Revert "Revert "Rollup merge of #98582 - oli-obk:unconstrained_opaque_type, r=estebank""

This reverts commit 4a742a691e7dd2522bad68b86fe2fd5a199d5561.

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -2146,7 +2146,9 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                             }
                             StorageDeadOrDrop::Destructor(_) => kind,
                         },
-                        ProjectionElem::Field(..) | ProjectionElem::Downcast(..) => {
+                        ProjectionElem::OpaqueCast { .. }
+                        | ProjectionElem::Field(..)
+                        | ProjectionElem::Downcast(..) => {
                             match place_ty.ty.kind() {
                                 ty::Adt(def, _) if def.has_dtor(tcx) => {
                                     // Report the outermost adt with a destructor

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -237,6 +237,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 }
                 ProjectionElem::Downcast(..) if opt.including_downcast => return None,
                 ProjectionElem::Downcast(..) => (),
+                ProjectionElem::OpaqueCast(..) => (),
                 ProjectionElem::Field(field, _ty) => {
                     // FIXME(project-rfc_2229#36): print capture precisely here.
                     if let Some(field) = self.is_upvar_field_projection(PlaceRef {
@@ -317,6 +318,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     PlaceRef { local, projection: proj_base }.ty(self.body, self.infcx.tcx)
                 }
                 ProjectionElem::Downcast(..) => place.ty(self.body, self.infcx.tcx),
+                ProjectionElem::OpaqueCast(ty) => PlaceTy::from_ty(*ty),
                 ProjectionElem::Field(_, field_type) => PlaceTy::from_ty(*field_type),
             },
         };

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -169,6 +169,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                         ..,
                         ProjectionElem::Index(_)
                         | ProjectionElem::ConstantIndex { .. }
+                        | ProjectionElem::OpaqueCast { .. }
                         | ProjectionElem::Subslice { .. }
                         | ProjectionElem::Downcast(..),
                     ],

compiler/rustc_borrowck/src/lib.rs

@@ -1781,6 +1781,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         for (place_base, elem) in place.iter_projections().rev() {
             match elem {
                 ProjectionElem::Index(_/*operand*/) |
+                ProjectionElem::OpaqueCast(_) |
                 ProjectionElem::ConstantIndex { .. } |
                 // assigning to P[i] requires P to be valid.
                 ProjectionElem::Downcast(_/*adt_def*/, _/*variant_idx*/) =>
@@ -2172,6 +2173,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     | ProjectionElem::Index(..)
                     | ProjectionElem::ConstantIndex { .. }
                     | ProjectionElem::Subslice { .. }
+                    | ProjectionElem::OpaqueCast { .. }
                     | ProjectionElem::Downcast(..) => {
                         let upvar_field_projection = self.is_upvar_field_projection(place);
                         if let Some(field) = upvar_field_projection {

compiler/rustc_borrowck/src/places_conflict.rs

@@ -250,6 +250,7 @@ fn place_components_conflict<'tcx>(
                 | (ProjectionElem::Index { .. }, _, _)
                 | (ProjectionElem::ConstantIndex { .. }, _, _)
                 | (ProjectionElem::Subslice { .. }, _, _)
+                | (ProjectionElem::OpaqueCast { .. }, _, _)
                 | (ProjectionElem::Downcast { .. }, _, _) => {
                     // Recursive case. This can still be disjoint on a
                     // further iteration if this a shallow access and
@@ -317,6 +318,17 @@ fn place_projection_conflict<'tcx>(
             debug!("place_element_conflict: DISJOINT-OR-EQ-DEREF");
             Overlap::EqualOrDisjoint
         }
+        (ProjectionElem::OpaqueCast(v1), ProjectionElem::OpaqueCast(v2)) => {
+            if v1 == v2 {
+                // same type - recur.
+                debug!("place_element_conflict: DISJOINT-OR-EQ-OPAQUE");
+                Overlap::EqualOrDisjoint
+            } else {
+                // Different types. Disjoint!
+                debug!("place_element_conflict: DISJOINT-OPAQUE");
+                Overlap::Disjoint
+            }
+        }
         (ProjectionElem::Field(f1, _), ProjectionElem::Field(f2, _)) => {
             if f1 == f2 {
                 // same field (e.g., `a.y` vs. `a.y`) - recur.
@@ -520,6 +532,7 @@ fn place_projection_conflict<'tcx>(
             | ProjectionElem::Field(..)
             | ProjectionElem::Index(..)
             | ProjectionElem::ConstantIndex { .. }
+            | ProjectionElem::OpaqueCast { .. }
             | ProjectionElem::Subslice { .. }
             | ProjectionElem::Downcast(..),
             _,

