Move abstract const to rustc_middle::ty

22 files changed, 728 insertions, 722 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 2bf07149cc8..147d4704407 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4514,6 +4514,7 @@ dependencies = [
  "rustc_data_structures",
  "rustc_errors",
  "rustc_hir",
+ "rustc_index",
  "rustc_infer",
  "rustc_middle",
  "rustc_session",
diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index f0ccf02c9fa..e5025121c4f 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -21,7 +21,6 @@ use rustc_index::vec::{Idx, IndexVec};
 use rustc_middle::metadata::ModChild;
 use rustc_middle::middle::exported_symbols::{ExportedSymbol, SymbolExportInfo};
 use rustc_middle::mir::interpret::{AllocDecodingSession, AllocDecodingState};
-use rustc_middle::thir;
 use rustc_middle::ty::codec::TyDecoder;
 use rustc_middle::ty::fast_reject::SimplifiedType;
 use rustc_middle::ty::GeneratorDiagnosticData;
@@ -638,7 +637,7 @@ impl<'a, 'tcx> Decodable<DecodeContext<'a, 'tcx>> for Span {
     }
 }
 
-impl<'a, 'tcx> Decodable<DecodeContext<'a, 'tcx>> for &'tcx [thir::abstract_const::Node<'tcx>] {
+impl<'a, 'tcx> Decodable<DecodeContext<'a, 'tcx>> for &'tcx [ty::abstract_const::Node<'tcx>] {
     fn decode(d: &mut DecodeContext<'a, 'tcx>) -> Self {
         ty::codec::RefDecodable::decode(d)
     }
diff --git a/compiler/rustc_metadata/src/rmeta/mod.rs b/compiler/rustc_metadata/src/rmeta/mod.rs
index a50eb2a71cf..a5a9dbb8da2 100644
--- a/compiler/rustc_metadata/src/rmeta/mod.rs
+++ b/compiler/rustc_metadata/src/rmeta/mod.rs
@@ -17,7 +17,6 @@ use rustc_middle::metadata::ModChild;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
 use rustc_middle::middle::exported_symbols::{ExportedSymbol, SymbolExportInfo};
 use rustc_middle::mir;
-use rustc_middle::thir;
 use rustc_middle::ty::fast_reject::SimplifiedType;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::{self, ReprOptions, Ty};
@@ -361,7 +360,7 @@ define_tables! {
     mir_for_ctfe: Table<DefIndex, LazyValue<mir::Body<'static>>>,
     promoted_mir: Table<DefIndex, LazyValue<IndexVec<mir::Promoted, mir::Body<'static>>>>,
     // FIXME(compiler-errors): Why isn't this a LazyArray?
-    thir_abstract_const: Table<DefIndex, LazyValue<&'static [thir::abstract_const::Node<'static>]>>,
+    thir_abstract_const: Table<DefIndex, LazyValue<&'static [ty::abstract_const::Node<'static>]>>,
     impl_parent: Table<DefIndex, RawDefId>,
     impl_polarity: Table<DefIndex, ty::ImplPolarity>,
     constness: Table<DefIndex, hir::Constness>,
diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index cbc45526e89..bdae7e5fcd6 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -351,7 +351,7 @@ rustc_queries! {
     /// Try to build an abstract representation of the given constant.
     query thir_abstract_const(
         key: DefId
-    ) -> Result<Option<&'tcx [thir::abstract_const::Node<'tcx>]>, ErrorGuaranteed> {
+    ) -> Result<Option<&'tcx [ty::abstract_const::Node<'tcx>]>, ErrorGuaranteed> {
         desc {
             |tcx| "building an abstract representation for {}", tcx.def_path_str(key),
         }
@@ -360,7 +360,7 @@ rustc_queries! {
     /// Try to build an abstract representation of the given constant.
     query thir_abstract_const_of_const_arg(
         key: (LocalDefId, DefId)
-    ) -> Result<Option<&'tcx [thir::abstract_const::Node<'tcx>]>, ErrorGuaranteed> {
+    ) -> Result<Option<&'tcx [ty::abstract_const::Node<'tcx>]>, ErrorGuaranteed> {
         desc {
             |tcx|
             "building an abstract representation for the const argument {}",
diff --git a/compiler/rustc_middle/src/thir.rs b/compiler/rustc_middle/src/thir.rs
index 3fe6394ad7e..d5700692b9d 100644
--- a/compiler/rustc_middle/src/thir.rs
+++ b/compiler/rustc_middle/src/thir.rs
@@ -30,7 +30,6 @@ use rustc_target::asm::InlineAsmRegOrRegClass;
 use std::fmt;
 use std::ops::Index;
 
-pub mod abstract_const;
 pub mod visit;
 
 newtype_index! {
diff --git a/compiler/rustc_middle/src/thir/abstract_const.rs b/compiler/rustc_middle/src/thir/abstract_const.rs
deleted file mode 100644
index 527dbd1cd09..00000000000
--- a/compiler/rustc_middle/src/thir/abstract_const.rs
+++ /dev/null
@@ -1,61 +0,0 @@
-//! A subset of a mir body used for const evaluatability checking.
-use crate::mir;
-use crate::ty::{self, Ty, TyCtxt};
-use rustc_errors::ErrorGuaranteed;
-
-rustc_index::newtype_index! {
-    /// An index into an `AbstractConst`.
-    pub struct NodeId {
-        derive [HashStable]
-        DEBUG_FORMAT = "n{}",
-    }
-}
-
-#[derive(Debug, Clone, Copy, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
-pub enum CastKind {
-    /// thir::ExprKind::As
-    As,
-    /// thir::ExprKind::Use
-    Use,
-}
-
-/// A node of an `AbstractConst`.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
-pub enum Node<'tcx> {
-    Leaf(ty::Const<'tcx>),
-    Binop(mir::BinOp, NodeId, NodeId),
-    UnaryOp(mir::UnOp, NodeId),
-    FunctionCall(NodeId, &'tcx [NodeId]),
-    Cast(CastKind, NodeId, Ty<'tcx>),
-}
-
-#[derive(Debug, Copy, Clone, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
-pub enum NotConstEvaluatable {
-    Error(ErrorGuaranteed),
-    MentionsInfer,
-    MentionsParam,
-}
-
-impl From<ErrorGuaranteed> for NotConstEvaluatable {
-    fn from(e: ErrorGuaranteed) -> NotConstEvaluatable {
-        NotConstEvaluatable::Error(e)
-    }
-}
-
-TrivialTypeTraversalAndLiftImpls! {
-    NotConstEvaluatable,
-}
-
-impl<'tcx> TyCtxt<'tcx> {
-    #[inline]
-    pub fn thir_abstract_const_opt_const_arg(
-        self,
-        def: ty::WithOptConstParam<rustc_hir::def_id::DefId>,
-    ) -> Result<Option<&'tcx [Node<'tcx>]>, ErrorGuaranteed> {
-        if let Some((did, param_did)) = def.as_const_arg() {
-            self.thir_abstract_const_of_const_arg((did, param_did))
-        } else {
-            self.thir_abstract_const(def.did)
-        }
-    }
-}
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index fe7f72024d3..955f2bdfa1d 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -10,7 +10,7 @@ mod structural_impls;
 pub mod util;
 
 use crate::infer::canonical::Canonical;
-use crate::thir::abstract_const::NotConstEvaluatable;
+use crate::ty::abstract_const::NotConstEvaluatable;
 use crate::ty::subst::SubstsRef;
 use crate::ty::{self, AdtKind, Ty, TyCtxt};
 
