Auto merge of #112984 - BoxyUwU:debug_with_infcx, r=compiler-errors

Introduce `trait DebugWithInfcx` to debug format types with universe info

Seeing universes of infer vars is valuable for debugging but currently we have no way of easily debug formatting a type with the universes of all the infer vars shown. In the future I hope to augment the new solver's proof tree output with a `DebugWithInfcx` impl so that it can show universes but I left that out of this PR as it would be non trivial and this is already large and complex enough.

The goal here is to make the various abstractions taking `T: Debug` able to use the codepath for printing out universes, that way we can do `debug!("{:?}", my_x)` and have `my_x` have universes shown, same for the `write!` macro. It's not possible to put the `Infcx: InferCtxtLike<I>` into the formatter argument to `Debug::fmt` so it has to go into the self ty. For this we introduce the type `OptWithInfcx<I: Interner, Infcx: InferCtxtLike<I>, T>` which has the data `T` optionally coupled with the infcx (more on why it's optional later).

Because of coherence/orphan rules it's not possible to write the impl `Debug for OptWithInfcx<..., MyType>` when `OptWithInfcx` is in a upstream crate. This necessitates a blanket impl in the crate defining `OptWithInfcx` like so: `impl<T: DebugWithInfcx> Debug for OptWithInfcx<..., T>`. It is not intended for people to manually call `DebugWithInfcx::fmt`, the `Debug` impl for `OptWithInfcx` should be preferred.

The infcx has to be optional in `OptWithInfcx` as otherwise we would end up with a large amount of code duplication. Almost all types that want to be used with `OptWithInfcx` do not themselves need access to the infcx so if we were to not optional we would end up with large `Debug` and `DebugWithInfcx` impls that were practically identical other than that when formatting their fields we wrap the field in `OptWithInfcx` instead of formatting it alone.

The only types that need access to the infcx themselves are ty/const/region infer vars, everything else is implemented by having the `Debug` impl defer to `OptWithInfcx` with no infcx available. The `DebugWithInfcx` impl is pretty much just the standard `Debug` impl except that instead of recursively formatting fields with `write!(f, "{x:?}")` we must do `write!(f, "{:?}", opt_infcx.wrap(x))`. This is some pretty rough boilerplate but I could not think of an alternative unfortunately.

`OptWithInfcx::wrap` is an eager `Option::map` because 99% of callsites were discarding the existing data in `OptWithInfcx` and did not need lazy evaluation.

A trait `InferCtxtLike` was added instead of using `InferCtxt<'tcx>` as we need to implement `DebugWithInfcx` for types living in `rustc_type_ir` which are generic over an interner and do not have access to `InferCtxt` since it lives in `rustc_infer`. Additionally I suspect that adding universe info to new solver proof tree output will require an implementation of `InferCtxtLike` for something that is not an `InferCtxt` although this is not the primary motivaton.

---

To summarize:
- There is a type `OptWithInfcx` which bundles some data optionally with an infcx with allows us to pass an infcx into a `Debug` impl. It's optional instead of being there unconditionally so that we can share code for `Debug` and `DebugWithInfcx` impls that don't care about whether there is an infcx available but have fields that might care.
- There is a trait `DebugWithInfcx` which allows downstream crates to add impls of the form `Debug for OptWithInfcx<...>` which would normally be forbidden by orphan rules/coherence.
- There is a trait `InferCtxtLike` to allow us to implement `DebugWithInfcx` for types that live in `rustc_type_ir`

This allows debug formatting various `ty::*` structures with universes shown by using the `Debug` impl for `OptWithInfcx::new(ty, infcx)`

---

This PR does not add `DebugWithInfcx` impls to absolutely _everything_ that should realistically have them, for example you cannot use `OptWithInfcx<Obligation<Predicate>>`. I am leaving this to a future PR to do so as it would likely be a lot more work to do.

