From 961c7463d8df79bef545af0c07a534aabfd258c2 Mon Sep 17 00:00:00 2001
From: Nadrieril <nadrieril+git@gmail.com>
Date: Sat, 29 Mar 2025 15:13:14 +0100
Subject: [PATCH] Add the inline const type annotation in pattern lowering

---
 .../src/builder/matches/match_pair.rs         | 30 +---------------
 .../rustc_mir_build/src/thir/pattern/mod.rs   | 36 +++++++++++++++++--
 2 files changed, 35 insertions(+), 31 deletions(-)

diff --git a/compiler/rustc_mir_build/src/builder/matches/match_pair.rs b/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
index c84664bd68e3..fcfacf1e1f86 100644
--- a/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
@@ -1,6 +1,5 @@
 use std::sync::Arc;
 
-use rustc_hir::def::DefKind;
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;
 use rustc_middle::ty::{self, Ty, TypeVisitableExt};
@@ -202,35 +201,8 @@ impl<'tcx> MatchPairTree<'tcx> {
                 None
             }
 
-            PatKind::ExpandedConstant { subpattern: ref pattern, def_id } => {
+            PatKind::ExpandedConstant { subpattern: ref pattern, .. } => {
                 MatchPairTree::for_pattern(place_builder, pattern, cx, &mut subpairs, extra_data);
-
-                // Apply a type ascription for the inline constant to the value at `match_pair.place`
-                if let Some(source) = place
-                    && matches!(cx.tcx.def_kind(def_id), DefKind::InlineConst)
-                {
-                    let span = pattern.span;
-                    let parent_id = cx.tcx.typeck_root_def_id(cx.def_id.to_def_id());
-                    let args = ty::InlineConstArgs::new(
-                        cx.tcx,
-                        ty::InlineConstArgsParts {
-                            parent_args: ty::GenericArgs::identity_for_item(cx.tcx, parent_id),
-                            ty: cx.infcx.next_ty_var(span),
-                        },
-                    )
-                    .args;
-                    let user_ty = cx.infcx.canonicalize_user_type_annotation(ty::UserType::new(
-                        ty::UserTypeKind::TypeOf(def_id, ty::UserArgs { args, user_self_ty: None }),
-                    ));
-                    let annotation = ty::CanonicalUserTypeAnnotation {
-                        inferred_ty: pattern.ty,
-                        span,
-                        user_ty: Box::new(user_ty),
-                    };
-                    let variance = ty::Contravariant;
-                    extra_data.ascriptions.push(super::Ascription { annotation, source, variance });
-                }
-
                 None
             }
 
diff --git a/compiler/rustc_mir_build/src/thir/pattern/mod.rs b/compiler/rustc_mir_build/src/thir/pattern/mod.rs
index 153a3b8a7298..7da0c872dfe9 100644
--- a/compiler/rustc_mir_build/src/thir/pattern/mod.rs
+++ b/compiler/rustc_mir_build/src/thir/pattern/mod.rs
@@ -13,12 +13,15 @@ use rustc_hir::def::{CtorOf, DefKind, Res};
 use rustc_hir::pat_util::EnumerateAndAdjustIterator;
 use rustc_hir::{self as hir, LangItem, RangeEnd};
 use rustc_index::Idx;
+use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::interpret::LitToConstInput;
 use rustc_middle::thir::{
     Ascription, FieldPat, LocalVarId, Pat, PatKind, PatRange, PatRangeBoundary,
 };
 use rustc_middle::ty::layout::IntegerExt;
-use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty, TyCtxt, TypeVisitableExt};
+use rustc_middle::ty::{
+    self, CanonicalUserTypeAnnotation, Ty, TyCtxt, TypeVisitableExt, TypingMode,
+};
 use rustc_middle::{bug, span_bug};
 use rustc_span::def_id::DefId;
 use rustc_span::{ErrorGuaranteed, Span};
@@ -605,7 +608,36 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
 
         let ct = ty::UnevaluatedConst { def: def_id.to_def_id(), args };
         let c = ty::Const::new_unevaluated(self.tcx, ct);
-        self.const_to_pat(c, ty, id, span).kind
+        let pattern = self.const_to_pat(c, ty, id, span);
+
+        // Apply a type ascription for the inline constant.
+        // FIXME: reusing the `args` above causes an ICE
+        let annotation = {
+            let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
+            let args = ty::InlineConstArgs::new(
+                tcx,
+                ty::InlineConstArgsParts {
+                    parent_args: ty::GenericArgs::identity_for_item(tcx, typeck_root_def_id),
+                    ty: infcx.next_ty_var(span),
+                },
+            )
+            .args;
+            infcx.canonicalize_user_type_annotation(ty::UserType::new(ty::UserTypeKind::TypeOf(
+                def_id.to_def_id(),
+                ty::UserArgs { args, user_self_ty: None },
+            )))
+        };
+        let annotation =
+            CanonicalUserTypeAnnotation { user_ty: Box::new(annotation), span, inferred_ty: ty };
+        PatKind::AscribeUserType {
+            subpattern: pattern,
+            ascription: Ascription {
+                annotation,
+                // Note that we use `Contravariant` here. See the `variance` field documentation
+                // for details.
+                variance: ty::Contravariant,
+            },
+        }
     }
 
     /// Lowers the kinds of "expression" that can appear in a HIR pattern: