test.rs
index 88f10775333d4..3b0e7b2e30afe 100644
--- a/src/bootstrap/src/core/build_steps/test.rs
+++ b/src/bootstrap/src/core/build_steps/test.rs
@@ -741,7 +741,13 @@ impl Step for Miri {
 
         // Run it again for mir-opt-level 4 to catch some miscompilations.
         if builder.config.test_args().is_empty() {
-            cargo.env("MIRIFLAGS", "-O -Zmir-opt-level=4 -Cdebug-assertions=yes");
+            cargo.env(
+                "MIRIFLAGS",
+                format!(
+                    "{} -O -Zmir-opt-level=4 -Cdebug-assertions=yes",
+                    env::var("MIRIFLAGS").unwrap_or_default()
+                ),
+            );
             // Optimizations can change backtraces
             cargo.env("MIRI_SKIP_UI_CHECKS", "1");
             // `MIRI_SKIP_UI_CHECKS` and `RUSTC_BLESS` are incompatible
@@ -3108,6 +3114,17 @@ impl Step for Crate {
             // does not set this directly, but relies on the rustc wrapper to set it, and we are not using
             // the wrapper -- hence we have to set it ourselves.
             cargo.rustflag("-Zforce-unstable-if-unmarked");
+            // Miri is told to invoke the libtest runner and bootstrap sets unstable flags
+            // for that runner. That only works when RUSTC_BOOTSTRAP is set. Bootstrap sets
+            // that flag but Miri by default does not forward the host environment to the test.
+            // Here we set up MIRIFLAGS to forward that env var.
+            cargo.env(
+                "MIRIFLAGS",
+                format!(
+                    "{} -Zmiri-env-forward=RUSTC_BOOTSTRAP",
+                    env::var("MIRIFLAGS").unwrap_or_default()
+                ),
+            );
             cargo
         } else {
             // Also prepare a sysroot for the target.