Port branch protection on aarch64

compiler/rustc_codegen_llvm/src/context.rs

@@ -3,6 +3,7 @@ use crate::back::write::to_llvm_code_model;
 use crate::callee::get_fn;
 use crate::coverageinfo;
 use crate::debuginfo;
+use crate::errors::BranchProtectionRequiresAArch64;
 use crate::llvm;
 use crate::llvm_util;
 use crate::type_::Type;
@@ -275,7 +276,7 @@ pub unsafe fn create_module<'ll>(
 
     if let Some(BranchProtection { bti, pac_ret }) = sess.opts.unstable_opts.branch_protection {
         if sess.target.arch != "aarch64" {
-            sess.err("-Zbranch-protection is only supported on aarch64");
+            sess.emit_err(BranchProtectionRequiresAArch64);
         } else {
             llvm::LLVMRustAddModuleFlag(
                 llmod,

compiler/rustc_codegen_llvm/src/errors.rs

@@ -55,3 +55,7 @@ pub(crate) struct SymbolAlreadyDefined<'a> {
     pub span: Span,
     pub symbol_name: &'a str,
 }
+
+#[derive(SessionDiagnostic)]
+#[diag(codegen_llvm::branch_protection_requires_aarch64)]
+pub(crate) struct BranchProtectionRequiresAArch64;

compiler/rustc_error_messages/locales/en-US/codegen_llvm.ftl

@@ -21,3 +21,6 @@ codegen_llvm_instrument_coverage_requires_llvm_12 =
 
 codegen_llvm_symbol_already_defined = 
     symbol `{$symbol_name}` is already defined
+
+codegen_llvm_branch_protection_requires_aarch64 =
+    -Zbranch-protection is only supported on aarch64