unconditionally allow shadow call-stack for AArch64 whenever fixed-x1…

…8 is applied

compiler/rustc_session/src/session.rs

@@ -1187,7 +1187,12 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
 
     // Sanitizers can only be used on platforms that we know have working sanitizer codegen.
     let supported_sanitizers = sess.target.options.supported_sanitizers;
-    let unsupported_sanitizers = sess.opts.unstable_opts.sanitizer - supported_sanitizers;
+    let mut unsupported_sanitizers = sess.opts.unstable_opts.sanitizer - supported_sanitizers;
+    // Niche: if `fixed-x18`, or effectively switching on `reserved-x18` flag, is enabled
+    // we should allow Shadow Call Stack sanitizer.
+    if sess.opts.unstable_opts.fixed_x18 && sess.target.arch == "aarch64" {
+        unsupported_sanitizers -= SanitizerSet::SHADOWCALLSTACK;
+    }
     match unsupported_sanitizers.into_iter().count() {
         0 => {}
         1 => {

tests/codegen/sanitizer/aarch64-shadow-call-stack-with-fixed-x18.rs

@@ -0,0 +1,18 @@
+//@ needs-llvm-components: aarch64
+//@ revisions: aarch64 android
+//@[aarch64] compile-flags: --target aarch64-unknown-none -Zfixed-x18 -Zsanitizer=shadow-call-stack
+//@[android] compile-flags: --target aarch64-linux-android -Zfixed-x18 -Zsanitizer=shadow-call-stack
+
+#![allow(internal_features)]
+#![crate_type = "rlib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}
+
+// CHECK: ; Function Attrs:{{.*}}shadowcallstack
+#[no_mangle]
+pub fn foo() {}
+
+// CHECK: attributes #0 = {{.*}}shadowcallstack{{.*}}+reserve-x18

tests/ui/abi/shadow-call-stack-without-fixed-x18.rs

@@ -0,0 +1,15 @@
+//@ compile-flags: --target aarch64-unknown-none -Zsanitizer=shadow-call-stack
+//@ error-pattern: shadow-call-stack sanitizer is not supported for this target
+//@ dont-check-compiler-stderr
+//@ needs-llvm-components: aarch64
+
+#![allow(internal_features)]
+#![crate_type = "rlib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}
+
+#[no_mangle]
+pub fn foo() {}