Auto merge of #97019 - b-naber:transition-to-valtrees-pt1, r=oli-obk

Transition to valtrees pt1

Compartmentalising https://github.com/rust-lang/rust/pull/96591 as much as possible.

r? `@oli-obk`

compiler/rustc_middle/src/mir/mod.rs

@@ -3,7 +3,7 @@
 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/mir/index.html
 
 use crate::mir::coverage::{CodeRegion, CoverageKind};
-use crate::mir::interpret::{ConstAllocation, ConstValue, GlobalAlloc, Scalar};
+use crate::mir::interpret::{ConstAllocation, ConstValue, GlobalAlloc, LitToConstInput, Scalar};
 use crate::mir::visit::MirVisitable;
 use crate::ty::adjustment::PointerCast;
 use crate::ty::codec::{TyDecoder, TyEncoder};
@@ -3075,6 +3075,58 @@ impl<'tcx> ConstantKind<'tcx> {
         Self::from_opt_const_arg_anon_const(tcx, ty::WithOptConstParam::unknown(def_id), param_env)
     }
 
+    #[instrument(skip(tcx), level = "debug")]
+    pub fn from_inline_const(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Self {
+        let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
+        let body_id = match tcx.hir().get(hir_id) {
+            hir::Node::AnonConst(ac) => ac.body,
+            _ => span_bug!(
+                tcx.def_span(def_id.to_def_id()),
+                "from_inline_const can only process anonymous constants"
+            ),
+        };
+        let expr = &tcx.hir().body(body_id).value;
+        let ty = tcx.typeck(def_id).node_type(hir_id);
+
+        let lit_input = match expr.kind {
+            hir::ExprKind::Lit(ref lit) => Some(LitToConstInput { lit: &lit.node, ty, neg: false }),
+            hir::ExprKind::Unary(hir::UnOp::Neg, ref expr) => match expr.kind {
+                hir::ExprKind::Lit(ref lit) => {
+                    Some(LitToConstInput { lit: &lit.node, ty, neg: true })
+                }
+                _ => None,
+            },
+            _ => None,
+        };
+        if let Some(lit_input) = lit_input {
+            // If an error occurred, ignore that it's a literal and leave reporting the error up to
+            // mir.
+            match tcx.at(expr.span).lit_to_mir_constant(lit_input) {
+                Ok(c) => return c,
+                Err(_) => {}
+            }
+        }
+
+        let typeck_root_def_id = tcx.typeck_root_def_id(def_id.to_def_id());
+        let parent_substs =
+            tcx.erase_regions(InternalSubsts::identity_for_item(tcx, typeck_root_def_id));
+        let substs =
+            ty::InlineConstSubsts::new(tcx, ty::InlineConstSubstsParts { parent_substs, ty })
+                .substs;
+        let uneval_const = tcx.mk_const(ty::ConstS {
+            val: ty::ConstKind::Unevaluated(ty::Unevaluated {
+                def: ty::WithOptConstParam::unknown(def_id).to_global(),
+                substs,
+                promoted: None,
+            }),
+            ty,
+        });
+        debug!(?uneval_const);
+        debug_assert!(!uneval_const.has_free_regions());
+
+        Self::Ty(uneval_const)
+    }
+
     #[instrument(skip(tcx), level = "debug")]
     fn from_opt_const_arg_anon_const(
         tcx: TyCtxt<'tcx>,