Check AArch64 branch-protection earlier in the pipeline.

compiler/rustc_codegen_llvm/src/context.rs

@@ -3,7 +3,6 @@ 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;
@@ -276,9 +275,7 @@ 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.emit_err(BranchProtectionRequiresAArch64);
-        } else {
+        if sess.target.arch == "aarch64" {
             llvm::LLVMRustAddModuleFlag(
                 llmod,
                 llvm::LLVMModFlagBehavior::Error,
@@ -304,6 +301,11 @@ pub unsafe fn create_module<'ll>(
                 "sign-return-address-with-bkey\0".as_ptr().cast(),
                 u32::from(pac_opts.key == PAuthKey::B),
             );
+        } else {
+            bug!(
+                "branch-protection used on non-AArch64 target; \
+                  this should be checked in rustc_session."
+            );
         }
     }
 

compiler/rustc_codegen_llvm/src/errors.rs

@@ -51,10 +51,6 @@ pub(crate) struct SymbolAlreadyDefined<'a> {
     pub symbol_name: &'a str,
 }
 
-#[derive(Diagnostic)]
-#[diag(codegen_llvm_branch_protection_requires_aarch64)]
-pub(crate) struct BranchProtectionRequiresAArch64;
-
 #[derive(Diagnostic)]
 #[diag(codegen_llvm_invalid_minimum_alignment)]
 pub(crate) struct InvalidMinimumAlignment {

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

@@ -17,9 +17,6 @@ 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
-
 codegen_llvm_invalid_minimum_alignment =
     invalid minimum global alignment: {$err}
 

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

@@ -41,6 +41,8 @@ session_unsupported_dwarf_version = requested DWARF version {$dwarf_version} is
 
 session_target_stack_protector_not_supported = `-Z stack-protector={$stack_protector}` is not supported for target {$target_triple} and will be ignored
 
+session_branch_protection_requires_aarch64 = `-Zbranch-protection` is only supported on aarch64
+
 session_split_debuginfo_unstable_platform = `-Csplit-debuginfo={$debuginfo}` is unstable on this platform
 
 session_file_is_not_writeable = output file {$file} is not writeable -- check its permissions

compiler/rustc_session/src/errors.rs

@@ -115,6 +115,10 @@ pub struct StackProtectorNotSupportedForTarget<'a> {
     pub target_triple: &'a TargetTriple,
 }
 
+#[derive(Diagnostic)]
+#[diag(session_branch_protection_requires_aarch64)]
+pub(crate) struct BranchProtectionRequiresAArch64;
+
 #[derive(Diagnostic)]
 #[diag(session_split_debuginfo_unstable_platform)]
 pub struct SplitDebugInfoUnstablePlatform {

compiler/rustc_session/src/session.rs

@@ -3,10 +3,10 @@ use crate::code_stats::CodeStats;
 pub use crate::code_stats::{DataTypeKind, FieldInfo, SizeKind, VariantInfo};
 use crate::config::{self, CrateType, InstrumentCoverage, OptLevel, OutputType, SwitchWithOptPath};
 use crate::errors::{
-    CannotEnableCrtStaticLinux, CannotMixAndMatchSanitizers, LinkerPluginToWindowsNotSupported,
-    NotCircumventFeature, ProfileSampleUseFileDoesNotExist, ProfileUseFileDoesNotExist,
-    SanitizerCfiEnabled, SanitizerNotSupported, SanitizersNotSupported, SkippingConstChecks,
-    SplitDebugInfoUnstablePlatform, StackProtectorNotSupportedForTarget,
+    BranchProtectionRequiresAArch64, CannotEnableCrtStaticLinux, CannotMixAndMatchSanitizers,
+    LinkerPluginToWindowsNotSupported, NotCircumventFeature, ProfileSampleUseFileDoesNotExist,
+    ProfileUseFileDoesNotExist, SanitizerCfiEnabled, SanitizerNotSupported, SanitizersNotSupported,
+    SkippingConstChecks, SplitDebugInfoUnstablePlatform, StackProtectorNotSupportedForTarget,
     TargetRequiresUnwindTables, UnleashedFeatureHelp, UnstableVirtualFunctionElimination,
     UnsupportedDwarfVersion,
 };
@@ -1542,6 +1542,10 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
         }
     }
 
+    if sess.opts.unstable_opts.branch_protection.is_some() && sess.target.arch != "aarch64" {
+        sess.emit_err(BranchProtectionRequiresAArch64);
+    }
+
     if let Some(dwarf_version) = sess.opts.unstable_opts.dwarf_version {
         if dwarf_version > 5 {
             sess.emit_err(UnsupportedDwarfVersion { dwarf_version });

src/doc/unstable-book/src/compiler-flags/branch-protection.md

@@ -1,7 +1,7 @@
 # `branch-protection`
 
 This option lets you enable branch authentication instructions on AArch64.
-This option is ignored for non-AArch64 architectures.
+This option is only accepted when targeting AArch64 architectures.
 It takes some combination of the following values, separated by a `,`.
 
 - `pac-ret` - Enable pointer authentication for non-leaf functions.

src/test/ui/invalid-compile-flags/branch-protection-missing-pac-ret.BADTARGET.stderr

@@ -1,4 +1,4 @@
-error: -Zbranch-protection is only supported on aarch64
+error: `-Zbranch-protection` is only supported on aarch64
 
 error: aborting due to previous error
 

src/test/ui/invalid-compile-flags/branch-protection-missing-pac-ret.rs

@@ -3,7 +3,7 @@
 // [BADFLAGS] check-fail
 // [BADFLAGS] needs-llvm-components: aarch64
 // [BADTARGET] compile-flags: --target=x86_64-unknown-linux-gnu -Zbranch-protection=bti
-// [BADTARGET] build-fail
+// [BADTARGET] check-fail
 // [BADTARGET] needs-llvm-components: x86
 
 #![crate_type = "lib"]