From 4b58c5034a59f58b9ba1a713a667b4d9ffc1df34 Mon Sep 17 00:00:00 2001
From: Alice Ryhl <aliceryhl@google.com>
Date: Mon, 5 May 2025 12:34:22 +0000
Subject: [PATCH] Make -Zfixed-x18 into a target modifier

---
 compiler/rustc_session/src/options.rs           |  2 +-
 .../ui/target_modifiers/auxiliary/fixed_x18.rs  |  7 +++++++
 ...incompatible_fixedx18.error_generated.stderr | 13 +++++++++++++
 .../target_modifiers/incompatible_fixedx18.rs   | 17 +++++++++++++++++
 4 files changed, 38 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/target_modifiers/auxiliary/fixed_x18.rs
 create mode 100644 tests/ui/target_modifiers/incompatible_fixedx18.error_generated.stderr
 create mode 100644 tests/ui/target_modifiers/incompatible_fixedx18.rs

diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 5f4695fb1841b..c3d922fc9c5ec 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -2208,7 +2208,7 @@ options! {
     fewer_names: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "reduce memory use by retaining fewer names within compilation artifacts (LLVM-IR) \
         (default: no)"),
-    fixed_x18: bool = (false, parse_bool, [TRACKED],
+    fixed_x18: bool = (false, parse_bool, [TRACKED TARGET_MODIFIER],
         "make the x18 register reserved on AArch64 (default: no)"),
     flatten_format_args: bool = (true, parse_bool, [TRACKED],
         "flatten nested format_args!() and literals into a simplified format_args!() call \
diff --git a/tests/ui/target_modifiers/auxiliary/fixed_x18.rs b/tests/ui/target_modifiers/auxiliary/fixed_x18.rs
new file mode 100644
index 0000000000000..32eff76ec54c4
--- /dev/null
+++ b/tests/ui/target_modifiers/auxiliary/fixed_x18.rs
@@ -0,0 +1,7 @@
+//@ no-prefer-dynamic
+//@ compile-flags: --target aarch64-unknown-none -Zfixed-x18
+//@ needs-llvm-components: aarch64
+
+#![feature(no_core)]
+#![crate_type = "rlib"]
+#![no_core]
diff --git a/tests/ui/target_modifiers/incompatible_fixedx18.error_generated.stderr b/tests/ui/target_modifiers/incompatible_fixedx18.error_generated.stderr
new file mode 100644
index 0000000000000..096d7cb5f2582
--- /dev/null
+++ b/tests/ui/target_modifiers/incompatible_fixedx18.error_generated.stderr
@@ -0,0 +1,13 @@
+error: mixing `-Zfixed-x18` will cause an ABI mismatch in crate `incompatible_fixedx18`
+  --> $DIR/incompatible_fixedx18.rs:12:1
+   |
+LL | #![feature(no_core)]
+   | ^
+   |
+   = help: the `-Zfixed-x18` flag modifies the ABI so Rust crates compiled with different values of this flag cannot be used together safely
+   = note: unset `-Zfixed-x18` in this crate is incompatible with `-Zfixed-x18=` in dependency `fixed_x18`
+   = help: set `-Zfixed-x18=` in this crate or unset `-Zfixed-x18` in `fixed_x18`
+   = help: if you are sure this will not cause problems, you may use `-Cunsafe-allow-abi-mismatch=fixed-x18` to silence this error
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/target_modifiers/incompatible_fixedx18.rs b/tests/ui/target_modifiers/incompatible_fixedx18.rs
new file mode 100644
index 0000000000000..6c13984f608ec
--- /dev/null
+++ b/tests/ui/target_modifiers/incompatible_fixedx18.rs
@@ -0,0 +1,17 @@
+//@ aux-build:fixed_x18.rs
+//@ compile-flags: --target aarch64-unknown-none
+//@ needs-llvm-components: aarch64
+
+//@ revisions:allow_match allow_mismatch error_generated
+//@[allow_match] compile-flags: -Zfixed-x18
+//@[allow_mismatch] compile-flags: -Cunsafe-allow-abi-mismatch=fixed-x18
+//@[error_generated] compile-flags:
+//@[allow_mismatch] check-pass
+//@[allow_match] check-pass
+
+#![feature(no_core)]
+//[error_generated]~^ ERROR mixing `-Zfixed-x18` will cause an ABI mismatch in crate `incompatible_fixedx18`
+#![crate_type = "rlib"]
+#![no_core]
+
+extern crate fixed_x18;