compiler/rustc_borrowck/src/prefixes.rs

@@ -81,6 +81,7 @@ impl<'cx, 'tcx> Iterator for Prefixes<'cx, 'tcx> {
                         }
                         ProjectionElem::Downcast(..)
                         | ProjectionElem::Subslice { .. }
+                        | ProjectionElem::OpaqueCast { .. }
                         | ProjectionElem::ConstantIndex { .. }
                         | ProjectionElem::Index(_) => {
                             cursor = cursor_base;

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -760,6 +760,19 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
                 }
                 PlaceTy::from_ty(fty)
             }
+            ProjectionElem::OpaqueCast(ty) => {
+                let ty = self.sanitize_type(place, ty);
+                let ty = self.cx.normalize(ty, location);
+                self.cx
+                    .eq_types(
+                        base.ty,
+                        ty,
+                        location.to_locations(),
+                        ConstraintCategory::TypeAnnotation,
+                    )
+                    .unwrap();
+                PlaceTy::from_ty(ty)
+            }
         }
     }
 
@@ -1166,10 +1179,11 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                 tcx,
                 self.param_env,
                 proj,
-                |this, field, ()| {
+                |this, field, _| {
                     let ty = this.field_ty(tcx, field);
                     self.normalize(ty, locations)
                 },
+                |_, _| unreachable!(),
             );
             curr_projected_ty = projected_ty;
         }
@@ -2501,6 +2515,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                 }
                 ProjectionElem::Field(..)
                 | ProjectionElem::Downcast(..)
+                | ProjectionElem::OpaqueCast(..)
                 | ProjectionElem::Index(..)
                 | ProjectionElem::ConstantIndex { .. }
                 | ProjectionElem::Subslice { .. } => {

compiler/rustc_codegen_cranelift/src/base.rs

@@ -850,6 +850,7 @@ pub(crate) fn codegen_place<'tcx>(
             PlaceElem::Deref => {
                 cplace = cplace.place_deref(fx);
             }
+            PlaceElem::OpaqueCast(ty) => cplace = cplace.place_opaque_cast(fx, ty),
             PlaceElem::Field(field, _ty) => {
                 cplace = cplace.place_field(fx, field);
             }

compiler/rustc_codegen_cranelift/src/value_and_place.rs

@@ -621,6 +621,14 @@ impl<'tcx> CPlace<'tcx> {
         }
     }
 
+    pub(crate) fn place_opaque_cast(
+        self,
+        fx: &mut FunctionCx<'_, '_, 'tcx>,
+        ty: Ty<'tcx>,
+    ) -> CPlace<'tcx> {
+        CPlace { inner: self.inner, layout: fx.layout_of(ty) }
+    }
+
     pub(crate) fn place_field(
         self,
         fx: &mut FunctionCx<'_, '_, 'tcx>,

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -400,6 +400,21 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
         downcast
     }
 
