Rollup merge of #110872 - Jules-Bertholet:err-67981, r=wesleywiser

Nicer ICE for #67981

Provides a slightly nicer ICE for #67981, documenting the problem. A proper fix will be necessary before `#![feature(unsized_fn_params)]` can be stabilized.

The problem is that the design of the `"rust-call"` ABI is fundamentally not compatible with `unsized_fn_params`. `"rust-call"` functions need to collect their arguments into a tuple, but if the arguments are not `Sized`, said tuple is potentially not even a valid type—and if it is, it requires `alloca` to create.

`@rustbot` label +A-abi +A-codegen +F-unboxed_closures +F-unsized_fn_params

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -304,7 +304,17 @@ fn arg_local_refs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
                     bug!("spread argument isn't a tuple?!");
                 };
 
-                let place = PlaceRef::alloca(bx, bx.layout_of(arg_ty));
+                let layout = bx.layout_of(arg_ty);
+
+                // FIXME: support unsized params in "rust-call" ABI
+                if layout.is_unsized() {
+                    span_bug!(
+                        arg_decl.source_info.span,
+                        "\"rust-call\" ABI does not support unsized params",
+                    );
+                }
+
+                let place = PlaceRef::alloca(bx, layout);
                 for i in 0..tupled_arg_tys.len() {
                     let arg = &fx.fn_abi.args[idx];
                     idx += 1;