diff --git a/compiler/rustc_middle/src/ty/abstract_const.rs b/compiler/rustc_middle/src/ty/abstract_const.rs
new file mode 100644
index 00000000000..3e376d54000
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/abstract_const.rs
@@ -0,0 +1,302 @@
+//! A subset of a mir body used for const evaluatability checking.
+use crate::mir;
+use crate::ty::{self, subst::Subst, DelaySpanBugEmitted, EarlyBinder, SubstsRef, Ty, TyCtxt};
+use rustc_errors::ErrorGuaranteed;
+use std::iter;
+use std::ops::ControlFlow;
+
+rustc_index::newtype_index! {
+    /// An index into an `AbstractConst`.
+    pub struct NodeId {
+        derive [HashStable]
+        DEBUG_FORMAT = "n{}",
+    }
+}
+
+/// A tree representing an anonymous constant.
+///
+/// This is only able to represent a subset of `MIR`,
+/// and should not leak any information about desugarings.
+#[derive(Debug, Clone, Copy)]
+pub struct AbstractConst<'tcx> {
+    // FIXME: Consider adding something like `IndexSlice`
+    // and use this here.
+    inner: &'tcx [Node<'tcx>],
+    substs: SubstsRef<'tcx>,
+}
+
+impl<'tcx> AbstractConst<'tcx> {
+    pub fn new(
+        tcx: TyCtxt<'tcx>,
+        uv: ty::Unevaluated<'tcx, ()>,
+    ) -> Result<Option<AbstractConst<'tcx>>, ErrorGuaranteed> {
+        let inner = tcx.thir_abstract_const_opt_const_arg(uv.def)?;
+        debug!("AbstractConst::new({:?}) = {:?}", uv, inner);
+        Ok(inner.map(|inner| AbstractConst { inner, substs: tcx.erase_regions(uv.substs) }))
+    }
+
+    pub fn from_const(
+        tcx: TyCtxt<'tcx>,
+        ct: ty::Const<'tcx>,
+    ) -> Result<Option<AbstractConst<'tcx>>, ErrorGuaranteed> {
+        match ct.kind() {
+            ty::ConstKind::Unevaluated(uv) => AbstractConst::new(tcx, uv.shrink()),
+            ty::ConstKind::Error(DelaySpanBugEmitted { reported, .. }) => Err(reported),
+            _ => Ok(None),
+        }
+    }
+
+    #[inline]
+    pub fn subtree(self, node: NodeId) -> AbstractConst<'tcx> {
+        AbstractConst { inner: &self.inner[..=node.index()], substs: self.substs }
+    }
+
+    #[inline]
+    pub fn root(self, tcx: TyCtxt<'tcx>) -> Node<'tcx> {
+        let node = self.inner.last().copied().unwrap();
+        match node {
+            Node::Leaf(leaf) => Node::Leaf(EarlyBinder(leaf).subst(tcx, self.substs)),
+            Node::Cast(kind, operand, ty) => {
+                Node::Cast(kind, operand, EarlyBinder(ty).subst(tcx, self.substs))
+            }
+            // Don't perform substitution on the following as they can't directly contain generic params
+            Node::Binop(_, _, _) | Node::UnaryOp(_, _) | Node::FunctionCall(_, _) => node,
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
+pub enum CastKind {
+    /// thir::ExprKind::As
+    As,
+    /// thir::ExprKind::Use
+    Use,
+}
+
+/// A node of an `AbstractConst`.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
+pub enum Node<'tcx> {
+    Leaf(ty::Const<'tcx>),
+    Binop(mir::BinOp, NodeId, NodeId),
+    UnaryOp(mir::UnOp, NodeId),
+    FunctionCall(NodeId, &'tcx [NodeId]),
+    Cast(CastKind, NodeId, Ty<'tcx>),
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
+pub enum NotConstEvaluatable {
+    Error(ErrorGuaranteed),
+    MentionsInfer,
+    MentionsParam,
+}
+
+impl From<ErrorGuaranteed> for NotConstEvaluatable {
+    fn from(e: ErrorGuaranteed) -> NotConstEvaluatable {
+        NotConstEvaluatable::Error(e)
+    }
+}
+
+TrivialTypeTraversalAndLiftImpls! {
+    NotConstEvaluatable,
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    #[inline]
+    pub fn thir_abstract_const_opt_const_arg(
+        self,
+        def: ty::WithOptConstParam<rustc_hir::def_id::DefId>,
+    ) -> Result<Option<&'tcx [Node<'tcx>]>, ErrorGuaranteed> {
+        if let Some((did, param_did)) = def.as_const_arg() {
+            self.thir_abstract_const_of_const_arg((did, param_did))
+        } else {
+            self.thir_abstract_const(def.did)
+        }
+    }
+}
+
+#[instrument(skip(tcx), level = "debug")]
+pub fn try_unify_abstract_consts<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    (a, b): (ty::Unevaluated<'tcx, ()>, ty::Unevaluated<'tcx, ()>),
+    param_env: ty::ParamEnv<'tcx>,
+) -> bool {
+    (|| {
+        if let Some(a) = AbstractConst::new(tcx, a)? {
+            if let Some(b) = AbstractConst::new(tcx, b)? {
+                let const_unify_ctxt = ConstUnifyCtxt { tcx, param_env };
+                return Ok(const_unify_ctxt.try_unify(a, b));
+            }
+        }
+
+        Ok(false)
+    })()
+    .unwrap_or_else(|_: ErrorGuaranteed| true)
+    // FIXME(generic_const_exprs): We should instead have this
+    // method return the resulting `ty::Const` and return `ConstKind::Error`
+    // on `ErrorGuaranteed`.
+}
+
+#[instrument(skip(tcx, f), level = "debug")]
+pub fn walk_abstract_const<'tcx, R, F>(
+    tcx: TyCtxt<'tcx>,
+    ct: AbstractConst<'tcx>,
+    mut f: F,
+) -> ControlFlow<R>
+where
+    F: FnMut(AbstractConst<'tcx>) -> ControlFlow<R>,
+{
+    #[instrument(skip(tcx, f), level = "debug")]
+    fn recurse<'tcx, R>(
+        tcx: TyCtxt<'tcx>,
+        ct: AbstractConst<'tcx>,
+        f: &mut dyn FnMut(AbstractConst<'tcx>) -> ControlFlow<R>,
+    ) -> ControlFlow<R> {
+        f(ct)?;
+        let root = ct.root(tcx);
+        debug!(?root);
+        match root {
+            Node::Leaf(_) => ControlFlow::CONTINUE,
+            Node::Binop(_, l, r) => {
+                recurse(tcx, ct.subtree(l), f)?;
+                recurse(tcx, ct.subtree(r), f)
+            }
+            Node::UnaryOp(_, v) => recurse(tcx, ct.subtree(v), f),
+            Node::FunctionCall(func, args) => {
+                recurse(tcx, ct.subtree(func), f)?;
+                args.iter().try_for_each(|&arg| recurse(tcx, ct.subtree(arg), f))
+            }
+            Node::Cast(_, operand, _) => recurse(tcx, ct.subtree(operand), f),
+        }
+    }
+
+    recurse(tcx, ct, &mut f)
+}
+
+pub struct ConstUnifyCtxt<'tcx> {
+    pub tcx: TyCtxt<'tcx>,
+    pub param_env: ty::ParamEnv<'tcx>,
+}
+
+impl<'tcx> ConstUnifyCtxt<'tcx> {
+    // Substitutes generics repeatedly to allow AbstractConsts to unify where a
+    // ConstKind::Unevaluated could be turned into an AbstractConst that would unify e.g.
+    // Param(N) should unify with Param(T), substs: [Unevaluated("T2", [Unevaluated("T3", [Param(N)])])]
+    #[inline]
+    #[instrument(skip(self), level = "debug")]
+    fn try_replace_substs_in_root(
+        &self,
+        mut abstr_const: AbstractConst<'tcx>,
+    ) -> Option<AbstractConst<'tcx>> {
+        while let Node::Leaf(ct) = abstr_const.root(self.tcx) {
+            match AbstractConst::from_const(self.tcx, ct) {
+                Ok(Some(act)) => abstr_const = act,
+                Ok(None) => break,
+                Err(_) => return None,
+            }
+        }
+
+        Some(abstr_const)
+    }
+
+    /// Tries to unify two abstract constants using structural equality.
+    #[instrument(skip(self), level = "debug")]
+    pub fn try_unify(&self, a: AbstractConst<'tcx>, b: AbstractConst<'tcx>) -> bool {
+        let a = if let Some(a) = self.try_replace_substs_in_root(a) {
+            a
+        } else {
+            return true;
+        };
+
+        let b = if let Some(b) = self.try_replace_substs_in_root(b) {
+            b
+        } else {
+            return true;
+        };
+
+        let a_root = a.root(self.tcx);
+        let b_root = b.root(self.tcx);
+        debug!(?a_root, ?b_root);
+
+        match (a_root, b_root) {
+            (Node::Leaf(a_ct), Node::Leaf(b_ct)) => {
+                let a_ct = a_ct.eval(self.tcx, self.param_env);
+                debug!("a_ct evaluated: {:?}", a_ct);
+                let b_ct = b_ct.eval(self.tcx, self.param_env);
+                debug!("b_ct evaluated: {:?}", b_ct);
+
+                if a_ct.ty() != b_ct.ty() {
+                    return false;
+                }
+
+                match (a_ct.kind(), b_ct.kind()) {
+                    // We can just unify errors with everything to reduce the amount of
+                    // emitted errors here.
+                    (ty::ConstKind::Error(_), _) | (_, ty::ConstKind::Error(_)) => true,
+                    (ty::ConstKind::Param(a_param), ty::ConstKind::Param(b_param)) => {
+                        a_param == b_param
+                    }
+                    (ty::ConstKind::Value(a_val), ty::ConstKind::Value(b_val)) => a_val == b_val,
+                    // If we have `fn a<const N: usize>() -> [u8; N + 1]` and `fn b<const M: usize>() -> [u8; 1 + M]`
+                    // we do not want to use `assert_eq!(a(), b())` to infer that `N` and `M` have to be `1`. This
+                    // means that we only allow inference variables if they are equal.
+                    (ty::ConstKind::Infer(a_val), ty::ConstKind::Infer(b_val)) => a_val == b_val,
+                    // We expand generic anonymous constants at the start of this function, so this
+                    // branch should only be taking when dealing with associated constants, at
+                    // which point directly comparing them seems like the desired behavior.
+                    //
+                    // FIXME(generic_const_exprs): This isn't actually the case.
+                    // We also take this branch for concrete anonymous constants and
+                    // expand generic anonymous constants with concrete substs.
+                    (ty::ConstKind::Unevaluated(a_uv), ty::ConstKind::Unevaluated(b_uv)) => {
+                        a_uv == b_uv
+                    }
+                    // FIXME(generic_const_exprs): We may want to either actually try
+                    // to evaluate `a_ct` and `b_ct` if they are are fully concrete or something like
+                    // this, for now we just return false here.
+                    _ => false,
+                }
+            }
+            (Node::Binop(a_op, al, ar), Node::Binop(b_op, bl, br)) if a_op == b_op => {
+                self.try_unify(a.subtree(al), b.subtree(bl))
+                    && self.try_unify(a.subtree(ar), b.subtree(br))
+            }
+            (Node::UnaryOp(a_op, av), Node::UnaryOp(b_op, bv)) if a_op == b_op => {
+                self.try_unify(a.subtree(av), b.subtree(bv))
+            }
+            (Node::FunctionCall(a_f, a_args), Node::FunctionCall(b_f, b_args))
+                if a_args.len() == b_args.len() =>
+            {
+                self.try_unify(a.subtree(a_f), b.subtree(b_f))
+                    && iter::zip(a_args, b_args)
+                        .all(|(&an, &bn)| self.try_unify(a.subtree(an), b.subtree(bn)))
+            }
+            (Node::Cast(a_kind, a_operand, a_ty), Node::Cast(b_kind, b_operand, b_ty))
+                if (a_ty == b_ty) && (a_kind == b_kind) =>
+            {
+                self.try_unify(a.subtree(a_operand), b.subtree(b_operand))
+            }
+            // use this over `_ => false` to make adding variants to `Node` less error prone
+            (Node::Cast(..), _)
+            | (Node::FunctionCall(..), _)
+            | (Node::UnaryOp(..), _)
+            | (Node::Binop(..), _)
+            | (Node::Leaf(..), _) => false,
+        }
+    }
+}
+
+// We were unable to unify the abstract constant with
+// a constant found in the caller bounds, there are
+// now three possible cases here.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
+pub enum FailureKind {
+    /// The abstract const still references an inference
+    /// variable, in this case we return `TooGeneric`.
+    MentionsInfer,
+    /// The abstract const references a generic parameter,
+    /// this means that we emit an error here.
+    MentionsParam,
+    /// The substs are concrete enough that we can simply
+    /// try and evaluate the given constant.
+    Concrete,
+}
diff --git a/compiler/rustc_middle/src/ty/codec.rs b/compiler/rustc_middle/src/ty/codec.rs
index 9a363914dc3..e6ea3d88853 100644
--- a/compiler/rustc_middle/src/ty/codec.rs
+++ b/compiler/rustc_middle/src/ty/codec.rs
@@ -12,7 +12,6 @@ use crate::mir::{
     self,
     interpret::{AllocId, ConstAllocation},
 };
-use crate::thir;
 use crate::traits;
 use crate::ty::subst::SubstsRef;
 use crate::ty::{self, AdtDef, Ty};
@@ -346,7 +345,7 @@ impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D>
 }
 
 impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D>
-    for [thir::abstract_const::Node<'tcx>]
+    for [ty::abstract_const::Node<'tcx>]
 {
     fn decode(decoder: &mut D) -> &'tcx Self {
         decoder.interner().arena.alloc_from_iter(
@@ -356,7 +355,7 @@ impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D>
 }
 
 impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D>
-    for [thir::abstract_const::NodeId]
+    for [ty::abstract_const::NodeId]
 {
     fn decode(decoder: &mut D) -> &'tcx Self {
         decoder.interner().arena.alloc_from_iter(
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index f15108fb750..d23019e1521 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -92,6 +92,7 @@ pub use self::sty::{
 pub use self::trait_def::TraitDef;
 
 pub mod _match;
+pub mod abstract_const;
 pub mod adjustment;
 pub mod binding;
 pub mod cast;
diff --git a/compiler/rustc_middle/src/ty/parameterized.rs b/compiler/rustc_middle/src/ty/parameterized.rs
index 54ba9e84fdb..e189ee2fc4d 100644
--- a/compiler/rustc_middle/src/ty/parameterized.rs
+++ b/compiler/rustc_middle/src/ty/parameterized.rs
@@ -3,7 +3,7 @@ use rustc_index::vec::{Idx, IndexVec};
 
 use crate::middle::exported_symbols::ExportedSymbol;
 use crate::mir::Body;
-use crate::thir::abstract_const::Node;
+use crate::ty::abstract_const::Node;
 use crate::ty::{
     self, Const, FnSig, GeneratorDiagnosticData, GenericPredicates, Predicate, TraitRef, Ty,
 };
diff --git a/compiler/rustc_privacy/src/lib.rs b/compiler/rustc_privacy/src/lib.rs
index 5560d44aa0d..4e73c26d35f 100644
--- a/compiler/rustc_privacy/src/lib.rs
+++ b/compiler/rustc_privacy/src/lib.rs
@@ -23,7 +23,7 @@ use rustc_middle::bug;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::middle::privacy::{AccessLevel, AccessLevels};
 use rustc_middle::span_bug;
-use rustc_middle::thir::abstract_const::Node as ACNode;
+use rustc_middle::ty::abstract_const::{walk_abstract_const, AbstractConst, Node as ACNode};
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::{self, Const, DefIdTree, GenericParamDefKind};
@@ -32,7 +32,6 @@ use rustc_session::lint;
 use rustc_span::hygiene::Transparency;
 use rustc_span::symbol::{kw, Ident};
 use rustc_span::Span;
-use rustc_trait_selection::traits::const_evaluatable::{self, AbstractConst};
 
 use std::marker::PhantomData;
 use std::ops::ControlFlow;
@@ -164,7 +163,7 @@ where
         tcx: TyCtxt<'tcx>,
         ct: AbstractConst<'tcx>,
     ) -> ControlFlow<V::BreakTy> {
-        const_evaluatable::walk_abstract_const(tcx, ct, |node| match node.root(tcx) {
+        walk_abstract_const(tcx, ct, |node| match node.root(tcx) {
             ACNode::Leaf(leaf) => self.visit_const(leaf),
             ACNode::Cast(_, _, ty) => self.visit_ty(ty),
             ACNode::Binop(..) | ACNode::UnaryOp(..) | ACNode::FunctionCall(_, _) => {
diff --git a/compiler/rustc_query_impl/src/on_disk_cache.rs b/compiler/rustc_query_impl/src/on_disk_cache.rs
index 4c25075327f..56fd90c9855 100644
--- a/compiler/rustc_query_impl/src/on_disk_cache.rs
+++ b/compiler/rustc_query_impl/src/on_disk_cache.rs
@@ -9,7 +9,6 @@ use rustc_index::vec::{Idx, IndexVec};
 use rustc_middle::dep_graph::{DepNodeIndex, SerializedDepNodeIndex};
 use rustc_middle::mir::interpret::{AllocDecodingSession, AllocDecodingState};
 use rustc_middle::mir::{self, interpret};
-use rustc_middle::thir;
 use rustc_middle::ty::codec::{RefDecodable, TyDecoder, TyEncoder};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_query_system::dep_graph::DepContext;
@@ -766,7 +765,7 @@ impl<'a, 'tcx> Decodable<CacheDecoder<'a, 'tcx>>
     }
 }
 
-impl<'a, 'tcx> Decodable<CacheDecoder<'a, 'tcx>> for &'tcx [thir::abstract_const::Node<'tcx>] {
+impl<'a, 'tcx> Decodable<CacheDecoder<'a, 'tcx>> for &'tcx [ty::abstract_const::Node<'tcx>] {
     fn decode(d: &mut CacheDecoder<'a, 'tcx>) -> Self {
         RefDecodable::decode(d)
     }
diff --git a/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs b/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
index 3a152eff485..38581538b17 100644
--- a/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
+++ b/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
@@ -8,22 +8,17 @@
 //! In this case we try to build an abstract representation of this constant using
 //! `thir_abstract_const` which can then be checked for structural equality with other
 //! generic constants mentioned in the `caller_bounds` of the current environment.
-use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def::DefKind;
-use rustc_index::vec::IndexVec;
 use rustc_infer::infer::InferCtxt;
-use rustc_middle::mir;
-use rustc_middle::mir::interpret::{ErrorHandled, LitToConstError, LitToConstInput};
-use rustc_middle::thir;
-use rustc_middle::thir::abstract_const::{self, Node, NodeId, NotConstEvaluatable};
-use rustc_middle::ty::subst::{Subst, SubstsRef};
-use rustc_middle::ty::{self, DelaySpanBugEmitted, EarlyBinder, TyCtxt, TypeVisitable};
+use rustc_middle::mir::interpret::ErrorHandled;
+use rustc_middle::ty::abstract_const::{
+    walk_abstract_const, AbstractConst, ConstUnifyCtxt, FailureKind, Node, NotConstEvaluatable,
+};
+use rustc_middle::ty::{self, TyCtxt, TypeVisitable};
 use rustc_session::lint;
-use rustc_span::def_id::LocalDefId;
 use rustc_span::Span;
 
 use std::cmp;
-use std::iter;
 use std::ops::ControlFlow;
 
 /// Check if a given constant can be evaluated.
@@ -42,21 +37,6 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
                 return Ok(());
             }
 
-            // We were unable to unify the abstract constant with
-            // a constant found in the caller bounds, there are
-            // now three possible cases here.
-            #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
-            enum FailureKind {
-                /// The abstract const still references an inference
-                /// variable, in this case we return `TooGeneric`.
-                MentionsInfer,
-                /// The abstract const references a generic parameter,
-                /// this means that we emit an error here.
-                MentionsParam,
-                /// The substs are concrete enough that we can simply
-                /// try and evaluate the given constant.
-                Concrete,
-            }
             let mut failure_kind = FailureKind::Concrete;
             walk_abstract_const::<!, _>(tcx, ct, |node| match node.root(tcx) {
                 Node::Leaf(leaf) => {
@@ -216,593 +196,3 @@ fn satisfied_from_param_env<'tcx>(
 
