rustc_mir::shim misuses ParamEnvs.

[eddyb]

(This example is about CloneShim, but DropGlue has a similar issue)

In this snippet, param_env is obtained from tcx.param_env(def_id) and used twice with self_ty:

1. for is_copy_modulo_regions
   • if self_ty doesn't involve type parameters, param_env doesn't matter, but if it does, they could be from anywhere (e.g. caller of clone, via MIR inliner), so the param_env being that of Clone::clone itself is not useful at all (all it contains is Self: Clone)
   • to get the right ParamEnv, the mir_shims query would need to take ParamEnvAnd<InstanceDef> instead of just InstanceDef (but we might not need this)
   • however it's a waste to generate one shim for every Copy type, we should instead do what DropShim does and have Option<Ty> in CloneShim, or split out of a CloneByCopyShim
2. for evaluating an array length
   • the length (a ty::Const) is converted to u64 only to be converted back to ty::Const, which is unnecessary and we could skip it entirely

rust/src/librustc_mir/shim.rs

Lines 330 to 343 in 7762131

	let param_env = tcx.param_env(def_id);
	let mut builder = CloneShimBuilder::new(tcx, def_id, self_ty);
	let is_copy = self_ty.is_copy_modulo_regions(tcx, param_env, builder.span);
	let dest = Place::return_place();
	let src = tcx.mk_place_deref(Place::from(Local::new(1 + 0)));
	match self_ty.kind {
	_ if is_copy => builder.copy_shim(),
	ty::Array(ty, len) => {
	let len = len.eval_usize(tcx, param_env);
	builder.array_shim(dest, src, ty, len)
	}