6 files changed, 207 insertions, 33 deletions

diff --git a/compiler/rustc_middle/src/ty/consts/kind.rs b/compiler/rustc_middle/src/ty/consts/kind.rs
index a6bf7491118..5f2b6d42bf8 100644
--- a/compiler/rustc_middle/src/ty/consts/kind.rs
+++ b/compiler/rustc_middle/src/ty/consts/kind.rs
@@ -8,7 +8,7 @@ use rustc_hir::def_id::DefId;
 use rustc_macros::HashStable;
 
 /// An unevaluated (potentially generic) constant used in the type-system.
-#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable, Lift)]
+#[derive(Copy, Clone, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable, Lift)]
 #[derive(Hash, HashStable, TypeFoldable, TypeVisitable)]
 pub struct UnevaluatedConst<'tcx> {
     pub def: DefId,
@@ -35,7 +35,7 @@ impl<'tcx> UnevaluatedConst<'tcx> {
     }
 }
 
-#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash)]
+#[derive(Copy, Clone, Eq, PartialEq, PartialOrd, Ord, Hash)]
 #[derive(HashStable, TyEncodable, TyDecodable, TypeVisitable, TypeFoldable)]
 pub enum Expr<'tcx> {
     Binop(mir::BinOp, Const<'tcx>, Const<'tcx>),
diff --git a/compiler/rustc_middle/src/ty/list.rs b/compiler/rustc_middle/src/ty/list.rs
index 71911a5a618..7a32cfb1085 100644
--- a/compiler/rustc_middle/src/ty/list.rs
+++ b/compiler/rustc_middle/src/ty/list.rs
@@ -1,6 +1,7 @@
 use crate::arena::Arena;
 use rustc_data_structures::aligned::{align_of, Aligned};
 use rustc_serialize::{Encodable, Encoder};
+use rustc_type_ir::{InferCtxtLike, OptWithInfcx};
 use std::alloc::Layout;
 use std::cmp::Ordering;
 use std::fmt;
@@ -119,6 +120,14 @@ impl<T: fmt::Debug> fmt::Debug for List<T> {
         (**self).fmt(f)
     }
 }
+impl<'tcx, T: super::DebugWithInfcx<TyCtxt<'tcx>>> super::DebugWithInfcx<TyCtxt<'tcx>> for List<T> {
+    fn fmt<InfCtx: InferCtxtLike<TyCtxt<'tcx>>>(
+        this: OptWithInfcx<'_, TyCtxt<'tcx>, InfCtx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
+        fmt::Debug::fmt(&this.map(|this| this.as_slice()), f)
+    }
+}
 
 impl<S: Encoder, T: Encodable<S>> Encodable<S> for List<T> {
     #[inline]
@@ -202,6 +211,8 @@ unsafe impl<T: Sync> Sync for List<T> {}
 // We need this since `List` uses extern type `OpaqueListContents`.
 #[cfg(parallel_compiler)]
 use rustc_data_structures::sync::DynSync;
+
+use super::TyCtxt;
 #[cfg(parallel_compiler)]
 unsafe impl<T: DynSync> DynSync for List<T> {}
 
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index 792ee8ebb25..519bdb01623 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -53,6 +53,7 @@ use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{ExpnId, ExpnKind, Span};
 use rustc_target::abi::{Align, FieldIdx, Integer, IntegerType, VariantIdx};
 pub use rustc_target::abi::{ReprFlags, ReprOptions};
+pub use rustc_type_ir::{DebugWithInfcx, InferCtxtLike, OptWithInfcx};
 pub use subst::*;
 pub use vtable::*;
 
diff --git a/compiler/rustc_middle/src/ty/structural_impls.rs b/compiler/rustc_middle/src/ty/structural_impls.rs
index 7220d133f68..c82ce9112fc 100644
--- a/compiler/rustc_middle/src/ty/structural_impls.rs
+++ b/compiler/rustc_middle/src/ty/structural_impls.rs
@@ -11,13 +11,15 @@ use crate::ty::{self, AliasTy, InferConst, Lift, Term, TermKind, Ty, TyCtxt};
 use rustc_hir::def::Namespace;
 use rustc_index::{Idx, IndexVec};
 use rustc_target::abi::TyAndLayout;
-use rustc_type_ir::ConstKind;
+use rustc_type_ir::{ConstKind, DebugWithInfcx, InferCtxtLike, OptWithInfcx};
 
-use std::fmt;
+use std::fmt::{self, Debug};
 use std::ops::ControlFlow;
 use std::rc::Rc;
 use std::sync::Arc;
 