     Ok(false)
 }
-
-/// A tree representing an anonymous constant.
-///
-/// This is only able to represent a subset of `MIR`,
-/// and should not leak any information about desugarings.
-#[derive(Debug, Clone, Copy)]
-pub struct AbstractConst<'tcx> {
-    // FIXME: Consider adding something like `IndexSlice`
-    // and use this here.
-    inner: &'tcx [Node<'tcx>],
-    substs: SubstsRef<'tcx>,
-}
-
-impl<'tcx> AbstractConst<'tcx> {
-    pub fn new(
-        tcx: TyCtxt<'tcx>,
-        uv: ty::Unevaluated<'tcx, ()>,
-    ) -> Result<Option<AbstractConst<'tcx>>, ErrorGuaranteed> {
-        let inner = tcx.thir_abstract_const_opt_const_arg(uv.def)?;
-        debug!("AbstractConst::new({:?}) = {:?}", uv, inner);
-        Ok(inner.map(|inner| AbstractConst { inner, substs: tcx.erase_regions(uv.substs) }))
-    }
-
-    pub fn from_const(
-        tcx: TyCtxt<'tcx>,
-        ct: ty::Const<'tcx>,
-    ) -> Result<Option<AbstractConst<'tcx>>, ErrorGuaranteed> {
-        match ct.kind() {
-            ty::ConstKind::Unevaluated(uv) => AbstractConst::new(tcx, uv.shrink()),
-            ty::ConstKind::Error(DelaySpanBugEmitted { reported, .. }) => Err(reported),
-            _ => Ok(None),
-        }
-    }
-
-    #[inline]
-    pub fn subtree(self, node: NodeId) -> AbstractConst<'tcx> {
-        AbstractConst { inner: &self.inner[..=node.index()], substs: self.substs }
-    }
-
-    #[inline]
-    pub fn root(self, tcx: TyCtxt<'tcx>) -> Node<'tcx> {
-        let node = self.inner.last().copied().unwrap();
-        match node {
-            Node::Leaf(leaf) => Node::Leaf(EarlyBinder(leaf).subst(tcx, self.substs)),
-            Node::Cast(kind, operand, ty) => {
-                Node::Cast(kind, operand, EarlyBinder(ty).subst(tcx, self.substs))
-            }
-            // Don't perform substitution on the following as they can't directly contain generic params
-            Node::Binop(_, _, _) | Node::UnaryOp(_, _) | Node::FunctionCall(_, _) => node,
-        }
-    }
-}
-
-struct AbstractConstBuilder<'a, 'tcx> {
-    tcx: TyCtxt<'tcx>,
-    body_id: thir::ExprId,
-    body: &'a thir::Thir<'tcx>,
-    /// The current WIP node tree.
-    nodes: IndexVec<NodeId, Node<'tcx>>,
-}
-
-impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
-    fn root_span(&self) -> Span {
-        self.body.exprs[self.body_id].span
-    }
-
-    fn error(&mut self, span: Span, msg: &str) -> Result<!, ErrorGuaranteed> {
-        let reported = self
-            .tcx
-            .sess
-            .struct_span_err(self.root_span(), "overly complex generic constant")
-            .span_label(span, msg)
-            .help("consider moving this anonymous constant into a `const` function")
-            .emit();
-
-        Err(reported)
-    }
-    fn maybe_supported_error(&mut self, span: Span, msg: &str) -> Result<!, ErrorGuaranteed> {
-        let reported = self
-            .tcx
-            .sess
-            .struct_span_err(self.root_span(), "overly complex generic constant")
-            .span_label(span, msg)
-            .help("consider moving this anonymous constant into a `const` function")
-            .note("this operation may be supported in the future")
-            .emit();
-
-        Err(reported)
-    }
-
-    #[instrument(skip(tcx, body, body_id), level = "debug")]
-    fn new(
-        tcx: TyCtxt<'tcx>,
-        (body, body_id): (&'a thir::Thir<'tcx>, thir::ExprId),
-    ) -> Result<Option<AbstractConstBuilder<'a, 'tcx>>, ErrorGuaranteed> {
-        let builder = AbstractConstBuilder { tcx, body_id, body, nodes: IndexVec::new() };
-
-        struct IsThirPolymorphic<'a, 'tcx> {
-            is_poly: bool,
-            thir: &'a thir::Thir<'tcx>,
-        }
-
-        use crate::rustc_middle::thir::visit::Visitor;
-        use thir::visit;
-
-        impl<'a, 'tcx> IsThirPolymorphic<'a, 'tcx> {
-            fn expr_is_poly(&mut self, expr: &thir::Expr<'tcx>) -> bool {
-                if expr.ty.has_param_types_or_consts() {
-                    return true;
-                }
-
-                match expr.kind {
-                    thir::ExprKind::NamedConst { substs, .. } => substs.has_param_types_or_consts(),
-                    thir::ExprKind::ConstParam { .. } => true,
-                    thir::ExprKind::Repeat { value, count } => {
-                        self.visit_expr(&self.thir()[value]);
-                        count.has_param_types_or_consts()
-                    }
-                    _ => false,
-                }
-            }
-
-            fn pat_is_poly(&mut self, pat: &thir::Pat<'tcx>) -> bool {
-                if pat.ty.has_param_types_or_consts() {
-                    return true;
-                }
-
-                match pat.kind.as_ref() {
-                    thir::PatKind::Constant { value } => value.has_param_types_or_consts(),
-                    thir::PatKind::Range(thir::PatRange { lo, hi, .. }) => {
-                        lo.has_param_types_or_consts() || hi.has_param_types_or_consts()
-                    }
-                    _ => false,
-                }
-            }
-        }
-
-        impl<'a, 'tcx> visit::Visitor<'a, 'tcx> for IsThirPolymorphic<'a, 'tcx> {
-            fn thir(&self) -> &'a thir::Thir<'tcx> {
-                &self.thir
-            }
-
-            #[instrument(skip(self), level = "debug")]
-            fn visit_expr(&mut self, expr: &thir::Expr<'tcx>) {
-                self.is_poly |= self.expr_is_poly(expr);
-                if !self.is_poly {
-                    visit::walk_expr(self, expr)
-                }
-            }
-
-            #[instrument(skip(self), level = "debug")]
-            fn visit_pat(&mut self, pat: &thir::Pat<'tcx>) {
-                self.is_poly |= self.pat_is_poly(pat);
-                if !self.is_poly {
-                    visit::walk_pat(self, pat);
-                }
-            }
-        }
-
-        let mut is_poly_vis = IsThirPolymorphic { is_poly: false, thir: body };
-        visit::walk_expr(&mut is_poly_vis, &body[body_id]);
-        debug!("AbstractConstBuilder: is_poly={}", is_poly_vis.is_poly);
-        if !is_poly_vis.is_poly {
-            return Ok(None);
-        }
-
-        Ok(Some(builder))
-    }
-
-    /// We do not allow all binary operations in abstract consts, so filter disallowed ones.
-    fn check_binop(op: mir::BinOp) -> bool {
-        use mir::BinOp::*;
-        match op {
-            Add | Sub | Mul | Div | Rem | BitXor | BitAnd | BitOr | Shl | Shr | Eq | Lt | Le
-            | Ne | Ge | Gt => true,
-            Offset => false,
-        }
-    }
-
-    /// While we currently allow all unary operations, we still want to explicitly guard against
-    /// future changes here.
-    fn check_unop(op: mir::UnOp) -> bool {
-        use mir::UnOp::*;
-        match op {
-            Not | Neg => true,
-        }
-    }
-
-    /// Builds the abstract const by walking the thir and bailing out when
-    /// encountering an unsupported operation.
-    fn build(mut self) -> Result<&'tcx [Node<'tcx>], ErrorGuaranteed> {
-        debug!("Abstractconstbuilder::build: body={:?}", &*self.body);
-        self.recurse_build(self.body_id)?;
-
-        for n in self.nodes.iter() {
-            if let Node::Leaf(ct) = n {
-                if let ty::ConstKind::Unevaluated(ct) = ct.kind() {
-                    // `AbstractConst`s should not contain any promoteds as they require references which
-                    // are not allowed.
-                    assert_eq!(ct.promoted, None);
-                    assert_eq!(ct, self.tcx.erase_regions(ct));
-                }
-            }
-        }
-
-        Ok(self.tcx.arena.alloc_from_iter(self.nodes.into_iter()))
-    }
-
-    fn recurse_build(&mut self, node: thir::ExprId) -> Result<NodeId, ErrorGuaranteed> {
-        use thir::ExprKind;
-        let node = &self.body.exprs[node];
-        Ok(match &node.kind {
-            // I dont know if handling of these 3 is correct
-            &ExprKind::Scope { value, .. } => self.recurse_build(value)?,
-            &ExprKind::PlaceTypeAscription { source, .. }
-            | &ExprKind::ValueTypeAscription { source, .. } => self.recurse_build(source)?,
-            &ExprKind::Literal { lit, neg} => {
-                let sp = node.span;
-                let constant =
-                    match self.tcx.at(sp).lit_to_const(LitToConstInput { lit: &lit.node, ty: node.ty, neg }) {
-                        Ok(c) => c,
-                        Err(LitToConstError::Reported) => {
-                            self.tcx.const_error(node.ty)
-                        }
-                        Err(LitToConstError::TypeError) => {
-                            bug!("encountered type error in lit_to_const")
-                        }
-                    };
-
-                self.nodes.push(Node::Leaf(constant))
-            }
-            &ExprKind::NonHirLiteral { lit , user_ty: _} => {
-                let val = ty::ValTree::from_scalar_int(lit);
-                self.nodes.push(Node::Leaf(ty::Const::from_value(self.tcx, val, node.ty)))
-            }
-            &ExprKind::ZstLiteral { user_ty: _ } => {
-                let val = ty::ValTree::zst();
-                self.nodes.push(Node::Leaf(ty::Const::from_value(self.tcx, val, node.ty)))
-            }
-            &ExprKind::NamedConst { def_id, substs, user_ty: _ } => {
