Rollup merge of #69036 - eddyb:monoshim, r=nikomatsakis

rustc: don't resolve Instances which would produce malformed shims.

There are some `InstanceDef` variants (shims and drop "glue") which contain a `Ty`, and that `Ty` is used in generating the shim MIR. But if that `Ty` mentions any generic parameters, the generated shim would refer to them (but they won't match the `Substs` of the `Instance`), or worse, generating the shim would fail because not enough of the type is known.

Ideally we would always produce a "skeleton" of the type, e.g. `(_, _)` for dropping any tuples with two elements, or `Vec<_>` for dropping any `Vec` value, but that's a lot of work, and they would still not match the `Substs` of the `Instance` as it exists today, so `Instance` would probably need to change.

By making `Instance::resolve` return `None` in the still-generic cases, we get behavior similar to specialization, where a default can only be used if there are no more generic parameters which would allow a more specialized `impl` to match.

<hr/>

This was found while testing the MIR inliner with #68965, because it was trying to inline shims.

cc @rust-lang/wg-mir-opt

src/librustc/ty/instance.rs

@@ -39,6 +39,10 @@ pub enum InstanceDef<'tcx> {
 
     /// `<fn() as FnTrait>::call_*`
     /// `DefId` is `FnTrait::call_*`.
+    ///
+    /// NB: the (`fn` pointer) type must currently be monomorphic to avoid double substitution
+    /// problems with the MIR shim bodies. `Instance::resolve` enforces this.
+    // FIXME(#69925) support polymorphic MIR shim bodies properly instead.
     FnPtrShim(DefId, Ty<'tcx>),
 
     /// `<dyn Trait as Trait>::fn`, "direct calls" of which are implicitly
@@ -57,9 +61,17 @@ pub enum InstanceDef<'tcx> {
     /// The `DefId` is for `core::ptr::drop_in_place`.
     /// The `Option<Ty<'tcx>>` is either `Some(T)`, or `None` for empty drop
     /// glue.
+    ///
+    /// NB: the type must currently be monomorphic to avoid double substitution
+    /// problems with the MIR shim bodies. `Instance::resolve` enforces this.
+    // FIXME(#69925) support polymorphic MIR shim bodies properly instead.
     DropGlue(DefId, Option<Ty<'tcx>>),
 
     ///`<T as Clone>::clone` shim.
+    ///
+    /// NB: the type must currently be monomorphic to avoid double substitution
+    /// problems with the MIR shim bodies. `Instance::resolve` enforces this.
+    // FIXME(#69925) support polymorphic MIR shim bodies properly instead.
     CloneShim(DefId, Ty<'tcx>),
 }
 

src/librustc_mir/shim.rs

@@ -2,13 +2,13 @@ use rustc::mir::*;
 use rustc::ty::layout::VariantIdx;
 use rustc::ty::query::Providers;
 use rustc::ty::subst::{InternalSubsts, Subst};
-use rustc::ty::{self, Ty, TyCtxt};
+use rustc::ty::{self, Ty, TyCtxt, TypeFoldable};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 
 use rustc_index::vec::{Idx, IndexVec};
 
-use rustc_span::{sym, Span};
+use rustc_span::Span;
 use rustc_target::spec::abi::Abi;
 
 use std::fmt;
@@ -39,6 +39,11 @@ fn make_shim<'tcx>(tcx: TyCtxt<'tcx>, instance: ty::InstanceDef<'tcx>) -> &'tcx
             None,
         ),
         ty::InstanceDef::FnPtrShim(def_id, ty) => {
+            // FIXME(eddyb) support generating shims for a "shallow type",
+            // e.g. `Foo<_>` or `[_]` instead of requiring a fully monomorphic
+            // `Foo<Bar>` or `[String]` etc.
+            assert!(!ty.needs_subst());
+
             let trait_ = tcx.trait_of_item(def_id).unwrap();
             let adjustment = match tcx.fn_trait_kind_from_lang_item(trait_) {
                 Some(ty::ClosureKind::FnOnce) => Adjustment::Identity,
@@ -81,17 +86,21 @@ fn make_shim<'tcx>(tcx: TyCtxt<'tcx>, instance: ty::InstanceDef<'tcx>) -> &'tcx
                 None,
             )
         }
-        ty::InstanceDef::DropGlue(def_id, ty) => build_drop_shim(tcx, def_id, ty),
+        ty::InstanceDef::DropGlue(def_id, ty) => {
+            // FIXME(eddyb) support generating shims for a "shallow type",
+            // e.g. `Foo<_>` or `[_]` instead of requiring a fully monomorphic
+            // `Foo<Bar>` or `[String]` etc.
+            assert!(!ty.needs_subst());
+
+            build_drop_shim(tcx, def_id, ty)
+        }
         ty::InstanceDef::CloneShim(def_id, ty) => {
-            let name = tcx.item_name(def_id);
-            if name == sym::clone {
-                build_clone_shim(tcx, def_id, ty)
-            } else if name == sym::clone_from {
-                debug!("make_shim({:?}: using default trait implementation", instance);
-                return tcx.optimized_mir(def_id);
-            } else {
-                bug!("builtin clone shim {:?} not supported", instance)
-            }
+            // FIXME(eddyb) support generating shims for a "shallow type",
+            // e.g. `Foo<_>` or `[_]` instead of requiring a fully monomorphic
+            // `Foo<Bar>` or `[String]` etc.
+            assert!(!ty.needs_subst());
+
+            build_clone_shim(tcx, def_id, ty)
         }
         ty::InstanceDef::Virtual(..) => {
             bug!("InstanceDef::Virtual ({:?}) is for direct calls only", instance)