+    pub fn project_type<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
+        &self,
+        bx: &mut Bx,
+        ty: Ty<'tcx>,
+    ) -> Self {
+        let mut downcast = *self;
+        downcast.layout = bx.cx().layout_of(ty);
+
+        // Cast to the appropriate type.
+        let variant_ty = bx.cx().backend_type(downcast.layout);
+        downcast.llval = bx.pointercast(downcast.llval, bx.cx().type_ptr_to(variant_ty));
+
+        downcast
+    }
+
     pub fn storage_live<Bx: BuilderMethods<'a, 'tcx, Value = V>>(&self, bx: &mut Bx) {
         bx.lifetime_start(self.llval, self.layout.size);
     }
@@ -442,6 +457,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 mir::ProjectionElem::Field(ref field, _) => {
                     cg_base.project_field(bx, field.index())
                 }
+                mir::ProjectionElem::OpaqueCast(ty) => cg_base.project_type(bx, ty),
                 mir::ProjectionElem::Index(index) => {
                     let index = &mir::Operand::Copy(mir::Place::from(index));
                     let index = self.codegen_operand(bx, index);

compiler/rustc_const_eval/src/interpret/projection.rs

@@ -350,6 +350,11 @@ where
     ) -> InterpResult<'tcx, PlaceTy<'tcx, M::Provenance>> {
         use rustc_middle::mir::ProjectionElem::*;
         Ok(match proj_elem {
+            OpaqueCast(ty) => {
+                let mut place = base.clone();
+                place.layout = self.layout_of(ty)?;
+                place
+            }
             Field(field, _) => self.place_field(base, field.index())?,
             Downcast(_, variant) => self.place_downcast(base, variant)?,
             Deref => self.deref_operand(&self.place_to_op(base)?)?.into(),
@@ -374,6 +379,11 @@ where
     ) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
         use rustc_middle::mir::ProjectionElem::*;
         Ok(match proj_elem {
+            OpaqueCast(ty) => {
+                let mut op = base.clone();
+                op.layout = self.layout_of(ty)?;
+                op
+            }
             Field(field, _) => self.operand_field(base, field.index())?,
             Downcast(_, variant) => self.operand_downcast(base, variant)?,
             Deref => self.deref_operand(base)?.into(),

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -656,6 +656,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
 
             ProjectionElem::ConstantIndex { .. }
             | ProjectionElem::Downcast(..)
+            | ProjectionElem::OpaqueCast(..)
             | ProjectionElem::Subslice { .. }
             | ProjectionElem::Field(..)
             | ProjectionElem::Index(_) => {}

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -308,6 +308,7 @@ where
 
             ProjectionElem::Deref
             | ProjectionElem::Field(_, _)
+            | ProjectionElem::OpaqueCast(_)
             | ProjectionElem::ConstantIndex { .. }
             | ProjectionElem::Subslice { .. }
             | ProjectionElem::Downcast(_, _)

compiler/rustc_const_eval/src/transform/promote_consts.rs

@@ -361,7 +361,7 @@ impl<'tcx> Validator<'_, 'tcx> {
                             return Err(Unpromotable);
                         }
                     }
-                    ProjectionElem::Downcast(..) => {
+                    ProjectionElem::OpaqueCast(..) | ProjectionElem::Downcast(..) => {
                         return Err(Unpromotable);
                     }
 

compiler/rustc_middle/src/mir/mod.rs

@@ -1380,6 +1380,7 @@ impl<V, T> ProjectionElem<V, T> {
 
             Self::Field(_, _)
             | Self::Index(_)
+            | Self::OpaqueCast(_)
             | Self::ConstantIndex { .. }
             | Self::Subslice { .. }
             | Self::Downcast(_, _) => false,
@@ -1574,7 +1575,9 @@ impl Debug for Place<'_> {
     fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
         for elem in self.projection.iter().rev() {
             match elem {
-                ProjectionElem::Downcast(_, _) | ProjectionElem::Field(_, _) => {
+                ProjectionElem::OpaqueCast(_)
+                | ProjectionElem::Downcast(_, _)
+                | ProjectionElem::Field(_, _) => {
                     write!(fmt, "(").unwrap();
                 }
                 ProjectionElem::Deref => {
@@ -1590,6 +1593,9 @@ impl Debug for Place<'_> {
 
         for elem in self.projection.iter() {
             match elem {
+                ProjectionElem::OpaqueCast(ty) => {
+                    write!(fmt, " as {})", ty)?;
+                }
                 ProjectionElem::Downcast(Some(name), _index) => {
                     write!(fmt, " as {})", name)?;
                 }

compiler/rustc_middle/src/mir/syntax.rs

@@ -829,6 +829,9 @@ pub type AssertMessage<'tcx> = AssertKind<Operand<'tcx>>;
 ///    generator has more than one variant, the parent place's variant index must be set, indicating
 ///    which variant is being used. If it has just one variant, the variant index may or may not be
 ///    included - the single possible variant is inferred if it is not included.
+///  - [`OpaqueCast`](ProjectionElem::OpaqueCast): This projection changes the place's type to the
+///    given one, and makes no other changes. A `OpaqueCast` projection on any type other than an
+///    opaque type from the current crate is not well-formed.
 ///  - [`ConstantIndex`](ProjectionElem::ConstantIndex): Computes an offset in units of `T` into the
 ///    place as described in the documentation for the `ProjectionElem`. The resulting address is
 ///    the parent's address plus that offset, and the type is `T`. This is only legal if the parent
@@ -928,6 +931,10 @@ pub enum ProjectionElem<V, T> {
     ///
     /// The included Symbol is the name of the variant, used for printing MIR.
     Downcast(Option<Symbol>, VariantIdx),
+
+    /// Like an explicit cast from an opaque type to a concrete type, but without
+    /// requiring an intermediate variable.
+    OpaqueCast(T),
 }
 
 /// Alias for projections as they appear in places, where the base is a place

compiler/rustc_middle/src/mir/tcx.rs

@@ -57,7 +57,7 @@ impl<'tcx> PlaceTy<'tcx> {
     /// `PlaceElem`, where we can just use the `Ty` that is already
     /// stored inline on field projection elems.
     pub fn projection_ty(self, tcx: TyCtxt<'tcx>, elem: PlaceElem<'tcx>) -> PlaceTy<'tcx> {
-        self.projection_ty_core(tcx, ty::ParamEnv::empty(), &elem, |_, _, ty| ty)
+        self.projection_ty_core(tcx, ty::ParamEnv::empty(), &elem, |_, _, ty| ty, |_, ty| ty)
     }
 
     /// `place_ty.projection_ty_core(tcx, elem, |...| { ... })`
@@ -71,6 +71,7 @@ impl<'tcx> PlaceTy<'tcx> {
         param_env: ty::ParamEnv<'tcx>,
         elem: &ProjectionElem<V, T>,
         mut handle_field: impl FnMut(&Self, Field, T) -> Ty<'tcx>,
+        mut handle_opaque_cast: impl FnMut(&Self, T) -> Ty<'tcx>,
     ) -> PlaceTy<'tcx>
     where
         V: ::std::fmt::Debug,
@@ -109,6 +110,7 @@ impl<'tcx> PlaceTy<'tcx> {
                 PlaceTy { ty: self.ty, variant_index: Some(index) }
             }
             ProjectionElem::Field(f, fty) => PlaceTy::from_ty(handle_field(&self, f, fty)),
+            ProjectionElem::OpaqueCast(ty) => PlaceTy::from_ty(handle_opaque_cast(&self, ty)),
         };
         debug!("projection_ty self: {:?} elem: {:?} yields: {:?}", self, elem, answer);
         answer