-                let uneval = ty::Unevaluated::new(ty::WithOptConstParam::unknown(def_id), substs);
-
-                let constant = self.tcx.mk_const(ty::ConstS {
-                                kind: ty::ConstKind::Unevaluated(uneval),
-                                ty: node.ty,
-                            });
-
-                self.nodes.push(Node::Leaf(constant))
-            }
-
-            ExprKind::ConstParam {param, ..} => {
-                let const_param = self.tcx.mk_const(ty::ConstS {
-                        kind: ty::ConstKind::Param(*param),
-                        ty: node.ty,
-                    });
-                self.nodes.push(Node::Leaf(const_param))
-            }
-
-            ExprKind::Call { fun, args, .. } => {
-                let fun = self.recurse_build(*fun)?;
-
-                let mut new_args = Vec::<NodeId>::with_capacity(args.len());
-                for &id in args.iter() {
-                    new_args.push(self.recurse_build(id)?);
-                }
-                let new_args = self.tcx.arena.alloc_slice(&new_args);
-                self.nodes.push(Node::FunctionCall(fun, new_args))
-            }
-            &ExprKind::Binary { op, lhs, rhs } if Self::check_binop(op) => {
-                let lhs = self.recurse_build(lhs)?;
-                let rhs = self.recurse_build(rhs)?;
-                self.nodes.push(Node::Binop(op, lhs, rhs))
-            }
-            &ExprKind::Unary { op, arg } if Self::check_unop(op) => {
-                let arg = self.recurse_build(arg)?;
-                self.nodes.push(Node::UnaryOp(op, arg))
-            }
-            // This is necessary so that the following compiles:
-            //
-            // ```
-            // fn foo<const N: usize>(a: [(); N + 1]) {
-            //     bar::<{ N + 1 }>();
-            // }
-            // ```
-            ExprKind::Block { body: thir::Block { stmts: box [], expr: Some(e), .. } } => {
-                self.recurse_build(*e)?
-            }
-            // `ExprKind::Use` happens when a `hir::ExprKind::Cast` is a
-            // "coercion cast" i.e. using a coercion or is a no-op.
-            // This is important so that `N as usize as usize` doesnt unify with `N as usize`. (untested)
-            &ExprKind::Use { source } => {
-                let arg = self.recurse_build(source)?;
-                self.nodes.push(Node::Cast(abstract_const::CastKind::Use, arg, node.ty))
-            }
-            &ExprKind::Cast { source } => {
-                let arg = self.recurse_build(source)?;
-                self.nodes.push(Node::Cast(abstract_const::CastKind::As, arg, node.ty))
-            }
-            ExprKind::Borrow{ arg, ..} => {
-                let arg_node = &self.body.exprs[*arg];
-
-                // Skip reborrows for now until we allow Deref/Borrow/AddressOf
-                // expressions.
-                // FIXME(generic_const_exprs): Verify/explain why this is sound
-                if let ExprKind::Deref {arg} = arg_node.kind {
-                    self.recurse_build(arg)?
-                } else {
-                    self.maybe_supported_error(
-                        node.span,
-                        "borrowing is not supported in generic constants",
-                    )?
-                }
-            }
-            // FIXME(generic_const_exprs): We may want to support these.
-            ExprKind::AddressOf { .. } | ExprKind::Deref {..}=> self.maybe_supported_error(
-                node.span,
-                "dereferencing or taking the address is not supported in generic constants",
-            )?,
-            ExprKind::Repeat { .. } | ExprKind::Array { .. } =>  self.maybe_supported_error(
-                node.span,
-                "array construction is not supported in generic constants",
-            )?,
-            ExprKind::Block { .. } => self.maybe_supported_error(
-                node.span,
-                "blocks are not supported in generic constant",
-            )?,
-            ExprKind::NeverToAny { .. } => self.maybe_supported_error(
-                node.span,
-                "converting nevers to any is not supported in generic constant",
-            )?,
-            ExprKind::Tuple { .. } => self.maybe_supported_error(
-                node.span,
-                "tuple construction is not supported in generic constants",
-            )?,
-            ExprKind::Index { .. } => self.maybe_supported_error(
-                node.span,
-                "indexing is not supported in generic constant",
-            )?,
-            ExprKind::Field { .. } => self.maybe_supported_error(
-                node.span,
-                "field access is not supported in generic constant",
-            )?,
-            ExprKind::ConstBlock { .. } => self.maybe_supported_error(
-                node.span,
-                "const blocks are not supported in generic constant",
-            )?,
-            ExprKind::Adt(_) => self.maybe_supported_error(
-                node.span,
-                "struct/enum construction is not supported in generic constants",
-            )?,
-            // dont know if this is correct
-            ExprKind::Pointer { .. } =>
-                self.error(node.span, "pointer casts are not allowed in generic constants")?,
-            ExprKind::Yield { .. } =>
-                self.error(node.span, "generator control flow is not allowed in generic constants")?,
-            ExprKind::Continue { .. } | ExprKind::Break { .. } | ExprKind::Loop { .. } => self
-                .error(
-                    node.span,
-                    "loops and loop control flow are not supported in generic constants",
-                )?,
-            ExprKind::Box { .. } =>
-                self.error(node.span, "allocations are not allowed in generic constants")?,
-
-            ExprKind::Unary { .. } => unreachable!(),
-            // we handle valid unary/binary ops above
-            ExprKind::Binary { .. } =>
-                self.error(node.span, "unsupported binary operation in generic constants")?,
-            ExprKind::LogicalOp { .. } =>
-                self.error(node.span, "unsupported operation in generic constants, short-circuiting operations would imply control flow")?,
-            ExprKind::Assign { .. } | ExprKind::AssignOp { .. } => {
-                self.error(node.span, "assignment is not supported in generic constants")?
-            }
-            ExprKind::Closure { .. } | ExprKind::Return { .. } => self.error(
-                node.span,
-                "closures and function keywords are not supported in generic constants",
-            )?,
-            // let expressions imply control flow
-            ExprKind::Match { .. } | ExprKind::If { .. } | ExprKind::Let { .. } =>
-                self.error(node.span, "control flow is not supported in generic constants")?,
-            ExprKind::InlineAsm { .. } => {
-                self.error(node.span, "assembly is not supported in generic constants")?
-            }
-
-            // we dont permit let stmts so `VarRef` and `UpvarRef` cant happen
-            ExprKind::VarRef { .. }
-            | ExprKind::UpvarRef { .. }
-            | ExprKind::StaticRef { .. }
-            | ExprKind::ThreadLocalRef(_) => {
-                self.error(node.span, "unsupported operation in generic constant")?
-            }
-        })
-    }
-}
-
-/// Builds an abstract const, do not use this directly, but use `AbstractConst::new` instead.
-pub(super) fn thir_abstract_const<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    def: ty::WithOptConstParam<LocalDefId>,
-) -> Result<Option<&'tcx [thir::abstract_const::Node<'tcx>]>, ErrorGuaranteed> {
-    if tcx.features().generic_const_exprs {
-        match tcx.def_kind(def.did) {
-            // FIXME(generic_const_exprs): We currently only do this for anonymous constants,
-            // meaning that we do not look into associated constants. I(@lcnr) am not yet sure whether
-            // we want to look into them or treat them as opaque projections.
-            //
-            // Right now we do neither of that and simply always fail to unify them.
-            DefKind::AnonConst | DefKind::InlineConst => (),
-            _ => return Ok(None),
-        }
-
-        let body = tcx.thir_body(def)?;
-
-        AbstractConstBuilder::new(tcx, (&*body.0.borrow(), body.1))?
-            .map(AbstractConstBuilder::build)
-            .transpose()
-    } else {
-        Ok(None)
-    }
-}
-
-#[instrument(skip(tcx), level = "debug")]
-pub(super) fn try_unify_abstract_consts<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    (a, b): (ty::Unevaluated<'tcx, ()>, ty::Unevaluated<'tcx, ()>),
-    param_env: ty::ParamEnv<'tcx>,
-) -> bool {
-    (|| {
-        if let Some(a) = AbstractConst::new(tcx, a)? {
-            if let Some(b) = AbstractConst::new(tcx, b)? {
-                let const_unify_ctxt = ConstUnifyCtxt { tcx, param_env };
-                return Ok(const_unify_ctxt.try_unify(a, b));
-            }
-        }
-
-        Ok(false)
-    })()
-    .unwrap_or_else(|_: ErrorGuaranteed| true)
-    // FIXME(generic_const_exprs): We should instead have this
-    // method return the resulting `ty::Const` and return `ConstKind::Error`
-    // on `ErrorGuaranteed`.
