Auto merge of #111913 - oli-obk:valtrees2, r=lcnr

Only rewrite valtree-constants to patterns and keep other constants opaque

Now that we can reliably fall back to comparing constants with `PartialEq::eq` to the match scrutinee, we can

1. eagerly try to convert constants to valtrees
2. then deeply convert the valtree to a pattern
3. if the to-valtree conversion failed, create an "opaque constant" pattern.

This PR specifically avoids any behavioral changes or major cleanups. What we can now do as follow ups is

* move the two remaining call sites to `destructure_mir_constant` off that query
* make valtree to pattern conversion infallible
    * this needs to be done after careful analysis of the effects. There may be user visible changes from that.

based on https://github.com/rust-lang/rust/pull/111768

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::interpret::{
-    AllocRange, ConstAllocation, ConstValue, ErrorHandled, GlobalAlloc, LitToConstInput, Scalar,
+    AllocRange, ConstAllocation, ConstValue, ErrorHandled, GlobalAlloc, Scalar,
 };
 use crate::mir::visit::MirVisitable;
 use crate::ty::codec::{TyDecoder, TyEncoder};
@@ -2461,51 +2461,6 @@ impl<'tcx> ConstantKind<'tcx> {
         Self::Val(val, ty)
     }
 
-    #[instrument(skip(tcx), level = "debug", ret)]
-    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 = UnevaluatedConst { def: def_id.to_def_id(), substs, promoted: None };
-        debug_assert!(!uneval.has_free_regions());
-
-        Self::Unevaluated(uneval, ty)
-    }
-
     /// Literals are converted to `ConstantKindVal`, const generic parameters are eagerly
     /// converted to a constant, everything else becomes `Unevaluated`.
     #[instrument(skip(tcx), level = "debug", ret)]