diff --git a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
index 568335f7dcb51..a47179c14d27b 100644
--- a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
@@ -347,7 +347,13 @@ extern "C" LLVMTargetMachineRef LLVMRustCreateTargetMachine(
     // option causes bugs in the LLVM WebAssembly backend. You should be able to
     // remove this check when Rust's minimum supported LLVM version is >= 18
     // https://github.com/llvm/llvm-project/pull/65876
-    if (!Trip.isWasm()) {
+    //
+    // Also keep traps after noreturn calls on Windows, because the trap is
+    // needed to keep the return address within the calling function's
+    // .pdata range. Without it, RtlLookupFunctionEntry resolves the wrong
+    // function and SEH unwinding (used for backtraces) terminates early.
+    // See https://github.com/rust-lang/rust/issues/140489
+    if (!Trip.isWasm() && !Trip.isOSWindows()) {
       Options.NoTrapAfterNoreturn = true;
     }
   }
@@ -840,11 +846,8 @@ extern "C" LLVMRustResult LLVMRustOptimize(
         }
         break;
       case LLVMRustOptStage::PreLinkThinLTO:
-        MPM = PB.buildThinLTOPreLinkDefaultPipeline(OptLevel);
-        NeedThinLTOBufferPasses = false;
-        break;
       case LLVMRustOptStage::PreLinkFatLTO:
-        MPM = PB.buildLTOPreLinkDefaultPipeline(OptLevel);
+        MPM = PB.buildThinLTOPreLinkDefaultPipeline(OptLevel);
         NeedThinLTOBufferPasses = false;
         break;
       case LLVMRustOptStage::ThinLTO:
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.
diff --git a/tests/ui/runtime/backtrace-debuginfo.rs b/tests/ui/runtime/backtrace-debuginfo.rs
index d3b4d057e6d0f..cc6132495f4ce 100644
--- a/tests/ui/runtime/backtrace-debuginfo.rs
+++ b/tests/ui/runtime/backtrace-debuginfo.rs
@@ -47,14 +47,9 @@ macro_rules! dump_and_die {
         // there, even on i686-pc-windows-msvc. We do the best we can in
         // rust-lang/rust to test it as well, but sometimes we just gotta keep
         // landing PRs.
-        //
-        // aarch64-msvc/arm64ec-msvc is broken as its backtraces are truncated.
-        // See https://github.com/rust-lang/rust/issues/140489
         if cfg!(any(target_os = "android",
                     all(target_os = "linux", target_arch = "arm"),
                     all(target_env = "msvc", target_arch = "x86"),
-                    all(target_env = "msvc", target_arch = "aarch64"),
-                    all(target_env = "msvc", target_arch = "arm64ec"),
                     target_os = "freebsd",
                     target_os = "dragonfly",
                     target_os = "openbsd")) {