-}
-
-#[instrument(skip(tcx, f), level = "debug")]
-pub fn walk_abstract_const<'tcx, R, F>(
-    tcx: TyCtxt<'tcx>,
-    ct: AbstractConst<'tcx>,
-    mut f: F,
-) -> ControlFlow<R>
-where
-    F: FnMut(AbstractConst<'tcx>) -> ControlFlow<R>,
-{
-    #[instrument(skip(tcx, f), level = "debug")]
-    fn recurse<'tcx, R>(
-        tcx: TyCtxt<'tcx>,
-        ct: AbstractConst<'tcx>,
-        f: &mut dyn FnMut(AbstractConst<'tcx>) -> ControlFlow<R>,
-    ) -> ControlFlow<R> {
-        f(ct)?;
-        let root = ct.root(tcx);
-        debug!(?root);
-        match root {
-            Node::Leaf(_) => ControlFlow::CONTINUE,
-            Node::Binop(_, l, r) => {
-                recurse(tcx, ct.subtree(l), f)?;
-                recurse(tcx, ct.subtree(r), f)
-            }
-            Node::UnaryOp(_, v) => recurse(tcx, ct.subtree(v), f),
-            Node::FunctionCall(func, args) => {
-                recurse(tcx, ct.subtree(func), f)?;
-                args.iter().try_for_each(|&arg| recurse(tcx, ct.subtree(arg), f))
-            }
-            Node::Cast(_, operand, _) => recurse(tcx, ct.subtree(operand), f),
-        }
-    }
-
-    recurse(tcx, ct, &mut f)
-}
-
-struct ConstUnifyCtxt<'tcx> {
-    tcx: TyCtxt<'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
-}
-
-impl<'tcx> ConstUnifyCtxt<'tcx> {
-    // Substitutes generics repeatedly to allow AbstractConsts to unify where a
-    // ConstKind::Unevaluated could be turned into an AbstractConst that would unify e.g.
-    // Param(N) should unify with Param(T), substs: [Unevaluated("T2", [Unevaluated("T3", [Param(N)])])]
-    #[inline]
-    #[instrument(skip(self), level = "debug")]
-    fn try_replace_substs_in_root(
-        &self,
-        mut abstr_const: AbstractConst<'tcx>,
-    ) -> Option<AbstractConst<'tcx>> {
-        while let Node::Leaf(ct) = abstr_const.root(self.tcx) {
-            match AbstractConst::from_const(self.tcx, ct) {
-                Ok(Some(act)) => abstr_const = act,
-                Ok(None) => break,
-                Err(_) => return None,
-            }
-        }
-
-        Some(abstr_const)
-    }
-
-    /// Tries to unify two abstract constants using structural equality.
-    #[instrument(skip(self), level = "debug")]
-    fn try_unify(&self, a: AbstractConst<'tcx>, b: AbstractConst<'tcx>) -> bool {
-        let a = if let Some(a) = self.try_replace_substs_in_root(a) {
-            a
-        } else {
-            return true;
-        };
-
-        let b = if let Some(b) = self.try_replace_substs_in_root(b) {
-            b
-        } else {
-            return true;
-        };
-
-        let a_root = a.root(self.tcx);
-        let b_root = b.root(self.tcx);
-        debug!(?a_root, ?b_root);
-
-        match (a_root, b_root) {
-            (Node::Leaf(a_ct), Node::Leaf(b_ct)) => {
-                let a_ct = a_ct.eval(self.tcx, self.param_env);
-                debug!("a_ct evaluated: {:?}", a_ct);
-                let b_ct = b_ct.eval(self.tcx, self.param_env);
-                debug!("b_ct evaluated: {:?}", b_ct);
-
-                if a_ct.ty() != b_ct.ty() {
-                    return false;
-                }
-
-                match (a_ct.kind(), b_ct.kind()) {
-                    // We can just unify errors with everything to reduce the amount of
-                    // emitted errors here.
-                    (ty::ConstKind::Error(_), _) | (_, ty::ConstKind::Error(_)) => true,
-                    (ty::ConstKind::Param(a_param), ty::ConstKind::Param(b_param)) => {
-                        a_param == b_param
-                    }
-                    (ty::ConstKind::Value(a_val), ty::ConstKind::Value(b_val)) => a_val == b_val,
-                    // If we have `fn a<const N: usize>() -> [u8; N + 1]` and `fn b<const M: usize>() -> [u8; 1 + M]`
-                    // we do not want to use `assert_eq!(a(), b())` to infer that `N` and `M` have to be `1`. This
-                    // means that we only allow inference variables if they are equal.
-                    (ty::ConstKind::Infer(a_val), ty::ConstKind::Infer(b_val)) => a_val == b_val,
-                    // We expand generic anonymous constants at the start of this function, so this
-                    // branch should only be taking when dealing with associated constants, at
-                    // which point directly comparing them seems like the desired behavior.
-                    //
-                    // FIXME(generic_const_exprs): This isn't actually the case.
-                    // We also take this branch for concrete anonymous constants and
-                    // expand generic anonymous constants with concrete substs.
-                    (ty::ConstKind::Unevaluated(a_uv), ty::ConstKind::Unevaluated(b_uv)) => {
-                        a_uv == b_uv
-                    }
-                    // FIXME(generic_const_exprs): We may want to either actually try
-                    // to evaluate `a_ct` and `b_ct` if they are are fully concrete or something like
-                    // this, for now we just return false here.
-                    _ => false,
-                }
-            }
-            (Node::Binop(a_op, al, ar), Node::Binop(b_op, bl, br)) if a_op == b_op => {
-                self.try_unify(a.subtree(al), b.subtree(bl))
-                    && self.try_unify(a.subtree(ar), b.subtree(br))
-            }
-            (Node::UnaryOp(a_op, av), Node::UnaryOp(b_op, bv)) if a_op == b_op => {
-                self.try_unify(a.subtree(av), b.subtree(bv))
-            }
-            (Node::FunctionCall(a_f, a_args), Node::FunctionCall(b_f, b_args))
-                if a_args.len() == b_args.len() =>
-            {
-                self.try_unify(a.subtree(a_f), b.subtree(b_f))
-                    && iter::zip(a_args, b_args)
-                        .all(|(&an, &bn)| self.try_unify(a.subtree(an), b.subtree(bn)))
-            }
-            (Node::Cast(a_kind, a_operand, a_ty), Node::Cast(b_kind, b_operand, b_ty))
-                if (a_ty == b_ty) && (a_kind == b_kind) =>
-            {
-                self.try_unify(a.subtree(a_operand), b.subtree(b_operand))
-            }
-            // use this over `_ => false` to make adding variants to `Node` less error prone
-            (Node::Cast(..), _)
-            | (Node::FunctionCall(..), _)
-            | (Node::UnaryOp(..), _)
-            | (Node::Binop(..), _)
-            | (Node::Leaf(..), _) => false,
-        }
-    }
-}
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 34f4a9f7902..0ef9d3af15a 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -24,8 +24,8 @@ use rustc_hir::Item;
 use rustc_hir::Node;
 use rustc_infer::infer::error_reporting::same_type_modulo_infer;
 use rustc_infer::traits::{AmbiguousSelection, TraitEngine};
-use rustc_middle::thir::abstract_const::NotConstEvaluatable;
 use rustc_middle::traits::select::OverflowError;
+use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::ExpectedFound;
 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::{
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index 78600652254..4aa62f8078d 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -6,7 +6,7 @@ use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProce
 use rustc_infer::traits::ProjectionCacheKey;
 use rustc_infer::traits::{SelectionError, TraitEngine, TraitEngineExt as _, TraitObligation};
 use rustc_middle::mir::interpret::ErrorHandled;
-use rustc_middle::thir::abstract_const::NotConstEvaluatable;
+use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::ToPredicate;
diff --git a/compiler/rustc_trait_selection/src/traits/mod.rs b/compiler/rustc_trait_selection/src/traits/mod.rs
index 74d2eb17b6b..f6a8dd6bbf9 100644
--- a/compiler/rustc_trait_selection/src/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/mod.rs
@@ -845,23 +845,9 @@ pub fn provide(providers: &mut ty::query::Providers) {
         vtable_entries,
         vtable_trait_upcasting_coercion_new_vptr_slot,
         subst_and_check_impossible_predicates,
-        thir_abstract_const: |tcx, def_id| {
-            let def_id = def_id.expect_local();
-            if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
-                tcx.thir_abstract_const_of_const_arg(def)
-            } else {
-                const_evaluatable::thir_abstract_const(tcx, ty::WithOptConstParam::unknown(def_id))
-            }
-        },
-        thir_abstract_const_of_const_arg: |tcx, (did, param_did)| {
-            const_evaluatable::thir_abstract_const(
-                tcx,
-                ty::WithOptConstParam { did, const_param_did: Some(param_did) },
-            )
-        },
         try_unify_abstract_consts: |tcx, param_env_and| {
             let (param_env, (a, b)) = param_env_and.into_parts();
-            const_evaluatable::try_unify_abstract_consts(tcx, (a, b), param_env)
+            rustc_middle::ty::abstract_const::try_unify_abstract_consts(tcx, (a, b), param_env)
         },
         ..*providers
     };
diff --git a/compiler/rustc_trait_selection/src/traits/object_safety.rs b/compiler/rustc_trait_selection/src/traits/object_safety.rs
index ac1811244ca..2921ce0ffef 100644
--- a/compiler/rustc_trait_selection/src/traits/object_safety.rs
+++ b/compiler/rustc_trait_selection/src/traits/object_safety.rs
@@ -11,12 +11,12 @@
 use super::elaborate_predicates;
 
