Make param-env optional in drop elaboration

Author: compiler-errors

compiler/rustc_mir_dataflow/src/elaborate_drops.rs

@@ -111,7 +111,11 @@ pub trait DropElaborator<'a, 'tcx>: fmt::Debug {
     fn patch(&mut self) -> &mut MirPatch<'tcx>;
     fn body(&self) -> &'a Body<'tcx>;
     fn tcx(&self) -> TyCtxt<'tcx>;
-    fn param_env(&self) -> ty::ParamEnv<'tcx>;
+
+    // A param-env that can be used to compute drop queries (such as `Ty::needs_drop`).
+    // If this is `None`, then the elaborator must handle the type as if it were unknown,
+    // i.e. emitting full drops.
+    fn param_env(&self) -> Option<ty::ParamEnv<'tcx>>;
 
     // Drop logic
 
@@ -276,9 +280,11 @@ where
                 let subpath = self.elaborator.field_subpath(variant_path, field);
                 let tcx = self.tcx();
 
-                assert_eq!(self.elaborator.param_env().reveal(), Reveal::All);
-                let field_ty =
-                    tcx.normalize_erasing_regions(self.elaborator.param_env(), f.ty(tcx, args));
+                let mut field_ty = f.ty(tcx, args);
+                if let Some(param_env) = self.elaborator.param_env() {
+                    assert_eq!(param_env.reveal(), Reveal::All);
+                    field_ty = tcx.normalize_erasing_regions(param_env, field_ty);
+                }
 
                 (tcx.mk_place_field(base_place, field, field_ty), subpath)
             })
@@ -374,9 +380,9 @@ where
         debug!("drop_ladder({:?}, {:?})", self, fields);
 
         let mut fields = fields;
-        fields.retain(|&(place, _)| {
-            self.place_ty(place).needs_drop(self.tcx(), self.elaborator.param_env())
-        });
+        if let Some(param_env) = self.elaborator.param_env() {
+            fields.retain(|&(place, _)| self.place_ty(place).needs_drop(self.tcx(), param_env));
+        }
 
         debug!("drop_ladder - fields needing drop: {:?}", fields);
 
@@ -548,10 +554,13 @@ where
                 have_otherwise = true;
 
                 let param_env = self.elaborator.param_env();
-                let have_field_with_drop_glue = variant
-                    .fields
-                    .iter()
-                    .any(|field| field.ty(tcx, args).needs_drop(tcx, param_env));
+
+                let have_field_with_drop_glue = param_env.is_none_or(|param_env| {
+                    variant
+                        .fields
+                        .iter()
+                        .any(|field| field.ty(tcx, args).needs_drop(tcx, param_env))
+                });
                 if have_field_with_drop_glue {
                     have_otherwise_with_drop_glue = true;
                 }
@@ -869,7 +878,10 @@ where
             ty::Adt(def, args) => self.open_drop_for_adt(*def, args),
             ty::Dynamic(..) => self.complete_drop(self.succ, self.unwind),
             ty::Array(ety, size) => {
-                let size = size.try_eval_target_usize(self.tcx(), self.elaborator.param_env());
+                let size = self
+                    .elaborator
+                    .param_env()
+                    .and_then(|param_env| size.try_eval_target_usize(self.tcx(), param_env));
                 self.open_drop_for_array(*ety, size)
             }
             ty::Slice(ety) => self.drop_loop_pair(*ety),

compiler/rustc_mir_dataflow/src/lib.rs

@@ -2,6 +2,7 @@
 #![feature(associated_type_defaults)]
 #![feature(box_patterns)]
 #![feature(exact_size_is_empty)]
+#![feature(is_none_or)]
 #![feature(let_chains)]
 #![feature(try_blocks)]
 // tidy-alphabetical-end

compiler/rustc_mir_transform/src/coroutine.rs

@@ -1178,7 +1178,8 @@ fn elaborate_coroutine_drops<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     let def_id = body.source.def_id();
     let param_env = tcx.param_env(def_id);
 
