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

1 files changed, 12 insertions, 0 deletions

diff --git a/src/librustc/ty/instance.rs b/src/librustc/ty/instance.rs
index 445df76cd32..c9ad8707a74 100644
--- a/src/librustc/ty/instance.rs
+++ b/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>),
 }