Skip parameter attribute deduction for MIR with spread_arg

When a MIR argument is spread at ABI level, deduced attributes are
potentially misapplied, since a spread argument can correspond to zero
or more arguments at ABI level.

Disable deduction for MIR using spread argument for the time being.

Author: tmiasko

compiler/rustc_mir_transform/src/deduce_param_attrs.rs

@@ -170,6 +170,11 @@ pub(super) fn deduced_param_attrs<'tcx>(
 
     // Grab the optimized MIR. Analyze it to determine which arguments have been mutated.
     let body: &Body<'tcx> = tcx.optimized_mir(def_id);
+    // Arguments spread at ABI level are currently unsupported.
+    if body.spread_arg.is_some() {
+        return &[];
+    }
+
     let mut deduce_read_only = DeduceReadOnly::new(body.arg_count);
     deduce_read_only.visit_body(body);
     tracing::trace!(?deduce_read_only.read_only);

tests/codegen-llvm/deduced-param-attrs.rs

@@ -3,6 +3,7 @@
 #![crate_type = "lib"]
 #![allow(internal_features)]
 #![feature(unsized_fn_params)]
+#![feature(unboxed_closures)]
 
 use std::cell::Cell;
 use std::hint;
@@ -79,3 +80,11 @@ pub fn forward_big_cell_container(big_cell_container: BigCellContainer) {
 pub fn forward_big_container(big_struct: BigStruct) {
     use_something_freeze(big_struct)
 }
+
+// Arguments spread at ABI level are unsupported.
+//
+// CHECK-LABEL: @spread_arg(
+// CHECK-NOT: readonly
+// CHECK-SAME: )
+#[no_mangle]
+pub extern "rust-call" fn spread_arg(_: (BigStruct, BigStruct, BigStruct)) {}