-    let mut elaborator = DropShimElaborator { body, patch: MirPatch::new(body), tcx, param_env };
+    let mut elaborator =
+        DropShimElaborator { body, patch: MirPatch::new(body), tcx, param_env: Some(param_env) };
 
     for (block, block_data) in body.basic_blocks.iter_enumerated() {
         let (target, unwind, source_info) = match block_data.terminator() {

compiler/rustc_mir_transform/src/elaborate_drops.rs

@@ -159,8 +159,8 @@ impl<'a, 'tcx> DropElaborator<'a, 'tcx> for Elaborator<'a, '_, '_, 'tcx> {
         self.ctxt.tcx
     }
 
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
-        self.ctxt.param_env()
+    fn param_env(&self) -> Option<ty::ParamEnv<'tcx>> {
+        Some(self.ctxt.param_env())
     }
 
     #[instrument(level = "debug", skip(self), ret)]

compiler/rustc_mir_transform/src/inline.rs

@@ -11,7 +11,7 @@ use rustc_middle::middle::codegen_fn_attrs::{CodegenFnAttrFlags, CodegenFnAttrs}
 use rustc_middle::mir::visit::*;
 use rustc_middle::mir::*;
 use rustc_middle::ty::TypeVisitableExt;
-use rustc_middle::ty::{self, Instance, InstanceKind, ParamEnv, Ty, TyCtxt, TypeFlags};
+use rustc_middle::ty::{self, Instance, InstanceKind, ParamEnv, Ty, TyCtxt};
 use rustc_session::config::{DebugInfo, OptLevel};
 use rustc_span::source_map::Spanned;
 use rustc_span::sym;
@@ -321,15 +321,6 @@ impl<'tcx> Inliner<'tcx> {
                 return Err("instance without MIR (intrinsic / virtual)");
             }
 
-            // 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.
-            InstanceKind::DropGlue(_, Some(ty)) if ty.has_type_flags(TypeFlags::HAS_CT_PARAM) => {
-                return Err("still needs substitution");
-            }
-
             // This cannot result in an immediate cycle since the callee MIR is a shim, which does
             // not get any optimizations run on it. Any subsequent inlining may cause cycles, but we
             // do not need to catch this here, we can wait until the inliner decides to continue

compiler/rustc_mir_transform/src/shim.rs

@@ -4,8 +4,8 @@ use rustc_hir::lang_items::LangItem;
 use rustc_index::{Idx, IndexVec};
 use rustc_middle::mir::*;
 use rustc_middle::query::Providers;
-use rustc_middle::ty::GenericArgs;
 use rustc_middle::ty::{self, CoroutineArgs, CoroutineArgsExt, EarlyBinder, Ty, TyCtxt};
+use rustc_middle::ty::{GenericArgs, TypeVisitableExt};
 use rustc_middle::{bug, span_bug};
 use rustc_span::{source_map::Spanned, Span, DUMMY_SP};
 use rustc_target::abi::{FieldIdx, VariantIdx, FIRST_VARIANT};
@@ -274,7 +274,8 @@ fn build_drop_shim<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, ty: Option<Ty<'tcx>>)
 
     if ty.is_some() {
         let patch = {
-            let param_env = tcx.param_env_reveal_all_normalized(def_id);
+            let param_env =
+                if ty.has_non_region_param() { None } else { Some(ty::ParamEnv::reveal_all()) };
             let mut elaborator =
                 DropShimElaborator { body: &body, patch: MirPatch::new(&body), tcx, param_env };
             let dropee = tcx.mk_place_deref(dropee_ptr);
@@ -331,7 +332,7 @@ pub struct DropShimElaborator<'a, 'tcx> {
     pub body: &'a Body<'tcx>,
     pub patch: MirPatch<'tcx>,
     pub tcx: TyCtxt<'tcx>,
-    pub param_env: ty::ParamEnv<'tcx>,
+    pub param_env: Option<ty::ParamEnv<'tcx>>,
 }
 
 impl fmt::Debug for DropShimElaborator<'_, '_> {
@@ -352,7 +353,7 @@ impl<'a, 'tcx> DropElaborator<'a, 'tcx> for DropShimElaborator<'a, 'tcx> {
     fn tcx(&self) -> TyCtxt<'tcx> {
         self.tcx
     }
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+    fn param_env(&self) -> Option<ty::ParamEnv<'tcx>> {
         self.param_env
     }