 use crate::infer::TyCtxtInferExt;
-use crate::traits::const_evaluatable::{self, AbstractConst};
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::{self, Obligation, ObligationCause};
 use rustc_errors::{FatalError, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
+use rustc_middle::ty::abstract_const::{walk_abstract_const, AbstractConst};
 use rustc_middle::ty::subst::{GenericArg, InternalSubsts, Subst};
 use rustc_middle::ty::{
     self, EarlyBinder, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor,
@@ -841,15 +841,13 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
             //
             // This shouldn't really matter though as we can't really use any
             // constants which are not considered const evaluatable.
-            use rustc_middle::thir::abstract_const::Node;
+            use rustc_middle::ty::abstract_const::Node;
             if let Ok(Some(ct)) = AbstractConst::new(self.tcx, uv.shrink()) {
-                const_evaluatable::walk_abstract_const(self.tcx, ct, |node| {
-                    match node.root(self.tcx) {
-                        Node::Leaf(leaf) => self.visit_const(leaf),
-                        Node::Cast(_, _, ty) => self.visit_ty(ty),
-                        Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
-                            ControlFlow::CONTINUE
-                        }
+                walk_abstract_const(self.tcx, ct, |node| match node.root(self.tcx) {
+                    Node::Leaf(leaf) => self.visit_const(leaf),
+                    Node::Cast(_, _, ty) => self.visit_ty(ty),
+                    Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
+                        ControlFlow::CONTINUE
                     }
                 })
             } else {
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 2bb53a466ca..7c5673c8632 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -32,7 +32,7 @@ use rustc_hir::def_id::DefId;
 use rustc_infer::infer::LateBoundRegionConversionTime;
 use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
 use rustc_middle::mir::interpret::ErrorHandled;
-use rustc_middle::thir::abstract_const::NotConstEvaluatable;
+use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::print::with_no_trimmed_paths;
diff --git a/compiler/rustc_ty_utils/Cargo.toml b/compiler/rustc_ty_utils/Cargo.toml
index d03d675bfd2..caad2ed4274 100644
--- a/compiler/rustc_ty_utils/Cargo.toml
+++ b/compiler/rustc_ty_utils/Cargo.toml
@@ -15,3 +15,4 @@ rustc_session = { path = "../rustc_session" }
 rustc_target = { path = "../rustc_target" }
 rustc_trait_selection = { path = "../rustc_trait_selection" }
 rustc_type_ir = { path = "../rustc_type_ir" }
+rustc_index = { path = "../rustc_index" }
diff --git a/compiler/rustc_ty_utils/src/consts.rs b/compiler/rustc_ty_utils/src/consts.rs
index 0b83cdb78dc..80957d64465 100644
--- a/compiler/rustc_ty_utils/src/consts.rs
+++ b/compiler/rustc_ty_utils/src/consts.rs
@@ -1,4 +1,12 @@
-use rustc_middle::ty::{self, TyCtxt};
+use rustc_errors::ErrorGuaranteed;
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::LocalDefId;
+use rustc_index::vec::IndexVec;
+use rustc_middle::mir::interpret::{LitToConstError, LitToConstInput};
+use rustc_middle::ty::abstract_const::{CastKind, Node, NodeId};
+use rustc_middle::ty::{self, TyCtxt, TypeVisitable};
+use rustc_middle::{mir, thir};
+use rustc_span::Span;
 use rustc_target::abi::VariantIdx;
 
