Auto merge of #127068 - compiler-errors:stall-drop, r=<try>

Stall dropaStall computing instance for drop shim until it has no unsubstituted const params

Stall resolving the drop shim instance for types that still have unsubstituted const params.

## Why?

#127030 ICEs because it tries to inline the drop shim for a type with an unsubstituted const param. In order to generate this shim, this requires calling the drop shim builder, which invokes the trait solver to compute whether constituent types need drop (since we compute if a type is copy to disqualify any `Drop` behavior):

https://github.com/rust-lang/rust/blob/9c3bc805dd9cb84019c124b9a50fdff1e62a7ec9/compiler/rustc_mir_dataflow/src/elaborate_drops.rs#L378

However, since we don't keep the param-env of the instance we resolved the item for, we use the wrong param-env:
https://github.com/rust-lang/rust/blob/9c3bc805dd9cb84019c124b9a50fdff1e62a7ec9/compiler/rustc_mir_transform/src/shim.rs#L278
(which is the param-env of `std::ptr::drop_in_place`)

This param-env is notably missing `ConstParamHasTy` predicates, and since we removed the type from consts in #125958, we literally cannot prove these predicates in this (relatively) empty param-env. This currently happens in places like the MIR inliner, but may happen elsewhere such as in lints that resolve terminators.

## What?

We delay the resolution (`Instance::resolve`) of calls for `drop_in_place` for types that have unsubstituted const params. This should be OK, since all cases that deal with polymorphic code should handle `Instance::resolve` returning `None` gracefully.

Author: bors

compiler/rustc_ty_utils/src/instance.rs

@@ -7,7 +7,7 @@ use rustc_middle::query::Providers;
 use rustc_middle::traits::{BuiltinImplSource, CodegenObligationError};
 use rustc_middle::ty::util::AsyncDropGlueMorphology;
 use rustc_middle::ty::GenericArgsRef;
-use rustc_middle::ty::{self, Instance, TyCtxt, TypeVisitableExt};
+use rustc_middle::ty::{self, Instance, TyCtxt, TypeFlags, TypeVisitableExt};
 use rustc_span::sym;
 use rustc_trait_selection::traits;
 use rustc_type_ir::ClosureKind;
@@ -53,6 +53,15 @@ fn resolve_instance<'tcx>(
                     _ => return Ok(None),
                 }
 
+                // FIXME(#127030): `ConstParamHasTy` has bad interactions with
+                // the drop shim builder, which does not evaluate predicates in
+                // the correct param-env for types being dropped. Stall resolving
+                // the MIR for this instance until all of its const params are
+                // substituted.
+                if ty.has_type_flags(TypeFlags::HAS_CT_PARAM) {
+                    return Ok(None);
+                }
+
                 ty::InstanceKind::DropGlue(def_id, Some(ty))
             } else {
                 debug!(" => trivial drop glue");

tests/ui/const-generics/polymorphic-drop-shim.rs

@@ -0,0 +1,18 @@
+//@ compile-flags: -Zinline-mir=yes --crate-type=lib
+//@ build-pass
+
+use std::mem::ManuallyDrop;
+
+pub struct Foo<T, const N: usize>([T; N]);
+
+pub struct Dorp {}
+
+impl Drop for Dorp {
+    fn drop(&mut self) {}
+}
+
+#[inline]
+// SAFETY: call this with a valid allocation idk
+pub unsafe fn drop<const M: usize>(x: *mut Foo<Dorp, M>) {
+    std::ptr::drop_in_place(x);
+}