+use super::{GenericArg, GenericArgKind, Region};
+
 impl fmt::Debug for ty::TraitDef {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         ty::tls::with(|tcx| {
@@ -89,7 +91,16 @@ impl fmt::Debug for ty::FreeRegion {
 
 impl<'tcx> fmt::Debug for ty::FnSig<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let ty::FnSig { inputs_and_output: _, c_variadic, unsafety, abi } = self;
+        OptWithInfcx::new_no_ctx(self).fmt(f)
+    }
+}
+impl<'tcx> DebugWithInfcx<TyCtxt<'tcx>> for ty::FnSig<'tcx> {
+    fn fmt<InfCtx: InferCtxtLike<TyCtxt<'tcx>>>(
+        this: OptWithInfcx<'_, TyCtxt<'tcx>, InfCtx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
+        let sig = this.data;
+        let ty::FnSig { inputs_and_output: _, c_variadic, unsafety, abi } = sig;
 
         write!(f, "{}", unsafety.prefix_str())?;
         match abi {
@@ -98,15 +109,15 @@ impl<'tcx> fmt::Debug for ty::FnSig<'tcx> {
         };
 
         write!(f, "fn(")?;
-        let inputs = self.inputs();
+        let inputs = sig.inputs();
         match inputs.len() {
             0 if *c_variadic => write!(f, "...)")?,
             0 => write!(f, ")")?,
             _ => {
-                for ty in &self.inputs()[0..(self.inputs().len() - 1)] {
-                    write!(f, "{ty:?}, ")?;
+                for ty in &sig.inputs()[0..(sig.inputs().len() - 1)] {
+                    write!(f, "{:?}, ", &this.wrap(ty))?;
                 }
-                write!(f, "{:?}", self.inputs().last().unwrap())?;
+                write!(f, "{:?}", &this.wrap(sig.inputs().last().unwrap()))?;
                 if *c_variadic {
                     write!(f, "...")?;
                 }
@@ -114,9 +125,9 @@ impl<'tcx> fmt::Debug for ty::FnSig<'tcx> {
             }
         }
 
-        match self.output().kind() {
+        match sig.output().kind() {
             ty::Tuple(list) if list.is_empty() => Ok(()),
-            _ => write!(f, " -> {:?}", self.output()),
+            _ => write!(f, " -> {:?}", &this.wrap(sig.output())),
         }
     }
 }
@@ -133,6 +144,14 @@ impl<'tcx> fmt::Debug for ty::TraitRef<'tcx> {
     }
 }
 
+impl<'tcx> ty::DebugWithInfcx<TyCtxt<'tcx>> for Ty<'tcx> {
+    fn fmt<InfCtx: InferCtxtLike<TyCtxt<'tcx>>>(
+        this: OptWithInfcx<'_, TyCtxt<'tcx>, InfCtx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
+        this.data.fmt(f)
+    }
+}
 impl<'tcx> fmt::Debug for Ty<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         with_no_trimmed_paths!(fmt::Display::fmt(self, f))
@@ -217,9 +236,17 @@ impl<'tcx> fmt::Debug for ty::PredicateKind<'tcx> {
 
