Use an error struct instead of a panic

compiler/rustc_codegen_llvm/messages.ftl

@@ -18,6 +18,8 @@ codegen_llvm_error_creating_import_library =
 codegen_llvm_error_writing_def_file =
     Error writing .DEF file: {$error}
 
+codegen_llvm_fixed_x18_invalid_arch = the `-Zfixed-x18` flag is not supported on the `{$arch}` architecture
+
 codegen_llvm_from_llvm_diag = {$message}
 
 codegen_llvm_from_llvm_optimization_diag = {$filename}:{$line}:{$column} {$pass_name} ({$kind}): {$message}

compiler/rustc_codegen_llvm/src/errors.rs

@@ -254,3 +254,9 @@ pub struct MismatchedDataLayout<'a> {
 pub(crate) struct InvalidTargetFeaturePrefix<'a> {
     pub feature: &'a str,
 }
+
+#[derive(Diagnostic)]
+#[diag(codegen_llvm_fixed_x18_invalid_arch)]
+pub(crate) struct FixedX18InvalidArch<'a> {
+    pub arch: &'a str,
+}

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -1,6 +1,6 @@
 use crate::back::write::create_informational_target_machine;
 use crate::errors::{
-    InvalidTargetFeaturePrefix, PossibleFeature, TargetFeatureDisableOrEnable,
+    FixedX18InvalidArch, InvalidTargetFeaturePrefix, PossibleFeature, TargetFeatureDisableOrEnable,
     UnknownCTargetFeature, UnknownCTargetFeaturePrefix, UnstableCTargetFeature,
 };
 use crate::llvm;
@@ -618,12 +618,11 @@ pub(crate) fn global_llvm_features(sess: &Session, diagnostics: bool) -> Vec<Str
     // -Zfixed-x18
     if sess.opts.unstable_opts.fixed_x18 {
         if sess.target.arch != "aarch64" {
-            // TODO: What's the correct way to return a error here?
-            panic!("-Zfixed-x18 only allowed on aarch64");
-        }
-
+            sess.dcx().emit_fatal(FixedX18InvalidArch { arch: &sess.target.arch });
+        } else {
             features.push("+reserve-x18".into());
         }
+    }
 
     if diagnostics && let Some(f) = check_tied_features(sess, &featsmap) {
         sess.dcx().emit_err(TargetFeatureDisableOrEnable {

tests/ui/abi/fixed_x18.aarch64.stderr

@@ -0,0 +1,2 @@
+error: the `-Zfixed-x18` flag is not supported on the `x86_64` architecture
+

tests/ui/abi/fixed_x18.arm.stderr

@@ -0,0 +1,2 @@
+error: the `-Zfixed-x18` flag is not supported on the `arm` architecture
+

tests/ui/abi/fixed_x18.i686.stderr

@@ -0,0 +1,2 @@
+error: the `-Zfixed-x18` flag is not supported on the `x86` architecture
+

tests/ui/abi/fixed_x18.riscv32.stderr

@@ -0,0 +1,2 @@
+error: the `-Zfixed-x18` flag is not supported on the `riscv32` architecture
+

tests/ui/abi/fixed_x18.riscv64.stderr

@@ -0,0 +1,2 @@
+error: the `-Zfixed-x18` flag is not supported on the `riscv64` architecture
+

tests/ui/abi/fixed_x18.rs

@@ -0,0 +1,23 @@
+// This tests that -Zfixed-x18 causes a compilation failure on targets other than aarch64.
+// Behavior on aarch64 is tested by tests/codegen/fixed-x18.rs.
+//
+//@ revisions: x64 i686 aarch64 arm riscv32 riscv64
+//
+//@ compile-flags: -Zfixed-x18
+//@ [x64] needs-llvm-components: x86
+//@ [x64] compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=rlib
+//@ [i686] needs-llvm-components: x86
+//@ [i686] compile-flags: --target=i686-unknown-linux-gnu --crate-type=rlib
+//@ [arm] needs-llvm-components: arm
+//@ [arm] compile-flags: --target=armv7-unknown-linux-gnueabihf --crate-type=rlib
+//@ [riscv32] needs-llvm-components: riscv
+//@ [riscv32] compile-flags: --target=riscv32i-unknown-none-elf --crate-type=rlib
+//@ [riscv64] needs-llvm-components: riscv
+//@ [riscv64] compile-flags: --target=riscv64gc-unknown-none-elf --crate-type=rlib
+
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}

tests/ui/abi/fixed_x18.x64.stderr

@@ -0,0 +1,2 @@
+error: the `-Zfixed-x18` flag is not supported on the `x86_64` architecture
+