it's called RUSTC_CTFE_BACKTRACE now

rust-version

@@ -1 +1 @@
-5aa3d9a7b5d3a46a7f158e8881146331a6bc9243
+00ee1b47f42129a0a6e33510578fbcf07c1e5382

src/bin/miri.rs

@@ -92,11 +92,11 @@ fn init_late_loggers() {
         }
     }
 
-    // If `MIRI_BACKTRACE` is set and `RUST_CTFE_BACKTRACE` is not, set `RUST_CTFE_BACKTRACE`.
-    // Do this late, so we really only apply this to miri's errors.
+    // If `MIRI_BACKTRACE` is set and `RUSTC_CTFE_BACKTRACE` is not, set `RUSTC_CTFE_BACKTRACE`.
+    // Do this late, so we ideally only apply this to Miri's errors.
     if let Ok(var) = env::var("MIRI_BACKTRACE") {
-        if env::var("RUST_CTFE_BACKTRACE") == Err(env::VarError::NotPresent) {
-            env::set_var("RUST_CTFE_BACKTRACE", &var);
+        if env::var("RUSTC_CTFE_BACKTRACE") == Err(env::VarError::NotPresent) {
+            env::set_var("RUSTC_CTFE_BACKTRACE", &var);
         }
     }
 }