 impl<'tcx> fmt::Debug for AliasTy<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        OptWithInfcx::new_no_ctx(self).fmt(f)
+    }
+}
+impl<'tcx> DebugWithInfcx<TyCtxt<'tcx>> for AliasTy<'tcx> {
+    fn fmt<InfCtx: InferCtxtLike<TyCtxt<'tcx>>>(
+        this: OptWithInfcx<'_, TyCtxt<'tcx>, InfCtx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
         f.debug_struct("AliasTy")
-            .field("substs", &self.substs)
-            .field("def_id", &self.def_id)
+            .field("substs", &this.map(|data| data.substs))
+            .field("def_id", &this.data.def_id)
             .finish()
     }
 }
@@ -232,13 +259,93 @@ impl<'tcx> fmt::Debug for ty::InferConst<'tcx> {
         }
     }
 }
+impl<'tcx> DebugWithInfcx<TyCtxt<'tcx>> for ty::InferConst<'tcx> {
+    fn fmt<InfCtx: InferCtxtLike<TyCtxt<'tcx>>>(
+        this: OptWithInfcx<'_, TyCtxt<'tcx>, InfCtx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
+        use ty::InferConst::*;
+        match this.infcx.and_then(|infcx| infcx.universe_of_ct(*this.data)) {
+            None => write!(f, "{:?}", this.data),
+            Some(universe) => match *this.data {
+                Var(vid) => write!(f, "?{}_{}c", vid.index, universe.index()),
+                Fresh(_) => {
+                    unreachable!()
+                }
+            },
+        }
+    }
+}
+
+impl<'tcx> fmt::Debug for ty::consts::Expr<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        OptWithInfcx::new_no_ctx(self).fmt(f)
+    }
+}
+impl<'tcx> DebugWithInfcx<TyCtxt<'tcx>> for ty::consts::Expr<'tcx> {
+    fn fmt<InfCtx: InferCtxtLike<TyCtxt<'tcx>>>(
+        this: OptWithInfcx<'_, TyCtxt<'tcx>, InfCtx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
+        match this.data {
+            ty::Expr::Binop(op, lhs, rhs) => {
+                write!(f, "({op:?}: {:?}, {:?})", &this.wrap(lhs), &this.wrap(rhs))
+            }
+            ty::Expr::UnOp(op, rhs) => write!(f, "({op:?}: {:?})", &this.wrap(rhs)),
+            ty::Expr::FunctionCall(func, args) => {
+                write!(f, "{:?}(", &this.wrap(func))?;
+                for arg in args.as_slice().iter().rev().skip(1).rev() {
+                    write!(f, "{:?}, ", &this.wrap(arg))?;
+                }
+                if let Some(arg) = args.last() {
+                    write!(f, "{:?}", &this.wrap(arg))?;
+                }
+
+                write!(f, ")")
+            }
+            ty::Expr::Cast(cast_kind, lhs, rhs) => {
+                write!(f, "({cast_kind:?}: {:?}, {:?})", &this.wrap(lhs), &this.wrap(rhs))
+            }
+        }
+    }
+}
+
+impl<'tcx> fmt::Debug for ty::UnevaluatedConst<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        OptWithInfcx::new_no_ctx(self).fmt(f)
+    }
+}
+impl<'tcx> DebugWithInfcx<TyCtxt<'tcx>> for ty::UnevaluatedConst<'tcx> {
+    fn fmt<InfCtx: InferCtxtLike<TyCtxt<'tcx>>>(
+        this: OptWithInfcx<'_, TyCtxt<'tcx>, InfCtx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
+        f.debug_struct("UnevaluatedConst")
+            .field("def", &this.data.def)
+            .field("substs", &this.wrap(this.data.substs))
+            .finish()
+    }
+}
 
 impl<'tcx> fmt::Debug for ty::Const<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        OptWithInfcx::new_no_ctx(self).fmt(f)
+    }
+}
+impl<'tcx> DebugWithInfcx<TyCtxt<'tcx>> for ty::Const<'tcx> {
+    fn fmt<InfCtx: InferCtxtLike<TyCtxt<'tcx>>>(
+        this: OptWithInfcx<'_, TyCtxt<'tcx>, InfCtx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
         // This reflects what `Const` looked liked before `Interned` was
         // introduced. We print it like this to avoid having to update expected
         // output in a lot of tests.
-        write!(f, "Const {{ ty: {:?}, kind: {:?} }}", self.ty(), self.kind())
+        write!(
+            f,
+            "Const {{ ty: {:?}, kind: {:?} }}",
+            &this.map(|data| data.ty()),
+            &this.map(|data| data.kind())
+        )
     }
 }
 
@@ -261,6 +368,66 @@ impl<T: fmt::Debug> fmt::Debug for ty::Placeholder<T> {
     }
 }
 
+impl<'tcx> fmt::Debug for GenericArg<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.unpack() {
+            GenericArgKind::Lifetime(lt) => lt.fmt(f),
+            GenericArgKind::Type(ty) => ty.fmt(f),
+            GenericArgKind::Const(ct) => ct.fmt(f),
+        }
+    }
+}
+impl<'tcx> DebugWithInfcx<TyCtxt<'tcx>> for GenericArg<'tcx> {
+    fn fmt<InfCtx: InferCtxtLike<TyCtxt<'tcx>>>(
+        this: OptWithInfcx<'_, TyCtxt<'tcx>, InfCtx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
+        match this.data.unpack() {
+            GenericArgKind::Lifetime(lt) => write!(f, "{:?}", &this.wrap(lt)),
+            GenericArgKind::Const(ct) => write!(f, "{:?}", &this.wrap(ct)),
+            GenericArgKind::Type(ty) => write!(f, "{:?}", &this.wrap(ty)),
+        }
+    }
+}
+
+impl<'tcx> fmt::Debug for Region<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:?}", self.kind())
+    }
+}
+impl<'tcx> DebugWithInfcx<TyCtxt<'tcx>> for Region<'tcx> {
+    fn fmt<InfCtx: InferCtxtLike<TyCtxt<'tcx>>>(
+        this: OptWithInfcx<'_, TyCtxt<'tcx>, InfCtx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
+        write!(f, "{:?}", &this.map(|data| data.kind()))
+    }
+}
+
+impl<'tcx> DebugWithInfcx<TyCtxt<'tcx>> for ty::RegionVid {
+    fn fmt<InfCtx: InferCtxtLike<TyCtxt<'tcx>>>(
+        this: OptWithInfcx<'_, TyCtxt<'tcx>, InfCtx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
+        match this.infcx.and_then(|infcx| infcx.universe_of_lt(*this.data)) {
+            Some(universe) => write!(f, "'?{}_{}", this.data.index(), universe.index()),
+            None => write!(f, "{:?}", this.data),
+        }
+    }
+}
+
+impl<'tcx, T: DebugWithInfcx<TyCtxt<'tcx>>> DebugWithInfcx<TyCtxt<'tcx>> for ty::Binder<'tcx, T> {
+    fn fmt<InfCtx: InferCtxtLike<TyCtxt<'tcx>>>(
+        this: OptWithInfcx<'_, TyCtxt<'tcx>, InfCtx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
+        f.debug_tuple("Binder")
+            .field(&this.map(|data| data.as_ref().skip_binder()))
+            .field(&this.data.bound_vars())
+            .finish()
+    }
+}
+
 ///////////////////////////////////////////////////////////////////////////
 // Atomic structs
 //
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 2c4ff575755..9f4515b95e7 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -35,9 +35,12 @@ use std::ops::{ControlFlow, Deref, Range};
 use ty::util::IntTypeExt;
 