src/librustc_ty/instance.rs

@@ -1,6 +1,7 @@
 use rustc::ty::subst::SubstsRef;
 use rustc::ty::{self, Instance, TyCtxt, TypeFoldable};
 use rustc_hir::def_id::DefId;
+use rustc_span::sym;
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits;
 
@@ -31,21 +32,26 @@ pub fn resolve_instance<'tcx>(
                 debug!(" => intrinsic");
                 ty::InstanceDef::Intrinsic(def_id)
             }
-            _ => {
-                if Some(def_id) == tcx.lang_items().drop_in_place_fn() {
-                    let ty = substs.type_at(0);
-                    if ty.needs_drop(tcx, param_env.with_reveal_all()) {
-                        debug!(" => nontrivial drop glue");
-                        ty::InstanceDef::DropGlue(def_id, Some(ty))
-                    } else {
-                        debug!(" => trivial drop glue");
-                        ty::InstanceDef::DropGlue(def_id, None)
+            ty::FnDef(def_id, substs) if Some(def_id) == tcx.lang_items().drop_in_place_fn() => {
+                let ty = substs.type_at(0);
+
+                if ty.needs_drop(tcx, param_env) {
+                    // `DropGlue` requires a monomorphic aka concrete type.
+                    if ty.needs_subst() {
+                        return None;
                     }
+
+                    debug!(" => nontrivial drop glue");
+                    ty::InstanceDef::DropGlue(def_id, Some(ty))
                 } else {
-                    debug!(" => free item");
-                    ty::InstanceDef::Item(def_id)
+                    debug!(" => trivial drop glue");
+                    ty::InstanceDef::DropGlue(def_id, None)
                 }
             }
+            _ => {
+                debug!(" => free item");
+                ty::InstanceDef::Item(def_id)
+            }
         };
         Some(Instance { def, substs })
     };
@@ -113,20 +119,44 @@ fn resolve_associated_item<'tcx>(
                 trait_closure_kind,
             ))
         }
-        traits::VtableFnPointer(ref data) => Some(Instance {
-            def: ty::InstanceDef::FnPtrShim(trait_item.def_id, data.fn_ty),
-            substs: rcvr_substs,
-        }),
+        traits::VtableFnPointer(ref data) => {
+            // `FnPtrShim` requires a monomorphic aka concrete type.
+            if data.fn_ty.needs_subst() {
+                return None;
+            }
+
+            Some(Instance {
+                def: ty::InstanceDef::FnPtrShim(trait_item.def_id, data.fn_ty),
+                substs: rcvr_substs,
+            })
+        }
         traits::VtableObject(ref data) => {
             let index = traits::get_vtable_index_of_object_method(tcx, data, def_id);
             Some(Instance { def: ty::InstanceDef::Virtual(def_id, index), substs: rcvr_substs })
         }
         traits::VtableBuiltin(..) => {
-            if tcx.lang_items().clone_trait().is_some() {
-                Some(Instance {
-                    def: ty::InstanceDef::CloneShim(def_id, trait_ref.self_ty()),
-                    substs: rcvr_substs,
-                })
+            if Some(trait_ref.def_id) == tcx.lang_items().clone_trait() {
+                // FIXME(eddyb) use lang items for methods instead of names.
+                let name = tcx.item_name(def_id);
+                if name == sym::clone {
+                    let self_ty = trait_ref.self_ty();
+
+                    // `CloneShim` requires a monomorphic aka concrete type.
+                    if self_ty.needs_subst() {
+                        return None;
+                    }
+
+                    Some(Instance {
+                        def: ty::InstanceDef::CloneShim(def_id, self_ty),
+                        substs: rcvr_substs,
+                    })
+                } else {
+                    assert_eq!(name, sym::clone_from);
+
+                    // Use the default `fn clone_from` from `trait Clone`.
+                    let substs = tcx.erase_regions(&rcvr_substs);
+                    Some(ty::Instance::new(def_id, substs))
+                }
             } else {
                 None
             }