compiler/rustc_middle/src/mir/visit.rs

@@ -1094,6 +1094,11 @@ macro_rules! visit_place_fns {
                     self.visit_ty(&mut new_ty, TyContext::Location(location));
                     if ty != new_ty { Some(PlaceElem::Field(field, new_ty)) } else { None }
                 }
+                PlaceElem::OpaqueCast(ty) => {
+                    let mut new_ty = ty;
+                    self.visit_ty(&mut new_ty, TyContext::Location(location));
+                    if ty != new_ty { Some(PlaceElem::OpaqueCast(new_ty)) } else { None }
+                }
                 PlaceElem::Deref
                 | PlaceElem::ConstantIndex { .. }
                 | PlaceElem::Subslice { .. }
@@ -1163,7 +1168,7 @@ macro_rules! visit_place_fns {
             location: Location,
         ) {
             match elem {
-                ProjectionElem::Field(_field, ty) => {
+                ProjectionElem::OpaqueCast(ty) | ProjectionElem::Field(_, ty) => {
                     self.visit_ty(ty, TyContext::Location(location));
                 }
                 ProjectionElem::Index(local) => {

compiler/rustc_mir_build/src/build/expr/as_place.rs

@@ -102,6 +102,8 @@ fn convert_to_hir_projections_and_truncate_for_capture<'tcx>(
                 variant = Some(*idx);
                 continue;
             }
+            // These do not affect anything, they just make sure we know the right type.
+            ProjectionElem::OpaqueCast(_) => continue,
             ProjectionElem::Index(..)
             | ProjectionElem::ConstantIndex { .. }
             | ProjectionElem::Subslice { .. } => {
@@ -168,7 +170,7 @@ fn find_capture_matching_projections<'a, 'tcx>(
 /// `PlaceBuilder` now starts from `PlaceBase::Local`.
 ///
 /// Returns a Result with the error being the PlaceBuilder (`from_builder`) that was not found.
-#[instrument(level = "trace", skip(cx))]
+#[instrument(level = "trace", skip(cx), ret)]
 fn to_upvars_resolved_place_builder<'tcx>(
     from_builder: PlaceBuilder<'tcx>,
     cx: &Builder<'_, 'tcx>,
@@ -213,7 +215,6 @@ fn to_upvars_resolved_place_builder<'tcx>(
                 &capture.captured_place.place.projections,
             );
             upvar_resolved_place_builder.projection.extend(remaining_projections);
-            trace!(?upvar_resolved_place_builder);
 
             Ok(upvar_resolved_place_builder)
         }
@@ -232,16 +233,21 @@ fn strip_prefix<'tcx>(
     prefix_projections: &[HirProjection<'tcx>],
 ) -> impl Iterator<Item = PlaceElem<'tcx>> {
     let mut iter = projections.into_iter();
+    let mut next = || match iter.next()? {
+        // Filter out opaque casts, they are unnecessary in the prefix.
+        ProjectionElem::OpaqueCast(..) => iter.next(),
+        other => Some(other),
+    };
     for projection in prefix_projections {
         match projection.kind {
             HirProjectionKind::Deref => {
-                assert!(matches!(iter.next(), Some(ProjectionElem::Deref)));
+                assert!(matches!(next(), Some(ProjectionElem::Deref)));
             }
             HirProjectionKind::Field(..) => {
                 if base_ty.is_enum() {
-                    assert!(matches!(iter.next(), Some(ProjectionElem::Downcast(..))));
+                    assert!(matches!(next(), Some(ProjectionElem::Downcast(..))));
                 }
-                assert!(matches!(iter.next(), Some(ProjectionElem::Field(..))));
+                assert!(matches!(next(), Some(ProjectionElem::Field(..))));
             }
             HirProjectionKind::Index | HirProjectionKind::Subslice => {
                 bug!("unexpected projection kind: {:?}", projection);
@@ -711,6 +717,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     }
                     ProjectionElem::Field(..)
                     | ProjectionElem::Downcast(..)
+                    | ProjectionElem::OpaqueCast(..)
                     | ProjectionElem::ConstantIndex { .. }
                     | ProjectionElem::Subslice { .. } => (),
                 }