 use rustc_type_ir::sty::TyKind::*;
+use rustc_type_ir::CollectAndApply;
+use rustc_type_ir::ConstKind as IrConstKind;
+use rustc_type_ir::DebugWithInfcx;
+use rustc_type_ir::DynKind;
+use rustc_type_ir::RegionKind as IrRegionKind;
 use rustc_type_ir::TyKind as IrTyKind;
-use rustc_type_ir::{CollectAndApply, ConstKind as IrConstKind};
-use rustc_type_ir::{DynKind, RegionKind as IrRegionKind};
 
 use super::GenericParamDefKind;
 
@@ -694,6 +697,15 @@ pub enum ExistentialPredicate<'tcx> {
     AutoTrait(DefId),
 }
 
+impl<'tcx> DebugWithInfcx<TyCtxt<'tcx>> for ExistentialPredicate<'tcx> {
+    fn fmt<InfCtx: rustc_type_ir::InferCtxtLike<TyCtxt<'tcx>>>(
+        this: rustc_type_ir::OptWithInfcx<'_, TyCtxt<'tcx>, InfCtx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
+        fmt::Debug::fmt(&this.data, f)
+    }
+}
+
 impl<'tcx> ExistentialPredicate<'tcx> {
     /// Compares via an ordering that will not change if modules are reordered or other changes are
     /// made to the tree. In particular, this ordering is preserved across incremental compilations.
@@ -1571,12 +1583,6 @@ impl<'tcx> Deref for Region<'tcx> {
     }
 }
 
-impl<'tcx> fmt::Debug for Region<'tcx> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{:?}", self.kind())
-    }
-}
-
 #[derive(Copy, Clone, PartialEq, Eq, Hash, TyEncodable, TyDecodable, PartialOrd, Ord)]
 #[derive(HashStable)]
 pub struct EarlyBoundRegion {
diff --git a/compiler/rustc_middle/src/ty/subst.rs b/compiler/rustc_middle/src/ty/subst.rs
index 4d5f5b8658c..e1a58b97557 100644
--- a/compiler/rustc_middle/src/ty/subst.rs
+++ b/compiler/rustc_middle/src/ty/subst.rs
@@ -17,7 +17,6 @@ use smallvec::SmallVec;
 
 use core::intrinsics;
 use std::cmp::Ordering;
-use std::fmt;
 use std::marker::PhantomData;
 use std::mem;
 use std::num::NonZeroUsize;
@@ -80,16 +79,6 @@ impl<'tcx> GenericArgKind<'tcx> {
     }
 }
 
-impl<'tcx> fmt::Debug for GenericArg<'tcx> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self.unpack() {
-            GenericArgKind::Lifetime(lt) => lt.fmt(f),
-            GenericArgKind::Type(ty) => ty.fmt(f),
-            GenericArgKind::Const(ct) => ct.fmt(f),
-        }
-    }
-}
-
 impl<'tcx> Ord for GenericArg<'tcx> {
     fn cmp(&self, other: &GenericArg<'tcx>) -> Ordering {
         self.unpack().cmp(&other.unpack())