 use std::iter;
@@ -72,6 +80,390 @@ pub(crate) fn destructure_const<'tcx>(
     ty::DestructuredConst { variant, fields }
 }
 
+pub struct AbstractConstBuilder<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    body_id: thir::ExprId,
+    body: &'a thir::Thir<'tcx>,
+    /// The current WIP node tree.
+    nodes: IndexVec<NodeId, Node<'tcx>>,
+}
+
+impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
+    fn root_span(&self) -> Span {
+        self.body.exprs[self.body_id].span
+    }
+
+    fn error(&mut self, span: Span, msg: &str) -> Result<!, ErrorGuaranteed> {
+        let reported = self
+            .tcx
+            .sess
+            .struct_span_err(self.root_span(), "overly complex generic constant")
+            .span_label(span, msg)
+            .help("consider moving this anonymous constant into a `const` function")
+            .emit();
+
+        Err(reported)
+    }
+    fn maybe_supported_error(&mut self, span: Span, msg: &str) -> Result<!, ErrorGuaranteed> {
+        let reported = self
+            .tcx
+            .sess
+            .struct_span_err(self.root_span(), "overly complex generic constant")
+            .span_label(span, msg)
+            .help("consider moving this anonymous constant into a `const` function")
+            .note("this operation may be supported in the future")
+            .emit();
+
+        Err(reported)
+    }
+
+    #[instrument(skip(tcx, body, body_id), level = "debug")]
+    pub fn new(
+        tcx: TyCtxt<'tcx>,
+        (body, body_id): (&'a thir::Thir<'tcx>, thir::ExprId),
+    ) -> Result<Option<AbstractConstBuilder<'a, 'tcx>>, ErrorGuaranteed> {
+        let builder = AbstractConstBuilder { tcx, body_id, body, nodes: IndexVec::new() };
+
+        struct IsThirPolymorphic<'a, 'tcx> {
+            is_poly: bool,
+            thir: &'a thir::Thir<'tcx>,
+        }
+
+        use crate::rustc_middle::thir::visit::Visitor;
+        use thir::visit;
+
+        impl<'a, 'tcx> IsThirPolymorphic<'a, 'tcx> {
+            fn expr_is_poly(&mut self, expr: &thir::Expr<'tcx>) -> bool {
+                if expr.ty.has_param_types_or_consts() {
+                    return true;
+                }
+
+                match expr.kind {
+                    thir::ExprKind::NamedConst { substs, .. } => substs.has_param_types_or_consts(),
+                    thir::ExprKind::ConstParam { .. } => true,
+                    thir::ExprKind::Repeat { value, count } => {
+                        self.visit_expr(&self.thir()[value]);
+                        count.has_param_types_or_consts()
+                    }
+                    _ => false,
+                }
+            }
+
+            fn pat_is_poly(&mut self, pat: &thir::Pat<'tcx>) -> bool {
+                if pat.ty.has_param_types_or_consts() {
+                    return true;
+                }
+
+                match pat.kind.as_ref() {
+                    thir::PatKind::Constant { value } => value.has_param_types_or_consts(),
+                    thir::PatKind::Range(thir::PatRange { lo, hi, .. }) => {
+                        lo.has_param_types_or_consts() || hi.has_param_types_or_consts()
+                    }
+                    _ => false,
+                }
+            }
+        }
+
+        impl<'a, 'tcx> visit::Visitor<'a, 'tcx> for IsThirPolymorphic<'a, 'tcx> {
+            fn thir(&self) -> &'a thir::Thir<'tcx> {
+                &self.thir
+            }
+
+            #[instrument(skip(self), level = "debug")]
+            fn visit_expr(&mut self, expr: &thir::Expr<'tcx>) {
+                self.is_poly |= self.expr_is_poly(expr);
+                if !self.is_poly {
+                    visit::walk_expr(self, expr)
+                }
+            }
+
+            #[instrument(skip(self), level = "debug")]
+            fn visit_pat(&mut self, pat: &thir::Pat<'tcx>) {
+                self.is_poly |= self.pat_is_poly(pat);
+                if !self.is_poly {
+                    visit::walk_pat(self, pat);
+                }
+            }
+        }
+
+        let mut is_poly_vis = IsThirPolymorphic { is_poly: false, thir: body };
+        visit::walk_expr(&mut is_poly_vis, &body[body_id]);
+        debug!("AbstractConstBuilder: is_poly={}", is_poly_vis.is_poly);
+        if !is_poly_vis.is_poly {
+            return Ok(None);
+        }
+
+        Ok(Some(builder))
+    }
+
+    /// We do not allow all binary operations in abstract consts, so filter disallowed ones.
+    fn check_binop(op: mir::BinOp) -> bool {
+        use mir::BinOp::*;
+        match op {
+            Add | Sub | Mul | Div | Rem | BitXor | BitAnd | BitOr | Shl | Shr | Eq | Lt | Le
+            | Ne | Ge | Gt => true,
+            Offset => false,
+        }
+    }
+
+    /// While we currently allow all unary operations, we still want to explicitly guard against
+    /// future changes here.
+    fn check_unop(op: mir::UnOp) -> bool {
+        use mir::UnOp::*;
+        match op {
+            Not | Neg => true,
+        }
+    }
+
+    /// Builds the abstract const by walking the thir and bailing out when
+    /// encountering an unsupported operation.
+    pub fn build(mut self) -> Result<&'tcx [Node<'tcx>], ErrorGuaranteed> {
+        debug!("Abstractconstbuilder::build: body={:?}", &*self.body);
+        self.recurse_build(self.body_id)?;
+
+        for n in self.nodes.iter() {
+            if let Node::Leaf(ct) = n {
+                if let ty::ConstKind::Unevaluated(ct) = ct.kind() {
+                    // `AbstractConst`s should not contain any promoteds as they require references which
+                    // are not allowed.
+                    assert_eq!(ct.promoted, None);
+                    assert_eq!(ct, self.tcx.erase_regions(ct));
+                }
+            }
+        }
+
+        Ok(self.tcx.arena.alloc_from_iter(self.nodes.into_iter()))
+    }
+
+    fn recurse_build(&mut self, node: thir::ExprId) -> Result<NodeId, ErrorGuaranteed> {
+        use thir::ExprKind;
+        let node = &self.body.exprs[node];
+        Ok(match &node.kind {
+            // I dont know if handling of these 3 is correct
+            &ExprKind::Scope { value, .. } => self.recurse_build(value)?,
+            &ExprKind::PlaceTypeAscription { source, .. }
+            | &ExprKind::ValueTypeAscription { source, .. } => self.recurse_build(source)?,
+            &ExprKind::Literal { lit, neg} => {
+                let sp = node.span;
+                let constant =
+                    match self.tcx.at(sp).lit_to_const(LitToConstInput { lit: &lit.node, ty: node.ty, neg }) {
+                        Ok(c) => c,
+                        Err(LitToConstError::Reported) => {
+                            self.tcx.const_error(node.ty)
+                        }
+                        Err(LitToConstError::TypeError) => {
+                            bug!("encountered type error in lit_to_const")
+                        }
+                    };
+
+                self.nodes.push(Node::Leaf(constant))
+            }
+            &ExprKind::NonHirLiteral { lit , user_ty: _} => {
+                let val = ty::ValTree::from_scalar_int(lit);
+                self.nodes.push(Node::Leaf(ty::Const::from_value(self.tcx, val, node.ty)))
+            }
+            &ExprKind::ZstLiteral { user_ty: _ } => {
+                let val = ty::ValTree::zst();
+                self.nodes.push(Node::Leaf(ty::Const::from_value(self.tcx, val, node.ty)))
+            }
+            &ExprKind::NamedConst { def_id, substs, user_ty: _ } => {
+                let uneval = ty::Unevaluated::new(ty::WithOptConstParam::unknown(def_id), substs);
+
+                let constant = self.tcx.mk_const(ty::ConstS {
+                                kind: ty::ConstKind::Unevaluated(uneval),
+                                ty: node.ty,
+                            });
+
+                self.nodes.push(Node::Leaf(constant))
+            }
+
+            ExprKind::ConstParam {param, ..} => {
+                let const_param = self.tcx.mk_const(ty::ConstS {
+                        kind: ty::ConstKind::Param(*param),
+                        ty: node.ty,
+                    });
+                self.nodes.push(Node::Leaf(const_param))
+            }
+
+            ExprKind::Call { fun, args, .. } => {
+                let fun = self.recurse_build(*fun)?;
+
+                let mut new_args = Vec::<NodeId>::with_capacity(args.len());
+                for &id in args.iter() {
+                    new_args.push(self.recurse_build(id)?);
+                }
+                let new_args = self.tcx.arena.alloc_slice(&new_args);
+                self.nodes.push(Node::FunctionCall(fun, new_args))
+            }
+            &ExprKind::Binary { op, lhs, rhs } if Self::check_binop(op) => {
+                let lhs = self.recurse_build(lhs)?;
+                let rhs = self.recurse_build(rhs)?;
+                self.nodes.push(Node::Binop(op, lhs, rhs))
+            }
+            &ExprKind::Unary { op, arg } if Self::check_unop(op) => {
+                let arg = self.recurse_build(arg)?;
+                self.nodes.push(Node::UnaryOp(op, arg))
+            }
+            // This is necessary so that the following compiles:
+            //
+            // ```
+            // fn foo<const N: usize>(a: [(); N + 1]) {
+            //     bar::<{ N + 1 }>();
+            // }
+            // ```
+            ExprKind::Block { body: thir::Block { stmts: box [], expr: Some(e), .. } } => {
+                self.recurse_build(*e)?
+            }
+            // `ExprKind::Use` happens when a `hir::ExprKind::Cast` is a
+            // "coercion cast" i.e. using a coercion or is a no-op.
+            // This is important so that `N as usize as usize` doesnt unify with `N as usize`. (untested)
+            &ExprKind::Use { source } => {
+                let arg = self.recurse_build(source)?;
+                self.nodes.push(Node::Cast(CastKind::Use, arg, node.ty))
+            }
+            &ExprKind::Cast { source } => {
+                let arg = self.recurse_build(source)?;
+                self.nodes.push(Node::Cast(CastKind::As, arg, node.ty))
+            }
+            ExprKind::Borrow{ arg, ..} => {
+                let arg_node = &self.body.exprs[*arg];
+
+                // Skip reborrows for now until we allow Deref/Borrow/AddressOf
+                // expressions.
+                // FIXME(generic_const_exprs): Verify/explain why this is sound
+                if let ExprKind::Deref {arg} = arg_node.kind {
+                    self.recurse_build(arg)?
+                } else {
+                    self.maybe_supported_error(
+                        node.span,
+                        "borrowing is not supported in generic constants",
+                    )?
+                }
+            }
+            // FIXME(generic_const_exprs): We may want to support these.
+            ExprKind::AddressOf { .. } | ExprKind::Deref {..}=> self.maybe_supported_error(
+                node.span,
+                "dereferencing or taking the address is not supported in generic constants",
+            )?,
+            ExprKind::Repeat { .. } | ExprKind::Array { .. } =>  self.maybe_supported_error(
+                node.span,
+                "array construction is not supported in generic constants",
+            )?,
+            ExprKind::Block { .. } => self.maybe_supported_error(
+                node.span,
+                "blocks are not supported in generic constant",
+            )?,
+            ExprKind::NeverToAny { .. } => self.maybe_supported_error(
+                node.span,
+                "converting nevers to any is not supported in generic constant",
+            )?,
+            ExprKind::Tuple { .. } => self.maybe_supported_error(
+                node.span,
+                "tuple construction is not supported in generic constants",
+            )?,
+            ExprKind::Index { .. } => self.maybe_supported_error(
+                node.span,
+                "indexing is not supported in generic constant",
+            )?,
+            ExprKind::Field { .. } => self.maybe_supported_error(
+                node.span,
+                "field access is not supported in generic constant",
+            )?,
+            ExprKind::ConstBlock { .. } => self.maybe_supported_error(
+                node.span,
+                "const blocks are not supported in generic constant",
+            )?,
+            ExprKind::Adt(_) => self.maybe_supported_error(
+                node.span,
+                "struct/enum construction is not supported in generic constants",
+            )?,
+            // dont know if this is correct
+            ExprKind::Pointer { .. } =>
+                self.error(node.span, "pointer casts are not allowed in generic constants")?,
+            ExprKind::Yield { .. } =>
+                self.error(node.span, "generator control flow is not allowed in generic constants")?,
+            ExprKind::Continue { .. } | ExprKind::Break { .. } | ExprKind::Loop { .. } => self
+                .error(
+                    node.span,
+                    "loops and loop control flow are not supported in generic constants",
+                )?,
+            ExprKind::Box { .. } =>
+                self.error(node.span, "allocations are not allowed in generic constants")?,
+
+            ExprKind::Unary { .. } => unreachable!(),
+            // we handle valid unary/binary ops above
+            ExprKind::Binary { .. } =>
+                self.error(node.span, "unsupported binary operation in generic constants")?,
+            ExprKind::LogicalOp { .. } =>
+                self.error(node.span, "unsupported operation in generic constants, short-circuiting operations would imply control flow")?,
+            ExprKind::Assign { .. } | ExprKind::AssignOp { .. } => {
+                self.error(node.span, "assignment is not supported in generic constants")?
+            }
+            ExprKind::Closure { .. } | ExprKind::Return { .. } => self.error(
+                node.span,
+                "closures and function keywords are not supported in generic constants",
+            )?,
+            // let expressions imply control flow
+            ExprKind::Match { .. } | ExprKind::If { .. } | ExprKind::Let { .. } =>
+                self.error(node.span, "control flow is not supported in generic constants")?,
+            ExprKind::InlineAsm { .. } => {
+                self.error(node.span, "assembly is not supported in generic constants")?
+            }
+
+            // we dont permit let stmts so `VarRef` and `UpvarRef` cant happen
+            ExprKind::VarRef { .. }
+            | ExprKind::UpvarRef { .. }
+            | ExprKind::StaticRef { .. }
+            | ExprKind::ThreadLocalRef(_) => {
+                self.error(node.span, "unsupported operation in generic constant")?
+            }
+        })
+    }
+}
+
+/// Builds an abstract const, do not use this directly, but use `AbstractConst::new` instead.
+pub fn thir_abstract_const<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    def: ty::WithOptConstParam<LocalDefId>,
+) -> Result<Option<&'tcx [Node<'tcx>]>, ErrorGuaranteed> {
+    if tcx.features().generic_const_exprs {
+        match tcx.def_kind(def.did) {
+            // FIXME(generic_const_exprs): We currently only do this for anonymous constants,
+            // meaning that we do not look into associated constants. I(@lcnr) am not yet sure whether
+            // we want to look into them or treat them as opaque projections.
+            //
+            // Right now we do neither of that and simply always fail to unify them.
+            DefKind::AnonConst | DefKind::InlineConst => (),
+            _ => return Ok(None),
+        }
+
+        let body = tcx.thir_body(def)?;
+
+        AbstractConstBuilder::new(tcx, (&*body.0.borrow(), body.1))?
+            .map(AbstractConstBuilder::build)
+            .transpose()
+    } else {
+        Ok(None)
+    }
+}
+
 pub fn provide(providers: &mut ty::query::Providers) {
-    *providers = ty::query::Providers { destructure_const, ..*providers };
+    *providers = ty::query::Providers {
+        destructure_const,
+        thir_abstract_const: |tcx, def_id| {
+            let def_id = def_id.expect_local();
+            if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
+                tcx.thir_abstract_const_of_const_arg(def)
+            } else {
+                thir_abstract_const(tcx, ty::WithOptConstParam::unknown(def_id))
+            }
+        },
+        thir_abstract_const_of_const_arg: |tcx, (did, param_did)| {
+            thir_abstract_const(
+                tcx,
+                ty::WithOptConstParam { did, const_param_did: Some(param_did) },
+            )
+        },
+        ..*providers
+    };
 }
diff --git a/compiler/rustc_ty_utils/src/lib.rs b/compiler/rustc_ty_utils/src/lib.rs
index 7624d31b40b..09f5c2a11aa 100644
--- a/compiler/rustc_ty_utils/src/lib.rs
+++ b/compiler/rustc_ty_utils/src/lib.rs
@@ -7,6 +7,8 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(control_flow_enum)]
 #![feature(let_else)]
+#![feature(never_type)]
+#![feature(box_patterns)]
 #![recursion_limit = "256"]
 
 #[macro_use]