stage2: Add `.save_err_return_trace_index` AIR op

This is encoded as a primitive AIR instruction to resolve one corner
case: A function may include a `catch { ... }` or `else |err| { ... }`
block but not call any errorable fn. In that case, there is no error
return trace to save the index of and codegen needs to avoid
interacting with the non-existing error trace.

By using a primitive AIR op, we can depend on Liveness to mark this
unused in this corner case.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/codegen/c.zig b/src/codegen/c.zig
index 072091d9b2..d6584d75ae 100644
--- a/src/codegen/c.zig
+++ b/src/codegen/c.zig
@@ -1935,6 +1935,7 @@ fn genBody(f: *Function, body: []const Air.Inst.Index) error{ AnalysisFail, OutO
             .errunion_payload_ptr_set    => try airErrUnionPayloadPtrSet(f, inst),
             .err_return_trace            => try airErrReturnTrace(f, inst),
             .set_err_return_trace        => try airSetErrReturnTrace(f, inst),
+            .save_err_return_trace_index => try airSaveErrReturnTraceIndex(f, inst),
 
             .wasm_memory_size => try airWasmMemorySize(f, inst),
             .wasm_memory_grow => try airWasmMemoryGrow(f, inst),
@@ -3625,6 +3626,11 @@ fn airSetErrReturnTrace(f: *Function, inst: Air.Inst.Index) !CValue {
     return f.fail("TODO: C backend: implement airSetErrReturnTrace", .{});
 }
 
+fn airSaveErrReturnTraceIndex(f: *Function, inst: Air.Inst.Index) !CValue {
+    _ = inst;
+    return f.fail("TODO: C backend: implement airSaveErrReturnTraceIndex", .{});
+}
+
 fn airWrapErrUnionPay(f: *Function, inst: Air.Inst.Index) !CValue {
     if (f.liveness.isUnused(inst))
         return CValue.none;