asm: Emit an error for asm goto with output operands

src/asm.rs

@@ -18,7 +18,7 @@ use rustc_target::asm::*;
 use crate::builder::Builder;
 use crate::callee::get_fn;
 use crate::context::CodegenCx;
-use crate::errors::{NulBytesInAsm, UnwindingInlineAsm};
+use crate::errors::{AsmGotoWithOutputs, NulBytesInAsm, UnwindingInlineAsm};
 use crate::type_of::LayoutGccExt;
 
 // Rust asm! and GCC Extended Asm semantics differ substantially.
@@ -545,6 +545,15 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
             return;
         }
 
+        // GCC's `asm goto` cannot have output operands (libgccjit rejects them with
+        // "cannot add output operand to asm goto"). Emit a clean error instead of
+        // letting libgccjit abort with an internal error.
+        if dest.is_some() && !outputs.is_empty() {
+            let err_sp = span.first().copied().unwrap_or(DUMMY_SP);
+            self.sess().dcx().emit_err(AsmGotoWithOutputs { span: err_sp });
+            return;
+        }
+
         // 4. Generate Extended Asm block
         let block = self.llbb();
         let extended_asm = if let Some(dest) = dest {

src/errors.rs

@@ -30,3 +30,10 @@ pub(crate) struct NulBytesInAsm {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag("GCC backend does not support `asm goto` with output operands")]
+pub(crate) struct AsmGotoWithOutputs {
+    #[primary_span]
+    pub span: Span,
+}

tests/compile/asm_goto_with_outputs.rs

@@ -0,0 +1,25 @@
+// Compiler:
+//   status: error
+//   stderr:
+//     error: GCC backend does not support `asm goto` with output operands
+//     ...
+
+// Test that an `asm goto` with output operands emits a clean error instead of an
+// ICE: libgccjit rejects output operands on an `asm goto` (issue #835).
+
+#![feature(asm_goto_with_outputs)]
+
+use std::arch::asm;
+
+fn main() {
+    let mut a: i32 = 0;
+    unsafe {
+        asm!(
+            "jmp {op}",
+            inout("eax") a,
+            op = label { a = 7; },
+            options(nostack, nomem),
+        );
+    }
+    let _ = a;
+}

tests/lang_tests.rs

@@ -201,7 +201,13 @@ fn compile_tests(tempdir: PathBuf, current_dir: String) {
         "lang compile",
         "tests/compile",
         TestMode::Compile,
-        &["simd-ffi.rs", "asm_nul_byte.rs", "global_asm_nul_byte.rs", "naked_asm_nul_byte.rs"],
+        &[
+            "simd-ffi.rs",
+            "asm_nul_byte.rs",
+            "asm_goto_with_outputs.rs",
+            "global_asm_nul_byte.rs",
+            "naked_asm_nul_byte.rs",
+        ],
     );
 }