compiler/rustc_mir_build/src/build/matches/util.rs

@@ -95,9 +95,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
 impl<'pat, 'tcx> MatchPair<'pat, 'tcx> {
     pub(in crate::build) fn new(
-        place: PlaceBuilder<'tcx>,
+        mut place: PlaceBuilder<'tcx>,
         pattern: &'pat Pat<'tcx>,
     ) -> MatchPair<'pat, 'tcx> {
+        // Force the place type to the pattern's type.
+        // FIXME(oli-obk): can we use this to simplify slice/array pattern hacks?
+        // FIXME(oli-obk): only add this projection if `place` actually had an opaque
+        // type before the projection.
+        place = place.project(ProjectionElem::OpaqueCast(pattern.ty));
         MatchPair { place, pattern }
     }
 }

compiler/rustc_mir_build/src/thir/pattern/usefulness.rs

@@ -842,7 +842,15 @@ fn is_useful<'p, 'tcx>(
             }
         }
     } else {
-        let ty = v.head().ty();
+        let mut ty = v.head().ty();
+
+        // Opaque types can't get destructured/split, but the patterns can
+        // actually hint at hidden types, so we use the patterns' types instead.
+        if let ty::Opaque(..) = ty.kind() {
+            if let Some(row) = rows.first() {
+                ty = row.head().ty();
+            }
+        }
         let is_non_exhaustive = cx.is_foreign_non_exhaustive_enum(ty);
         debug!("v.head: {:?}, v.span: {:?}", v.head(), v.head().span());
         let pcx = &PatCtxt { cx, ty, span: v.head().span(), is_top_level, is_non_exhaustive };

compiler/rustc_mir_dataflow/src/move_paths/abs_domain.rs

@@ -48,6 +48,7 @@ impl<'tcx> Lift for PlaceElem<'tcx> {
         match *self {
             ProjectionElem::Deref => ProjectionElem::Deref,
             ProjectionElem::Field(f, ty) => ProjectionElem::Field(f, ty.lift()),
+            ProjectionElem::OpaqueCast(ty) => ProjectionElem::OpaqueCast(ty.lift()),
             ProjectionElem::Index(ref i) => ProjectionElem::Index(i.lift()),
             ProjectionElem::Subslice { from, to, from_end } => {
                 ProjectionElem::Subslice { from, to, from_end }

src/test/ui/type-alias-impl-trait/cross_inference_pattern_bug.rs

@@ -1,8 +1,9 @@
 // compile-flags: --edition=2021
+// check-pass
 #![feature(type_alias_impl_trait)]
 
 fn main() {
-    type T = impl Copy; //~ ERROR unconstrained opaque type
+    type T = impl Copy;
     let foo: T = (1u32, 2u32);
     let (a, b): (u32, u32) = foo;
 }

src/test/ui/type-alias-impl-trait/cross_inference_pattern_bug.stderr

@@ -1,10 +0,0 @@
-error: unconstrained opaque type
-  --> $DIR/cross_inference_pattern_bug.rs:5:14
-   |
-LL |     type T = impl Copy;
-   |              ^^^^^^^^^
-   |
-   = note: `T` must be used in combination with a concrete type within the same module
-
-error: aborting due to previous error
-

src/test/ui/type-alias-impl-trait/cross_inference_pattern_bug_no_type.rs

@@ -1,13 +1,13 @@
-// known-bug: #96572
 // compile-flags: --edition=2021 --crate-type=lib
 // rustc-env:RUST_BACKTRACE=0
+// check-pass
 
 // tracked in https://github.com/rust-lang/rust/issues/96572
 
 #![feature(type_alias_impl_trait)]
 
 fn main() {
-    type T = impl Copy;  // error: unconstrained opaque type
+    type T = impl Copy;
     let foo: T = (1u32, 2u32);
-    let (a, b) = foo; // removing this line makes the code compile
+    let (a, b) = foo; // this line used to make the code fail
 }

src/test/ui/type-alias-impl-trait/cross_inference_pattern_bug_no_type.stderr

@@ -1,10 +0,0 @@
-error: unconstrained opaque type
-  --> $DIR/cross_inference_pattern_bug_no_type.rs:10:14
-   |
-LL |     type T = impl Copy;  // error: unconstrained opaque type
-   |              ^^^^^^^^^
-   |
-   = note: `T` must be used in combination with a concrete type within the same module
-
-error: aborting due to previous error
-

src/test/ui/type-alias-impl-trait/issue-96572-unconstrained-mismatch.rs

@@ -0,0 +1,10 @@
+#![feature(type_alias_impl_trait)]
+
+fn main() {
+    type T = impl Copy;
+    let foo: T = Some((1u32, 2u32));
+    match foo {
+        None => (),
+        Some((a, b, c)) => (), //~ ERROR mismatched types
+    }
+}

src/test/ui/type-alias-impl-trait/issue-96572-unconstrained-mismatch.stderr

@@ -0,0 +1,15 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-96572-unconstrained-mismatch.rs:8:14
+   |
+LL |     match foo {
+   |           --- this expression has type `T`
+LL |         None => (),
+LL |         Some((a, b, c)) => (),
+   |              ^^^^^^^^^ expected a tuple with 2 elements, found one with 3 elements
+   |
+   = note: expected tuple `(u32, u32)`
+              found tuple `(_, _, _)`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/type-alias-impl-trait/issue-96572-unconstrained.rs

@@ -0,0 +1,92 @@
+#![feature(type_alias_impl_trait)]
+// check-pass
+
+fn main() {
+    type T = impl Copy;
+    let foo: T = Some((1u32, 2u32));
+    match foo {
+        None => (),
+        Some((a, b)) => (),
+    }
+}
+
+fn upvar() {
+    #[derive(Copy, Clone)]
+    struct Foo((u32, u32));
+
+    type T = impl Copy;
+    let foo: T = Foo((1u32, 2u32));
+    let x = move || {
+        let Foo((a, b)) = foo;
+    };
+}
+
+fn enum_upvar() {
+    type T = impl Copy;
+    let foo: T = Some((1u32, 2u32));
+    let x = move || {
+        match foo {
+            None => (),
+            Some((a, b)) => (),
+        }
+    };
+}
+
+fn r#struct() {
+    #[derive(Copy, Clone)]
+    struct Foo((u32, u32));
+
+    type U = impl Copy;
+    let foo: U = Foo((1u32, 2u32));
+    let Foo((a, b)) = foo;
+}
+
+mod only_pattern {
+    type T = impl Copy;
+
+    fn foo(foo: T) {
+        let (mut x, mut y) = foo;
+        x = 42;
+        y = "foo";
+    }
+
+    type U = impl Copy;
+
+    fn bar(bar: Option<U>) {
+        match bar {
+            Some((mut x, mut y)) => {
+                x = 42;
+                y = "foo";
+            }
+            None => {}
+        }
+    }
+}
+
+mod only_pattern_rpit {
+    #[allow(unconditional_recursion)]
+    fn foo(b: bool) -> impl Copy {
+        let (mut x, mut y) = foo(false);
+        x = 42;
+        y = "foo";
+        if b {
+            panic!()
+        } else {
+            foo(true)
+        }
+    }
+
+    fn bar(b: bool) -> Option<impl Copy> {
+        if b {
+            return None;
+        }
+        match bar(!b) {
+            Some((mut x, mut y)) => {
+                x = 42;
+                y = "foo";
+            }
+            None => {}
+        }
+        None
+    }
+}

src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs

@@ -260,6 +260,7 @@ fn check_place<'tcx>(tcx: TyCtxt<'tcx>, place: Place<'tcx>, span: Span, body: &B
                 }
             },
             ProjectionElem::ConstantIndex { .. }
+            | ProjectionElem::OpaqueCast(..)
             | ProjectionElem::Downcast(..)
             | ProjectionElem::Subslice { .. }
             | ProjectionElem::Deref